1. Discover Biology SIXTH EDITION
Anu Singh-Cundy • Gary Shin
Discover Biology SIXTH EDITION
CHAPTER 9 Patterns of Inheritance
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2. CHAPTER 9 Patterns of Inheritance, Part 1
The Lost Princess
9.1 Principles of Genetics: An Overview
Genes determine traits
Diploid cells have two copies of every gene
Genotype directs phenotype
Some phenotypes are controlled by dominant alleles
Gene mutations are the source of new alleles
Controlled crosses help us understand patterns of inheritance
9.2 Basic Patterns of Inheritance
Mendel’s genetic experiments began with true-breeding pea plants
Mendel inferred that inherited traits are determined by genes
9.3 Mendel’s Laws of Inheritance
Mendel’s single-trait crosses revealed the law of segregation
Mendel’s two-trait experiments led to the law of independent assortment
Mendel’s insights rested on a sound understanding of probability

3. CHAPTER 9 Patterns of Inheritance, Part 2
9.4 Extensions of Mendel’s Laws
Many alleles display incomplete dominance
The alleles of some genes are codominant
A pleiotropic gene affects multiple traits
Alleles for one gene can alter the effects of another gene
The environment can alter the effects of a gene
Most traits are determined by two or more genes
Complex traits are polygenic and potentially influenced by the environment
BIOLOGY MATTERS: KNOW YOUR TYPE
APPLYING WHAT WE LEARNED: Solving the Mystery of the Lost Princess

4. The Lost Princess
The last Russian czar and his family were executed in 1918.
Could Mendelian genetics help settle persistent rumors that one of his daughters, Anastasia, had escaped the murder of the Romanov family?
A Royal Mystery
Czar Nicholas II, seen here with the czarina and their five children, was the last czar of Russia. His youngest daughter, Princess Anastasia, is on the right in this 1910 photo.

5. Humans Have Used the Principles of Inheritance for Thousands of Years to Domesticate Plants and Animals
Shar-pei, one of the
oldest dog breeds
The field of genetics originated in 1866 after Gregor Mendel published a paper on inheritance in pea plants.
Mendel’s work was largely ignored for 30 years before it was adopted as the foundation for modern genetics.
Genetics is the study of inherited characteristics (genetic traits) and the genes that affect those traits.
Figure 9.1 Artificial Selection from Wolf Ancestors
You would not guess it from looks, but comparisons with wolf DNA shows the the shar-pei is one of the oldest dog breeds in existence. Bred as fighting dogs in China, shar-peis have a bristly coat and wrinkles that make them difficult to hold on to.
Figure 9.2 Gregor Mendel (1822–1884)
Mendel used mathematics to analyze the inheritance of seven genetic traits in pea plants.
Gregor Mendel (1822–1884)

6. A Genetic Trait Is Any Inherited Characteristic of an Organism That Can Be Observed or Detected
Figure 9.3 Genetic Traits in Humans
A phenotype is a particular version of a genetic trait in a given individual. How many genetic traits can you identify in the photo? Do you think the traits you have identified are controlled by genes exclusively, or might they be influenced by environmental factors also?
Invariant genetic traits are the same in all individuals in a population.
Variant genetic traits come in two or more different versions, or phenotypes.
The display of a particular version of a genetic trait in a specific individual is the phenotype of that genetic trait in that individual.

7. Diploid Cells Have Two Copies of Every Gene
Figure 9.4 Somatic Cells Have Two Copies of Most Genes
The majority of cells in a multicellular organism are somatic cells. In plants and animals, somatic cells are diploid: each type of chromosome occurs as a pair, known as a homologous pair. Alternative versions of a gene are known as alleles. In a pair of homologous chromosomes, a particular gene is found at the same location on each chromosome, but the paternal and maternal homologues may carry different alleles of that gene. (The X and Y chromosomes are an exception.)
Somatic cells (nonsex cells) in the body of plants and animals are diploid: they contain two copies of each type of chromosome, which together make up a homologous pair.
Each homologous pair contains one paternal homologue and one maternal homologue.
Humans have 23 pairs of homologous chromosomes, making a total of 46 chromosomes.

