1. Chapter 11 Introduction to Genetics
SC.912.L.16.1 Use Mendel’s Laws of segregation and independent assortment to analyze patterns of inheritance

2. 11.1 The work of Gregor Mendel Objectives
Describe Mendel’s studies and conclusions about inheritance.
Describe the role of fertilization.

3. Analyzing Inheritance
Offspring resemble their parents. Offspring inherit genes for characteristics from their parents. To learn about inheritance, scientists have experimented with breeding various plants and animals.
In each experiment shown in the table on the next slide, two pea plants with different characteristics were bred. Then, the offspring produced were bred to produce a second generation of offspring. Consider the data and answer the questions that follow.

4. Analyzing Inheritance
Interest Grabber continued
Parents
Long stems  short stems
Red flowers  white flowers
Green pods  yellow pods
Round seeds  wrinkled seeds
Yellow seeds  green seeds
First Generation
All long
All red
All green
All round
All yellow
Second Generation
787 long: 277 short
705 red: 224 white
428 green: 152 yellow
5474 round: 1850 wrinkled
6022 yellow: 2001 green
1. In the first generation of each experiment, how do the characteristics of the offspring compare to the parents’ characteristics?
2. How do the characteristics of the second generation compare to the characteristics of the first generation?

5. Gregor Mendel
Documented a particulate mechanism of inheritance through his experiments with garden peas
Figure 11.1

6. 11.1 The work of Gregor Mendel
Mendel discovered the basic principles of heredity
By breeding garden peas in carefully planned experiments
Why do you think Mendel chose to work with pea plants?
Because they are available in many varieties
Reproduce fast, and
Because he could strictly control which plants mated with which

7. Crossing pea plants Figure 11.25 4 3 2
Removed stamens
from purple flower
Transferred sperm-
bearing pollen from
stamens of white
flower to egg-
bearing carpel of
purple flower
Parental
generation
(P)
Pollinated carpel
matured into pod
Carpel
(female)
Stamens
(male)
Planted seeds
from pod
Examined
offspring:
all purple
flowers
First
offspring
(F1)
APPLICATION By crossing (mating) two true-breeding
varieties of an organism, scientists can study patterns of
inheritance. In this example, Mendel crossed pea plants
that varied in flower color.
TECHNIQUE
When pollen from a white flower fertilizes
eggs of a purple flower, the first-generation hybrids all have purple
flowers. The result is the same for the reciprocal cross, the transfer
of pollen from purple flowers to white flowers.
RESULTS
Figure 11.2

8. Some genetic vocabulary
Character: a heritable feature, such as flower color
Trait: a variant of a character, such as purple or white flowers

9. Mendel also made sure that
Mendel chose to track
Only those characters that varied in an “either-or” manner
Mendel also made sure that
He started his experiments with varieties that were “true-breeding”

10. In a typical breeding experiment
Mendel mated two contrasting, true-breeding varieties, a process called hybridization
The true-breeding parents
Are called the P generation

11. The hybrid offspring of the P generation
Are called the F1 generation
When F1 individuals self-pollinate
The F2 generation is produced

12. The Law of Segregation
When Mendel crossed contrasting, true-breeding white and purple flowered pea plants
All of the offspring were purple
When Mendel crossed the F1 plants
Many of the plants had purple flowers, but some had white flowers

13. Mendel discovered
A ratio of about three to one, purple to white flowers, in the F2 generation
P Generation
(true-breeding
parents)
Purple
flowers
White 
F1 Generation
(hybrids)
All plants had
purple flowers
F2 Generation

14. Mendel reasoned that
In the F1 plants, only the purple flower factor was affecting flower color in these hybrids
Purple flower color was dominant, and white flower color was recessive

15. Mendel observed the same pattern
In many other pea plant characters

16. Mendel’s Model Mendel developed a hypothesis
To explain the 3:1 inheritance pattern that he observed among the F2 offspring
Four related concepts make up this model

17. First, alternative versions of genes
Account for variations in inherited characters, which are now called alleles
Allele for purple flowers
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers

18. Second, for each character
An organism inherits two alleles, one from each parent
A genetic locus is actually represented twice

19. Third, if the two alleles at a locus differ
Then one, the dominant allele, determines the organism’s appearance
The other allele, the recessive allele, has no noticeable effect on the organism’s appearance

20. Fourth, the law of segregation
The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

21. Does Mendel’s segregation model account for the 3:1 ratio he observed in the F2 generation of his numerous crosses?
We can answer this question using a Punnett square

22. Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses
The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup
A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele
© 2011 Pearson Education, Inc.

