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Sex determination and sex linkage

Determination of sex

While some species are hermaphroditic (each individual has two sexes), others have sex determination in separate individuals based on environmental cues during development such as temperature and resource availability, or based on genetic factors e.g. chromosomes.

Some factors that can shift an individual's sex in some species include their size, parasitic infection or competition. For example, crustaceans such as shrimp are born without a sex, and are prone to being directed by various cues. A paramyxean parasite was found to drive shrimps to turn female, causing an imbalance in the population and disrupting marine habitats.

The Y chromosome in mammals and some insects is the male sex determinant through a specific locus called SRY that enables subsequent cascades of expression of the male sex developmental path. All the products are contained on other chromosomes, including the X chromosome, so the Y chromosome merely carries the on switch for all of this.

(Chromosomes 1 through 22 are typically present in two copies in all individuals of mammalian species. These are autosomes.)

This means that all genes on the X chromosome (which has a common ancestry with the Y chromosome resembling homologous chromosomes) are one copy in XY individuals, and 2 copies in XX individuals. In diploid organisms where all chromosomes are represented twice, this leaves a vulnerable loophole for recessive conditions carried on the X chromosome.

Sex linkage

Sex linkage refers to a trait that is carried on a sex chromosome like the X chromosome. Because someone might have a different number or combination of sex chromosomes such as a single X chromosome or two X chromosomes, the expression of various traits can differ.

If for example the allele on the affected X chromosome means that an essential protein isn't being made, the carrier XX child has another unaffected X chromosome to fall back on and be able to produce the essential protein. The same loci (genes) on the second X chromosome are randomly inactivated to prevent double the dose of their products in the cell. This means that as a carrier, half the cells in tissues will be expressing one X, and the other half the other X. This prevents the expression of the illness.

The carrier XY child only has the affected X chromosome and cannot make the protein. This results in an illness for example, such as haemophilia. Haemophilia is a blood clotting disorder in which excessive bleeding takes place because the platelet plug and fibrin which are supposed to stop bleeding do not work fully.

Another example of an X-linked condition is Duchenne muscular dystrophy which affects around 1 in every few thousand children with an XY genotype. It is a progressive muscle degeneration condition that affects mobility, and in the long term requires assistive ventilation devices, with an overall impact on life expectancy. 

The inheritance pattern matches that of haemophilia.

Dominant examples of sex-linked conditions include Alport syndrome. These can affect both XX and XY individuals because just one allele is enough to cause the disease. Since XY children inherit their Y chromosome only from their XY parent (who might be carrying the allele on their X), they will not be affected. However, all XX children will be affected.

If the affected allele is on one of the XX parent chromosomes, there is a 50% probability that the condition will be passed on to any of the children.

Alport syndrome is a genetic disorder that produces kidney failure, hearing loss and later on vision loss. Symptoms are caused by a defect in the production of type IV collagen. Despite the condition being inherited in a dominant manner, differences in symptoms between XY and XX individuals exist, where the presence of the additional unaffected X chromosome can prevent kidney failure, with only bloody urine present. Therefore, there are varying degrees of severity of Alport syndrome.

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