8. An Allele Is a Variant Form of a Gene
Alleles are different versions of a given gene.
Allelic variation is what makes a population genetically diverse.
Mutations are the source of new alleles.
A mutation is a change in the DNA sequence of a gene.
Gene mutations occur at random.
Mutations are commonly neutral, sometimes harmful, and occasionally beneficial, to the individual.
Only mutations that are present in the gametes, or the cells that produce gametes, can be passed to offspring.
(The illustration is Figure 10.2 from Chapter 10.)

9. Genotype Directs Phenotype
The genotype of an individual is the allelic makeup of that individual with respect to the specified genetic trait(s):
The genotype completely or partially controls an individual’s phenotype.
An individual who carries two copies of the same allele is said to be homozygous for that gene.
An individual whose genotype consists of two different alleles for a given phenotype is said to be heterozygous for that gene.
(The illustration is Figure 10.2 from Chapter 10.)
The baby’s genotype for the genes shown
here is rrHhDDeeGg.

10. Some Phenotypes Are Controlled by Dominant Alleles
FIGURE 9.5 Hair Texture in Dogs Is Controlled by a Single Gene
The W allele produces a roughtextured, or wiry, coat in dogs. Dachshunds, terriers, and schnauzers are some of the breeds in which the wiry hair phenotype is accepted under
standards set by kennel clubs. The W allele is dominant over the w allele, so dogs with WW or Ww genotype have wiry hair. The smooth-hair phenotype appears only in the ww
homozygote.
The allele that exerts a controlling influence on the phenotype in a heterozygote is said to be dominant.
An allele that has no effect on the phenotype when paired with a dominant allele in a heterozygote is said to be recessive.

11. Breeding Trials Help Us Understand Patterns of Inheritance
A genetic cross is a controlled mating experiment performed to examine how a particular trait is inherited.
The parents, or P generation, are crossed to produce offspring, called the F1 generation.
Two individuals from the F1 generation are then crossed to produce the F2 generation.
Figure 9.6 Inheritance of a Single Trait over Three Generations
Mendel crossed parent plants that were true-breeding (homozygous) for two discrete phenotypes (purple or white) of a particular trait (flower color). Such breeding trials are described as monohybrid crosses because the F1 plants are hybrid (heterozygous) for a single trait (flower color).

12. Mendel’s Genetic Experiments Began with True-Breeding Pea Plants
Mendel proposed that offspring inherit two separate units of genetic information (two copies of each gene), one from each parent.
Mendel used true-breeding lines of pea plants to conduct highly controlled experiments .
True-breeding, or purebred, individuals have a homogenous genotype.
Mendel crossed two lines of pure-breeding plants to produce two generations of hybrid plants and recorded the phenotypic data.
(The figure is from one of the problem sets on page 214, Chapter 9.)
(Prior to Mendel’s work, people thought that offspring were a blend of parental traits that produced an intermediate (“in-between”) phenotype, meaning that lost traits could not appear in later generations, an idea known as blending inheritance.)

13. Mendel Began By Studying the Inheritance of Single Traits
In a single-trait
cross, the experimenter
tracks the inheritance
of the two alleles of a
single gene.
If all F2 offspring are hybrids for that one trait, as they were in all of Mendel’s experiments, this type of cross is a monohybrid cross.
Figure 9.6 True-Breeding Traits Have a Homozygous Genotype
Flower color in peas is controlled by a gene with two alleles (P and p). Although there are three genotypes (PP, Pp, and pp), there are only two phenotypes (purple flowers and white flowers). Genotypes PP and Pp both produce purple flowers, and only pp produces white flowers.
For the P generation: Mendel crossed true-breeding pea plants with contrasting phenotypes for a particular genetic trait, such as flower color.
He performed many such crosses and recorded the phenotypes of the resulting F1 generation; next he crossed individuals of the F1 generation to raise the F2 generation.

14. Mendel Observed a 3:1 Ratio of Dominant to Recessive Phenotypes in the F2 Generation
Mendel’s observations:
Odds that the dominant phenotype will be seen in the F1 generation: 100 percent (4 in 4)
Odds that the recessive phenotype will reappear in the F2 generation:
1 in 4 (25 percent)
Phenotypic ratio (dominant to
recessive phenotype) is 3:1
Genotypic ratios:
1:4 (25 percent) PP
1:2 (50 percent) Pp
1:4 (25 percent) pp
Figure 9.7 Inheritance of a Single Trait over Three Generations
Mendel crossed parent plants that were true-breeding (homozygous) for two discrete phenotypes of a particular trait. In the example here, pea plants with purple flowers (PP) were crossed with pea plants with white flowers (pp). In the F1 generation, all hybrids were purple (Pp). In the F2 generation, the majority of the plants were purple, but some were white.

