Types of biological variation

Discontinuous (qualitative) variation: simple alternativeforms; alternative phenotypes; usually due to alternative genotypes

often due to interactions of dominant and recessive alleles of genes

common alternatives due to polymorphism

rare alternatives due to mutation (vs. wild type)

Continuously variable (quantitative) traits: no distinct increments; most common variation; due to polygenes and/or significant non-genetic influence.

Development that is genetically drivenFig. 1-17

Development that is environmentally drivenFig. 1-18

Development that is driven by interactions between genes and the environmentFig. 1-19

Norm of reaction: phenotypic outcome of the interactions of genotype and environment; characteristic for each genotype

Developmental noise: random influences on phenotype that result in random individual variations

Drosophila melanogaster (wild-type)Fig. 1-20

Fig. 1-20

Development resulting from interactions between genes, environment and noiseFig. 1-23

Chapter 2 OverviewFig. 2-1

Simple monohybrid inheritance

single gene (allele pair)simple dominance of one allele

Fig. 2-7Genes are particulate

2. Genes in pairs and can be different forms (alleles)

3. Halving of pairs in gametogenesis

4. Alleles separate (segregate) in gametogenesis

5. Fertilization is random

Mendels explanation of simple monohybrid inheritance

Fig. 2-8Testcross to test/demonstrate heterozygosityTestcross: cross possible heterozygote to homozygous recessive

Fig. 2-10Dihybrid inheritance

Fig. 2-11Dihybrid inheritance

Estimating the likelihoods of events

Independent events:

Compute the likelihood of each event Compute the product of those likelihoods

Dependent (mutually exclusive) events:

Compute the likelihood of each event Compute the sum of those likelihoods

Problem: predict the phenotypic ratios expected among theprogeny of the cross A/a ; b/b X A/a ; B/b

Solution: use a branch diagram

Fig. 2-11Dihybrid inheritance

p. 155Problem: predict the phenotypic ratios expected among theprogeny of the cross A/a ; b/b X A/a ; B/b

Solution: use a branch diagram

Conventional symbols used in pedigree analysisFig. 2-12

Fig. 2-13Analysis of a rare autosomal, recessive phenotypeTypical: affected males and females; affected individuals have unaffected parents

Analysis of a autosomal dominant phenotypeFig. 2-16Typical: affected males and females; about half of progeny of affected individual are affected

T.H. Morgans analysis of the sex linkage of whiteFig. 2-24

Fig. 2-24Repeat from previous slide

Fig. 2-25Analysis of a rare sex-linked, recessive phenotypeTypical: almost exclusively affected males; mothers of affected sons are carriers; appears to skip generations

Fig. 2-30Mirabilis jalapa

Fig. 2-31Schematic of organellar/cytoplasmic inheritance

X2 (Chi-square) test: assesses the likelihood that a deviation from expectations can be accepted

Example: Do results of a dihybrid cross reflect linkage?

Products of a dihybrid (A/a B/b) testcross

AB 142ab 133Ab 113aB 112 Parental types

Recombinant types

X2 (Chi-square) test: assesses the likelihood that a deviation from expectations can be accepted

Example: Do results of a dihybrid cross reflect linkage?

1st step: Make null hypothesis genes are not linked Predicts 1:1:1:1 ratio of gamete genotypes

2nd step: Compute X2 = (O-E)2 / E

3rd step: Determine degrees of freedom (number of independent measurements)

4th step: Consult X2 chart of critical values