Patterns of Inheritance

Chapter 12

2

Mystery of heredity

• Before the 20th century, 2 concepts were the basis for ideas about heredity– Heredity occurs within species– Traits are transmitted directly from parent to

offspring

• Thought traits were borne through fluid and blended in offspring

• Paradox – if blending occurs why don’t all individuals look alike?

3

Early work

• Josef Kolreuter – 1760 – crossed tobacco strains to produce hybrids that differed from both parents– Additional variation observed in 2nd generation

offspring contradicts direct transmission

• T.A. Knight – 1823 – crossed 2 varieties of garden pea, Pisum sativa– Crossed 2 true-breeding strains– 1st generation resembled only 1 parent strain– 2nd generation resembled both

4

Gregor Mendel

• Chose to study pea plants because:

1.Other research showed that pea hybrids could be produced

2.Many pea varieties were available

3.Peas are small plants and easy to grow

4.Peas can self-fertilize or be cross-fertilized

5

Mendel’s experimental method

• Usually 3 stages1.Produce true-breeding strains for each trait

he was studying2.Cross-fertilize true-breeding strains having

alternate forms of a trait – Also perform reciprocal crosses

3.Allow the hybrid offspring to self-fertilize for several generations and count the number of offspring showing each form of the trait

6

7

Monohybrid crosses

• Cross to study only 2 variations of a single trait

• Mendel produced true-breeding pea strains for 7 different traits– Each trait had 2 variants

8

9

10

Conclusions

• His plants did not show intermediate traits– Each trait is intact, discrete

• For each pair, one trait was dominant, the other recessive

• Pairs of alternative traits examined were segregated among the progeny of a particular cross

• Alternative traits were expressed in the F2 generation in the ratio of ¾ dominant to ¼ recessive

11

5 element model

1. Parents transmit discrete factors (genes)

2. Each individual receives one copy of a gene from each parent

3. Not all copies of a gene are identical– Allele – alternative form of a gene– Homozygous – 2 of the same allele– Heterozygous – different alleles

4. Alleles remain discrete – no blending

5. Presence of allele does not guarantee expression

– Dominant allele – expressed– Recessive allele – hidden by dominant allele

• Genotype – total set of alleles an individual contains

• Phenotype – physical appearance

12

13

Principle of Segregation

14

15

Human traits

• Some human traits are controlled by a single gene– Some of these exhibit dominant and recessive

inheritance

• Pedigree analysis is used to track inheritance patterns in families

• Dominant pedigree – juvenile glaucoma– Disease causes degeneration of optic nerve

leading to blindness– Dominant trait appears in every generation

16

– In 1909, British physician Archibald Garrod first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions

– Inborn errors of metabolism– alkaptonuria (alkapton)

Linking genes to enzymes : a metabolic pathway

18

• Recessive pedigree – albinism– Condition in which the pigment melanin is not

produced– Pedigree for form of albinism due to a

nonfunctional allele of the enzyme tyrosinase– Males and females affected equally– Most affected individuals have unaffected

parents

19

20

21

Dihybrid crosses

• Examination of 2 separate traits in a single cross

• Produced true-breeding lines for 2 traits

• RR YY x rryy

• The F1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait

• Allow F1 to self-fertilize to produce F2

22

23

24

Principle of independent assortment

• In a dihybrid cross, the alleles of each gene assort independently

• The segregation of different allele pairs is independent

• Independent alignment of different homologous chromosome pairs during metaphase I leads to the independent segregation of the different allele pairs

25

Testcross

• Cross used to determine the genotype of an individual with dominant phenotype

• Cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp)

• Phenotypic ratios among offspring are different, depending on the genotype of the unknown parent

26

27

Extensions to Mendel

• Mendel’s model of inheritance assumes that– Each trait is controlled by a single gene– Each gene has only 2 alleles– There is a clear dominant-recessive

relationship between the alleles

• Most genes do not meet these criteria

28

Polygenic inheritance

• Occurs when multiple genes are involved in controlling the phenotype of a trait

• The phenotype is an accumulation of contributions by multiple genes

• These traits show continuous variation and are referred to as quantitative traits– For example – human height– Histogram shows normal distribution

29

30

Pleiotropy

• Refers to an allele which has more than one effect on the phenotype

• Pleiotropic effects are difficult to predict, because a gene that affects one trait often performs other, unknown functions

• This can be seen in human diseases such as cystic fibrosis or sickle cell anemia– Multiple symptoms can be traced back to one

defective allele

31

Multiple alleles

• May be more than 2 alleles for a gene in a population

• ABO blood types in humans– 3 alleles

• Each individual can only have 2 alleles• Number of alleles possible for any gene is

constrained, but usually more than two alleles exist for any gene in an outbreeding population

32

• Incomplete dominance– Heterozygote is intermediate in phenotype

between the 2 homozygotes– Red flowers x white flowers = pink flowers

• Codominance– Heterozygote shows some aspect of the

phenotypes of both homozygotes– Type AB blood

33

34

Human ABO blood group

• The system demonstrates both– Multiple alleles

• 3 alleles of the I gene (IA, IB, and i)

– Codominance• IA and IB are dominant to i but codominant to each

other

35

Environmental influence• Coat color in

Himalayan rabbits and Siamese cats– Allele produces

an enzyme that allows pigment production only at temperatures below 30oC

36

37

Epistasis

• Behavior of gene products can change the ratio expected by independent assortment, even if the genes are on different chromosomes that do exhibit independent assortment

• R.A. Emerson crossed 2 white varieties of corn– F1 was all purple

– F2 was 9 purple:7 white – not expected

38