15. A Punnett Square Can Be Used to Show All the Possible Ways in Which Two Alleles Can Recombine Through Fertilization
Figure 9.8 The Punnett Square Method Is Used to Predict All Possible Outcomes of a Genetic Cross
Punnett squares chart the segregation (separation) of alleles into gametes and all the possible ways in which the alleles borne by these gametes can be combined to produce offspring.

16. Mendel’s Single-Trait Crosses Revealed the Law of Segregation
Mendel concluded that the results were best explained by assuming that the two copies of a gene separate into different cells during the formation of egg or sperm (meiosis).
Thus, according to Mendel’s law of segregation, the two copies of a gene are separated during meiosis and end up in different gametes.
Therefore, each offspring receives one copy of the gene (one allele) from the egg and the other copy (other allele) from the sperm.

17. Mendel’s Experiments with Two Traits (Dihybrid Crosses)
Next, Mendel sought to determine if a particular phenotype of one trait is always inherited together with a particular phenotype of a different trait.
Is the yellow seed color always inherited with the round seed shape? Or would he find combinations of phenotypes (yellow color and wrinkled shape) among the offspring that were not present in the P generation?
Figure 9.9 Inheritance of Two Distinct Traits
In his dihybrid breeding trials, Mendel was asking if phenotypes of two different traits (seed color and seed shape) would be transmitted as a “package deal” from parent to offspring. This is rather like asking whether a particular eye color phenotype (say, blue eyes) always goes with a particular hair color phenotype (say, blond hair). Individuals who, like actor Daniel Radcliffe, have blue eyes and dark hair illustrate Mendel’s law of independent assortment: eye color phenotypes are inherited completely separately from hair color phenotypes.
Mendel’s question was rather like asking whether a particular eye color phenotype (say, blue eyes) always goes with a particular hair color phenotype (say, blond hair).

18. Two Trait Cross between True-Breeding Parents: Recombinant Phenotypes Appear in the F2 Generation
Mendel crossed dihybrids, individuals that are heterozygous for two traits.
He observed two nonparental combinations of phenotypes (recombinant phenotypes) in the F2 :
- Round shape; green color (RRyy and Rryy)
- Wrinkled shape, yellow color (rrYY
and rrYy)
The phenotypic ratios were 9:3:3:1
-9/16 dominant for both traits
-3/16 dominant for seed shape, recessive for seed color
-3/16 recessive for seed shape, dominant for seed color
-1/16 recessive for both seed shape and color
Figure Inheritance of Two Traits over Three Generations
A two-trait breeding experiment in which the F1 plants are double heterozygotes (heterozygous for both traits) is a dihybrid cross. Mendel used dihybrid crosses to test the hypothesis that the alleles of two different genes are inherited independently from each other. He tracked the seed shape trait controlled by the R/r alleles and the seed color trait controlled by the Y/y alleles. Two new phenotypic combinations were found among the F2 offspring: plants that made round, green seeds (R-yy) and plants that made wrinkled, yellow seeds (rrY-). The bottom panel summarizes the ratio of the two parental phenotypes and the two novel, nonparental phenotypes.

19. Predicting Genotypes of Gametes Produced by Dihybrids of the F1 Generation
Figure The Alleles of Two Genes Are Sorted into Gametes Independently
The 9:3:3:1 phenotypic ratios in the F2 generation of Mendel’s dihybrid cross (see Figure 9.10)  are best explained by the law of independent assortment, according to which the alleles of the R gene segregate independently of the alleles of the Y gene during meiosis.
Mendel deduced that the alleles of one gene (R/r) are sorted into gametes independently of the alleles of the other gene (Y/y).
The gametes that a dihybrid produces have genotypes that include all possible combinations:
RY, Ry, rY, and ry.

