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

Explain the inheritance of dark brown, orange, and tortoiseshell coat colors in cats.

Key Concepts

X-Linked Inheritance

In cats, coat color is determined by genes located on the X chromosome. Female cats have two X chromosomes (XX), allowing them to express two different color alleles, resulting in the tortoiseshell pattern. In contrast, male cats have one X and one Y chromosome (XY), which means they can only express one color allele, leading to either dark brown or orange fur.

Codominance

The tortoiseshell coat color in female cats is an example of codominance, where both alleles for color (dark brown and orange) are expressed simultaneously. This results in a unique patchwork appearance, as the fur displays both colors in distinct areas rather than blending into a single color. This phenomenon is crucial for understanding how the tortoiseshell phenotype arises.

Genetic Crosses and Punnett Squares

Genetic crosses, such as those described in the question, can be analyzed using Punnett squares to predict the offspring's genotypes and phenotypes. By crossing a dark brown male with an orange female, and vice versa, we can visualize the inheritance patterns and determine the expected ratios of male and female offspring, including the occurrence of tortoiseshell females.