Question

Please answer the question thoroughly and show all work. The answer to the question is option A, so I just need a full explanation of why that is.

36) A geneticist is mapping the chromosomes of the newly captured gremlin. Stripe is heterozygous for three linked genes with alleles Bb, Ee, and Hh that determine if gremlins have biting teeth (B), are evil (E), and have hair (H). In order to determine if the three genes are linked, a standard testcross was done, and the 1000 offspring had the following genotypes:

48 bb ee Hh
36 Bb ee hh
400 Bb ee Hh
4 Bb Ee Hh
426 bb Ee hh
46 Bb Ee hh
38 bb Ee Hh
2 bb ee hh

Which of the following is a genotype produced by a double cross over:

A) Bb Ee Hh
B) bb ee Hh
C) bb Ee Hh
D) bb Ee hh
E) Bb Ee hh

Answer 1

Answer: “A” Bb Ee Hh

Explanation:The highest frequency of offspring is of parental type. The double cross over is the lowest frequency genotype, in this cross the two double cross overs are 4 Bb Ee Hh and 2 bb ee hh.

If for simplification we take the phenotypes of the given genotypes, we can show the cross overs as follows: P is for parental and R is for recombination or cross over.

Phenotype	observed	Loci B,E	B,H	E,H
BeH	400	P	P	P
bEh	426	P	P	P
beH	48	R	R	P
Beh	36	P	R	R
BEH	4	R	P	R
BEh	46	R	R	P
bEH	38	P	R	R
beh	2	R	P	R

As it can be observed from the above table the rare phenotypic classes generated because of double cross over event will show the middle locus flanked by cross over on either side of it, it can be observed by comparing it with the parental type and hence the answer.