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Albia Dugger • Miami Dade College

Cecie StarrChristine EversLisa Starr

Chapter 13Observing Patterns in

Inherited Traits(Sections 13.4 - 13.6)

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

Fig. 13.7, p. 194

meiosis II

meiosis I

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.

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.

D Thus, when sister chromatids separate during meiosis II, the gametes that result have one of four possible combinationsof maternal and paternal chromosomes.

gamete genotype:

meiosis II

meiosis I

or

PtpTPTpt

Independent Assortment

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

Fig. 13.7, p. 194

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 I

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.

meiosis II

D Thus, when sister chromatids separate during meiosis II, the gametes that result have one of four possible combinationsof maternal and paternal chromosomes.

gamete genotype:

meiosis II

PtpTPTpt

Stepped Art

Independent Assortment

A Dihybrid Cross

Fig. 13.8, p. 195

parent plant homozygous

for purple flowers and long stems

PPTT pptt

dihybridPpTt

four types of gametes

parent plant homozygous

for white flowers and short stems

1

2

3

4

PPTT PPTt PpTT PpTt

PPTt PPtt PpTt Pptt

PpTT PpTt ppTT ppTt

PpTt Pptt ppTt pptt

pt

PT Pt pT pt

PP

Pt

pT

pt

A Dihybrid Cross

PT Pt pT pt

PT

Fig. 13.8.1-3, p. 195

A Dihybrid Cross

Fig. 13.8.4, p. 195

A Dihybrid Cross

Fig. 13.8, p. 195

parent plant homozygous

for purple flowers and long stemsPPTT pptt

dihybridPpTt

four types of gametes

parent plant homozygous

for white flowers and short stems

1

2

3

4

PPTT PPTt PpTT PpTt

PPTt PPtt PpTt Pptt

PpTT PpTt ppTT ppTt

PpTt Pptt ppTt ppttPT Pt pT pt

PT pt

PT Pt pT pt

PP

Pt

pT

pt

Stepped Art

A Dihybrid Cross

ANIMATION: Dihybrid cross

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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

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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

Fig. 13.9, p. 196

Phenotypes (blood type):

Genotypes:

O

OO

BABA

AA

or

AO AB

BB

or

BO

Codominance: ABO Blood Types

ANIMATION: Codominance: ABO blood types

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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

Fig. 13.10, p. 196

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.

B If two of the pink heterozygotesare crossed, the phenotypesof the resulting offspring will occur in a 1:2:1 ratio.

Incomplete Dominance: Snapdragons

Fig. 13.10a, p. 196

Incomplete Dominance: Snapdragons

Fig. 13.10a, p. 196

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.

Incomplete Dominance: Snapdragons

Fig. 13.10b, p. 196

Incomplete Dominance: Snapdragons

Fig. 13.10b, p. 196

B If two of the pink heterozygotes are crossed, the phenotypes of the resulting offspring will occur in a 1:2:1 ratio.

Incomplete Dominance: Snapdragons

ANIMATION: Incomplete dominance

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• 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

Epistasis: Labrador Retriever

• Labrador retriever coat color, can be black, brown, or yellow

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

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

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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.)

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

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

Environmental Effects on Phenotype

• Yarrow plant (Achillea millefolium) grew to different heights at three different elevations

Fig. 13.16, p. 199

C Plant grown at low elevation (30 meters above sea level)

A Plant grown at high elevation (3,060 meters above sea level)

B Plant grown at mid-elevation (1,400 meters above sea level)

Environmental Effects on Phenotype

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