Question

The gene for coat color in some breeds of cats is found on the X chromosome. Calico cats are mottled with orange and black colorings. What is a possible explanation for the fact that true calico cats are only female? (A) The allele for coat color is randomly chosen by Xinactivation. (B) The coat color is linked with genes that are lethal to male cats, so male calico cats never live past birth. (C) Coat orange and black color is expressed only in female cats; males have only one color. (D) Male cats have Y chromosomes.

Short answer

The most plausible explanation for why true calico cats are only female is Option A: The allele for coat color is randomly chosen by X inactivation.

Step-by-step solution

Option A: The allele for coat color is randomly chosen by X inactivation

X inactivation is a process that happens in female mammals where one of the X chromosomes is inactivated to create a mosaic pattern of gene expression. As female cats have two X chromosomes, they will have two different alleles for coat color present on each chromosome. If the allele for coat color is randomly chosen by X inactivation, it's possible that this could explain why true calico cats are only female. This option seems plausible.

Option B: The coat color is linked with genes that are lethal to male cats

If the genes for coat color were linked to genes that are lethal for males, then it wouldn't be possible for a male cat to have the combination of genes required to be a true calico cat. While this option could be theoretically possible, it's not as likely as option A because it does not provide any evidence that specifically links coat color to genes that may be lethal to males.

Option C: Coat orange and black color is expressed only in female cats

This option suggests that the coat colors (orange and black) are expressed only in female cats, which could explain why only female cats can be true calico cats. However, there is no detailed information or rationale provided in this option that explains why coat colors are expressed only in females. It may be a plausible explanation, but option A is still a more convincing explanation.

Option D: Male cats have Y chromosomes

While this option is a fact, male cats having Y chromosomes doesn't necessarily explain why true calico cats are only females. This statement does not provide any direct evidence to link male cats having Y chromosomes to the fact that true calico cats are females, making it an unlikely explanation compared to option A. Based on the analysis above, it's clear that the most plausible explanation for why true calico cats are only female is: Option A: The allele for coat color is randomly chosen by X inactivation.

Key concepts

Sex-Linked Traits

Sex-linked traits are qualities or disorders that are determined by genes located on the sex chromosomes. In mammals, the sex chromosomes are known as X and Y. Males have one X and one Y chromosome (XY), while females have two X chromosomes (XX). Because of this difference, males and females can exhibit certain traits differently.

For instance, if a trait is carried on the X chromosome, it is termed 'X-linked'. Since males have only one X chromosome, any allele present on it will be expressed, even if it is recessive. Females, with two X chromosomes, can be carriers of X-linked traits without them being externally visible. In this way, certain traits or disorders can affect one sex more commonly than the other.

X-linked inheritance is central to understanding the patterning of coat colors in calico cats. The genes for coat color, specifically for the black and orange colors, are located on the X chromosome. This means that the varied and distinct pattern seen in calico cats has a genetic basis tied to their sex chromosome configuration.

X Chromosome

The X chromosome is one of the two sex chromosomes, the other being the Y chromosome. Females typically have a pair of X chromosomes while males have one X and one Y, determining their sex. It is of considerable size and contains many genes that are crucial for the normal functioning of cells. For a long time, the X chromosome was thought to mainly carry genes related to sex and reproduction, but research has shown that it also includes genes integral to brain function and the development of other body tissues.

In the context of calico cats, the X chromosome carries multiple alleles for coat color. A female cat has two of these chromosomes, and therefore, two opportunities to express the coloration genes. The phenomenon of X inactivation decides which of the cat's X chromosomes will be

Genetic Mosaicism

Genetic mosaicism is a condition where an individual has cells with different genetic makeups. This occurs when there is a change in the genetic material in a cell after conception, leading to a situation in which a single individual has cells with different genetic codes. This phenomenon can result in notable variation within the tissues of an organism. For calico cats, mosaicism is visible in their skin coloration patterns.

X inactivation, a key component to creating this patchwork of colors, is a common example of genetic mosaicism. Since female mammals, including cats, have two X chromosomes, one of them is randomly inactivated in each cell early in development. This inactivation process causes the cells to express one allele while the other remains silent, resulting in distinct patches of color on the coat of calico cats.

Alleles for Coat Color

Alleles are different versions of a gene that determine distinct traits that can be passed from parents to offspring. The color of a cat's coat is directly determined by the alleles present in its genetic makeup. In calico cats, there are typically two alleles involved in coat color that are located on the X chromosome: one for black fur and another for orange fur.

Since male cats have only one X chromosome, they can either be black or orange, but not a combination of both, which is necessary for calico patterning. Female cats, with their two X chromosomes, can have one chromosome with each allele. When X inactivation occurs randomly in different cells, some cells express the black allele while others express the orange allele. The result is the beautiful and unique mosaic-like calico pattern that is seen only in female cats.