20. Mendel’s Two-Trait Experiments Led to the Law of Independent Assortment
The law of independent assortment states that when gametes form, the two copies of any given allele are sorted independently of any two alleles of other genes.
The law of independent assortment applies to the inheritance of two genes that are physically separated on different chromosomes.
The 9:3:3:1 phenotypic ratio is best explained by assuming that the alleles of one gene (RR/Rr/rr) are sorted into gametes independently of the alleles of the other gene (YY/Yy/yy).

21. Chromosomal Basis of Mendel’s Law of Independent Assortment
(The illustration is from Chapter 7. Figure The Random Assortment of Homologous Chromosomes Generates Chromosomal Diversity among Gametes.)
The random assortment of different homologous pairs explains Mendel’s law of
independent assortment: the alleles (R/r) of one genetic locus on a certain pair of homologues are sorted independently of the alleles (Y/y) of another genetic locus that is located on a different pair of homologous chromosomes.

22. What Mendel Inferred from His Breeding Experiments: A Summary
Alternative versions of genes (alleles) cause variation in inherited traits.
Offspring inherit one copy (one allele) of a gene from each parent.
An allele is dominant if, when paired with a different allele, it has exclusive control over an individual’s phenotype.
The two copies (alleles) of a gene segregate during meiosis and end up in different gametes.
The alleles of one gene (such as R/r) are sorted independently of the alleles of another gene (Y/y).*
Gametes fuse randomly, without regard to the particular alleles they carry.
*A modern caveat: this principle will hold if the two genes
are located on a different pair of homologous chromosomes.

23. Mendel’s Insights Rested on a Sound Understanding of Probability
To deduce the patterns of inheritance, Mendel used probability to analyze the data he collected from the offspring of the genetic crosses.
We can predict the probability that a particular offspring will have a certain phenotype or genotype, but we cannot predict the actual phenotype or genotype of a particular individual.
The probability that a particular offspring will display a specific phenotype is completely unaffected by the number of offspring.

24. Extensions of Mendel’s Laws of Inheritance
Mendel’s work was based on genetic traits controlled by a single gene with a dominant and a recessive allele.
Mendel’s laws have been expanded to help explain more complex patterns of inheritance.
(The photo is from Figure 9.3. The graph of normal distribution is from Figure 9.19.)
Most human traits are non-Mendelian (their inheritance patterns cannot be explained by Mendel’s laws alone).

25. Modern Variations on the Theme by Mendel
Incomplete dominance
of alleles produces
an intermediate phenotype in the heterozygote.
Figure Incomplete Dominance in Snapdragons
Incomplete dominance leads to an intermediate phenotype in the heterozygote.
We can still predict the genotypes and phenotypes of F1 and F2 offspring using Mendelian laws of inheritance.

26. Another Example of Incomplete Dominance: Coat Color in Horses and Other Mammals
Figure Incomplete Dominance in Horses
Palominos (heterozygous genotype HCHCr) are intermediate in color to chestnuts (HCHC) and cremellos (HCrHCr) because in the heterozygote, the HCr allele “dilutes” the effect of the HC allele to produce the intermediate phenotype.
Intermediate
phenotype

27. The Alleles of Some Genes Are Codominant
Codominance occurs when the effect of both alleles is equally visible in the phenotype of the heterozygote.
Neither allele is diminished or diluted in a heterozygote that displays codominance.
The ABO blood groups provide an example of codominance:
-The I/i gene controls the type of cell surface sugars that are found on a person’s red blood cells
The I allele is dominant over the i allele
However, the IA and IB alleles are codominant
Figure Genetic Basis of the ABO Blood Types in Humans
The ABO blood types are determined by chains of sugars covalently attached to certain cell surface proteins on red blood cells. Type A red blood cells have a distinctive “A type” of sugar (chemically, N-acetylgalactosamine), whereas type B blood cells have a “B type” of sugar (galactose) at the end of the chain. The AB blood type reflects codominance: the red blood cells in type AB blood have about equal amounts of A-type sugars and B-type sugars on their cell surface. The I gene encodes an enzyme that comes in at least two allelic forms: the form encoded by the A allele adds A-type sugars to red blood cell surfaces, and the form encoded by the B allele adds B-type sugars. The i allele codes for a nonfunctional version of the enzyme that cannot attach any sugar to the cell surface protein. People who have two copies of the i allele (homozygotes) are said to have blood type O.

