Coat color in cats is determined by genes at several different loci. At one locu
ID: 141772 • Letter: C
Question
Coat color in cats is determined by genes at several different loci. At one locus on the X chromosome, one allele (X+) encodes black fur, another allele (X0) encodes orange fur. Another autosomal locus determines whether the fur has pigment deposited or not; one allele encodes for white fur (dominant, variable expressivity) and the other allele encodes for color (recessive). A Calico female cat (white, orange and black) mates with a black and white male cat. Using this information answer the following questions.
A. Explain how barr bodies relate to copper and black coloring in female cats. Include a definition of barr bodies and why they are present.
B. Provide a possible explanation for how expression of the gene that allows pigmentation is variably expressed in different individuals.
C. List all possible male and female cat phenotypes and genotypes.
Explanation / Answer
A. The genotypes of allosomes (sex chromosomes) in males is XY and that in females is XX. Thus, the female chromosome is having an exact duplicate locus for each gene in the sex chromosomes. Whereas, the males have different sets of genes in the X and Y chromosomes. The manly characters are mostly controlled by the genes in the Y chromosome. In case of autosomes, both in male and female, alleles are present for every gene in a pair of chromosomes.
In females, one of the X chromosomes gets inactivated (for compensating the lack of double X chromosomes in males) and the other X becomes dominant and is expressed. This inactivated X chromosome condenses by a process called lyonization and forms the Barr body. They are deeply stained and can be seen in the rims of the nucleus in a female somatic cell. Presence of barr body is a basis of sex determination tests.
In the female cats, black and orange fur are determined by the alleles X+ and X0. The possible genotypes are X+X+, X+X0 and X0X0 and phenotypes pure black, black & orange and pure orange respectively. The patchwork of orange and black fur in a heterozygous is known as tortoise shell.
In the males, the possible phenotypes are pure orange and pure black since there is no chance of a heterozygous condition. In the case of females, in the X+X+ genotype, one allele of the gene becomes inactivated and forms the barr body and the other active one expresses. In this case, since both are identical, black is the colour. Same is the case with X0X0 leading to orange cats.
The lyonization process occurs during embryonal development and is a random process differing from cell to cell. In a heterozygous female cat (X+X0), in some cells the X+ is inactivated and in others, it may be X0. Thus, the resulting phenotype being a mixture of black and orange fur in the cat.
B. Pigmentation is a polygenic trait, meaning multiple genes control the expression of the same. These multiple gene loci may be placed on the autosomes or on the allosomes. Genes on allosomes are sex dependant whereas those on the autosomes are not. In the above case two loci are given. In case of human skin pigmentation, there are 378 gene loci determining the skin colour.
C. In the male cats, genotypes in allosomes are X+Y and X0Y and hence they can be either black or else orange. In case of autosomal genes, let dominant white allele be W and the recessive color allele be w, then possible genotypes are WW, Ww and ww, corresponding phenotypes being white, white and color. The possible outcomes are:
WW X+Y white and black
WW X0Y white and orange
Ww X+Y white and black
Ww X0Y white and orange
ww X+Y color and black
ww X0Y color and orange
In female cats, allosomal genotypes are X+X+, X+X0 and X0X0 and autosomal genotypes WW, Ww and ww. The possible combinations are:
WW X+X+ white and black
WW X+X0 white, black and orange
WW X0X0 white and orange
Ww X+X+ white and black
Ww X+X0 white, black and orange
Ww X0X0 white and orange
ww X+X+ color and black
ww X+X0 color, black and orange
ww X0X0 color and orange