Consider two hypothetical recessive
autosomal genes a and b, where a
heterozygote is testcrossed to a double
homozygous mutant. Predict the phenotypic
ratios under the following conditions:
(a) a and b are located on separate
autosomes.
(b) a and b are linked on the same
autosome but are so far apart that a
crossover occurs between them.
(c) a and b are linked on the same
autosome but are so close together that
a crossover almost never occurs.
Answers
Explanation:
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Question: Genetics: Consider Two Hypothetical Recessive Autosomal Genes A And B, Where A Heterozygote Is Testcrossed To A Doublehomozygous Mutant. Predict The Phenotypic Ratios Under The Following Conditions: (a) A And B Are Located On Separate Autosomes. (b) A And B Are Linked On The Same Autosome But Are So Far Apart That A Crossover Always Occurs Between Them. ...
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Genetics: Consider two hypothetical recessive autosomal genes a and b, where a heterozygote is testcrossed to a doublehomozygous mutant. Predict the phenotypic ratios under the following conditions:
(a) a and b are located on separate autosomes.
(b) a and b are linked on the same autosome but are so far apart that a crossover always occurs between them.
(c) a and b are linked on the same autosome but are so close together that a crossover almost never occurs.
HINT: This problem involves an understanding of linkage, crossing over, and independent assortment. The key to its solution is to be aware that results are indistinguishable when two genes are unlinked compared to the case where they are linked but so far apart that crossing over always intervenes between them during meiosis.
I'm having a difficult time visualizing what's happening. For example, when they say the two loci are linked and on the same chromosome, what does that visually look like? Can someone draw them out the chromosomes being crossed? It's fine if you only do this for one of them.
ANSWERS:
a) If the two loci are on different chromosomes, independent assortment would occur and the following distribution (1:1:1:1) is expected:
1/4 AaBb
1/4 Aabb
1/4 aaBb
1/4 aabb
b) Even though th two loci are linked and on the same chromosomes, the frequency of the crossing over is so high that crossovers always occur. Under that condition, independent assortment would occur, and the following distribution (1:1:1:1) is expected:
1/4 AaBb
1/4 Aabb
1/4 aaBb
1/4 aabb
c) If crossovers never occur, then all of the gametes from the heterozygous parent are parental. If the arrangement is AB/ab x ab/ab, then the two types of offspring will be:
1/2 AB/ab
1/2 ab/ab
Under this condition, AB are coupled. If, however, A and B are not couples, then the symbolism would be Ab/aB x ab/ab.
The offspring would occur as follows:
1/2 Ab/ab
1/2 aB/ab
Answer:
Refer to the attached screenshots
Explanation:
Refer to the attached screenshots
