Types of biological variation
Discontinuous (qualitative) variation: simple alternative
forms; 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 driven
Fig. 1-17
Development that is environmentally driven
Fig. 1-18
Development that is driven by interactions
between genes and the environment
Fig. 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 “noise”
Fig. 1-23
Chapter 2 Overview
Fig. 2-1
Simple monohybrid inheritance
•
•
single gene (allele pair)
simple dominance of one allele
Mendel’s explanation of simple monohybrid inheritance
1. Genes 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
Fig. 2-7
Testcross to test/demonstrate heterozygosity
Testcross: cross possible heterozygote to homozygous
recessive
Fig. 2-8
Dihybrid inheritance
Fig. 2-10
Dihybrid inheritance
Fig. 2-11
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 the
progeny of the cross A/a ; b/b X A/a ; B/b
Solution: use a branch diagram
Dihybrid inheritance
Fig. 2-11
Problem: predict the phenotypic ratios expected among the
progeny of the cross A/a ; b/b X A/a ; B/b
Solution: use a branch diagram
p. 155
Conventional symbols used in pedigree analysis
Fig. 2-12
Analysis of a rare autosomal, recessive phenotype
Fig. 2-13
Typical: affected males and females;
affected individuals have unaffected parents
Analysis of a autosomal dominant phenotype
Typical: affected males and females;
about half of progeny of affected individual are affected
Fig. 2-16
T.H. Morgan’s analysis of the sex linkage of white
Fig. 2-24
Repeat from
previous slide
Fig. 2-24
Analysis of a rare sex-linked, recessive phenotype
Fig. 2-25
Typical: almost exclusively affected males;
mothers of affected sons are carriers;
appears to “skip” generations
Mirabilis jalapa
Fig. 2-30
Schematic of organellar/cytoplasmic inheritance
Fig. 2-31
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
ab
Ab
aB
142
133
113
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