1. Patterns of Inheritance
Chapter 9
Part 1

2. 9.1 Impacts/Issues Menacing Mucus
Cystic fibrosis, the most common fatal genetic disorder in the US, is caused by a deletion in the CFTR gene
The CF allele persists at high frequency despite its devastating effects
Only those homozygous for the CF allele have the disorder

3. Video: One bad transporter and cystic fibrosis

4. Victims of Cystic Fibrosis
At least one young person in the US dies every day from cystic fibrosis

5. 9.2 Tracking Traits
Mid-1800s: Genes and chromosomes were unknown; Gregor Mendel’s experiments with pea plants established principles of inheritance

6. Breeding Garden Peas

7. 1 2 3
Figure 9.3: Animated!
Breeding garden pea plants. 1 In this example, pollen from a plant that breeds true for purple flowers is brushed onto the flower of a plant that breeds true for white flowers. 2 Seeds develop inside pods of the cross-fertilized plant. The seeds grow into new plants. 3 Every plant that arises from this cross has purple flowers. Predictable patterns such as this offer evidence of how inheritance works.
Fig. 9-3, p. 157

8. Animation: Crossing garden pea plants

9. Phenotype
Gene expression results in phenotype, which refers to an individual’s observable traits
Phenotype
An individual’s observable traits

10. Alleles and Phenotype
Diploid cells have homologous chromosomes (usually one inherited from each parent) with pairs of genes that may or may not be identical
Individuals that carry two identical alleles are homozygous for the gene

11. Paired Genes on Homologous Chromosomes
Homozygous
Having identical alleles of a gene
Heterozygous
Having two different alleles of a gene

12. Genes on Homologous Chromosomes

13. Genes occur in pairs on homologous chromosomes.
The members of each pair of genes may be identical in DNA sequence, or they may be slightly different (alleles).
Figure 9.2: Animated!
Diploid species have pairs of genes, on pairs of homologous chromosomes. Any gene on one chromosome may or may not be identical with its partner on the homologous chromosome.
Fig. 9-2, p. 157

14. Animation: Genetic terms

15. Genotype An individual’s alleles constitute genotype Genotype Hybrid
Hybrids (heterozygotes) have two nonidentical alleles
Genotype
The particular alleles carried by an individual
Hybrid
The offspring of a cross between two individuals that breed true for different forms of a trait

16. Dominant and Recessive Alleles
Refers to an allele that masks the effect of a recessive allele paired with it
Recessive
Refers to an allele with an effect that is masked by a dominant allele paired with it

17. Gene Segregation
Homologous chromosomes separate during meiosis, separating the pairs of genes they carry
Each resulting gamete carries only one of the two members of each gene pair
All gametes of a homozygous parent have the same allele for a trait

18. Gene Segregation

19. DNA replication DNA replication meiosis I meiosis I meiosis II
Figure 9.4
Gene segregation. Homologous chromosomes separate during meiosis, so the pairs of genes they carry separate too. Each of the resulting gametes carries only one of the two members of each gene pair. For clarity, we show nuclei of reproductive cells that carry only one chromosome. 1 All gametes produced by a parent homozygous for a dominant allele carry the dominant allele. 2 All gametes produced by a parent homozygous for a recessive allele carry the recessive allele. 3 If these two parents are crossed, the union of any of their gametes at fertilization produces a zygote with both alleles. Thus, all of the offspring of this cross will be heterozygous.
(gametes)
(gametes)
(zygote)
Fig. 9-4, p. 158

20. DNA replication meiosis I meiosis II (gametes) (zygote) Figure 9.4
Gene segregation. Homologous chromosomes separate during meiosis, so the pairs of genes they carry separate too. Each of the resulting gametes carries only one of the two members of each gene pair. For clarity, we show nuclei of reproductive cells that carry only one chromosome. 1 All gametes produced by a parent homozygous for a dominant allele carry the dominant allele. 2 All gametes produced by a parent homozygous for a recessive allele carry the recessive allele. 3 If these two parents are crossed, the union of any of their gametes at fertilization produces a zygote with both alleles. Thus, all of the offspring of this cross will be heterozygous.
Stepped Art
Fig. 9-4, p. 158