28. A Pleiotropic Gene Affects Multiple Traits
The situation in which a single gene influences two or more distinctly different traits is called pleiotropy.
A mutation in a pleiotropic gene can cause changes in many different traits.
Albinism is an example of a pleiotropic disorder.
Figure A Child with Albinism
A mother and her son, who has albinism, in their living room in Douala, Cameroon, Africa. Because it affects eyesight as well as pigmentation in skin, eyes, and hair, albinism is an example of a pleiotropic condition.
Albinism is caused by a single recessive allele affecting pigment formation, but other traits such as vision are also affected.

29. Another Example Of Pleitropy: Marfan Syndrome
[Marfan syndrome is a connective tissue disorder with a dominant pattern of inheritance. Connective tissue is a mixture of cell types that binds and strengthens organs and other tissue types. Bone, cartilage, tendons, and the sheath surrounding blood vessels are examples of specialized connective tissue.]
Figure Flo Hyman
U.S. Olympic volleyball silver medalist Flora Jean (“Flo”) Hyman, shown during practice in Complications from Marfan syndrome, a pleiotropic genetic disorder, contributed to her death in 1986.
(Marfan syndrome is a pleiotropic disorder in which many organ systems are affected because a single gene, coding for a protein called fibrillin-1, does not work properly. The protein is crucial for the normal function of connective tissues, which act as a gluing and scaffolding system for all types of organs, from bones to the walls of blood vessels. The one in 5,000 Americans diagnosed each year with Marfan syndrome show a wide range of phenotypes, depending on which allele of the fibrillin-1 gene they possess and what other genetic characteristics they have. Many are tall and gangly, with long arms, legs, fingers, and toes. Weakening of the aorta, the largest blood vessel carrying blood away from the heart, is the most serious phenotype associated with the disorder.)
Flo Hyman was diagnosed with Marfan syndrome only after her death, shortly after she and her teammates won the silver at the 1984 Olympic games.

30. Alleles for One Gene Can Alter the Effects of Another Gene
Figure Alleles of One Gene May Affect the Phenotype Produced by Alleles of Another Gene
In this example the c allele of gene C masks the alleles of another gene, B. In mice with the CC or Cc genotype, the dominant B allele directs the production of melanin, resulting in black fur, while the recessive bb genotype “dilutes” melanin accumulation so that brown fur results. Mice with the cc genotype are albinos because melanin production gets blocked at an early point in the pathway, before the B gene exerts its influence. The cc genotype always results in albinism, regardless of whether the genotype with respect to the B gene is BB, Bb, or bb.
The term epistasis applies when the phenotypic effect of the alleles of one gene depends on the presence of certain alleles for another, independently inherited gene.
Epistasis can be seen in the coat color of numerous animals, whose many genes code for enzymes that convert the amino acid tyrosine into melanin in a multistep pathway.

31. The Environment Can Alter the Phenotype
Chemicals, nutrition, sunlight, and other internal and external environmental factors can alter the effects of certain genes.
The production of melanin in Siamese cats is sensitive to temperature—cooler temperatures produce dark fur on the extremities.
Figure The Environment Can Alter the Effects of Genes
Coat color in Siamese cats is controlled by a temperature-sensitive allele. The Ct allele of the C gene directs melanin production only at lower temperatures (below 37°C). Melanin therefore accumulates only in the colder extremities—the snout, tail, lower legs, and edges of the ears—and not in the main trunk, which is at the core body temperature (37°C).

32. Polygenic Inheritance of a Genetic Trait Leads to a Range of Phenotypes in the Population
Traits governed by the action of more than one gene are polygenic traits.
Skin color, running speed, blood pressure, and body size are all polygenic traits in humans.
Skin color in humans, and many other mammals, is controlled by multiple genes.
Assuming control by three genes, each with two incompletely dominant alleles, seven different phenotypes are possible in the offspring of triple-heterozygote parents.
Figure Three Genes Can Produce a Range of Skin Color in Humans
In this model, skin color is influenced by the total number of melanin “units” specified by the person’s genotype. Alleles that do not contribute to melanin production (A0, B0, and C0) are represented by open circles, while alleles that do contribute to melanin production (A1, B1, and C1) are represented by solid circles. The bar graph depicts the seven phenotypic outcomes arranged from lightest skin to darkest skin. Bar heights indicate the relative proportions of children of each phenotype.

