In cats, tortoiseshell coat color appears in females. A tortoiseshell coat has patches of dark brown fur and patches of orange fur that each in total cover about half the body but have a unique pattern in each female. Male cats can be either dark brown or orange, but a male cat with tortoiseshell coat is rarely produced. Two sample crosses between males and females from pure-breeding lines produced the tortoiseshell females shown.

Cross I          P: dark brown male × orange female
                      F₁: orange males and tortoiseshell females
Cross II         P: orange male × dark brown female
                      F₁: dark brown males and tortoiseshell females

Why are tortoiseshell cats female?

Key Concepts

X-Linked Inheritance

Tortoiseshell coat color in cats is a result of X-linked inheritance, where the genes responsible for the color are located on the X chromosome. Female cats have two X chromosomes, allowing them to express both color traits (orange and dark brown) simultaneously. In contrast, male cats have only one X chromosome, which limits their coat color to either orange or dark brown, making tortoiseshell males extremely rare.

Dosage Compensation

In female mammals, one of the two X chromosomes is randomly inactivated in a process called X-inactivation or dosage compensation. This results in a mosaic expression of the genes on the X chromosomes, leading to the characteristic patchwork of colors seen in tortoiseshell females. This phenomenon explains why tortoiseshell coloration is typically observed only in females.

Genetic Crosses and Phenotypes

The genetic crosses described in the question illustrate how different parental genotypes can produce specific offspring phenotypes. In the provided crosses, the combination of dark brown and orange parents results in tortoiseshell females due to the inheritance of different alleles from each parent. Understanding these crosses helps clarify how the tortoiseshell phenotype arises and why it is predominantly female.