21. Mendelian Inheritance Patterns
A cross, or mating, between heterozygous individuals can reveal dominance relationships among the alleles under study
Monohybrid cross
Cross in which individuals with different alleles of a gene are crossed
Dominant trait will have a 3:1 phenotype ratio

22. Punnett Squares
Punnett squares are used to calculate the probability of the genotype and phenotype of the offspring of crosses
Punnett square
Diagram used to predict the outcome of a cross

23. Punnett Squares: A Monohybrid Cross

24. Figure 9.5: Animated!
Example of a monohybrid cross.
Figure It Out: How many genotypes are possible among the offspring of this cross?
Answer: Three: AA, Aa, and aa.
Fig. 9-5a, p. 159

25. gametes in the corresponding row and column met up.
female gametes
male gametes
Figure 9.5: Animated!
Example of a monohybrid cross.
Figure It Out: How many genotypes are possible among the offspring of this cross?
Answer: Three: AA, Aa, and aa.
A Making a Punnett square. The possible parental gametes are listed on the top and left sides of the
grid (in circles). Each square is filled in with the combination of alleles that would result if the
gametes in the corresponding row and column met up.
Fig. 9-5a, p. 159

26. Figure 9.5: Animated!
Example of a monohybrid cross.
Figure It Out: How many genotypes are possible among the offspring of this cross?
Answer: Three: AA, Aa, and aa.
Fig. 9-5b, p. 159

27. offspring Heterozygous parent Heterozygous parent
Figure 9.5: Animated!
Example of a monohybrid cross.
Figure It Out: How many genotypes are possible among the offspring of this cross?
Answer: Three: AA, Aa, and aa.
Heterozygous parent
B A Punnett square shows that the ratio of dominant-to-recessive phenotypes among offspring of a monohybrid cross is about 3:1 (3 purple to 1 white).
Fig. 9-5b, p. 159

28. female gametes offspring male gametes Heterozygous parent
A) Making a Punnett square. The possible parental gametes are listed on the top and left sides of the grid (in circles). Each square is filled in with the combination of alleles that would result if the gametes in the corresponding row and column met up.
offspring
Heterozygous parent
B) A Punnett square shows that the ratio of dominant-to-recessive phenotypes among offspring of a monohybrid cross is about 3:1 (3 purple to 1 white).
Figure 9.5: Animated!
Example of a monohybrid cross.
Figure It Out: How many genotypes are possible among the offspring of this cross?
Answer: Three: AA, Aa, and aa.
Stepped Art
Fig. 9-5b, p. 159

29. Animation: Monohybrid cross

30. Independent Assortment
Mendel’s dihybrid crosses showed inheritance of one trait did not affect inheritance of other traits
Dihybrid cross
Experiment in which individuals with different alleles of two genes are crossed (9:3:3:1 ratio)
Independent assortment
A gene tends to be distributed independently of how other genes are distributed

31. Independent Assortment During Meiosis
During meiosis, the genes of each pair separate
Each gamete gets one or the other gene
Meiosis assorts gene pairs on homologous chromosomes independently of gene pairs on other chromosomes
Homologous chromosomes randomly attach to opposite spindle poles during prophase I

32. Independent Assortment During Meiosis

33. blue, have already been duplicated.
A This example shows just two pairs of homologous chromosomes in the nucleus of
a diploid (2n) reproductive cell. Maternal and paternal chromosomes, shown in pink and
blue, have already been duplicated.
B Either chromosome of a pair may get attached
to either spindle pole during meiosis I. With two
pairs of homologous chromosomes, there are
two different ways that the maternal and paternal homologues can get attached to opposite
spindle poles. or
meiosis I
meiosis I
C Two nuclei form with each scenario, so
there are a total of four possible combinations of parental chromosomes in the nuclei that
form after meiosis I.
meiosis II
meiosis II
D Thus, when sister chromatids separate during meiosis II, the gametes that result have one of four possible combinations of maternal and paternal chromosomes.
Figure 9.6
Independent assortment during meiosis. Here, we track how alleles of two gene pairs on two chromosomes are distributed into gametes.
gamete genotype:
Fig. 9-6, p. 160