33. A Three-Trait Punnett Square to Predict the Phenotypic and Genotypic Outcomes with Respect to the Skin Color Trait
If you imagine that skin color is controlled by just three genetic loci, with only two alleles that display incomplete dominance, you would predict seven possible phenotypes in the offspring of a couple that both have an intermediate phenotype (are heterozygotes for each of the three genetic loci hypothetically controlling skin color).
Figure Three Genes Can Produce a Range of Skin Color in Humans
In this model, skin color is influenced by the total number of melanin “units” specified by the person’s genotype. Alleles that do not contribute to melanin production (A0, B0, and C0) are represented by open circles, while alleles that do contribute to melanin production (A1, B1, and C1) are represented by solid circles. The Punnett square shows the proportions of gametes with specific genotypes that are produced by two heterozygote (A1A0B1B0C1C0) parents and all the possible ways in which the eight different genotypes can come together during fertilization. In some cases, several different genotypes can produce the same phenotype. Additional variation in skin color would result from different levels of sun exposure.

34. Polygenic Traits, Combined with Environmental Influences, Produce a Smoothly Graded Range of Phenotypic Classes or Continuous Variation
Now, consider the influence of sun exposure on the seven
Predicted phenotypes.
An even broader range of phenotypes,
from very pale to very dark, is possible.
Geneticists estimate there are more than a dozen genes that control melanin production in our skin, which, when coupled with environmental influences, results in continuous variation in the trait.

35. Most Traits That Are Essential for Survival Are Complex Traits
Figure Most Phenotypes Are Shaped by Interactions among Multiple Genes and the Environment
The effect of a gene on an organism’s phenotype can depend on a combination of the gene’s own function, the function of other genes with which it interacts, and the impact of environmental factors. As a result, two individuals with the same genotype for a gene may show very different phenotypes, as illustrated by the differences in growth between the two pea plants depicted here.
Complex traits are those that cannot be predicted using Mendel’s laws of inheritance; complex traits display often display continuous variation in a population.
According to one hypothesis, the evolutionary benefit of continuous variation in phenotypes is that if the environment changes, there are good odds that one out of the many phenotypes will be adaptive under the new conditions.

36. BIOLOGY MATTERS: KNOW YOUR TYPE
Molecules foreign to the body are recognized as antigens
by the immune system.
The cell surface sugars responsible for the A/B/AB blood groups are potential antigens.
If type A whole blood is given to a patient with blood type B or O, the recipient’s immune system produces specific antigen-fighting proteins
(antibodies) that attack their
target antigen (red blood cells
with A-type sugars, in this
example). The transfused cells
clump together when attacked,
leading to life-threatening clots.
In a blood transfusion, the donor and recipient blood types must match.

37. APPLYING WHAT WE LEARNED: Solving the Mystery of the Lost Princess
In 1991, the grave of Czar Nicholas and some of his family members was discovered.
Five alleles of one gene—A1, A2, A3, A4, and A5—are common in people of European descent.
The czar’s genotype: A1A2; czarina’s genotype :A2A3
Anna Anderson’s genotype: A4A5
Mendelian genetics shows that Anna
Anderson could not be the offspring of
Czar Nicholas and Czarina Alexandra.
In 2009, DNA recovered from a skeleton buried near the Romanov family was confirmed to be that of Anastasia and her brother Alexei.
Anna Anderson, a Polish factory worker,
claimed she was Anastasia.
Figure Anastasia and One of Her Three Sisters
Grand Duchesses Maria (left) and Anastasia visiting a hospital for soldiers.
Figure Anna Anderson
Franziska Schanzkowska, also known as Anna Anderson, in about Anna Anderson was a Polish factory work who was 5 years older than Grand Duchess Anastasia and spoke no Russian. Yet, many people believed the two women were the same person.
Grand Duchesses Maria (left) and Anastasia visiting a hospital for soldiers.

