1. 4.3 Theoretical genetics

2. 4.3 Theoretical genetics: Objectives
1- Define genotype, phenotype, dominant allele, recessive allele, codominant alleles, locus, homozygous, heterozygous, carrier and test cross.
2- Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a Punnett grid.
3- State that some genes have more than two alleles (multiple alleles).
4- Describe ABO blood groups as an example of codominance and multiple alleles.
5- Explain how the sex chromosomes control gender by referring to the inheritance of X and Y chromosomes in humans.

3. 6- State that some genes are present on the X chromosome and absent from the shorter Y chromosome in humans.
7- Define sex linkage.
8- Describe the inheritance of color blindness and hemophilia as examples of sex linkage.
9- State that a human female can be homozygous or heterozygous with respect to sex-linked genes.
10- Explain that female carriers are heterozygous for X-linked recessive alleles.
11- Predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving any of the above patterns of inheritance.
12- Deduce the genotypes and phenotypes of individuals in pedigree charts.
Heredity SL.pptx
Heredity Student version.pptx
heredity.ppt

4. Definitions
Allele: an alternative form of a gene, occupying a specific locus. Genotype: the genetic constitution of an organism Phenotype: the characteristics or appearance (structural, biochemical ..) of an organism Dominant allele: is the allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state. recessive allele: an allele that has an effect on the phenotype only when present in the homozygous state. codominant alleles: Pairs of allele that both affect phenotype when present in a heterozygous state. locus: particular position on the homologous chromosomes of a gene Homozygous: having two identical alleles of a gene heterozygous: having two different alleles of a gene. carrier: an individual that has one copy of a recessive allele that causes a genetic diseases in individuals that are homozygous. test cross: testing a suspected heterozygote by crossing it with a known homozygous recessive.

5. Flower color Purple White Flower position Axial Terminal Seed color
Flower color
Purple
White
Flower position
Axial
Terminal
Seed color
Yellow
Green
Seed shape
Round
Wrinkled
Pod shape
Inflated
Constricted
Figure 9.2D The seven pea characters studied by Mendel.
Mendel studied seven characteristics for pea plants. Later studies have shown that pea plants have seven pairs of chromosomes, and each of these characteristics is on a different chromosome. This explains why Mendel’s results were not affected by genetic recombination.
Pod color
Green
Yellow
Stem length
Tall
Dwarf

6. Mendelain Genetics Example of a monohybrid cross
Example of a monohybrid cross
Parental generation: purple flowers  white flowers
F1 generation: all plants with purple flowers
F2 generation: of plants with purple flowers of plants with white flowers
Mendel needed to explain
Why one trait seemed to disappear in the F1 generation
Why that trait reappeared in one quarter of the F2 offspring
For the BLAST Animation Single-Trait Crosses, go to Animation and Video Files.
Student Misconceptions and Concerns
1. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling will reflect expected ratios.
Teaching Tips
1. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress.
2. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP pp and (b) Pp pp.
3. For students struggling with basic terminology, an analogy between a genetic trait and a pair of shoes might be helpful. A person might wear a pair of shoes in which both shoes match (homozygous), or less likely, a person might wear shoes that do not match (heterozygous).
4. Another analogy that might help struggling students is a pair of people trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. This (heterozygous) couple eats at home (the dominant allele “wins”).
5. Figure 9.4 can be of great benefit when introducing genetic terminology of genes. For students struggling to think abstractly, such a visual aid may be essential when describing these features in lecture.
Copyright © 2009 Pearson Education, Inc.

7. of plants have purple flowers of plants have white flowers
P generation
(true-breeding
parents)
Purple flowers
White flowers
F1 generation
All plants have
purple flowers
Fertilization
among F1 plants
(F1 ´ F1)
Figure 9.3A Crosses tracking one character (flower color).
F2 generation
of plants
have purple flowers 3 – 4
of plants
have white flowers 1 – 4

8. Mendel’s law of segregation describes the inheritance of a single character
Four Hypotheses
Genes are found in alternative versions called alleles; a genotype is the listing of alleles an individual carries for a specific gene
For each characteristic, an organism inherits two alleles, one from each parent; the alleles can be the same or different
A homozygous genotype has identical alleles
A heterozygous genotype has two different alleles
There is an allele for purple flower color and a different allele for white flower color.
Student Misconceptions and Concerns
1. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling will reflect expected ratios.
Teaching Tips
1. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress.
2. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP pp and (b) Pp pp.
3. For students struggling with basic terminology, an analogy between a genetic trait and a pair of shoes might be helpful. A person might wear a pair of shoes in which both shoes match (homozygous), or less likely, a person might wear shoes that do not match (heterozygous).
4. Another analogy that might help struggling students is a pair of people trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. This (heterozygous) couple eats at home (the dominant allele “wins”).
5. Figure 9.4 can be of great benefit when introducing genetic terminology of genes. For students struggling to think abstractly, such a visual aid may be essential when describing these features in lecture.
Copyright © 2009 Pearson Education, Inc.