34. blue, have already been duplicated.
A) This example shows just two pairs of homologous chromosomes in the nucleus of
a diploid (2n) reproductive cell. Maternal and paternal chromosomes, shown in pink and
blue, have already been duplicated.
B) Either chromosome of a pair may get attached
to either spindle pole during meiosis I. With two
pairs of homologous chromosomes, there are
two different ways that the maternal and paternal homologues can get attached to opposite
spindle poles. or
C) Two nuclei form with each scenario, so
there are a total of four possible combinations of parental chromosomes in the nuclei that
form after meiosis I.
meiosis I
D) Thus, when sister chromatids separate during meiosis II, the gametes that result have one of four possible combinations of maternal and paternal chromosomes.
meiosis II
gamete genotype:
Figure 9.6
Independent assortment during meiosis. Here, we track how alleles of two gene pairs on two chromosomes are distributed into gametes.
Stepped Art
Fig. 9-6, p. 160

35. A Dihybrid Cross

36. Figure 9.7: Animated!
A dihybrid cross between plants that differ in flower color and plant height. A and a stand for dominant and recessive alleles for flower color. B and b stand for dominant and recessive alleles for height. 1 Meiosis in a homozygous individual results in one type of gamete. 2 A cross between two homozygous individuals yields one type of gamete. All offspring that form in this example are dihybrids (heterozygous for two genes). 3 Meiosis in dihybrid individuals results in four kinds of gametes. 4 If two dihybrid individuals are crossed (a dihybrid cross), the four types of gametes can meet up in 16 possible ways. Out of 16 possible genotypes of the offspring, 9 will result in plants that are purple-flowered and tall; 3, purple-flowered and short; 3, white-flowered and tall; and 1, white-flowered and short. The ratio of phenotypes in a dihybrid cross is 9:3:3:1.
Fig. 9-7a, p. 161

37. parent plant homozygous for purple flowers and long stems
for white flowers and short stems
Figure 9.7: Animated!
A dihybrid cross between plants that differ in flower color and plant height. A and a stand for dominant and recessive alleles for flower color. B and b stand for dominant and recessive alleles for height. 1 Meiosis in a homozygous individual results in one type of gamete. 2 A cross between two homozygous individuals yields one type of gamete. All offspring that form in this example are dihybrids (heterozygous for two genes). 3 Meiosis in dihybrid individuals results in four kinds of gametes. 4 If two dihybrid individuals are crossed (a dihybrid cross), the four types of gametes can meet up in 16 possible ways. Out of 16 possible genotypes of the offspring, 9 will result in plants that are purple-flowered and tall; 3, purple-flowered and short; 3, white-flowered and tall; and 1, white-flowered and short. The ratio of phenotypes in a dihybrid cross is 9:3:3:1.
dihybrid
four types of gametes
Fig. 9-7a, p. 161

38. Figure 9.7: Animated!
A dihybrid cross between plants that differ in flower color and plant height. A and a stand for dominant and recessive alleles for flower color. B and b stand for dominant and recessive alleles for height. 1 Meiosis in a homozygous individual results in one type of gamete. 2 A cross between two homozygous individuals yields one type of gamete. All offspring that form in this example are dihybrids (heterozygous for two genes). 3 Meiosis in dihybrid individuals results in four kinds of gametes. 4 If two dihybrid individuals are crossed (a dihybrid cross), the four types of gametes can meet up in 16 possible ways. Out of 16 possible genotypes of the offspring, 9 will result in plants that are purple-flowered and tall; 3, purple-flowered and short; 3, white-flowered and tall; and 1, white-flowered and short. The ratio of phenotypes in a dihybrid cross is 9:3:3:1.
Fig. 9-7b, p. 161

