One of the genes for coat color in cats is found on the X chromosome. Male cats are either black (B) or orange (O). Females are black, orange, or calico, which has patches of black and orange. Calico is formed from the codominance between the two alleles in the heterozygote so a calico cat is (X X ). Give the genotype and phenotype ratio of a cross between an orange male and a calico cat?
Answers
Answer:
Explanation:
In mammals, sex is determined by two sex chromosomes, known as the X and the Y chromosomes. Genes located on either the X or the Y chromosome are known as "sex-linked" genes. Genes on any chromosomes other than the X or Y are known as autosomal genes.
The Karyotype: A Visualization of the Chromosomes
Normal female mammals have two X chromosomes. Normal males have one X and one Y chromosome. This can be seen in this human male karyotype:
The X and Y chromosomes appear at the bottom right corner of the image. If this were a female, the two sex chromosomes would both be relatively larger X chromosomes. As you can see, compared to the X chromosome, the Y chromosome is small and carries fewer genes.
The exact genes carried on the X chromosome varies among species. In humans, for example, the gene coding for normal clotting factors and the gene coding for normal cone photoreceptor pigment are located on the X chromosome. Abnormal mutant forms of these genes can result in hemophilia (a potentially fatal disorder in which the blood fails to clot) in the former case, and red-green color blindness in the latter.
Cats: One X-linked Gene with Two Alleles for Coat Color
In cats, one of several genes controlling fur color is located on the X chromosome. The gene has two versions, or alleles. One form of the gene codes for orange fur (XB), and the other form codes for black fur (Xb). While it's difficult to say which allele is dominant and which is recessive, since only one is ever expressed (see below), let's say, for the sake of argument, that orange is dominant to black. Ordinarily, this would mean that an animal inheriting one copy of each gene (genotype XBXb) should have orange fur. Surprisingly, a female cat heterozygous at that locus (XBXb) will not be orange. Instead, her coat will be a patchwork of orange and black, a condition known as tortoiseshell. Why is she not just orange?
Female Genotypes
There are three possible female genotypes for this locus:
Male Genotypes
Males, having only one X chromosome, carry only one allele for the orange vs. black fur color trait. The Y does not carry this allele at all, being much smaller and having fewer loci than the X chromosome. The condition of carrying only one allele for a particular trait is known as hemizygosity (from the Greek hemi, meaning "half" and "zygote", referring to the fertilized egg).