What phenotypic ratio is expected when a double heterozygote

What phenotypic ratio is expected when a double heterozygote in the cis configuration is crossed with a double heterozygote in the trans configuration and the gene are tightly linked? 3:1 1:1 2:1:1 1:1:1:1 9:3:3:1 If two genes are 40 m.u. apart, what % of meiocytes would be expected to experience a crossover between the two genes (i.e. what is the frequency of crossing over 10 20 40 80 The alleles for gene 1 are A and a. The alleles for a second gene are B and b. Crossing over between these two genes occurs 20% of the time. What is the corresponding recombination frequency? 10% 20% 30% 40% 50%

Solution

1.The double heterozygote in cis configuration consist of parents that are eiter wild type or mutant for both the traits. The double heterozygote in trans configuration consist of parents one trait with wild type and other trait mutant. The progeny produced from such parents would be all wild type. The cross between cis and trans double heterozygote thus would be in 1:1 ratio. Option b is correct.

AA x Aa

A

A

A

AA

AA

a

Aa

Aa

2. When the two genes are 40mu apart 40% of gametes undergo crossover to produce recombinants and 60% gametes produce nonrecombinants. Option c is correct.

3. The crossing over between gene 1 and gene 2 with corresponding alleles A,a and B,b occurs 20% of the time. The recombination frequency is 40%. This is because the genes are 20 mu apart and recombination between the genes is

20/100 = 0.2

1/2 (1 - 0.2) = 1/2 (0.8) = 0.4 or 40%. Option d is correct.

	A	A
A	AA	AA
a	Aa	Aa