Today’s topics:
1. Principle of segregation
2. The testcross and the backcross
3. Independent assortment
Principle of segregation
Punnett square
3:1 ratio of dominant: recessive phenotypes in F2
see Figure 2.6
Summary of key points1. Each individual is heterozygous or homozygous.
2. Each gamete contains only one allele.
3. Every gamete is equally likely to contain the allele from either of the 2 parental gene copies.
4. Union of male and female gametes during reproduction reunites the alleles in pairs.
The F3 generationsee Figure 2.8
The testcross and the backcross
The "testcross" for an individual suspected to be heterozygous.
see Figure 2.9
The "backcross" with a parent
Segregation of two genes
"Independence" in genetics (Figure 2.16)
1) The genotype of each offspring is independent.
2) Principle of Independent Assortment:
Alleles for different genes sort independently.
Linkage"Linked genes" are on the same chromosome.
Basic probability theory
Joint probability for two alleles occuring together during gamete formation:Probability of at least one independent event:
Independent assortment of unlinked loci:
1. The multilocus gamete composition can be determined by multiplying the independent segregation probabilities.
The segregation of two genes can be demonstrated with a "dihybrid cross"
2. For the F2, the multilocus composition of a cross is found by multiplying the single locus proportions
example: the F2 generation from the dihybrid cross
For next time: pages 46-49, 52-53