9. Mendel’s law of segregation describes the inheritance of a single character
Four Hypotheses
If the alleles differ, the dominant allele determines the organism’s appearance, and the recessive allele has no noticeable effect
The phenotype is the appearance or expression of a trait
The same phenotype may be determined by more than one genotype
Law of segregation: Allele pairs separate (segregate) from each other during the production of gametes so that a sperm or egg carries only one allele for each gene
In reference to hypothesis 3, in the F1 generation, plants received a purple allele from one parent and a white allele from the other. The purple allele is dominant since all the F1 plants have purple petals.
In reference to hypothesis 4, in general, a dominant allele provides instructions for producing a functional protein. A recessive allele may lead to a nonfunctional protein or the complete absence of the protein.
Mendel needed to explain:
Why one trait seemed to disappear in the F1 generation—This is explained by dominance of the purple allele over the white allele.
Why that trait reappeared in one fourth of the F2 offspring—This is explained by the segregation of alleles, the generation of all possible combinations of gametes, and the dominance of the purple allele.
Student Misconceptions and Concerns
1. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling will reflect expected ratios.
Teaching Tips
1. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress.
2. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP pp and (b) Pp pp.
3. For students struggling with basic terminology, an analogy between a genetic trait and a pair of shoes might be helpful. A person might wear a pair of shoes in which both shoes match (homozygous), or less likely, a person might wear shoes that do not match (heterozygous).
4. Another analogy that might help struggling students is a pair of people trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. This (heterozygous) couple eats at home (the dominant allele “wins”).
5. Figure 9.4 can be of great benefit when introducing genetic terminology of genes. For students struggling to think abstractly, such a visual aid may be essential when describing these features in lecture.
Copyright © 2009 Pearson Education, Inc.

10. Genetic makeup (alleles) P plants PP ppPP pp
Gametes
All P
All p
F1 plants
(hybrids)
All Pp
Gametes 1 – 2 P 1 – 2 p
Sperm
Figure 9.3B Explanation of the crosses in Figure 9.3A.
A Punnett square is used to show all possible fertilization events for parental gametes. In three quarters of the F2 genotypes, there will be at least one dominant allele. P p
F2 plants
Phenotypic ratio
3 purple : 1 white P PP Pp
Eggs
Genotypic ratio
1 PP : 2 Pp : 1 pp p Pp pp

11. Homologous chromosomes show the alleles for each character
For a pair of homologous chromosomes, alleles of a gene locate at the same locus
Homozygous individuals have the same allele on both homologues
Heterozygous individuals have a different allele on each homologue
Student Misconceptions and Concerns
1. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling will reflect expected ratios.
Teaching Tips
1. Figure 9.4 can be of great benefit when introducing genetic terminology of genes. For students struggling to think abstractly, such a visual aid may be essential when describing these features in lecture.
Copyright © 2009 Pearson Education, Inc.

12. Gene loci Dominant allele P a B P a b Recessive allele Genotype: PP aa
Gene loci
Dominant
allele P a B P a b
Recessive
allele
Figure 9.4 Matching gene loci on homologous chromosomes.
Genotype: PP aa Bb
Homozygous
for the
dominant allele
Homozygous
for the
recessive allele
Heterozygous

13. Geneticists use the testcross to determine unknown genotypes
Testcross
Mating between an individual of unknown genotype and a homozygous recessive individual
Will show whether the unknown genotype includes a recessive allele
Used by Mendel to confirm true-breeding genotypes
Student Misconceptions and Concerns
1. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling will reflect expected ratios.
Teaching Tips
1. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. (If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three dark and one brown or all dark. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype.)
Copyright © 2009 Pearson Education, Inc.

14. Two possibilities for the black dog:
Testcross:
Genotypes B_ bb
Two possibilities for the black dog: BB or Bb
Figure 9.6 Using a testcross to determine genotype.
If the black Labrador retriever has the Bb genotype, chocolate offspring are expected.
To test whether a round-seeded pea plant is true breeding, what plant would you use for the testcross? A wrinkled-seeded plant.
What result would show that the round-seeded plant was NOT true breeding? Having wrinkled-seeded plants among the offspring of the testcross.
Gametes B B b b Bb b Bb bb
Offspring
All black
1 black : 1 chocolate

15. Genetic traits in humans can be tracked through family pedigrees
A pedigree
Shows the inheritance of a trait in a family through multiple generations
Demonstrates dominant or recessive inheritance
Can also be used to deduce genotypes of family members
Student Misconceptions and Concerns
1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in the lecture.
Teaching Tips
1. Students also seem to learn much from Figure 9.8b by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people.
Copyright © 2009 Pearson Education, Inc.

16. Dominant Traits Recessive Traits Freckles No freckles Widow’s peak
Freckles
No freckles
Figure 9.8A Examples of single-gene inherited traits in humans.
This figure shows three traits that are determined by single genes with alternate alleles. The role of the dominant allele in influencing the phenotype when at least one copy is present can be emphasized here.
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe

17. First generation (grandparents) Ff Ff ff Ff Second generation
First generation
(grandparents) Ff Ff ff Ff
Second generation
(parents, aunts,
and uncles) FF ff ff Ff Ff ff or Ff
Third generation
(two sisters)
Figure 9.8B Pedigree showing inheritance of attached versus free earlobe in a hypothetical family. ff FF or
Female
Male Ff
Affected
Unaffected

