chapter 13 observing patterns in inherited traits

Albia Dugger • Miami Dade College

Chapter 13Observing Patterns in Inherited Traits

13.1 Menacing Mucus

• Cystic fibrosis (CF) is the most common fatal genetic disorder in the United States

• The CFTR gene encodes CFTR protein which maintains a thin film of water on the surface of the epithelial sheets

• A deletion of three base pairs (ΔF508, deletion) prevents proper membrane trafficking of CFTR so it can’t do its job

• Mucus obstructs the airways and bacteria infect the intestine and lungs – most CF patients live no longer than thirty years

Figure 13-1a p203

ATP

ΔF508

13.2 Mendel, Pea Plants, and Inheritance Patterns

• Recurring inheritance patterns are observable evidence of how heredity works

• Before the discovery of genes, it was thought that inherited traits resulted from a blend of parental characters

Mendel’s Experimental Approach

• Gregor Mendel was a monk with training in plant breeding and mathematics

• He studied the garden pea (Pisum sativum), which breeds true for a number of traits

• Mendel discovered that traits of offspring of cross-fertilized pea plants often appear in predictable patterns

• Mendel’s work led him to conclude that hereditary information passes from one generation to the next in discrete units

Gregor Mendel

Garden Pea Plant: Self Fertilization and Cross-Fertilization

carpelA

anther

BC

D

E

ANIMATED FIGURE: Crossing garden pea plants

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Terms Used in Modern Genetics

• Genes are heritable units of information about traits

• Each gene has a specific locus on a chromosome

• Diploid cells (chromosome number 2n) have pairs of genes on homologous chromosomes

• Alleles are different molecular forms of a gene

Loci of Some Human Genes

Figure 13-3a p205

ribosomal RNA

skin pigmentation

15

(Tay–Sachs disease)

fibrillin 1 (Marfan syndrome)

Figure 13-3b p205

17

BRCA1 (breast, ovarian cancer)

NF1 (neurofibromatosis)

(Canavan disease)

serotonin transporter

Growth hormone

p53 tumor antigen

Figure 13-3c p205

19 LH, β chain

HCG, β chain

(Warfarin resistance)

green/blue eye color

brown hair color

insulin receptor

LDL receptor (coronary artery disease)

Figure 13-3d p205

20

GHRH (acromegaly)

prion protein(Creutzfeldt–Jakob disease)

oxytocin

Figure 13-3e p205

(green-deficient color blind)X(red-deficient color blind)

(hemophilia A)

(hemophilia B)

XIST X chromosomeinactivation control

IL2RG (SCID-X1)

dystrophin (muscular dystrophy)

(anhidrotic ectodermal dysplasia)

Terms Used in Modern Genetics

• The particular set of alleles that an individual carries is the individual’s genotype

• An individual with two identical alleles of a gene is homozygous for that gene

• An individual with nonidentical alleles of a gene is heterozygous for that gene

Terms Used in Modern Genetics

• A hybrid is the heterozygote offspring of a cross between two individuals that breed true for different forms of a trait

• An individual’s genotype determines its phenotype, which refers to an individual’s observable traits

• Any mutated gene is a new allele, whether or not it affects phenotype

Terms Used in Modern Genetics

• An allele is dominant if its effect masks the effect of a recessive allele paired with it• Capital letters (P) signify dominant alleles; lowercase

letters (p) signify recessive alleles• Homozygous dominant (PP)• Homozygous recessive (pp)• Heterozygous (Pp)

Genotypes Give Rise to Phenotypes

Pp(heterozygous atthe P gene locus)

genotype:

phenotype:

PP(homozygous fordominant allele P)

pp(homozygous forrecessive allele p)

Take-Home Message:

How do alleles contribute to traits?

