Question

1) a plant that is heterozygous for a trait that displays incomplete dominance. if it is used in a test cross, what proportion of the progeny will be homozygous recessive?

2)Two species of the same genus have a different number of chromosome. Species A has 8 chromosomes while species B has 22.. In which species will one see more genetic variation among the progeny? Human cells have 46 chromosomes, how many different chromosome combination in the gamete are possible?

Answer 1

1. The proportion of the progeny with homozygous recessive will be 50%. In a test cross the the plant is crossed with a homozygous recessive plant. Here the plant is heterozygous for a trait. It means the genotype will be Aa. To perform a test cross this plant will be crossed with a plant having trait aa. The gamete produced by Aa will be A and a while the gamete produced by aa will be a. Thus the genotype of obtaned progeny will be Aa and aa. Here one out of 2 progeny is with aa. Thus the proportion of homozygous recessive is 50%.

2. The organism with 22 chromosomes will have more genetic variations. Genetic variation is caused by mutation and recombination. More the number of chromosomes more the chances of mutation and recombination and therefore more genetic variation. In human (46) the number of different chromosome cobination in the gamete will be 8388608. (The possible combination with 2 chromosome is 2 and the numer of possible combination is doubled for each 2 chromosome addition).