18. Many inherited disorders in humans are controlled by a single gene
Inherited human disorders show
Recessive inheritance
Two recessive alleles are needed to show disease
Heterozygous parents are carriers of the disease-causing allele
Probability of inheritance increases with inbreeding, mating between close relatives
Dominant inheritance
One dominant allele is needed to show disease
Dominant lethal alleles are usually eliminated from the population
Students may assume that dominant alleles are “better” and most common. The role of dominant alleles in causing some diseases shows that dominant alleles are not always advantageous. In addition, the incidence of these disease-causing dominant alleles is low, showing that recessive alleles can be more common than dominant ones.
In general, a dominant allele leads to the production of a functional protein. In the case of a homozygous recessive genotype, a nonfunctional protein may be produced or the protein may be entirely absent. Cystic fibrosis is caused by a mutation in the gene for a membrane protein (CFTR, cystic fibrosis transmembrane conductance regulator) that controls chloride ion balance across cell membranes. Individuals with at least one copy of the dominant allele produce sufficient amounts of CFTR and are unaffected by the disease. In the homozygous recessive genotype, the CFTR is produced in an altered form that is not inserted in the cell membrane. The resulting ion imbalance leads to the buildup of sticky mucus that causes cystic fibrosis symptoms.
Teaching Tips
1. The 2/3 fraction noted in the discussion of carriers of a recessive disorder for deafness often catches students off guard as they are expecting odds of 1/4, 1/2, or 3/4. However, when we eliminate the dd (deaf) possibility, as it would not be a carrier, we have three possible genotypes. Thus, the odds are based out of the remaining three genotypes Dd, dD, and DD.
2. As a simple test of comprehension, ask students to explain why lethal alleles are not eliminated from a population. Several possibilities exist: The lethal allele might be recessive, persisting in the population due to the survival of carriers, or the lethal allele might be dominant, but is not expressed until after the age of reproduction.
3. Ask your class (a) what the odds are of developing Huntington’s disease if a parent has this disease (50%) and (b) whether they would want this genetic test if they were at risk. The Huntington Disease Society website, offers many additional details. It is a good starting point for those who want to explore this disease in more detail.
Copyright © 2009 Pearson Education, Inc.

19. Parents Normal Dd Normal Dd ´ Sperm D d Dd Normal (carrier) DD Normal
Parents
Normal Dd
Normal Dd
Sperm D d Dd
Normal
(carrier) DD
Normal D
Offspring
Eggs
Figure 9.9A Offspring produced by parents who are both carriers for a recessive disorder.
This diagram shows the inheritance of deafness, a recessive trait, from two heterozygous parents. Dd
Normal
(carrier) dd
Deaf d

20. Book page 163
Table 9.9 Some Autosomal Disorders in Humans.

21. Incomplete (codominance) dominance results in intermediate phenotypes
Incomplete (codominance) dominance
Neither allele is dominant over the other
Expression of both alleles is observed as an intermediate phenotype in the heterozygous individual
Phenotype ratio = genotype ratio= 1:2:1
Student Misconceptions and Concerns
1. After reading the preceding modules, students might expect all traits to be governed by a single gene with two alleles, one dominant over the other. Modules 9.11–9.15 describe deviations from this simplistic model of inheritance.
2. As these variations of Mendel’s laws are introduced, students are likely to get confused and become uncertain about the prior definitions. Consider keeping a clear definition of these different patterns of inheritance available for the class to refer to as
new patterns are discussed (perhaps as a handout for student reference).
3. As your class size increases, the chances increase that at least one student will have a family member with one of the genetic disorders discussed. Some students may find this embarrassing, but others might have a special interest in learning more about these topics, and may even be willing to share some of their family’s experiences with the class.
Teaching Tips
1. Incomplete dominance is analogous to a compromise, or a gray shade. The key concept is that both “sides” have input. Complete dominance is more analogous to an authoritarian style, overruling others and insisting on things being a certain way. Although these analogies might seem obvious to instructors, many students new to genetics appreciate them.
2. Another analogy for cholesterol receptors is fishing poles. The more fishing poles you use, the more fish you can catch. Heterozygotes for hypercholesterolemia have fewer “fishing poles” for cholesterol. Thus, fewer “fish” are caught and more “fish” remain in the water.
Copyright © 2009 Pearson Education, Inc.

22. P generation Red RR White rr Gametes R r F1 generation Pink Rr Gametes
Red RR
White rr
Gametes R r
F1 generation
Pink Rr
Gametes 1 – 2 R 1 – 2 r
Figure 9.11A Incomplete dominance in snapdragon color.
This figure shows incomplete dominance in flower color. The difference in color between the RR and Rr genotypes is proposed to be a dosage effect, where the presence of one allele allows the production of half as much pigment as the presence of two alleles.
Sperm 1 – 2 R 1 – 2 r
F2 generation RR rR 1 – 2 R
Eggs Rr rr 1 – 2 r