• Gregor Mendel discovered the role of alleles in inheritance by breeding pea plants and tracking traits of their offspring

• Genotype refers to the particular set of alleles carried by an individual’s somatic cell; phenotype refers to the individual’s set of observable traits; genotype is the basis of phenotype

• A homozygous individual has two identical alleles at a particular locus; a heterozygous individual has nonidentical alleles at the locus

• Dominant alleles mask the effects of recessive ones in heterozygous individuals

13.3 Mendel’s Law of Segregation

• Pairs of genes on homologous chromosomes separate during meiosis, so they end up in different gametes

• Mendel showed that garden pea plants inherit two “units” of information for a trait, one from each parent

Gene Segregation

• Homologous chromosomes (and all the alleles they carry) segregate into separate gametes during meiosis

• Plants homozygous for the dominant allele (PP) can only make gametes that carry the allele P

• Plants homozygous for the recessive allele (pp) can only make gametes that carry the allele p

• Heterozygous plants produce both type of gametes

Calculating Probabilities

• Probability• A measure of the chance that a particular outcome will

occur

• Punnett square• A grid used to calculate the probability of genotypes and

phenotypes in offspring

Stepped Art

gametes (p)

meiosis II

gametes (P)

DNA replication

meiosis I

1 2

zygote (Pp)

3

female gametes

mal

e g

amet

es

4

Figure 13-5 p206

Figure 13-5b p206

mal

e g

amet

es

female gametes

Testcrosses

• A testcross is a method of determining if an individual is heterozygous or homozygous dominant

• An individual with unknown genotype is crossed with one that is homozygous recessive (PP x pp) or (Pp x pp)

Monohybrid Crosses

• A monohybrid cross is a testcross that checks for a dominance relationship between two alleles at a single locus

• May be a cross between true breeding (homozygous) individuals (PP x pp), or between identical heterozygotes (Pp x Pp)

Generations in a Monohybrid Cross

• P stands for parents, F for filial (offspring)

• F1: First generation offspring of parents

• F2: Second generation offspring of parents

Mendel’s Monohybrid Crosses

• Mendel used monohybrid crosses to find dominance relationships among pea plant traits

• When he crossed plants that bred true for white flowers with plants that bred true for purple flowers, all F1 plants had purple flowers

• When he crossed two F1 plants, ¾ of the F2 plants had purple flowers, ¼ had white flowers

Table 13-1 p207

Mendel’s Dihybrid Cross

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

Offspring of Mendel’s Monohybrid Cross

Mendel’s Law of Segregation

• Mendel observed a phenotype ratio of 3:1 in the F2 offspring of his monohybrid crosses• Consistent with the probability of the pp genotype in the

offspring of a heterozygous cross (Pp x Pp)

• This is the basis of Mendel’s law of segregation• Diploid cells have pairs of genes on pairs of homologous

chromosomes • The two genes of each pair separate during meiosis, and

end up in different gametes

Take-Home Message:

What is Mendel’s law of segregation?

• Diploid cells carry pairs of genes, on pairs of homologous chromosomes

• The two genes of each pair are separated from each other during meiosis, so they end up in different gametes

• Mendel discovered patterns of inheritance in pea plants by tracking the results of many monohybrid crosses

ANIMATED FIGURE: Monohybrid cross

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13.4 Mendel’s Law of Independent Assortment

• Mendel’s law of independent assortment• During meiosis, members of a pair of genes on

homologous chromosomes get distributed into gametes independently of other gene pairs

Dihybrid Crosses

• Dihybrid crosses test for dominance relationships between alleles at two loci

• Individuals that breed true for two different traits are crossed (PPTT x pptt)

• F2 phenotype ratio is 9:3:3:1 (four phenotypes)

• Individually, each dominant trait has an F2 ratio of 3:1 – inheritance of one trait does not affect inheritance of the other

The Contribution of Crossovers

• Independent assortment also occurs when the genes are on the same chromosome, but far enough apart that crossing over occurs between them very frequently

• Genes that have loci very close to one another on a chromosome tend to stay together during meiosis and not assort independently

Linkage Groups

• All genes on one chromosome are called a linkage group

• The farther apart two genes are on a chromosome, the more often crossing over occurs between them

• Linked genes are very close together; crossing over rarely occurs between them

• The probability that a crossover will separate alleles of two genes is proportional to the distance between those genes

Take-Home Message: What is Mendel’s law of independent assortment?

