Chapter 5, Problem 1CS

CASE STUDY| Doggone it!

A dog breeder discovers one of her male puppies has abnormal genitalia. After a visit to the veterinary clinic at a nearby university, the breeder learns that the dog's karyotype lacks a Y chromosome, but instead has an XX chromosome pair, with one of the X chromosomes slightly larger than usual (being mammals, male dogs are normally XY and females are XX). The veterinarian tells her that in other breeds, some females display an XY chromosome pair, with the Y chromosome being slightly shorter than normal. These observations raise several interesting questions:

Can you offer a chromosomal explanation of these two cases?

Summary Introduction

To review:

Normally, the breeds of dogs have homozygous females (XX) and heterozygous males (XY), but in other dog breeds, females are heterozygous (XY) with Y chromosome a little shorter than its normal size. In the given case, the male puppy is found to have abnormal genitalia and its karyotype showed that it lacks a Y chromosome and possess an XX chromosome pair. Give the chromosomal explanation of the given cases.

Introduction:

From the chromosomal theory of inheritance, it is known that genes that are units of the genetic material, which is DNA (deoxyribonucleic acid), are present on the chromosome (condensed DNA). These chromosomes are passed down to the daughter cells during cell division. When the genetic material in the parent cell is mutated, then the mutated form of the same will be inherited by the progeny.

Explanation of Solution

Dogs have a total of 39 pairs of chromosomes with 38 pairs of autosomes that define the somatic characters and 1 pair of sex chromosomes that are also called allosomes that control the development of reproductive organs. Generally, in mammals, females are a homogametic organisms having 2 X chromosome, out of which one X chromosome is inactivated. The males are heterogametic with one X and one Y chromosome. The Y chromosome carries genes that define the development of male reproductive organs. In females, out of the two X chromosomes, one is almost entirely inactive as one X chromosome is enough to define the characters of a female.

In some dog breeds, the females carry XY chromosomes, with Y chromosome a little shorter than the normal size. In this case, it would be possible that the factor responsible for determining the development of testes is absent in such females as the Y chromosome is of shorter length.

The dog exhibiting male characteristics but having XX karyotype have one slightly larger X chromosome. In this case, the X chromosome that is donated by the father would have combined with the Y chromosome. Thus, X chromosome would be larger than the normal as it has extra genes from the Y chromosome. These genes from Y chromosomes would control male sexual characteristics. Thus, the genitalia would be abnormal.

Conclusion

Thus, it can be concluded that generally, dogs have an XX chromosome pair for females and XY pair for males. In some breeds, the female has X and a shorter Y then that Y chromosome must be lacking the gene that determines the development of testes. Also, in XX males, one X chromosome would have Y chromosome genes that resulted in abnormal genitalia.