Sex+Linkage

**Sex Linkage (Sex-linked Inheritance)**
__**Collaborative Notes:**__

There are some key terms: __Sex Chrosmosome & Autosome:__ Chromosone involoved in the determination of sex. In Human, there are 23 pairs of chromosome. The first 22 pairs are autosomal chromosome, the 23th pair of chromsome (ie XX for female or XY for male) is the sex chromosome.Autosome is said to be non-sex chromosome. __Heterogametic Sex:__ The parental sex which determines the sex of the offspring. For human, heterogametic sex is said to be male who has XY (two different gamets) __Homogametic Sex:__ The sex that play no part in sex determination. For human, homogametic sex is female ( two identical gametes).

Sex-linked Traits

Sex-linked traits are caused by genes on the sex chromosomes (almost always the X-chromosome, in which case the condition is said to be X-linked.)
 * X-Linked Inheritance: **


 * The Y-chromosome is much shorter than the X, so it cannot carry as many genes as X-chromosome. There must therefore be many loci on the X-chromosome that are not represented on the Y-chromosome.
 * Male have only one X-chromosome,inherited from the mother. Males therefore cannot be the heterozygous for X linked genes, which are always expressed. As male only have one X, males are said to be hemizygous for X-linked genes
 * SInce females have two X-chromosomes, a sex-linked recessive allele will only be expressed if she is homozygous (ie,xx ). For this to happen, //both// her parents must carry an X-chromosome with the defective allele.
 * This is why sex-linked recessive alleles are much more rarely expressed in females thatn in males


 * Y-linked Inheritance: **


 * The Y-chromosome contains only about 60 genes. The X and Y chromosomes are homologous nevertheless because they pair and subsequently segregate during meiosis.
 * The most important gene is the sex-determining region of the Y-chromosome, The SRY gene, which triggers the development of male characteristics during development.
 * This gene of found on a segment of the Y chromosome that is not found on the X chromosome, so is said to be Y-linked.
 * No crossing over occurs between the X and Y chromosome at this point. A Y-linked gene is only found in males (unless as result of translocation mutation it shifts to the X-chromosome and is inherited by female offspring and must therefore be passed on to any male offspring.) There are very few Y-linked genes known for certain to be functional.
 * The X- and Y-chromosomes do undergo crossing over and recombination, but only in the short regions near the ends. These regions contain the 20 or so genes that are equally inherited by both sexes and are said to be //pseudo-autosomal.// They show that normal diploid inheritance patterns that are seen for other genes found on autosomes.

It is often possible to tell from a family tree whether an allele is sex-linked. The following are the important points to bear in mind:
 * Pedigrees and Sex-linked Inheritance **
 * In autosomal recessive and dominant traits, males are affected as often as females.
 * X-linked recessive traits are more common in males that in females, since a male only has to have one parent (his mother ) with the allele, whereas for a female to develop the trait she must have both parents with the allele.
 * A male showing an X-linked condition receives the allele from his mother and passes it on to his daughter; he cannot pass it to his sons.
 * X-linked dominant traits are more common among females that males. This is because, statistically speaking, an affected male passes on the allele to all his daughters. but an affected female passes it on to half her sons and half her daughter.

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