• Each member of a pair of genes on homologous chromosomes tends to be distributed into gametes independently of how other genes are distributed during meiosis

ANIMATED FIGURE: Independent assortment

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ANIMATED FIGURE: Dihybrid cross

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13.5 Beyond Simple Dominance

• Mendel focused on traits based on clearly dominant and recessive alleles; however, the expression patterns of genes for some traits are not as straightforward

Codominance

• Codominance• Two nonidentical alleles of a gene are both fully expressed

in heterozygotes, so neither is dominant or recessive• May occur in multiple allele systems

• Multiple allele systems• Genes with three or more alleles in a population• Example: ABO blood types

Codominance in ABO Blood Types

Phenotypes (blood type):

Genotypes:

O

OO

BABA

AA

or

AO AB

BB

or

BO

Incomplete Dominance

• Incomplete dominance• One allele is not fully dominant over its partner• The heterozygote’s phenotype is somewhere between the

two homozygotes, resulting in a 1:2:1 phenotype ratio in F2 offspring

• Example: Snapdragon color• RR is red• Rr is pink• rr is white

Figure 13-10 p210homozygous (RR) homozygous (rr)heterozygous (Rr)

Figure 13-10b p210

INTERACTION: Incomplete dominance

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Epistasis

• Epistasis• Two or more gene products influence a trait• Typically, one gene product suppresses the effect of

another

• Example: Coat color in dogs• Alleles B and b designate colors (black or brown)• Two recessive alleles ee suppress color

Coat Colors in Labrador Retrievers

Figure 13-11b p211

Pleiotropy

• A pleiotropic gene influences multiple traits

• Example: Some tall, thin athletes have Marfan syndrome, a potentially fatal genetic disorder

Take-Home Message

Are all alleles dominant or recessive?

• An allele may be fully dominant, incompletely dominant, or codominant with its partner on a homologous chromosome

• In epistasis, two or more gene products influence a trait

• The product of a pleiotropic gene influences two or more traits

13.6 Nature and Nurture

• Variations in traits aren’t always the result of differences in alleles – many traits are influenced by environmental factors

Environment and Gene Expression

• The environment affects the expression of many genes, which in turn affects phenotype – including behavioral traits

• We can summarize this relationship as:

genotype + environment → phenotype

Environment and Epigenetics

• Environmentally driven changes in gene expression patterns can be permanent and heritable

• Such changes are implemented by gene controls such as chromatin modifications and RNA interference that act on DNA itself

• Example: Many environmental factors affect DNA methylation patterns, enhancing or suppressing gene expression

Effects of Temperature on Gene Expression

Figure 13-14a p212

c Maturecutting at lowelevation (30meters abovesea level)

a Maturecutting athigh elevation(3,060 metersabove sealevel)

b Maturecutting atmid-elevation(1,400 metersabove sealevel)

Figure 13-15a p213A Light micrograph of a living water flea.

Figure 13-15b p213

B Electron micrographs comparing Daphnia body form that develops in the presence of few predators (left) with the form that develops in the presence of many predators (right). Note the difference in the length of the tail spine and the pointiness of the head. Chemicals emitted by thewater flea’s insect predators provoke the change.

Mood Disorders in Humans

• Environment is a factor in schizophrenia, bipolar disorder, depression, and other mood disorders

• Example: Stress-induced depression causes methylation-based silencing of a particular nerve growth factor – some antidepressants work by reversing this methylation

• Future treatments for many disorders may involve deliberate modification of epigenetic marks in one’s DNA

Take-Home Message: Is genotype the only factor that gives rise to phenotype?

• The environment influences gene expression, and therefore can alter phenotype

• Cell-signaling pathways link environmental cues with epigenetic marks such as methylation and other chromatin modifications

ANIMATION: Coat color in the Himalayan rabbit

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13.7 Complex Variations in Traits

• Individuals of most species vary in some of their shared traits

• Many traits (such as eye color) show a continuous range of variation

Continuous Variation

• Continuous variation• Traits with a range of small differences• The more factors that influence a trait, the more

continuous the distribution of phenotype

• Bell curve• When continuous phenotypes are divided into measurable

categories and plotted as a bar chart, they form a bell-shaped curve

Continuous Variation in Height (Females)

Continuous Variation in Height (Males)

The Bell Curve

Regarding the Unexpected Phenotype

• Phenotype results from complex interactions among gene products and the environment

• Enzymes and other gene products control steps of most metabolic pathways

• Mutations, interactions among genes, and environmental conditions may result in unpredictable traits

• Example: Camptodactyly can affect any fingers on either or both hands

Camptodactyly

Take-Home Message:Do all traits occur in distinct forms?

• The more genes and other factors that influence a trait, the more continuous is its range of variation

• Unpredictable phenotypes can be caused by interactions among genes with a range of expression among individuals