Chapter 13 Observing Patterns in Inherited Traits (Sections 13.4 - 13.6) 1

13.4 Mendel’s Theory of Independent Assortment When homologous chromosomes separate during meiosis, either one of the pair can end up in a particular nucleus Thus, gene pairs on one chromosome get sorted into gametes independently of gene pairs on other chromosomes Punnett squares can be used to predict inheritance patterns of two or more genes simultaneously

Dihybrid Cross In a dihybrid cross, individuals identically heterozygous for alleles of two genes (dihybrids) are crossed, and the traits of the offspring are observed dihybrid cross Breeding experiment in which individuals identically heterozygous for two genes are crossed The frequency of traits among the offspring offers information about the dominance relationships between the paired alleles

A Dihybrid Cross Start with one parent plant that breeds true for purple flowers and tall stems (PPTT ) and one that breeds true for white flowers and short stems (pptt) Each plant makes only one type of gamete (PT or pt) All F1 offspring will be dihybrids (PpTt) and have purple flowers and tall stems

A Dihybrid Cross (cont.) Then cross two F1 plants: a dihybrid cross (PpTt X PpTt) Four types of gametes can combine in sixteen possible ways In F2 plants, four phenotypes result in a ratio of 9:3:3:1 9 tall with purple flowers 3 short with purple flowers 3 tall with white flowers 1 short with white flowers

Law of Independent Assortment Mendel discovered the 9:3:3:1 ratio in his dihybrid experiments – and noted that each trait still kept its individual 3:1 ratio Each trait (gene pair) sorted into gametes independently of other traits (gene pairs) law of independent assortment During meiosis, members of a pair of genes on homologous chromosomes get distributed into gametes independently of other gene pairs

Independent Assortment

Independent Assortment 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 chromosomes 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. Figure 13.7 Independent assortment. 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. gamete genotype: pt PT pT Pt Fig. 13.7, p. 194

Independent Assortment 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 chromosomes can get attached to opposite spindle poles. or 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. Figure 13.7 Independent assortment. 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. gamete genotype: Pt pT PT pt Stepped Art Fig. 13.7, p. 194

A Dihybrid Cross

A Dihybrid Cross parent plant homozygous 4 PT Pt pT pt PP PPTT PPTt PpTT PpTt parent plant homozygous for purple flowers and long stems parent plant homozygous for white flowers and short stems Pt PPTt PPtt PpTt Pptt PPTT pptt PT pt pT PpTT PpTt ppTT ppTt 1 Figure 13.8 A dihybrid cross between plants that differ in flower color and plant height. P and p stand for dominant and recessive alleles for flower color. T and t stand for dominant and recessive alleles for height. 2 PpTt dihybrid pt PpTt Pptt ppTt pptt PT Pt pT pt 3 four types of gametes Fig. 13.8, p. 195

A Dihybrid Cross Figure 13.8 A dihybrid cross between plants that differ in flower color and plant height. P and p stand for dominant and recessive alleles for flower color. T and t stand for dominant and recessive alleles for height. Fig. 13.8.1-3, p. 195

A Dihybrid Cross Figure 13.8 A dihybrid cross between plants that differ in flower color and plant height. P and p stand for dominant and recessive alleles for flower color. T and t stand for dominant and recessive alleles for height. Fig. 13.8.4, p. 195

A Dihybrid Cross parent plant homozygous 4 PT Pt pT pt PP PPTT PPTt PpTT PpTt parent plant homozygous for purple flowers and long stems parent plant homozygous for white flowers and short stems Pt PPTt PPtt PpTt Pptt PPTT pptt PT pt pT PpTT PpTt ppTT ppTt 1 Figure 13.8 A dihybrid cross between plants that differ in flower color and plant height. P and p stand for dominant and recessive alleles for flower color. T and t stand for dominant and recessive alleles for height. 2 PpTt dihybrid pt PpTt Pptt ppTt pptt PT Pt pT pt 3 four types of gametes Stepped Art Fig. 13.8, p. 195

ANIMATION: Dihybrid cross To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

The Contribution of Crossovers Genes that are far apart on a chromosome tend to assort into gametes independently because crossing over occurs between them very frequently Genes that are very close together on a chromosome are linked, they do not assort independently because crossing over rarely happens between them linkage group All genes on a chromosome