23. P Generation Appearance: Purple flowers White flowers Genetic makeup:
Figure
P Generation
Appearance:
Purple flowers
White flowers
Genetic makeup: PP pp
Gametes: P p
Figure 14.5 Mendel’s law of segregation.

24. P Generation Appearance: Purple flowers White flowers Genetic makeup:
Figure
P Generation
Appearance:
Purple flowers
White flowers
Genetic makeup: PP pp
Gametes: P p
F1 Generation
Appearance:
Purple flowers
Genetic makeup: Pp
Gametes:
1/2 P
1/2 p
Figure 14.5 Mendel’s law of segregation.

25. P Generation Appearance: Purple flowers White flowers Genetic makeup:
Figure
P Generation
Appearance:
Purple flowers
White flowers
Genetic makeup: PP pp
Gametes: P p
F1 Generation
Appearance:
Purple flowers
Genetic makeup: Pp
Gametes:
1/2 P
1/2 p
Sperm from F1 (Pp) plant
F2 Generation P p
Figure 14.5 Mendel’s law of segregation. P
Eggs from F1 (Pp) plant PP Pp p Pp pp 3
: 1

26. Useful Genetic Vocabulary
An organism that is homozygous for a particular gene
Has a pair of identical alleles for that gene
Exhibits true-breeding
An organism that is heterozygous for a particular gene
Has a pair of alleles that are different for that gene

27. An organism’s phenotype An organism’s genotype
Is its physical appearance
An organism’s genotype
Is its genetic makeup

28. Phenotype versus genotype3 1 2
Phenotype
Purple
White
Genotype PP
(homozygous) Pp
(heterozygous) pp
Ratio 3:1
Ratio 1:2:1

29. The Testcross In pea plants with purple flowers
The genotype is not immediately obvious
How can we tell the genotype of an individual with the dominant phenotype?
Such an individual could be either homozygous dominant or heterozygous

30. A testcross
Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype
Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait

31. Dominant phenotype, unknown genotype: PP or Pp?
Figure 14.7
TECHNIQUE
Dominant phenotype, unknown genotype: PP or Pp?
Recessive phenotype, known genotype: pp
Predictions
If purple-flowered parent is PP or
If purple-flowered parent is Pp
Sperm
Sperm p p p p P P Pp Pp Pp Pp
Eggs
Eggs
Figure 14.7 Research Method: The Testcross P p Pp Pp pp pp
RESULTS or
All offspring purple
1/2 offspring purple and 1/2 offspring white

32. The Law of Independent Assortment
Mendel derived the law of segregation by following a single character
The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one character
A cross between such heterozygotes is called a monohybrid cross
© 2011 Pearson Education, Inc.

33. 11.2 Probabilities and Punnett Squares
Solve these genetics problems. Be sure to complete the Punnett square to show how you derived your solution. 1. In humans the allele for albinism is recessive to the allele for normal skin pigmentation. If two heterozygotes have children, what is the chance that a child will have normal skin pigment? What is the chance that a child will be albino?
If the child is normal, what is the chance that it is a carrier (heterozygous) for the albino allele? (CAREFUL!) _______________________

34. 2. In purple people eaters, one-horn is dominant and no horns is recessive. Show the cross of a purple people eater that is heterozygous for horns with a purple people eater that does not have horns. Summarize the genotypes & phenotypes of the possible offspring?

35. 3. In humans, the brown-eye (B) allele is dominant to the blue-eye allele (b).
If two heterozygotes mate, what will be the likely genotype and phenotype ratios of the offspring. Show your work.
Genotypic Ratio:__________
Phenotypic Ratio:_________

36. 4. In seals, the gene for the length of the whiskers has two alleles
4. In seals, the gene for the length of the whiskers has two alleles. The dominant allele (W) codes long whiskers & the recessive allele (w) codes for short whiskers.
What percentage of offspring would be expected to have short whiskers from the cross of two long-whiskered seals, one that is homozygous dominant and one that is heterozygous?
Percentage of short whiskers:_____

37. Mendel identified his second law of inheritance by following two characters at the same time
Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters
A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently
© 2011 Pearson Education, Inc.

