Question

in his analysis of F2 individuals in his one-factor crosses, Mendel inferred the genotypes associated with dominant phenotypes by examining F3 produced by selfing the F2. You decide instead to test cross the dominant phenotype F2, crossing them to recessive parents. Because you are busy with classes, you examine only 8 progeny from each test cross to determine whether an individual F2 is pure breeding or hybrid like. What fraction of the dominant phenotype F2 would you identify as pure breeding? Show your work

Answer 1

• If we consider genotypes of the pure parent ----- TT [tall] and tt [dwarf]
• the F1 generation of this cross given all genotype as----- Tt [tall but heterozygous]
• IF by crossing F2 dominant phenotye to reccessive parent -------- the dominant phynotype is tall which show two types of genetic combination 1. TT [homozygous] . 2. Tt [heterozygous]. so we have to consider both cases on by one.
• 1. TT [homozygous] :- when this F2 is crossed with parent tt then -----------

TT x tt -------- male x female

T t ------------ gametes

Tt -------------- all four are heterozygous tall .

• Tt [heterozygous]. :- when this F2 is crossed with parent tt then -----------

Tt x tt -------- male x female

T and t T and t ------------ gametes

The genotype combinations are TT and Tt in 1;1 ratio. ------------ out of four progeny 2 are tall [heterozygous] and 2 are dwarf.

• What fraction of the dominant phenotype F2 would you identify as pure breeding?
• answer :- pure breeding dominant fraction is 1/4.