Y linkage

Y linkage, which can also be known as sex linkage or holandric inheritance, describes traits that are produced by genes located on the Y chromosome.

Y linkage can be very hard to determine and prove. This is due in part because the Y chromosome is small and does not contain as many genes as autosomal genes or the X chromosome. However, the human Y chromosome is no longer considered to be as barren as it once was;[1] it is estimated to contain about 200 genes. The Y-chromosome is sex determining in some species but not others, so not all genes that play a role in sex determination are Y-linked. The Y-chromosome for the most part does not undergo genetic recombination; only small regions called the pseudoautosomal regions exhibit recombination. The majority of the Y-chromosome that does not recombine is called the "non-recombining region".[2]

For a trait to exhibit Y linkage it must follow three requirements: the trait occurs in males only, it reoccurs in all sons of affected males, and that the daughters of affected men are not only phenotypically normal but also do not have affected offspring.[3] These requirements were established by the pioneer of Y linkage, Curt Stern. Stern detailed in his paper a plethora of genes he suspected to be y-linked.[3] His requirements at first made Y linkage very hard to determine and prove. In the 1950s using human pedigrees, many genes were determined incorrectly to be Y-linked.[4] This has led to new research with more advanced techniques and more complicated statistical analysis.[5] Hairy ears is an example of a gene once thought to be Y-linked in humans; however, that hypothesis has been discredited.[4] Due to advancements with sequencing, Y linkage is getting easier to determine and prove. The field of genomics will help advance this topic due to the fact that it is getting cheaper to sequence entire genomes. In recent years, the Y-chromosome has been almost entirely mapped revealing many traits that are y-linked.

Y linkage is similar but different than X linkage; however, both are forms of sex linkage. X linkage can be genetically linked and sex linked, while Y linkage can only be genetically linked. This is because there is only one copy of the Y-chromosome in males. X-chromosomes have two copies, one from each parent and recombination can occur. The X chromosome also contains more genes and the chromosome is substantially larger.

Some traits are thought to be Y-linked but have not been confirmed. One example is hearing impairment. Hearing impairment was tracked in one specific family and through seven generations all males were affected by this trait. However, this trait has only occurred very rarely and has not been entirely confirmed or disproved.[6]

Pedigree tree showing the inheritance of a Y-linked trait

In guppies, Y-linked genes help determine sex selection. This is done indirectly by traits that allow the guppy to appear more attractive to a prospective mate. These traits were shown to be on the Y-chromosome and thus Y-linked.[7] Also in guppies, it appears that the four measures of sexual activity is Y-linked.[8]

Y-chromosome deletions are a frequent genetic cause of male infertility.

Y linkage in rats

Hypertension or high blood pressure appears to be Y-linked in the hypertensive rat. One of the loci was autosomal however a second component of this trait appeared to be Y-linked. This stayed true in the third generation of rats as well as the second. Male offspring with a hypertensive father had significantly higher blood pressure than male offspring with a hypertensive mother indicating a component of the trait being Y-linked. The results were not the same in females as in males further symbolizing it having a Y-component. This research could have implications in helping fight blood pressure in human males. This is useful in genomics due to the fact that if the gene can be sequenced it may be possible to determine if a person will develop hypersensitivity before they actually do.[9]

Hairy ears Y-linked discussion

Many genes were at once thought to be Y-linked traits, including hairy ears. The discussion on whether Hairy Ears are a y-linked trait has been a debated issue in recent papers on Y linkage. The primary analysis was that Hairy Ears were a y-linked trait.[10] However, that paper was done through pedigrees and visual observations[11] and has since been debunked. Since then, a paper has emerged using sequencing to determine the validity of the original paper. This paper used DNA Sequencing and binary markers to determine statistically that there was no correlation between individuals with hairy ears and individuals without hairy ears.[4] This proved that hairy ears were not a Y-linked trait. Genomics and sequencing allowed for the final determination that this trait is not Y-linked. Without DNA-mini-seqs and other methods it would have proved impossible to settle this argument. Sequencing was a way to statistically disprove what pedigrees only showed.

Genes known to be contained on the human Y chromosome

In general, traits that exist on the Y chromosome are Y-linked because they only occur on that chromosome and do not change in recombination.

As of 2000, a number of genes were known to be Y-linked, including:[12]

See also

References

  1. ↑ Sayres, Wilson (2012). "Gene survival and death on the human Y chromosome". Mol Biol Evol. 30: 781–87. doi:10.1093/molbev/mss267. PMC 3603307. PMID 23223713.
  2. ↑ Skaletsky, Helen (2003). "The male-specific region of the human Y chromosome is a mosaic of discrete sequence classes". Nature. 423: 825–837. doi:10.1038/nature01722.
  3. 1 2 Curt, Stern (1957). "The Problem of Complete Y-Linkage in Man". American Journal of Human Genetics. 9.3: 147–166 – via Google Scholar.
  4. 1 2 3 Lee, Andrew (2004). "Molecular evidence for absence of Y-linkage of the Hairy Ears trait". European Journal of Human Genetics. 112: 1077–1079. doi:10.1038/sj.ejhg.5201271.
  5. ↑ Ott, J (1986). "Y-linkage and pseudoautosomal linkage". Am J Hum Genet. 38: 891–7. PMC 1684847. PMID 3728465.
  6. ↑ Wang, Qiuju (2013). "Genetic Basis of Y-Linked Hearing Impairment". Am J Hum Genet. 92: 301–6. doi:10.1016/j.ajhg.2012.12.015. PMC 3567277. PMID 23352258.
  7. ↑ Postma, Erik (2011). "SEX-DEPENDENT SELECTION DIFFERENTIALLY SHAPES GENETIC VARIATION ON AND OFF THE GUPPY Y CHROMOSOME". Society for the Study of Evolution. 65: 2145–2156. doi:10.1111/j.1558-5646.2011.01314.x.
  8. ↑ Farr, James (1983). "The Inheritance of Quantitative Fitness Traits in Guppies, Poecilia reticulata". Evolution. 37: 1193–1209. doi:10.2307/2408841.
  9. ↑ Ely, D. (1990). "Hypertension in the spontaneously hypertensive rat is linked to the Y chromosome". American Heart Association.
  10. ↑ Stern, Curt (1964). "New Data on the Problem of Y-Linkage of Hairy Pinnae". Am J Hum Genet. 16: 455–71. PMC 1932324. PMID 14250426.
  11. ↑ Dronameraju, K. (1960). "Hypertrichosis of the pinna of the human ear, Y-linked pedigrees". Journal of Genetics. 57: 230–243. doi:10.1007/bf02987230.
  12. ↑ "Y-linked gene definition - Medical Dictionary: Definitions of Popular Terms Defined on MedTerms". Medterms.com. 2012-09-20. Retrieved 2014-06-29.
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