Which chromosomes do males have

This page appears in the following eBook. Aa Aa Aa. In humans and many other animal species, sex is determined by specific chromosomes. How did researchers discover these so-called sex chromosomes? The path from the initial discovery of sex chromosomes in to an understanding of their true function was paved by the diligent efforts of multiple scientists over the course of many years.

As often happens during a lengthy course of discovery, scientists observed and described sex chromosomes long before they knew their function.

An idea inspired by the "X element". Before Her Time. The Life of Nettie Stevens. Figure 2: The darkling beetle, Tenebrio molitor. The variety of sex determination systems. Figure 3: Example set of male human chromosomes. In the image, the X and Y chromosomes are indicated by arrows. Figure 4: Sex determination in humans. On the upper left, the female parent genotype is XX. The first X is labeled to the left of the top quadrant, while the second X is labeled outside the left quadrant. On the upper right, the male parent genotype is XY.

The X is labeled to the right of the top quadrant, while the Y is labeled outside the right quadrant. In this case, two of the four quadrants contain the genotype XX, resulting in female offspring; the remaining two quadrants contain the genotype XY, resulting in male offspring.

Figure 5: Sex determination in insects. On the upper right, the male parent genotype is X-. The X is labeled to the right of the top quadrant, while the minus sign is labeled outside the right quadrant.

Deprived of the benefits of recombination, Y chromosomal genes degenerate over time and are eventually lost from the genome. Looking to other species Y chromosomes exist in mammals and some other species , a growing body of evidence indicates that Y-chromosome gene amplification is a general principle across the board. These amplified genes play critical roles in sperm production and at least in rodents in regulating offspring sex ratio.

Writing in Molecular Biology and Evolution recently, researchers give evidence that this increase in gene copy number in mice is a result of natural selection. The latter group argues that its defence mechanisms do a great job and have rescued the Y chromosome. But the leavers say that all they are doing is allowing the Y chromosome to cling on by its fingernails, before eventually dropping off the cliff.

The debate therefore continues. Sex is determined by the SRY gene, which is responsible for the development of a fetus into a male. Other genes on the Y chromosome are important for enabling men to father biological children male fertility.

Many genes are unique to the Y chromosome, but genes in areas known as pseudoautosomal regions are present on both sex chromosomes. As a result, men and women each have two functional copies of these genes.

Many genes in the pseudoautosomal regions are essential for normal development. The following chromosomal conditions are associated with changes in the structure or number of copies of y chromosome. In most individuals with 46,XX testicular disorder of sex development, the condition results from an abnormal exchange of genetic material between chromosomes translocation.

This exchange occurs as a random event during the formation of sperm cells in the affected person's father. In the translocation that causes 46,XX testicular disorder of sex development, the SRY gene, which is normally found on the Y chromosome, is misplaced, almost always onto an X chromosome. An individual with an X chromosome that carries the SRY gene will develop as a male despite not having a Y chromosome, but will not be able to produce sperm to father biological children.

Males with 47,XYY syndrome have one X chromosome and two Y chromosomes in each cell, for a total of 47 chromosomes. An extra copy of the genes contained in the pseudoautosomal region of the Y chromosome may explain the tall stature and other features that can affect boys and men with this condition. Some males with 47,XYY syndrome have an extra Y chromosome in only some of their cells. Extra genetic material from the X chromosome interferes with male sexual development, preventing the testes from functioning normally and reducing the levels of testosterone a hormone that directs male sexual development in adolescent and adult males.

Extra copies of genes from the pseudoautosomal region of the extra X and Y chromosomes contribute to the signs and symptoms of 48,XXYY syndrome; however, the specific genes have not been identified. Deletions of small amounts of genetic material in certain areas of the Y chromosome lead to a condition called Y chromosome infertility. This condition affects the production of sperm and makes it difficult or impossible for affected men to father children.

The deletions occur in areas of the Y chromosome called azoospermia factor AZF regions. The father can contribute an X or a Y chromosome, while the mother always contributes an X. The Y chromosome is one-third the size of the X chromosome and contains about 55 genes while the X chromosome has about genes. In genealogy, the male lineage is often traced using the Y chromosome because it is only passed down from the father.

All individuals carrying a Y chromosome are related through a single XY ancestor who likely lived around , years ago.

The Y chromosome contains a "male-determining gene," the SRY gene , that causes testes to form in the embryo and results in development of external and internal male genitalia. If there is a mutation in the SRY gene, the embryo will develop female genitalia despite having XY chromosomes.