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Chapter 5: Problem 21

Cat breeders are aware that kittens expressing the X-linked calico coat pattern and tortoiseshell pattern (Figure 5.6 ) are almost invariably females. Why?

Short Answer

Expert verified
Answer: X-linked calico and tortoiseshell coat pattern kittens are almost always females because their chromosomal makeup allows them to carry both the Orange (O) and non-orange (o) genes on their two X chromosomes, resulting in the expression of both black and orange fur. Males, having only one X chromosome, cannot carry both genes simultaneously, making it highly unlikely for them to exhibit these coat patterns.

Step by step solution

01

Understanding X-linked inheritance and chromosomes

X-linked inheritance refers to genes that are present on the X chromosome. In mammals, including cats and humans, gender is determined by the presence of X and Y chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). Because the calico and tortoiseshell coat patterns are X-linked, they are determined by genes on the X chromosome.
02

Explaining coat color genes in cats

In cats, there are two primary genes responsible for coat color, and they are both X-linked genes. The Orange gene (O) codes for orange fur, while the non-orange gene (o) codes for black fur. Since cats have two X chromosomes, a female cat can have two different color genes, one on each X chromosome. This can result in three possible combinations: OO (orange fur), oo (black fur), and Oo (calico or tortoiseshell, where both black and orange fur are expressed).
03

Expression of coat color genes in males and females

Since male cats only have one X chromosome, they can only carry either the Orange (O) or the non-orange (o) gene but not both. Hence male cats can have either orange or black fur but usually cannot exhibit both colors like in calico or tortoiseshell patterns. Female cats, however, can carry both genes because they have two X chromosomes. So if a female cat inherits one Orange (O) gene and one non-orange (o) gene, she can have the calico or tortoiseshell coat pattern.
04

Conclusion

In conclusion, the reason why kittens expressing the X-linked calico coat pattern and tortoiseshell pattern are almost always females is due to their chromosomal makeup. Females, having two X chromosomes, can carry both the Orange (O) and non-orange (o) genes, and thus express both black and orange fur in these coat patterns. Males can only have one X chromosome, limiting their ability to express both genes simultaneously, making it highly unlikely for them to have calico or tortoiseshell coat patterns.

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Most popular questions from this chapter

In chickens, a key gene involved in sex determination has recently been identified. Called \(D M R T 1\), it is located on the \(Z\) chromosome and is absent on the W chromosome. Like SRY in humans, it is male determining. Unlike \(S R Y\) in humans, however, female chickens (ZW) have a single copy while males (ZZ) have two copies of the gene. Nevertheless, it is transcribed only in the developing testis. Working in the laboratory of Andrew sinclair (a co- discoverer of the human \(S R Y\) gene), Craig Smith and colleagues were able to "knock down" expression of \(D M R T 1\) in \(Z Z\) embryos using RNA interference techniques (see Chapter 16 ). In such cases, the developing gonads look more like ovaries than testes [Nature 461: 267 (2009)]. What conclusions can you draw about the role that the DMRT1 gene plays in chickens in contrast to the role the SRY gene plays in humans?

How do mammals, including humans, solve the "dosage problem" caused by the presence of an \(\mathrm{X}\) and \(\mathrm{Y}\) chromosome in one sex and two X chromosomes in the other sex?

It is believed that any male-determining genes contained on the Y chromosome in humans are not located in the limited region that synapses with the X chromosome during meiosis. What might be the outcome if such genes were located in this region?

As related to sex determination, what is meant by (a) homomorphic and heteromorphic chromosomes; and (b) homogametic sex and heterogametic sex?

What specific observations (evidence) support the conclusions about sex determination in Drosophila and humans?
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