23. Many genes have more than two alleles in the population
Multiple alleles
More than two alleles are found in the population(to control one trait)
A diploid individual can carry any two of these alleles
The ABO blood group has three alleles, leading to four phenotypes: type A, type B, type AB, and type O blood
An example for blood type inheritance: Mr. Jones has type A blood. His wife has type AB blood. Their first child has type B blood. What are the possible phenotypes for future offspring and the probabilities for each one?
The type B child has the genotype IBi, receiving the IB allele from Mrs. Jones and the i allele from Mr. Jones. Therefore Mr. Jones has a heterozygous genotype, IAi. If he were homozygous for the IA allele, then he could only have children with type A or type AB blood. Mrs. Jones has the genotype IAIB.
IAi  IAIB  ¼ type AB, ¼ type B, ½ type A
Student Misconceptions and Concerns
1. After reading the preceding modules, students might expect all traits to be governed by a single gene with two alleles, one dominant over the other. Modules 9.11–9.15 describe deviations from this simplistic model of inheritance.
2. As these variations of Mendel’s laws are introduced, students are likely to get confused and become uncertain about the prior definitions. Consider keeping a clear definition of these different patterns of inheritance available for the class to refer to as
new patterns are discussed (perhaps as a handout for student reference).
3. As your class size increases, the chances increase that at least one student will have a family member with one of the genetic disorders discussed. Some students may find this embarrassing, but others might have a special interest in learning more about these topics, and may even be willing to share some of their family’s experiences with the class.
Teaching Tips
1. Students can think of blood types as analogous to socks on their feet. You can have socks that match, a sock on one foot but not the other, you can wear two socks that do not match, or you can even go barefoot (type O blood)! Developed further, think of Amber (A) and Blue (B) socks. Type A blood can have an Amber sock with either another Amber sock or a bare foot (or “zero” sock). Blue socks work the same way. One amber and one blue sock is the AB blood type. No socks, as already noted, represent type O.
2. Consider specifically comparing the principles of codominance (expression of both alleles) and incomplete dominance (expression of one intermediate trait). Students will likely benefit from this direct comparison.
Copyright © 2009 Pearson Education, Inc.

24. Many genes have more than two alleles in the population
Codominance
Neither allele is dominant over the other
Expression of both alleles is observed as a distinct phenotype in the heterozygous individual
Observed for type AB blood
An example for blood type inheritance: Mr. Jones has type A blood. His wife has type AB blood. Their first child has type B blood. What are the possible phenotypes for future offspring and the probabilities for each one?
The type B child has the genotype IBi, receiving the IB allele from Mrs. Jones and the i allele from Mr. Jones. Therefore Mr. Jones has a heterozygous genotype, IAi. If he were homozygous for the IA allele, then he could only have children with type A or type AB blood. Mrs. Jones has the genotype IAIB.
IAi  IAIB  ¼ type AB, ¼ type B, ½ type A
Student Misconceptions and Concerns
1. After reading the preceding modules, students might expect all traits to be governed by a single gene with two alleles, one dominant over the other. Modules 9.11–9.15 describe deviations from this simplistic model of inheritance.
2. As these variations of Mendel’s laws are introduced, students are likely to get confused and become uncertain about the prior definitions. Consider keeping a clear definition of these different patterns of inheritance available for the class to refer to as
new patterns are discussed (perhaps as a handout for student reference).
3. As your class size increases, the chances increase that at least one student will have a family member with one of the genetic disorders discussed. Some students may find this embarrassing, but others might have a special interest in learning more about these topics, and may even be willing to share some of their family’s experiences with the class.
Teaching Tips
1. Students can think of blood types as analogous to socks on their feet. You can have socks that match, a sock on one foot but not the other, you can wear two socks that do not match, or you can even go barefoot (type O blood)! Developed further, think of Amber (A) and Blue (B) socks. Type A blood can have an Amber sock with either another Amber sock or a bare foot (or “zero” sock). Blue socks work the same way. One amber and one blue sock is the AB blood type. No socks, as already noted, represent type O.
2. Consider specifically comparing the principles of codominance (expression of both alleles) and incomplete dominance (expression of one intermediate trait). Students will likely benefit from this direct comparison.
Copyright © 2009 Pearson Education, Inc.

25. Figure 9.12 Multiple alleles for the ABO blood groups.
Blood
Group
(Phenotype)
Antibodies
Present in
Blood
Reaction When Blood from Groups Below Is Mixed
with Antibodies from Groups at Left
Genotypes
Red Blood Cells O A B AB
Anti-A
Anti-B O ii
IAIA or
IAi A
Carbohydrate A
Anti-B
IBIB or
IBi B
Carbohydrate B
Anti-A
Figure 9.12 Multiple alleles for the ABO blood groups.
This figure emphasizes the phenotypic manifestation of codominance. Alleles IA and IB are codominant. Allele IA causes the production of carbohydrate A, while allele IB causes the production of carbohydrate B. For the genotype IAIB, both types of carbohydrates are found on the cell surface. The phenotype differs from either of the IAIA or IBIB homozygotes and does not appear as an intermediate as would be seen for intermediate dominance.
Another human example of codominance involves the allele for sickle cell anemia (see Module 9.13). HbS codes for an altered form of the beta-hemoglobin molecule; HbA is the nonmutant form. Individuals with the HbS HbS genotype have only the mutant form of beta-hemoglobin on their red blood cells and suffer from sickle cell anemia. Individuals with HbA HbS have both types of hemoglobin on their red blood cells and have a milder condition called sickle cell trait. Individuals with HbA HbA have only the nonmutant form of beta-hemoglobin and neither the anemia nor the trait.
Sometimes the distinction between incomplete dominance and codominance depends on the level at which observations are made. One example of incomplete dominance is coat color in cattle, where roan is the heterozygous intermediate to homozygotes red and white. But roan cattle have distinct white hairs overlaying distinct dark hairs so this could be viewed as a codominant phenotype.
Students are often interested in the universal donor, universal recipient descriptions for blood type. Type O is called the universal donor since the cells do not have either carbohydrate A or carbohydrate B so that antibodies in the other blood types will not cause a reaction with these cells. But type O blood contains antibodies to type A and type B. In general, the patient is given his or her own blood type in a transfusion in order to avoid adverse reactions. In an emergency, the universal donor type may be used. AB
IAIB —

