Normally, men have an X and a Y chromosome, and women have two X chromosomes. However, things do not always go as planned. Sometimes there is an extra X or an extra Y. Sometimes there is a missing X or a missing Y.
Men have one X chromosome. Likewise, they only have one copy of each of the genes on the X chromosome. This allows certain recessive genes on the X chromosome to act dominant, as in the case of X-linked disorders.
Females have two copies of the X chromosomes and its genes. Fortunately, there is an ingenious way of preventing extra protein from two active X chromosomes from problematizing normal development. Women deactivate all of the genes on one of their X chromosomes. This is true of every one of the female human's ceclls.
The XIST (X Inactive-Specific Transcript) gene causes one of the X chromosomes to deactivate in women. After the beginning of a female zygote's development, the zygote becomes about 16 cells large before the XIST gene activates, binding to one of the X chromosomes in order to inactivate all of its genes. '
The inactivated X chromosome is called a Barr body. Only one is inactivated. The other remains active. The female gets one X chromosome from her mother and the other from her father. Which is activated is up to chance. Since both X chromosomes are from different parents, the one which activates determines the phenotypes that are expressed in the woman (either maternal or paternal phenotypes).
Some humans have a chromosomal abnormality in which they have more than the normal number of X chromosomes for their sex. Some women, for example, have three, rather than two, X's. Some of these suffer from mental retardation, although most simply tend to be taller and thinner, but are in other respects, normal. 1 out of 1,000 girls are XXX. In cases of mental retardation, the severity is worsened by extra X's.
Men with more than one X chromosome are XXY. One out of 500 boys are XXY. This is called Klinefelter syndrome. Like women, some men can have up to 4 X's. While only one of these X's are active, they do produce an effect before X inactivation occurs, which can cause these men to be unusually tall, and sometimes, sterile, as well as feminizing secondary sex characteristics to a degree. They tend, for example, to have less facial hair, and some experience breast enlargement.
Some men have an extra Y (XYY). They tend to be taller and grow faster. They can also experience some learning disabilities. Some women, on the other hand, lack a Y chromosome. They only possess an X chromosome. This is called Turner syndrome. This affects one in every 2,500 girls.
They frequently fail to acquire feminine secondary sex characteristics, such as breast development and menstruation. They tend to be shorter than usual, and sometimes exhibit heart and kidney problems. In other respects, however, they are normal.
Inheritance can be sex-linked. This means that the genes which produce certain traits are on the X or Y chromosomes themselves. In some cases, genes located on autosomal chromosomes may express themselves differently depending upon whether they occur in men or women. That is, some phenotypes are controlled or influenced by the sex of the human individual.
A major revolution in understanding the sex-linked nature of some traits occurred in 1910, when Thomas H. Morgan discovered X-linked inheritance in the eye color of fruit flies. The inheritance patterns with respect to red and white eyes seemed to contradict standard Mendelian inheritance patterns until he realized that the gene for eye color resided on the X chromosome. White eye color in the bugs is recessive, only expressing itself in female flies when it is homozygous.
In males, however, the trait expresses itself whenever they inherit the gene, even though it is recessive. It behaves in men as though it is dominant. That is, it behaves in a dominant manner when it is "hemizygous," or one copy. Such X-linked recessive disorders rarely show up in women, but they show up in men whenever the men inherit the traits. Female carriers do not exhibit the phenotype, but they pass it on to their sons, who do express the phenotype.
Other traits are sex-limited. That means that while their inheritance patterns are normal, they only express a phenotype in one sex. It does not matter if they are heterozygous or homozygous in a certain sex, they will never express themselves. In the language of genetics, they exhibit 100 percent penetrance in one sex and 0 percent penetrance in the other sex.
Men and women both possess the gene responsible for milk production, but only women express these genes. Gene expression, in this case, is controlled by hormone levels. In one case, a gene on chromosome 2 causes premature puberty, but it only has this effect in men.
Some traits are sex-influenced. Theye xist on autosomal chromosomes, and express phenotypes in both sexes, but how they express themselves depends upon the sex of the individual with the gene. The gene for male-pattern baldness exists on chromosome 15.
It is autosomal dominant in men, and women only experience this phenotype when they are homozygous for this gene. Women who are heterozygous for this gene are predisposed to ovarian disease, such as polycystic ovary disease. Thus, men who are heterozygous for this gene become bald, whereas women who are heterozygous for the disease may have ovary problems, and only women who are homozygous for the disease become bald.
Y-linked traits are passed from father to son, unsurprisingly, since the majority of Y-linked traits have to do with male sex traits. Since Y is hemizygous, Y-linked traits are always expressed. One Y-linked trait is hairy ears. This trait is incompletely penetrant. So just because you have a hairy-eared dad does not mean that you (a male) will necessarily be hairy-eared.
Robinson, Tara Rodden (2010-04-13). Genetics For Dummies (Kindle Location 1745). Wiley. Kindle Edition.