Question

2. You have a fly strain heterozygous for a reciprocal translocation between an acrocentric chromosome (solid) and a metacentric chromosome (wavy). The normal homologues have dominant alleles for the genes shown, while the re-arranged chromosomes have recessive alleles. A B F G NOR a bf an anong A. (3pts) What kind of gametes can a male of this strain produce? (note that there is no recombination in Drosophila males) B. (3pts) If you performed a test cross of a male from this strain (to a homozygous recessive stock with normal chromosomes), and normal copy number of the G gene is essential for survival, what would be the expected phenotypes and their frequencies in the offspring? Explain, using at least one of Mendel's Laws in your answer. (The g allele is a hypomorph but provides sufficient function for survival.) C. (3pts) If you performed a chromosome in situ on one of the progeny from the test cross and detected the A BF G IN Q R following combination of chromosomes, what sort of a bf g nona meiotic error could be abfqur responsible?

Answer 1

A) Note that if recombination is not an option then the gametes will be each chromosome per se. These are the resulting gametes:

ABFG/KNQR

abfqr/KNQR

ABFG/kng

abfqr/kng

B) We have to make a punnet square for this, then eliminate any individual without the two G/g copies and calculate frequencies for such resulting individuals.

	ABFG/KNQR	abfqr/KNQR	ABFG/kng	abfqr/kng
abfg/knqr	AaBbFfGg/KkNnQqRr	aabbffgqr/KkNnQqRr	AaBbFfGg/kknngqr	aabbffgqr/kknngqr

Note that the highlighted offspring lacks the two copies for gene G, that means such individual will never develop. So the observed frequencies will be:

1/3 = AaBbFfGg/KkNnQqRr

1/3 = AaBbFfGg/kknngqr

1/3 = aabbffgqr/kknngqr

We can state this because we know that alleles segregate in reproductive events, just like the second law of Mendel states (Law of segregation). We can actually get to this conclusions because we know that some of these genes are linked because they are in the same chromosome. That actually goes against the third law that states all alleles segregate independently (Law of independent assortment), but science actually refused such law around a century ago

C) We can see there is a trisomy for the first pair in question. Trisomies occur when there are problems during segregation in either Anaphase I or Anaphase II. If the issue in question is about segregating the homologous chromosomes (which actually is the case in this exercise) then it occured during Anaphase I, if the issue is about segregating sister chromatides then it occured during Anaphase II.

In this case a spermatocyte had a problem segregating homologous chromosomes during Anaphase I, leading to a final sperm cell with both homologous chromosomes for that pair. Such sperms managed to fecundate an egg and produced this individual with such trisomy.