Final answer:
If a heterozygous cat for orange and black alleles has a deletion of the XIST gene on the X chromosome with the black allele, it would typically show a predominantly black fur color rather than the characteristic tortoiseshell pattern.
Step-by-step explanation:
If a cat is heterozygous for orange and black alleles, typically you would expect it to be a tortoiseshell cat with patches of both colors. This occurs due to a phenomenon called X inactivation, where one of the two X chromosomes in female mammals is inactivated early in embryonic development, leading to a mixture of cells with either the orange or the black allele being expressed.
However, the X chromosome that carries the black allele also contains the XIST gene, which is essential for X inactivation. This gene produces an RNA molecule that coats the X chromosome from which it is transcribed and causes it to be inactivated. If there is a deletion of the XIST gene on the black allele's X chromosome, X inactivation wouldn’t occur normally on that chromosome, potentially leading to that X chromosome being active in all cells.
In this scenario, the cat would predominantly express the black fur color, and it might not have the characteristic tortoiseshell patterning even though it’s heterozygous for orange and black coat colors.