Key Concepts Insights From Dihybrid Crosses Pairs of genes on different chromosomes are typically distributed into gametes independently of how other gene pairs are distributed Breeding experiments with alternative forms of two unrelated traits can be used as evidence of such independent assortment

ANIMATION: Crossover review To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

13.5 Beyond Simple Dominance Mendel studied inheritance patterns that are examples of simple dominance, in which a dominant allele fully masks the expression of a recessive one Other patterns of inheritance are not so simple: Codominance Incomplete dominance Epistasis Pleiotropy

Codominance Codominant alleles are both expressed at the same time in heterozygotes, as in multiple allele systems such as the one underlying ABO blood typing codominant Refers to two alleles that are both fully expressed in heterozygous individuals multiple allele system Gene for which three or more alleles persist in a population

Codominance: ABO Blood Types Which two of the three alleles of the ABO gene you have determines your blood type The A and the B allele are codominant when paired Genotype AB = blood type AB The O allele is recessive when paired with either A or B Genotype AA or AO = blood type A Genotype BB or BO= type B Genotype OO = type O

Codominance: ABO Blood Types

Codominance: ABO Blood Types AA or AO BB or BO Genotypes: AB OO Figure 13.9 Combinations of alleles that are the basis of human blood type. Phenotypes (blood type): A AB B O Fig. 13.9, p. 196

ANIMATION: Codominance: ABO blood types To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

Incomplete Dominance With incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypes incomplete dominance Condition in which one allele is not fully dominant over another, so the heterozygous phenotype is between the two homozygous phenotypes

Incomplete Dominance: Snapdragons In snapdragons, one allele (R) encodes an enzyme that makes a red pigment, and allele (r) makes no pigment RR = red; Rr = pink; rr = white A cross between red and white (RR X rr) yields pink (Rr) A cross between two pink (Rr X Rr) yields red, pink, and white in a 1:2:1 ratio

Incomplete Dominance: Snapdragons

Incomplete Dominance: Snapdragons homozygous (RR) x homozygous (rr) heterozygous (Rr) A Cross a red-flowered with a white-flowered plant, and all of the offspring will be pink heterozygotes. Figure 13.10 Incomplete dominance in heterozygous (pink) snapdragons. An allele that affects red pigment is paired with a ‘white’ allele. B If two of the pink heterozygotes are crossed, the phenotypes of the resulting offspring will occur in a 1:2:1 ratio. Fig. 13.10, p. 196

Incomplete Dominance: Snapdragons Figure 13.10 Incomplete dominance in heterozygous (pink) snapdragons. An allele that affects red pigment is paired with a ‘white’ allele. Fig. 13.10a, p. 196

Incomplete Dominance: Snapdragons Figure 13.10 Incomplete dominance in heterozygous (pink) snapdragons. An allele that affects red pigment is paired with a ‘white’ allele. homozygous (RR) x homozygous (rr) heterozygous (Rr) A Cross a red-flowered with a white-flowered plant, and all of the offspring will be pink heterozygotes. Fig. 13.10a, p. 196

Incomplete Dominance: Snapdragons Figure 13.10 Incomplete dominance in heterozygous (pink) snapdragons. An allele that affects red pigment is paired with a ‘white’ allele. Fig. 13.10b, p. 196

Incomplete Dominance: Snapdragons B If two of the pink heterozygotes are crossed, the phenotypes of the resulting offspring will occur in a 1:2:1 ratio. Figure 13.10 Incomplete dominance in heterozygous (pink) snapdragons. An allele that affects red pigment is paired with a ‘white’ allele. Fig. 13.10b, p. 196

ANIMATION: Incomplete dominance To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

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

Epistasis: Labrador Retriever Labrador retriever coat color, can be black, brown, or yellow Figure 13.11 Epistasis in dogs. Epistatic interactions among products of two gene pairs affect coat color in Laborador retrievers. Left, all dogs with an E and B allele have black fur. Those with an E and two recessive b alleles have brown fur. All dogs homozygous for the recessive e allele have yellow fur. Right: black, chocolate, and yellow Laborador retrievers.