26. Blood Group (Phenotype) Genotypes Red Blood Cells O ii IAIA or IAi A
Blood
Group
(Phenotype)
Genotypes
Red Blood Cells O ii
IAIA or
IAi A
Carbohydrate A
IBIB or
IBi
Figure 9.12 Multiple alleles for the ABO blood groups.
This figure emphasizes the phenotypic manifestation of codominance. Alleles IA and IB are codominant. Allele IA causes the production of carbohydrate A, while allele IB causes the production of carbohydrate B. For the genotype IAIB, both types of carbohydrates are found on the cell surface. The phenotype differs from either of the IAIA or IBIB homozygotes and does not appear as an intermediate as would be seen for intermediate dominance.
Another human example of codominance involves the allele for sickle cell anemia (see Module 9.13). HbS codes for an altered form of the beta-hemoglobin molecule; HbA is the nonmutant form. Individuals with the HbS HbS genotype have only the mutant form of beta-hemoglobin on their red blood cells and suffer from sickle cell anemia. Individuals with HbA HbS have both types of hemoglobin on their red blood cells and have a milder condition called sickle cell trait. Individuals with HbA HbA have only the nonmutant form of beta-hemoglobin and neither the anemia nor the trait.
Sometimes the distinction between incomplete dominance and codominance depends on the level at which observations are made. One example of incomplete dominance is coat color in cattle, where roan is the heterozygous intermediate to homozygotes red and white. But roan cattle have distinct white hairs overlaying distinct dark hairs so this could be viewed as a codominant phenotype.
Students are often interested in the universal donor, universal recipient descriptions for blood type. Type O is called the universal donor since the cells do not have either carbohydrate A or carbohydrate B so that antibodies in the other blood types will not cause a reaction with these cells. But type O blood contains antibodies to type A and type B. In general, the patient is given his or her own blood type in a transfusion in order to avoid adverse reactions. In an emergency, the universal donor type may be used. B
Carbohydrate B AB
IAIB

27. Reaction When Blood from Groups Below Is Mixed
Blood
Group
(Phenotype)
Antibodies
Present in
Blood
Reaction When Blood from Groups Below Is Mixed
with Antibodies from Groups at Left O A B AB
Anti-A
Anti-B O A
Anti-B
Figure 9.12 Multiple alleles for the ABO blood groups.
This figure emphasizes the phenotypic manifestation of codominance. Alleles IA and IB are codominant. Allele IA causes the production of carbohydrate A, while allele IB causes the production of carbohydrate B. For the genotype IAIB, both types of carbohydrates are found on the cell surface. The phenotype differs from either of the IAIA or IBIB homozygotes and does not appear as an intermediate as would be seen for intermediate dominance.
Another human example of codominance involves the allele for sickle cell anemia (see Module 9.13). HbS codes for an altered form of the beta-hemoglobin molecule; HbA is the nonmutant form. Individuals with the HbS HbS genotype have only the mutant form of beta-hemoglobin on their red blood cells and suffer from sickle cell anemia. Individuals with HbA HbS have both types of hemoglobin on their red blood cells and have a milder condition called sickle cell trait. Individuals with HbA HbA have only the nonmutant form of beta-hemoglobin and neither the anemia nor the trait.
Sometimes the distinction between incomplete dominance and codominance depends on the level at which observations are made. One example of incomplete dominance is coat color in cattle, where roan is the heterozygous intermediate to homozygotes red and white. But roan cattle have distinct white hairs overlaying distinct dark hairs so this could be viewed as a codominant phenotype.
Students are often interested in the universal donor, universal recipient descriptions for blood type. Type O is called the universal donor since the cells do not have either carbohydrate A or carbohydrate B so that antibodies in the other blood types will not cause a reaction with these cells. But type O blood contains antibodies to type A and type B. In general, the patient is given his or her own blood type in a transfusion in order to avoid adverse reactions. In an emergency, the universal donor type may be used. B
Anti-A AB —

28. SEX CHROMOSOMES AND SEX-LINKED GENES
Copyright © 2009 Pearson Education, Inc.

29. Figure 9.21A Fruit fly eye color, a sex-linked characteristic.

30. Female Male XR XR Xr Y Sperm Xr Y Eggs XR XR Xr XR Y
XR XR
Xr Y
Sperm Xr Y
Figure 9.21B Homozygous, red-eyed female  white-eyed male.
Since the female fly is homozygous for the dominant R allele, both female and male offspring will have red eyes.
Eggs XR
XR Xr
XR Y
R = red-eye allele
r = white-eye allele

31. Then he crossed F1 X F1
F2 generation did conform the expected 3:1 ratio but there was a twist; the white eye trait showed up only in males. All the females had red eyes, whereas half of the males had white half of them had red.
Why were there no white eyed females??

