Sex Chromosomes Definition
Sex chromosomes are chromosomes that determine whether the individual is male or female. Though these two chromosomes pair with each other during meiosis, there is usually very minimal homology or recombination between them, primarily because of a large difference in their genetic content and size. Often one chromosome is smaller, and appears to retain only those genes that are necessary for sex determination.
In evolutionary timescales, the appearance of distinctly different sex chromosomes, or heteromorphic sex chromosomes, is a relatively recent event. The first instances of sexual dimorphism, where male and female reproductive organs are in different individuals, are thought to have arisen through temperature-dependent sex determination, where some genes are turned on or off depending on the ambient temperature. These genes give rise to the external male or female characteristics and some species of lizards continue to use this method. Over time, this is supposed to have evolved into the system of different sex chromosomes.
Other methods of sex determination include haplodiploidy – where males develop from unfertilized eggs, and therefore have only one set of chromosomes, and females are diploid. Bees, ants and wasps are all common examples where male drones are haploid and the female worker bees are diploid. Komodo dragons can even preferentially produce males through parthenogenesis.
Types of Sex Chromosomes
There is a wide variety of forms that sex chromosomes can take and a number of ways in which sex determination can occur. The two major ways in which heteromorphic sex chromosomes can determine sex are known as the XY and ZW systems.
X and Y Chromosomes
In the XY system, males contain one X and one Y chromosome, while females contain two X chromosomes. Thus, males are considered heterogametic – they can produce two different types of gametes, depending on whether the sperm carries an X or a Y chromosome. Females are homogametic – all their eggs carry one X chromosome. Many primates, including humans, use the XY sex-determination system.
A variant of this is the method used by some grasshoppers. Here, males have only one X chromosome and no Y chromosome. In such systems, it is believed that a male or female develops based on the ratio between X chromosomes and the number of sets of autosomes. For instance, if a diploid individual has two X chromosomes, it develops into a female, while males arise from diploids who have one X chromosome. The impact of ratios is particularly important in sex determination in fruit flies Drosophila melanogaster and roundworms C. elegans where XXY or XXYY individuals are females and XO individuals are male. This is in contrast to the case in humans where the mere presence of one Y chromosome confers maleness, irrespective of the number of X chromosomes or the ratio between sex chromosomes and autosomes.
Until recently, there was widespread belief that the Y chromosome in primates is undergoing rapid gene loss and that the chromosome would completely disappear in about ten million years, leading primates to an XX/XO system of sex determination. This is now being contested and research into the evolution of sex chromosomes is leading to new discoveries.
W and Z Chromosomes
Birds, some fishes, reptiles and even some invertebrates undergo sex determination by the ZW method. Here the males are homogametic (ZZ) and the females carry two different sex chromosomes (ZW). Occasionally the W chromosome can be completely absent, such as in some species of butterflies, and ZO develop into females. In others, the presence of a W chromosome isn’t even necessary for the development into females.
Sex Chromosomes in Flowering Plants
For most flowering plants, or angiosperms, the male and female sex organs are present on the same flower. Occasionally, a single plant may produce separate male and female flowers to enhance cross fertilization or the male and female sex organs may mature at different times. However, the presence of distinct male plants and female plants is relatively rare and only six percent of angiosperms show this characteristic, which is called dioecy. Even those that have this kind of sexual dimorphism rise due to male sterile or female sterile mutations and therefore distinct sex chromosomes are known only in four plant families.
It appears as if plants are in the early stages of evolution of heteromorphic sex chromosomes. Therefore they can be used as models to study the events that lead to chromosomal sex determination.
Accumulation of Harmful Mutations on the Y-Chromosome
Evolution of sex chromosomes is thought to arise through mutation of autosomes that carry sex determination genes. At some point, when there is clustering of genes for sex determination on one of the two autosomes, there is a suppression of recombination to ensure that the gene cluster is inherited in one block. Once this occurs, however, an incipient Y chromosome begins to form, by accumulating transposable elements, chromosomal rearrangements and other harmful mutations, that hitchhike with favorable sex-determination genes. This is said to lead to fully heteromorphic sex chromosomes and sex determination.
The inability of the Y-chromosome to auto-correct mutations through recombination during meiosis makes it particularly prone to accumulating errors. In addition, sperms are formed in large numbers, involving many cell division events, all of which enhance the chances of error accumulation. Sperm is also stored in a highly oxidative environment in the testes, again increasing the possibility of genetic mutation. One hypothesis states that these factors have contributed to a situation where the Y chromosome has lost most of its genes, except those that are crucial for sex determination and survival of the fetus. This leads to homogametic females having nearly double the number of genes on their sex chromosomes compared to their heterogametic partners. In some animals where the males are XY, gene expression on one of the X chromosomes in females is silenced through the formation of heterochromatin. Alternatively, some insects choose to over express the genes on their X chromosome in the heterogametic individuals. This modification of gene expression is called dosage compensation. Recently, dosage compensation has also been observed for the first time in the dioecious plant S.latifolia, or while campion.
Genetic Disorders in Sex Chromosomes
Heterogametic individuals are more susceptible to genetic disorders involving the sex chromosome because they receive only one copy of each gene. For instance, if the mother is a carrier for a recessive genetic disorder, she has a fifty percent chance of passing on the illness to her male child, depending on which allele gets retained in the egg. On the other hand, none of her daughters is likely to be affected, because they would inherit another X chromosome from their father which would have the normal allele.
One major historical event was precipitated by an X-linked genetic disorder. The hemophilia of Alexis Nikolaevich, the son of the last Tsar of Russia, was inherited from his mother. The initial mutation originated in Queen Victoria and, through her daughters, it spread to many royal families in Europe. It is believed that Alexis’ ailment contributed to the fall of Russian monarchy.
In this image, carriers are colored pink and hemophiliacs are colored red. It shows the transfer of the hemophilia allele from Queen Victoria to one son and two daughters. Thereafter, three grandsons and four granddaughters inherited the allele. Through marriage, the diseased allele was passed on to royal families in Germany, Spain and Russia.
Other X-linked disorders include color blindness which is seen much more often in males. Additionally, any mutations on the Y chromosome are also inherited by male progeny without any chance of recombination or change. Y-linked inheritance is associated with reduced fertility and baldness.
On the other hand, X-linked dominant disorders affect male and female progeny. A mother who carries an X-linked dominant disorder can pass on her illness to fifty percent of her daughters and fifty percent of her sons. A father will pass on his condition to all his daughters and none of his sons. However, these are rare events because the presence of dominant genetic anomalies severely reduces reproductive opportunities.
Occasionally, a nondisjunction event during meiosis leads to aneuploidy – the fertilized egg has an irregular set of chromosomes with some being present in multiple copies or being absent altogether. When this happens with sex chromosomes, it can lead to individuals who have an unusual set of X and Y chromosomes. For instance, some females have three X chromosomes and this condition is usually discovered when there are other symptoms, such as poor muscle tone or learning difficulties. On the other hand, females with only one X chromosome have Turner’s syndrome and suffer from a number of physical, reproductive, and neurological deficiencies. Klinefelter’s males are people with two X and one Y chromosome. This is among the most common sex chromosome aneuploidies in humans with a number of subtle and gross symptoms. The men are often sterile, are taller than average but have poor muscle tone and coordination. On the other hand, men who have an extra Y chromosome show increased height but no other symptoms.
Most cases of sex chromosome aneuploidy are discovered when the individuals show neurological symptoms, learning difficulties or infertility.