39. parent plant homozygous for purple flowers and long stems
for white flowers and short stems
2) dihybrid
Figure 9.7: Animated!
A dihybrid cross between plants that differ in flower color and plant height. A and a stand for dominant and recessive alleles for flower color. B and b stand for dominant and recessive alleles for height. 1 Meiosis in a homozygous individual results in one type of gamete. 2 A cross between two homozygous individuals yields one type of gamete. All offspring that form in this example are dihybrids (heterozygous for two genes). 3 Meiosis in dihybrid individuals results in four kinds of gametes. 4 If two dihybrid individuals are crossed (a dihybrid cross), the four types of gametes can meet up in 16 possible ways. Out of 16 possible genotypes of the offspring, 9 will result in plants that are purple-flowered and tall; 3, purple-flowered and short; 3, white-flowered and tall; and 1, white-flowered and short. The ratio of phenotypes in a dihybrid cross is 9:3:3:1.
3) four types of gametes
Stepped Art
Fig. 9-7b, p. 161

40. Animation: Testcross

41. The Contribution of Crossovers
Two genes located close together on the same chromosome tend to be inherited together
When two genes on the same chromosome are far apart, crossing over occurs more frequently between them; they tend to assort independently

42. A Human Example: Skin Color
Variations in skin color depend on the kinds and amounts of melanins produced
More than 100 gene products affect production and deposition of melanins
Independent assortment of these genes produces a wide variety of phenotypes

43. Variation in Skin Color

44. Animation: Dihybrid cross

45. Animation: F2 ratios interaction

46. Video: ABC News: All in the family: mixed race twins

47. Animation: Genotypes variation calculator

48. Animation: Chromosome segregation

49. 9.3 Beyond Simple Dominance
An allele may be fully dominant, incompletely dominant, or codominant with its partner on a homologous chromosome

50. Codominance and Incomplete Dominance
Codominant
Refers to two alleles that are both fully expressed in heterozygous individuals
Incomplete dominance
Condition in which one allele is not fully dominant over another, so the heterozygous phenotype is between the two homozygous phenotypes

51. Codominance: Blood Type

52. Phenotypes (blood type): A AB B O
Genotypes:
Figure 9.9: Animated!
Combinations of alleles that are the basis of blood type.
Phenotypes (blood type): A AB B O
Fig. 9-9, p. 163

53. Animation: Codominance: ABO blood types

54. Incomplete Dominance

55. homozygous parent (RR) homozygous parent (rr)
Figure 9.10
Incomplete dominance of flower color alleles in snapdragon plants.
Figure It Out: Is the experiment in (B) a monohybrid or dihybrid cross?
Answer: A monohybrid cross.
homozygous parent (RR)
homozygous parent (rr)
heterozygous offspring (Rr) x
A Cross a red-flowered with a white-flowered plant, and all of the offspring will be pink heterozygotes.
Fig. 9-10a, p. 163

56. B If two of the heterozygotes are crossed, the phenotypes
of the resulting offspring will occur in a 1:2:1 ratio.
Figure 9.10
Incomplete dominance of flower color alleles in snapdragon plants.
Figure It Out: Is the experiment in (B) a monohybrid or dihybrid cross?
Answer: A monohybrid cross.
Fig. 9-10b, p. 163

57. Animation: Incomplete dominance

58. Epistasis
Some traits are affected by multiple gene products, an effect called polygenic inheritance or epistasis
Epistasis
Effect in which a trait is influenced by the products of multiple genes
Example: labrador retriever coat color

59. Epistasis

60. Pleiotropy Pleiotropic genes influence two or more traits Pleiotropic
Mutations in pleiotropic genes are associated with sickle cell anemia, cystic fibrosis, and Marfan syndrome
Pleiotropic
Refers to a gene whose product influences multiple traits

61. Pleiotropy
Marfan syndrome, caused by mutation in a pleiotropic gene encoding fibrillin
Basketball star Haris Charalambous, age 21, died of a burst aorta during exercise

62. Animation: Pleiotropic effects of Marfan syndrome

63. Animation: Comb shape in chickens

64. Animation: Coat color in Labrador retrievers

65. Animation: ABO compatibilities