Epistasis: Labrador Retriever A dominant allele (B) specifies black fur, and its recessive partner (b) specifies brown fur A dominant allele of a different gene (E ) causes color to be deposited in fur, and its recessive partner (e) reduces color A dog with an E and a B allele has black fur A dog with an E allele and homozygous for b is brown A dog homozygous for the e allele has yellow fur regardless of its B or b alleles

Epistasis: Labrador Retriever Figure 13.11 Epistasis in dogs. Epistatic interactions among products of two gene pairs affect coat color in Laborador retrievers. Left, all dogs with an E and B allele have black fur. Those with an E and two recessive b alleles have brown fur. All dogs homozygous for the recessive e allele have yellow fur. Right: black, chocolate, and yellow Laborador retrievers.

Animation: Coat Color in Labrador Retrievers

Pleiotropy A pleiotropic gene influences multiple traits Mutations in pleiotropic genes are associated with complex genetic disorders such as sickle-cell anemia, cystic fibrosis, and Marfan syndrome pleiotropic Refers to a gene whose product influences multiple traits

Pleiotropy: Marfan Syndrome In Marfan syndrome, mutations affect elasticity of tissues of the heart, skin, blood vessels, tendons, and other body parts Haris Charalambous died when his aorta burst – he was 21

ANIMATION: Pleiotropic effects of Marfan syndrome To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

Animation: Comb Shape in Chickens

13.6 Complex Variation in Traits Phenotype often results from complex interactions among gene products and the environment Many traits show a continuous range of variation

Continuous Variation Some traits appear in two or three forms; others occur in a range of small differences (continuous variation) The more genes and environmental factors that influence a trait, the more continuous is its variation continuous variation In a population, a range of small differences in a shared trait

Continuous Variation (Cont.) If a graph line drawn around the top of the bars showing the distribution of values for a trait is bell-shaped (a bell curve) the trait varies continuously bell curve Bell-shaped curve Typically results from graphing frequency versus distribution for a trait that varies continuously

Continuous Variation (Cont.) Human height and eye color are traits that vary continuously

Continuous Variation (Cont.) Figure 13.13 Continuous variation in height among male biology students at the University of Florida. The students were divided into categories of one-inch increments in height and counted (bottom). A graph of the resulting data produces a bell-shaped curve (top), an indication that height varies continuously.

Environmental Effects on Phenotype Environmental factors often affect gene expression, which in turn affects phenotype: Seasonal change in animal fur colors Spines grow in presence of predators Different plant heights when grown at different altitudes

Environmental Effects on Phenotype In summer, the snowshoe hare’s fur is brown; in winter, white – offering seasonal camouflage from predators Figure 13.14 Example of environmental effects on animal phenotype. The color of the snowshoe hare’s fur varies by season. In summer, the fur is brown (left); in winter, white (right). Both forms offer seasonally appropriate camouflage from predators.

Animation: Continuous Variation in Height

Environmental Effects on Phenotype Daphnia at right developed a longer tail spine and a pointy head in response to chemicals emitted by predatory insects Figure 13.15 Environmental effects on the body form of Daphnia. In this animal, the form of the individual on the left develops in environments with few predators. The Daphnia on the right has a longer tail spine and a pointy head—traits that develop in response to chemicals emitted by predatory insects.

Environmental Effects on Phenotype Yarrow plant (Achillea millefolium) grew to different heights at three different elevations Figure 13.16 Environmental effects on plant phenotype. Cuttings from the same yarrow plant (Achillea millefolium) grow to different heights at three different elevations.

Environmental Effects on Phenotype Figure 13.16 Environmental effects on plant phenotype. Cuttings from the same yarrow plant (Achillea millefolium) grow to different heights at three different elevations. A Plant grown at high elevation (3,060 meters above sea level) B Plant grown at mid-elevation (1,400 meters above sea level) C Plant grown at low elevation (30 meters above sea level) Fig. 13.16, p. 199

Key Concepts Variations on Mendel’s Theme Not all traits appear in Mendelian inheritance patterns An allele may be partly dominant over a nonidentical partner, or codominant with it Multiple genes may influence a trait; some genes influence many traits The environment also influences gene expression

Menacing Mucus (revisited) The ΔF508 allele that causes cystic fibrosis in homozygotes may persist because it offers heterozygous individuals a survival advantage against certain deadly infectious diseases People who carry it may have a decreased susceptibility to typhoid fever and other bacterial diseases that begin in the intestinal tract