32. Female Male XR Xr XR Y Sperm XR Y XR XR XR XR Y Eggs Xr Xr XR Xr Y
XR Xr
XR Y
Sperm XR Y
Figure 9.21C Heterozygous female  red-eyed male.
All of the female offspring will have red eyes, since the father will pass the X-linked R allele to all of his daughters.
For the sons, there is a one-half probability of receiving the R allele or the r allele from the mother fly. XR
XR XR
XR Y
Eggs Xr
Xr XR
Xr Y

33. Because all of the females took(inherit) at least one dominant allele from their father, so they had either XRXR or XRXr genotypes(all red phenotype). But for males, since they took their X chromosome only from mother, half of them had recessive half had dominant alleles(half white half red phenotype).
Then he made another experiment to prove his idea of eye color gene is carried on the X chromosome

34. Female Male XR Xr Xr Y Sperm Xr Y XR XR XR XR Y Eggs Xr Xr Xr Xr Y
XR Xr
Xr Y
Sperm Xr Y
Figure 9.21D Heterozygous female  white-eyed male.
White-eyed female offspring receive an r allele from the mother fly and an r allele from the father fly. XR
XR XR
XR Y
Eggs Xr
Xr Xr
Xr Y

35. Sex-linked genes exhibit a unique pattern of inheritance
Sex-linked genes are located on either of the sex chromosomes
X-linked genes are passed from mother to son and mother to daughter
X-linked genes are passed from father to daughter
Y-linked genes are passed from father to son
The X and Y chromosomes separate during meiosis I so that the alleles linked to them segregate from each other.
The Y chromosome does not have the same genes as the X chromosome so it is not able to exert dominance over X-linked alleles. XY males will show both dominant and recessive x-linked traits even though the alleles are present in only one copy. XX females must inherit two copies of an X-linked recessive allele to express the trait.
Let R = red and r = white
White-eyed female  red-eyed male XrXr  XRY  XRXr and XrY (red-eyed females and white-eyed males)
Red-eyed female  white-eyed male XRXR  XrY  XRXr and XRY (red-eyed females and red-eyed males)
Student Misconceptions and Concerns
1. The prior discussion of linked genes addresses a different relationship than the use of the similar term sex-linked genes. Consider emphasizing this distinction for your students.
Teaching Tips
1. An analogy can be drawn between sex-linked genes and the risk of not having a backup copy of a file on your computer. If you only have one copy, and it is damaged, you have to live with the damaged file. Females, who have two X chromosomes, thus have a “backup copy” that can function if one of the sex-linked genes is damaged.
Copyright © 2009 Pearson Education, Inc.

36. HUMAN SEX LINKED TRAITS
In humans, as in drosophilas, the Y chromosome carries much less genetic information than the X chromosome.
One of the X linked inheritance is color blindness called Daltonism. They cannot distinguish red&green colors
XRXR  homozygote normal vision female
XRXr  heterozygote normal vision female
XrXr  homozygote color blind female
XRY  normal vision male
XrY  color blind male
Colorblindness is due to the X-linked recessive allele b, while the X-linked dominant allele B leads to full color vision. Predict the ratio of offspring phenotypes for each of the following:
Colorblind female x male with full color vision
XbXb  XBY  ½ XBXb and ½ XbY = ½ females with full color vision + ½ colorblind males
Female heterozygous for colorblindness  colorblind male
XBXb  XbY  ¼ XBXb, ¼ XbXb, ¼ XBY, ¼ XbY = ¼ females with full color vision + ¼ colorblind females + ¼ males with full color vision + ¼ colorblind males
Female homozygous for full color vision  colorblind male
XBXB  XbY  ½ XBXb and ½ XBY = ½ females with full color vision + ½ males with full color vision
Student Misconceptions and Concerns
1. The likelihood that at least some students in larger classes are color-blind is very high. Some of these students might find this interesting and want to discuss it further. However, others might be embarrassed by what might be perceived as a defect.
Teaching Tips
1. For additional information about hemophilia, consider visiting the website of the National Hemophilia Foundation at
Copyright © 2009 Pearson Education, Inc.

37. Predict the ratio of offspring phenotypes for the following:
Female heterozygous for colorblindness  colorblind male?
XBXb  XbY
¼ XBXb, ¼ XbXb, ¼ XBY, ¼ XbY = ¼ females with full color vision + ¼
colorblind females + ¼ males with full color vision + ¼ colorblind males

38. 2) Haemophilia is a disease in which blood clotting doesn’t occur.
Clotting is a complex series of reactions. Each step of it requires specific enzymes. One of them is controlled by a gene on the X chromosome called factor VIII(antihaemophilic globin)
XHXH  normal female
XHXh  carrier female
XhXh  haemophilic female
XHY  normal male
XhY  haemophilic male

39. Queen Victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia
There is no prior history of hemophilia in Queen Victoria’s family, so it is thought that the allele arose by spontaneous mutation in the sex cells of one of her parents
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Figure 9.22 Hemophilia in the royal family of Russia (half-filled symbols represent heterozygous carriers).
There is no prior history of hemophilia in Queen Victoria’s family, so it is thought that the allele arose by spontaneous mutation in the sex cells of one of her parents.
Alexis

40. HL TOPICS

41. 10.2 Dihybrid crosses and gene linkage
Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.
Distinguish between autosomes and sex chromosomes.
Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles.
Define linkage
Explain an example of a cross between two linked genes.
Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.

