4. Peter (an orange male cat (solid colour) is mated to Tinkerbell (a black and

Question

4. Peter (an orange male cat (solid colour) is mated to Tinkerbell (a black and white female cat). They have a female tortoiseshell kitten which was named Leia and a black & white male kitten named Luke. Leia and Luke were separated soon after birth and adopted by two different families several blocks away from one another. About a year later the two cats cross paths and mate. a. What is Peter's genotype (for coat colour on X-chromosome (XB or X and autosomal Spotting gene (SS, Ss, ss)? xo Y 5 b. What are Leia's and Luke's genotypes? c. What proportion of Leia's and Luke's kittens will be Calico? 2 d. What proportion of Leia's and Luke's kittens will be Tortoishell?

Explanation / Answer

a) Peter is an orange XY male. The chromosome X carries the gene for orange coat. Hence, genotype for coat color would be XOY.

Gene S or the spotting gene is responsible whether the coat color gets white patches. Gene s is recessive to S. A cat with solid color implies there are no white patches. Hence, the genotype should be ss.

Hence, Peter’s genotype for coat color on X-chromosome and autosomal spotting gene will be XOYss.

b) In order to estimate the genotype of Luke and Leia, we must perform the cross of parents, Peter and Tinkerbell.

Peter: XOYss (orange, solid male)

Tinkerbell: XBXBSs (Black and white female)

XOYss x XBXBSs

XB

XB

XO

XBXO

Tortoise shell(female)

XBXO

Tortoise shell(female)

Y

XBY

Black and white(male)

XBY

Black and white(male)

Autosomal crossing of Ss x ss, will yield Ss and ss.

Therefore, genotype of Leia would be XBXOss (tortoise shell female) and

genotype of Luke would be XBYSs (black and white male).

c) A calico is a tortoise shell coat color which also expresses an additional white spotting gene whose genotype would be XBXOSS and XBXOSs. Hence, we can say tortoise shell cats are almost exclusively female.

Let us see what proportion will be calico.

Crossing Leia and Luke

XBXOss x XBYSs

XB

Y

XB

XBXB

Black and white (female)

XBY

Black and white (male)

XO

XBXO

tortoise shell (female)

XOY

Orange (male)

On crossing sex chromosomes, the proportion of tortoise shell female is 1/4.

Let us perform the cross of autosomes of Luke and Leia.

Ss x ss

s

s

S

Ss

White patches

Ss

White patches

s

ss

solid clor

ss

Solid color

Hence, the proportion of white patches would be 2/4.

Since calico is a tortoise shell coat color which also expresses an additional white spotting gene, the proportion of calico would be 1/4 x 2/4 = 2/16 = 1/8.

d) The genotype of tortoise shell is XBXOss, since it lacks white patches. From the cross performed above, proportion of tortoise shell in sex chromosome cross is 1/4 and proportion of solid coat color on autosome is 2/4.

Therefore, the proportion of tortoise shell would be 1/4 x 2/4 = 2/16 = 1/8.

XB

XB

XO

XBXO

Tortoise shell(female)

XBXO

Tortoise shell(female)

Y

XBY

Black and white(male)

XBY

Black and white(male)

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