38. Hypothesis of dependent assortment
Figure 14.8
EXPERIMENT
YYRR
P Generation
yyrr
Gametes YR yr
F1 Generation
YyRr
Predictions
Hypothesis of dependent assortment
Hypothesis of independent assortment
Sperm
Predicted offspring of F2 generation or
1/4 YR
1/4 Yr
1/4 yR
1/4 yr
Sperm
1/2 YR
1/2 yr
1/4 YR
YYRR
YYRr
YyRR
YyRr
1/2 YR
YYRR
YyRr
1/4 Yr
Eggs
YYRr
YYrr
YyRr
Yyrr
Eggs
Figure 14.8 Inquiry: Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character?
1/2 yr
YyRr
yyrr
1/4 yR
YyRR
YyRr
yyRR
yyRr
3/4
1/4
1/4 yr
Phenotypic ratio 3:1
YyRr
Yyrr
yyRr
yyrr
9/16
3/16
3/16
1/16
Phenotypic ratio 9:3:3:1
RESULTS
315
108
101 32
Phenotypic ratio approximately 9:3:3:1

39. Using a dihybrid cross, Mendel developed the law of independent assortment
The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation
Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome
Genes located near each other on the same chromosome tend to be inherited together
© 2011 Pearson Education, Inc.

40. Concept Map Gregor Mendel Pea plants “Factors” determine traits Law of
Section 11-3
Gregor
Mendel
experimented with
concluded that
Pea
plants
“Factors”
determine
traits
Some alleles
are dominant,
and some alleles
are recessive
Alleles are
separated during
gamete formation
which is called the
which is called the
Law of
Dominance
Segregation

41. 11.3 Other Patterns of Inheritance
Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:
When alleles are not completely dominant or recessive
When a gene has more than two alleles
When a gene produces multiple phenotypes
© 2011 Pearson Education, Inc.

42. The Spectrum of Dominance
Complete dominance
Occurs when the phenotypes of the heterozygote and dominant homozygote are identical
PP and Pp

43. In incomplete dominance
The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties (neither-nor)
P Generation
F1 Generation
F2 Generation
Red
CRCR
Gametes CR CW 
White
CWCW
Pink
CRCW
Sperm Cw
1⁄2
Eggs
CR CR
CR CW
CW CW

44. Answer the following question
In radishes, the gene that controls color exhibits incomplete dominance. Pure-breeding red radishes crossed with pure-breeding white radishes make purple radishes. What are the genotypic and phenotypic ratios when you cross a purple radish with a white radish?
Genotypic Ratio:__________
Phenotypic Ratio:_________

45. In codominance
Two dominant alleles affect the phenotype in separate, distinguishable ways

46. In a certain fish, blue scales (B) and red scales (R) are codominant.
1. Write the genotype for the fish shown below.

47. When a fish has the genotype BR, it has a patchwork of blue and red scales. (from previous slide)
What happens if you breed this fish with a fish that only has Blue Scales.
Genotypic Ratio:__________
Phenotypic Ratio:_________

48. Frequency of Dominant Alleles Dominant alleles
Are not necessarily more common in populations than recessive alleles

49. Most genes exist in populations
Multiple Alleles
Most genes exist in populations
In more than two allelic forms

50. The ABO blood group in humans
Multiple Allele
The ABO blood group in humans
Is determined by multiple alleles
Table 14.2

51. Extending Mendelian Genetics for Two or More Genes
Some traits
May be determined by two or more genes

52. Polygenic Inheritance
Many human characters
Vary in the population along a continuum and are called quantitative characters

53. Quantitative variation usually indicates polygenic inheritance
An additive effect of two or more genes on a single phenotype 
AaBbCc
aabbcc
Aabbcc
AaBbcc
AABbCc
AABBCc
AABBCC
20⁄64
15⁄64
6⁄64
1⁄64
Fraction of progeny
Figure 14.12