42. The law of independent assortment is revealed by tracking two characters at once
Example of a dihybrid cross
Parental generation: round yellow seeds  wrinkled green seeds
F1 generation: all plants with round yellow seeds
F2 generation: of plants with round yellow seeds of plants with round green seeds of plants with wrinkled yellow seeds of plants with wrinkled green seeds
Mendel needed to explain
Why nonparental combinations were observed
Why a 9:3:3:1 ratio was observed among the F2 offspring
Student Misconceptions and Concerns
1. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling will reflect expected ratios.
Teaching Tips
1. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses.
Copyright © 2009 Pearson Education, Inc.

43. The law of independent assortment is revealed by tracking two characters at once
Law of independent assortment
Each pair of alleles segregates independently of the other pairs of alleles during gamete formation
For genotype RrYy, four gamete types are possible: RY, Ry, rY, and ry
For the BLAST Animation Genetic Variation: Independent Assortment, go to Animation and Video Files.
For the BLAST Animation Two-Trait Crosses, go to Animation and Video Files.
Student Misconceptions and Concerns
1. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling will reflect expected ratios.
Teaching Tips
1. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses.
Copyright © 2009 Pearson Education, Inc.

44. Hypothesized (not actually seen) Actual results (support hypothesis)
Hypothesis: Dependent assortment
Hypothesis: Independent assortment P
generation
RRYY
rryy
RRYY
rryy
Gametes RY ry
Gametes RY ry F1
generation
RrYy
RrYy
Sperm
Sperm 1 – 4 RY 1 4 – rY 1 4 – Ry 1 4 – ry 1 – 2 RY 1 – 2 ry F2
generation 1 – 4 RY 1 – 2 RY
RRYY
RrYY
RRYy
RrYy
Eggs
Figure 9.5A Two hypotheses for segregation in a dihybrid cross.
This figure compares dependent assortment to independent assortment. For dependent assortment, allele combinations present in the original parents are the only ones observed in the F2 offspring. Mendel’s results support independent assortment with nonparental combinations of round with green and wrinkled with yellow produced in the F2 generation.
Note that both characteristics, seed shape and seed color, show 3:1 ratios when considered separately. Yellow is seen in 12/16 of the offspring and green is observed in 4/16 of the offspring. This confirms that these alleles are segregating from each other as predicted by the law of segregation, without interference from the other allele pair. 1 4 – rY 1 2 – ry
RrYY
rrYY
RrYy
rrYy
Eggs
Yellow
round 1 – 4 Ry 16 –– 9
RRYy
RrYy
RRyy
Rryy
Green
round 16 –– 3
Hypothesized
(not actually seen) 1 – 4 ry
Yellow
wrinkled
RrYy
rrYy
Rryy
rryy 16 –– 3
Actual results
(support hypothesis)
Green
wrinkled 1 16 ––

45. Genes on the same chromosome tend to be inherited together
Linked Genes
Are located close together on the same chromosome
Tend to be inherited together
Example studied by Bateson and Punnett
Parental generation: plants with purple flowers, long pollen crossed to plants with red flowers, round pollen
The F2 generation did not show a 9:3:3:1 ratio
Most F2 individuals had purple flowers, long pollen or red flowers, round pollen
Student Misconceptions and Concerns
1. This section of the chapter relies upon a good understanding of the chromosome sorting process of meiosis. If students were not assigned Chapter 8, and meiosis has not otherwise been addressed, it will be difficult for students to understand the chromosomal basis of inheritance or linked genes.
Teaching Tips
1. Building on the shoe analogy developed in Chapter 8, linked genes are like a shoe and its shoelaces. The two are usually transferred together but can be moved separately under special circumstances.
Copyright © 2009 Pearson Education, Inc.

46. Not accounted for: purple round and red long
Experiment
Purple flower
PpLl
PpLl
Long pollen
Observed
offspring
Prediction
(9:3:3:1)
Phenotypes
Purple long
Purple round
Red long
Red round
284 21 55
215 71 24
Explanation: linked genes
Parental
diploid cell
PpLl PL pl
Meiosis
Most
gametes PL pl
Figure 9.17 Experiment involving linked genes in the sweet pea.
Fertilization
Sperm PL pl PL PL PL
Most
offspring PL pl
Eggs pl pl pl PL pl
3 purple long : 1 red round
Not accounted for: purple round and red long

