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I am confused about definition of "sex linkage". I have found different definitions but they do no match.

From P.S Verma, V.K Agarwal cell biology, genetics, molecular biology, evolution and ecology:

If the genes are situated in the same chromosome and are fairly close to each other, they tend to be inherited together. This type of coexistence of two or more gene in the same chromosome is known as linkage.

From: Anthony J.F Griffith "Introduction to genetic analysis

In general, genes in the differential regions are said to show inheritance patterns called sex linkage.

From: Wikipedia

Sex linkage is phenotypic expression of an allele related to the allosome (sex chromosome) of the individual.

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  • $\begingroup$ Related post. $\endgroup$
    – Tyto alba
    Commented Feb 14, 2017 at 15:44
  • $\begingroup$ This question is being bumped often(4 times from your post history). If any of the entries below have answered your question, please consider accepting an answer or asking a follow up question instead. Bumping of a post that has a good but unupvoted answer removes a post that deserves attention off the main page. $\endgroup$
    – Tyto alba
    Commented Jun 17, 2017 at 6:53

2 Answers 2

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Linkage is the tendency of two genes present on the same chromosome to be inherited together. The degree of linkage depends on the distance between the two genes, i.e two genes that are physically close are less likely to be separated onto two different chromosomes during recombination/crossover.

So this is what your first definition trying to convey.

Sex linkage simply means that a gene is to be found on the differential region of a sex chromosome (and not an autosome). See it as (gene) Linked to sex chromosome.

Note: Linkage and sex linkage are not the same.

An Introduction to Genetic Analysis, 7th edition makes it a bit easier:

Sex linkage is the location of a gene on a sex chromosome.

So to sum up the definition:

Sex linkage is the location of a gene on the differential region of a sex chromosome. (this is applicable to human beings because we have PARs)

For animals in general having sex chromosomes the definition from Genetic analysis is alright.

Note: Sex chromosomes are chromosomes that have genes that have roles in determining the sex of an individual of a species.

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  • $\begingroup$ Good answer. +1. Why are is the PAR important in order to be allowed to talk about sex linkage? Note your answer make it feel like only animals have sexual chromosomes which would be wrong. Also, I am not sure that a sex chromosome is solely defined by the presence of loci explaining variance for sex. If I am not mistaken, there are GSD species that are not considered to have sex chromosomes (typically when sex is a polygenic trait). $\endgroup$
    – Remi.b
    Commented Jun 18, 2017 at 4:39
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The first definition is a definition of linkage as in "linkage disequilibrium" - that one has nothing to do with sex linkage.

Sex linked means that the gene or genes involved in a trait are inherited on one of the sex chromosomes (X or Y) and that, as a result, the disease may occur at different rates in different sexes. A good example of this is color blindness caused by a recessive genetic variant found on the X chromosome. Men only have one X chromosome, so if they inherit the variant, they will be colorblind. Women have two X chromosomes, so they will not be colorblind unless both of their X chromosomes have this problem.

In addition to saying "sex-linked", you can also be more specific and say that color-blindness is "X-linked". There are also Y-linked disorders, although they are much rarer since they usually cause infertility and don't get passed on. The term "sex-linked" seems to be gradually dying off as people adopt these more specific terms.

There are also some other terms you may come across, which sound similar but mean different things:

Sex-limited trait: This has nothing to do with genetics and instead refers to whether a person is biologically capable of manifesting a disease. For example, prostate cancer would be sex-limited because only men have prostates.

Sex-influenced trait: A genetically-influenced trait where, although the genes influencing the trait are not on a sex chromosome, a person's sex may still influence whether/how a trait manifests. An example of this would be breast cancer - certain variations in the BRCA genes increase risk of breast cancer in both women and men, but women with a BRCA mutation are still much more likely to get breast cancer then men with a BRCA mutation are.

Unfortunately, some people use the term "sex-linked" to refer to sex-limited or sex-influenced traits, which can make things very confusing.

To read more about these three categories, see: http://www.eplantscience.com/index/genetics/sex_linked_sex_influenced_and_sex_limited_traits/sex_linked_sex_influenced_and_sex_limited_traits.php

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  • $\begingroup$ can you add some references so others can read about your answer, etc? $\endgroup$
    – Vance L Albaugh
    Commented Feb 15, 2017 at 4:04
  • $\begingroup$ The first definition is a definition of "linkage (physical linkage)", not "linkage disequilibrium" $\endgroup$
    – Remi.b
    Commented Jun 18, 2017 at 4:32

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