38. List of Key Terms: Chapter 9
allele (p. 194) codominance (p. 204) complex trait (p. 210) dihybrid (p. 200) dominant allele (p. 195) epistasis (p. 207) F¹ generation (p. 196) F² generation (p. 196) gene (p. 192) genetic cross (p. 196) genetic trait (p. 192) genetics (p. 192) genotype (p. 194) heterozygote (p. 195) homozygote (p. 195) hybrid (p. 198) incomplete dominance (p. 203) law of independent assortment (p. 202)
law of segregation (p. 199)
monohybrid (p. 199)
mutation (p. 195)
P generation (p. 196)
phenotype (p. 193)
pleiotropy (p. 206)
polygenic (p. 208)
Punnett square (p. 199)
recessive allele (p. 195)
Phenotypic Diversity in Guppies
Male guppies (Poecilia reticulata) come in such a rainbow of color phenotypes that in some habitats, no two males are alike. The flashier colors are well suited for attracting mates by showing off, but drab colors are adaptive for avoiding predators and for sneak mating.
Phenotypic variation in guppies

39. Class Quiz, Part 1
The orange gene (O) controls the production of a pigment (pheomelanin) that gives Bengal tigers their orange coat color. Felines that are homozygous for the inactive version of the gene (oo) fail to make pheomelanin and have white fur. If a white tiger (oo) mates with a carrier for the trait (Oo), what are the odds that they will have a white tiger cub?
(The O gene controls orange fur. Black pigment is controlled by other genes, expressed in a stripelike pattern of skin cells; in white tigers (oo), no orange pigment is made anywhere, but the black pigment is produced in the normal pattern, so these tigers keep their stripes.)
4/4
1/2
1/4
0/4
The correct answer is B. In presentation mode, the next mouse click will bring up the animated arrow pointing to the correct answer.
(See page 212, Biology in the News, for more on the genetics of white tigers.)

40. Class Quiz, Part 2 A red carnation and a white carnation
produce offspring that are all pink. The alleles controlling these flower phenotypes show
complete dominance.
incomplete dominance.
codominance.
epistasis.
The correct answer is B.
Pink is the intermediate color between red and white.
Red and white are homozygous and pink is heterozygous.
If complete dominance were occurring, then there would only be two phenotypes, red and white.
If codominance were occurring, then the heterozygote would have both red and white patches on the same flower.

41. Class Quiz, Part 3 Fur color in rabbits shows incomplete dominance.
FBFB individuals are brown, FBFW individuals are
cream, FWFW individuals are white. What is the
expected ratio of a FBFW x FWFW cross?
3 white : 1 brown
3 white : 1 cream
2 white : 2 cream
The correct answer is C.
In order to produce brown rabbits, each parent would have to have at least one copy of the brown allele, since brown is homozygous (FBFB).
Although a 3:1 ratio is possible of the offspring, the expected ratio cannot be 3:1.

42. Class Quiz, Part 4
Marfan syndrome is caused by a single allele, M, that codes for an abnormal version of a protein called fibrillin-1 that is produced by many different cell types and plays an important role in gluing cells together. The symptoms of Marfan syndrome include unusual tallness, with long arms, legs, fingers, and toes; weakening of the aorta, the largest blood vessel carrying blood away from the heart; and eye problems. From this information we can conclude that
A. the M allele is codominant with m allele.
B. the M and m allele is incompletely dominant.
C. Marfan syndrome shows polygenic inheritance.
D. the gene that codes for fibrillin-1 displays pleitropy.
E. the M and m alleles display epistasis.
The correct answer is D.
In Marfan syndrome, a single allele (M) of the gene that codes for fibrillin-1 affects a wide variety of genetic traits, a phenomenon known as pleiotropy.

43. Relevant Art from Other Chapters
All art files from the book are available in JPEG and PPT formats online and on the Instructor Resource Disc

44. A fluorescence in situ hybridization of chromosomes
Figure Genes Are Located on Chromosomes
A technique called fluorescence in situ hybridization (FISH) was used to show the location of three different genes on these chromosomes prepared from tumor cells in mitosis. Each of the three genes (HER2, green; p16, pink; znk217, gold) is known to be involved in cancer.

45. Genes Are Located on Chromosomes
Figure Genes Are Located on Chromosomes
The genes shown here take up a larger portion of the chromosome than they would if they were drawn to scale. The average human chromosome has more than a thousand different genes interspersed with large stretches of noncoding DNA.

46. Autosomes and Sex Chromosomes
Figure Autosomes and Sex Chromosomes
These chromosomes have been stained using a technique called FISH (fluorescence in situ hybridization).