54. Nature and Nurture: The Environmental Impact on Phenotype
Another departure from simple Mendelian genetics arises
When the phenotype for a character depends on environment as well as on genotype

55. The norm of reaction
Is the phenotypic range of a particular genotype that is influenced by the environment
Figure 14.13

56. Multifactorial characters
Are those that are influenced by both genetic and environmental factors

57. Integrating a Mendelian View of Heredity and Variation
An organism’s phenotype
Includes its physical appearance, internal anatomy, physiology, and behavior
Reflects its overall genotype and unique environmental history

58. Even in more complex inheritance patterns
Mendel’s fundamental laws of segregation and independent assortment still apply

59. Pedigree Analysis A pedigree
Is a family tree that describes the interrelationships of parents and children across generations

60. Inheritance patterns of particular traits
Can be traced and described using pedigrees Ww ww WW or
First generation
(grandparents)
Second generation
(parents plus aunts
and uncles)
Third
generation
(two sisters) Ff ff
FF or Ff FF
Widow’s peak
No Widow’s peak
Attached earlobe
Free earlobe
(a) Dominant trait (widow’s peak)
(b) Recessive trait (attached earlobe)
Figure A, B

61. Pedigrees
Can also be used to make predictions about future offspring

62. Recessively Inherited Disorders
Many genetic disorders
Are inherited in a recessive manner

63. Recessively inherited disorders
Show up only in individuals homozygous for the allele
Carriers
Are heterozygous individuals who carry the recessive allele but are phenotypically normal

64. Symptoms of cystic fibrosis include
Mucus buildup in the some internal organs
Abnormal absorption of nutrients in the small intestine

65. Sickle-Cell Disease Sickle-cell disease Symptoms include
Affects one out of 400 African-Americans
Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
Symptoms include
Physical weakness, pain, organ damage, and even paralysis

66. Mating of Close Relatives
Matings between relatives
Can increase the probability of the appearance of a genetic disease
Are called consanguineous matings

67. Dominantly Inherited Disorders
Some human disorders
Are due to dominant alleles

68. One example is achondroplasia
A form of dwarfism that is lethal when homozygous for the dominant allele
Figure 14.15

69. Huntington’s disease Is a degenerative disease of the nervous system
Has no obvious phenotypic effects until about 35 to 40 years of age
Figure 14.16

70. Multifactorial Disorders
Many human diseases
Have both genetic and environment components
Examples include
Heart disease and cancer

71. Genetic Testing and Counseling
Genetic counselors
Can provide information to prospective parents concerned about a family history for a specific disease

72. Counseling Based on Mendelian Genetics and Probability Rules
Using family histories
Genetic counselors help couples determine the odds that their children will have genetic disorders

73. Tests for Identifying Carriers
For a growing number of diseases
Tests are available that identify carriers and help define the odds more accurately

74. In chorionic villus sampling (CVS)
Fetal Testing
In amniocentesis
The liquid that bathes the fetus is removed and tested
In chorionic villus sampling (CVS)
A sample of the placenta is removed and tested

75. (b) Chorionic villus sampling (CVS)
Fetal testing
(a) Amniocentesis
Amniotic
fluid
withdrawn
Fetus
Placenta
Uterus
Cervix
Centrifugation
A sample of
amniotic fluid can
be taken starting at
the 14th to 16th
week of pregnancy.
(b) Chorionic villus sampling (CVS)
Fluid
Fetal
cells
Biochemical tests can be
Performed immediately on
the amniotic fluid or later
on the cultured cells.
Fetal cells must be cultured
for several weeks to obtain
sufficient numbers for
karyotyping.
Several
weeks
Biochemical
tests
hours
Chorionic viIIi
A sample of chorionic villus
tissue can be taken as early
as the 8th to 10th week of
pregnancy.
Suction tube
Inserted through
cervix
Karyotyping and biochemical
tests can be performed on
the fetal cells immediately,
providing results within a day
or so.
Karyotyping
Figure A, B

76. Some genetic disorders can be detected at birth
Newborn Screening
Some genetic disorders can be detected at birth
By simple tests that are now routinely performed in most hospitals in the United States