47. Crossing over produces new combinations of alleles
Linked alleles can be separated by crossing over
Recombinant chromosomes are formed
Thomas Hunt Morgan demonstrated this in early experiments
Geneticists measure genetic distance by recombination frequency Or
The probability that two genes will be seperated by crossing over is proportional to the physical distance between them in the chromosome.
For the Bateson-Punnett experiment described in Module 9.17, the parental chromosomes have alleles P-----L and p------l. Crossing over between the P and L loci would yield recombinant chromosomes P-----l and p------L.
Genetic distance was measured in map units where 1% recombination = 1 map unit. The map unit is also called the centimorgan, in honor of Thomas Hunt Morgan.
Student Misconceptions and Concerns
1. This section of the chapter relies upon a good understanding of the chromosome sorting process of meiosis. If students were not assigned Chapter 8, and meiosis has not otherwise been addressed, it will be difficult for students to understand the chromosomal basis of inheritance or linked genes.
2. The nature of linked genes builds upon our natural expectations that items that are closely together are less likely to be separated. Yet, students may find such concepts initially foreign. Whether it is parents holding the hands of children or people and their pets, we generally know that separation is more likely when things are farther apart.
Teaching Tips
1. Crossing over (from Chapter 8) is like randomly editing out a minute of film from two movies and swapping them. Perhaps the fifth minute of Bambi is swapped for the fifth minute of Texas Chainsaw Massacre. Clearly, the closer together two frames of film are, the more likely they are to move or remain together.
2. Challenge students to explain why Sturtevant and Morgan studied the genetics of fruit flies. As the text notes, their small size, ease of care, and ability to produce several generations in a matter of weeks or months were important factors.
Copyright © 2009 Pearson Education, Inc.

48. A B A b A B a b a B a b Tetrad Crossing over GametesA B A b A B a b a B a b
Tetrad
Crossing over
Figure 9.18A Review: Production of recombinant gametes.
Gametes

49. GgLl (female) ggll (male)
Explanation
G L
g l
GgLl
(female)
ggll
(male)
g l
g l
G L
g l
G l
g L
g l
Eggs
Sperm
G L
g l
G l
g L
Figure 9.18C Fruit fly experiment demonstrating the role of crossing over in inheritance.
This figure shows Morgan’s observations of crossing over. Parental types are the most frequent offspring, and the percentage of recombinants can be used to estimate the genetic distance.
g l
g l
g l
g l
Offspring

50. Mutant phenotypes Short aristae Black body (g) Cinnabar eyes (c)
Mutant phenotypes
Short
aristae
Black
body
(g)
Cinnabar
eyes
(c)
Vestigial
wings
(l)
Brown
eyes
Figure 9.19B A partial genetic map of a fruit fly chromosome.
Long aristae
(appendages
on head)
Gray
body
(G)
Red
eyes
(C)
Normal
wings
(L)
Red
eyes
Wild-type phenotypes

51. A single character may be influenced by many genes
Polygenic inheritance
Many genes influence one trait
Skin color is affected by at least three genes
Student Misconceptions and Concerns
1. After reading the preceding modules, students might expect all traits to be governed by a single gene with two alleles, one dominant over the other. Modules 9.11–9.15 describe deviations from this simplistic model of inheritance.
2. As these variations of Mendel’s laws are introduced, students are likely to get confused and become uncertain about the prior definitions. Consider keeping a clear definition of these different patterns of inheritance available for the class to refer to as
new patterns are discussed (perhaps as a handout for student reference).
3. As your class size increases, the chances increase that at least one student will have a family member with one of the genetic disorders discussed. Some students may find this embarrassing, but others might have a special interest in learning more about these topics, and may even be willing to share some of their family’s experiences with the class.
Teaching Tips
1. Polygenic inheritance makes it possible for children to inherit genes to be taller or shorter than either parent. Similarly, skin tones can be darker or lighter than either parent. The environment also contributes significantly to the final phenotype for both of these traits.
Copyright © 2009 Pearson Education, Inc.

52. Figure 9.14 A model for polygenic inheritance of skin color.
P generation
aabbcc
(very light)
AABBCC
(very dark)
F1 generation
AaBbCc
AaBbCc
Sperm 8 – 1 8 – 1 8 – 1 8 – 1 8 – 1 8 – 1 8 – 1 8 – 1
F2 generation 8 – 1 8 – 1 8 – 1 64 –– 20 8 – 1
Figure 9.14 A model for polygenic inheritance of skin color.
Polygenic inheritance is implicated in susceptibility to certain diseases including insulin-dependent diabetes mellitus, multiple sclerosis, psoriasis, and schizophrenia.
Eggs 8 – 1 64 –– 15 8 – 1 8 – 1
Fraction of population 8 – 1 64 –– 6 64 –– 1 64 –– 1 64 –– 6 64 –– 15 64 –– 20 64 –– 15 64 –– 6 64 –– 1
Skin color

53. aabbcc (very light) AABBCC (very dark) AaBbCc AaBbCc
P generation
aabbcc
(very light)
AABBCC
(very dark)
F1 generation
AaBbCc
AaBbCc
Sperm
F2 generation 8 – 1 8 – 1 8 – 1 8 – 1 8 – 1 8 – 1 8 – 1 8 – 1 8 – 1 8 – 1 8 – 1
Figure 9.14 A model for polygenic inheritance of skin color.
Polygenic inheritance is implicated in susceptibility to certain diseases including insulin-dependent diabetes mellitus, multiple sclerosis, psoriasis, and schizophrenia. 8 – 1
Eggs 8 – 1 8 – 1 8 – 1 8 – 1 64 –– 1 64 –– 6 64 –– 15 64 –– 20 64 –– 15 64 –– 6 64 –– 1

54. Fraction of population20 –– 64 15 –– 64
Fraction of population 6 –– 64
Figure 9.14 A model for polygenic inheritance of skin color.
Polygenic inheritance is implicated in susceptibility to certain diseases including insulin-dependent diabetes mellitus, multiple sclerosis, psoriasis, and schizophrenia. 1 –– 64
Skin color