47. Most Chronic Diseases Are Complex Traits
(Chapter 10, page 226. BIOLOGY MATTERS. Most Chronic Diseases Are Complex Traits)

48. Crossing-Over
Figure Segments of Chromosomes Are Exchanged in Crossing-Over
Crossing-over takes place during prophase I. As a result of crossover between A/a and B/b, half of the gametes have a parental genotype (ABC or abc), while the other half have a nonparental genotype (Abc or aBC). In this example, there is no crossing-over between B/b and C/c.

49. The Sorting of One Pair of Homologous Chromosomes Is Independent of the Sorting of Any Other Pair of Homologous Chromosomes
[The line drawings are from a previous edition of Discover Biology.]
(The independent assortment of chromosomes—that is, the random distribution of the different homologous chromosome pairs into daughter cells during meiosis I—contributes to the genetic variety of the gametes produced. It comes about because each homologous chromosome pair orients itself independently—without regard to the alignment of any other homologous pair—when it lines up at the metaphase plate during meiosis I.)
Which homologue of each pair goes to which pole is essentially random.

50. The Flow of Genetic Information
Figure From Gene to Phenotype at the Molecular Level
The readout of information coded in DNA produces the phenotype, such as the activity of lactase enzyme in the small intestine of humans.

51. Messenger RNA Directs Protein Synthesis
Figure Genetic Information Flows from DNA to RNA to Protein during Transcription and Translation
The transcription of a protein-coding gene produces an mRNA molecule, which is then transported to the cytoplasm, where translation occurs and the protein is made with the help of ribosomes. Different amino acids in the protein being constructed at the ribosome are represented here by different colors and shapes.

52. 9.1 Concept Check, Part 1
1. What is an allele? How do new alleles arise? How many different alleles can a single person carry for a particular gene?
ANSWER: Alleles are different versions of a particular gene. New alleles arise by mutation, a change to the gene’s DNA code. Normally, an individual carries only two alleles for any gene.

53. 9.1 Concept Check, Part 2
2. For a single genetic trait, what is the difference between a phenotype and a genotype?
ANSWER: The phenotype is the particular version of a genetic trait that is displayed by an individual, such as wiry hair or smooth hair in dogs. The genotype specifies the allelic makeup that determines the phenotype. Dogs with wiry hair (a dominant trait) have either the genotype WW or Ww; those with smooth hair (recessive) have the genotype ww.

54. 9.2 Concept Check, Part 1
1. Hair length in cats is controlled by a gene that has at least two alleles, L and l. Short-haired cats are LL or Ll. Long hair is a breed standard for Maine coon cats. Are these cats homozygous for the hair length allele? What is their genotype?
ANSWER: Purebred Maine coon cats are homozygous recessive for the hair length gene, with the ll genotype.

55. 9.2 Concept Check, Part 2
2. Which of Mendel’s observations demonstrated that the theory of blending inheritance was false?
ANSWER: The F1 generation did not have an intermediate phenotype, and moreover, the parental phenotypes reappeared in the F2 generation instead of vanishing forever into a blended phenotype.

56. 9.3 Concept Check, Part 1
1. For the offspring of a cross between an Rr plant and an rr plant, with R being dominant, predict the number and ratio of genotypes and phenotypes.
ANSWER: Two genotypes (Rr and rr) and two phenotypes are predicted, each in a ratio of 1:1. Constructing a Punnett square confirms this.

57. 9.3 Concept Check, Part 2
2. Explain Mendel’s law of segregation and law of independent assortment.
ANSWER: The law of segregation states that during gamete formation, two alleles separate into different gametes so that each carries only one allele; the law of independent assortment states that alleles of two different genes are sorted into gametes independently of each other.

58. 9.4 Concept Check, Part 1
1. Allele H produces straight hair; allele HH' produces curly hair. Individuals with the HH' genotype have wavy hair, somewhere between straight and curly. Are alleles H and HH' codominant?
ANSWER: No. They display incomplete dominance
because they produce an intermediate phenotype in the heterozygote.

59. 9.4 Concept Check, Part 2 2. What is pleiotropy?
ANSWER: In pleiotropy, a single gene affects several to many different genetic traits.

60. 9.4 Concept Check, Part 3
3. The ABO blood groups are determined by several alleles of the I gene. Is ABO blood type a polygenic trait?
ANSWER: No. Polygenic means “many genes.” ABO blood groups are controlled by a single gene with multiple alleles.