Question

Tortoise shell or calico cats are always ________________ for O locus that is responsible for orange color development.

(a) Homozygous

(b) Heterozygous

(c) Deleterious

(d) Duplication

I have been asked this question in a national level competition.

I'd like to ask this question from Dosage Compensation Mechanism for X linked Genes in section Chromosomes and Sex Determination of Cytogenetics

Answer 1

Correct answer is (b) Heterozygous

Easy explanation: The O gene is responsible for orange color development. Only dominant O can express it over the mutant B gene that causes black color. O gene is located in X chromosome. Thus in heterozygous condition if O carrying X chromosome is inactivated it leads to black patch as seen here.