10.4 Sex Linkage: An Exception to Mendel’s First Law

We have introduced sex chromosomes and autosomes (non-sex-linked chromosomes). For loci on autosomes, the alleles follow the classic Mendelian pattern of inheritance. However, for loci on the sex chromosomes this doesn’t follow because most (not all) of the loci on the typical X-chromosome are absent from the Y-chromosome, even though they act as a homologous pair during meiosis. Instead, they will follow a sex-linked pattern of inheritance. An X-linked allele in the father will always be passed on to his daughters only, but an X-linked allele in the mother will be passed on to both daughters and sons equally.

X-Linked Genes: The White Gene in Drosophila Melanogaster

A well-studied, sex-linked gene is the white gene on the X chromosome of Drosophila melanogaster. Normally flies have red eyes, but flies with a mutant allele of this gene called white (w) have white eyes because the red pigments are absent. Because this mutation is recessive to the wild type w+ allele, females that are heterozygous have normal red eyes. Female flies that are homozygous for the mutant allele have white eyes. Because there is no white gene on the Y chromosome, male flies can only be hemizygous for the wild type allele or the mutant allele.

A researcher may not know beforehand whether a novel mutation is sex-linked. The definitive method to test for sex-linkage is reciprocal crosses (Figure 10.4.2). This means to cross a male and a female that have different phenotypes, and then conduct a second set of crosses, in which the phenotypes are reversed relative to the sex of the parents in the first cross. For example, if you were to set up reciprocal crosses with flies from pure-breeding w+ and w strains the results would be as shown in Figure 10.4.2. Whenever reciprocal crosses give different results in the F1 and F2 and whenever the male and female offspring have different phenotypes the usual explanation is sex-linkage. Remember, if the locus were autosomal, the F1 and F2 progeny would be different from either of these crosses.

A similar pattern of sex-linked inheritance is seen for X-chromosome loci in other species with an XX-XY sex chromosome system, including mammals and humans.

 

Five flies showing various genotypes and phenotypes for eye colour
Figure 10.4.1 – Relationship between genotype and phenotype for the white gene on the X-linked gene in Drosophila melanogaster. The Y chromosome is indicated with a capital Y because it does not have a copy of the white gene.

 

Image shows reciprocal crosses which involving an X-linked gene in fruit flies and the F1 and F2 generations
Figure 10.4.2 – Reciprocal crosses involving an X-linked gene in Drosophila melanogaster. In the first cross (left) all of the offspring have red eyes. In second (reciprocal) cross (right) all of the female offspring have red eyes and the male offspring all have white eyes. If the F1 progeny are crossed (to make the P2), the F2 progeny will be different in each cross. The first cross has all red-eyed females and half red-eyed males. The reciprocal cross has half red-eyed males and females.
Thomas Morgan was awarded the Nobel Prize, in part, for using these crosses to demonstrate that genes (such as white) were on chromosomes (in this case the X-chromosome).

Take a look at the video below, Inheritance of X-Linked Genes, presented by Professor Dave Explains (2020), which discusses the inheritance of sex-linked traits.

Media Attributions

Reference

Deyholos, M., Harrington, M., & Locke, J. (2017). Figures: 4. Relationship between genotype and phenotype…; and 5. Reciprocal crosses involving an X-linked gene [digital image]. In Locke, J., Harrington, M., Canham, L. and Min Ku Kang (Eds.), Open Genetics Lectures, Fall 2017 (Chapter 20, p. 3). Dataverse/ BCcampus. http://solr.bccampus.ca:8001/bcc/file/7a7b00f9-fb56-4c49-81a9-cfa3ad80e6d8/1/OpenGeneticsLectures_Fall2017.pdf

Professor Dave Explains. (2020, March 11). Inheritance of X-linked genes (video file). YouTube. https://www.youtube.com/watch?v=IShS60Azqjg

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Open Genetics by Natasha Ramroop Singh, Thompson Rivers University is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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