Patterns of Inheritance Chapter 9 Part 1

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

Video: One bad transporter and cystic fibrosis

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

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

Breeding Garden Peas

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

Animation: Crossing garden pea plants

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

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

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

Genes on Homologous Chromosomes

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

Animation: Genetic terms

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

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

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

Gene Segregation

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

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

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

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

Punnett Squares: A Monohybrid Cross

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

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

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

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

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

Animation: Monohybrid cross

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

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

Independent Assortment During Meiosis

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

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

A Dihybrid Cross

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

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

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

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

Animation: Testcross

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

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

Variation in Skin Color

Animation: Dihybrid cross

Animation: F2 ratios interaction

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

Animation: Genotypes variation calculator

Animation: Chromosome segregation

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

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

Codominance: Blood Type

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

Animation: Codominance: ABO blood types

Incomplete Dominance

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

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

Animation: Incomplete dominance

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

Epistasis

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

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

Animation: Pleiotropic effects of Marfan syndrome

Animation: Comb shape in chickens

Animation: Coat color in Labrador retrievers

Animation: ABO compatibilities