Question

The alleles c and C are for albinism and color, respectively; A and a are for agouti and black patterns, respectively. Two black parents produce an albino offspring. Which of the following could be the genotype of a parent?

	a.	aaCC
	b.	AaCc
	c.	aaCc
	d.	AAcc

Following recombination:

	a.	cells are haploid
	b.	sister chromatids are not identical
	c.	homologous chromosomes are not identical
	d.	there are two copies of each gene on each chromatid
	e.	chromosomes are different sizes

Scientists studying a recessive disease have found that some individuals who inherit two recessive alleles do not show any disease symptoms. This indicates that the gene responsible for the disease is

	a.	recessive.
	b.	a modifier.
	c.	pleiotropic.
	d.	incompletely penetrant.
	e.	conditionally lethal.

Examination of testcross data in Drosophila showed that two linked genes, p and q, had a recombination rate of 25%. A third gene, d, had recombination rates of 15% and 10% with p and q, respectively. What is the order and distances of these three genes on the chromosome?

	a.	p--15--d--10--q
	b.	p--25--q--10--d
	c.	q--10--p--15--d
	d.	d--15--p--25--q
	e.	d--10--q--15--p

Answer 1

The following is the genotype of both parents:

(c) aaCc

Albino is recessive to color and black is recessive to agouti. Since both parents are black they both are homozygous recessive for black allele (aa). To produce color, they require at least one dominant C allele. However to produce an albino offspring it is only possible if they are both heterozygous (Cc).

aa × aa = aa (1:0, all albino)

Cc × Cc =

	C	c
C	CC	Cc
c	Cc	cc

3:1 (agouti:black)

Following recombination:

(b) sister chromatids are not identical

Recombination occurs after each homologous chromosome has undergone replication to produce 2 sister chromatids. Sister chromatids are exact replicas of the chromosome from a single parent. Unlike homologous chromosomes which carry genes for the same trait but from different parents (eg. Chromosome 2 of mother and father). Recombination occurs between the sister chromatids of two different homologous chromosomes. Hence after recombination, the sister chromatids which were previously identical are now partly different.

Scientists studying a recessive disease have found that some individuals who inherit two recessive alleles do not show disease symptoms. This indicates that the gene responsible for the disease is:

(d) incompletely penetrant

A modifier gene influences the expression of other genes. For that there is no evidence. A pleiotrophic gene affects the more that one phenotype outcome. There is no evidence that this gene affects many phenotypes. It is not lethal since the individuals are alive. It is incompletely penetrant because not all individuals who get the genotype for disease actually get the disease.

The distance between the three genes is:

(a) p ---15--- d ---10--- q

This arrangement satisfies all conditions. 15+10=25 which is the distance between p and q.