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Griffiths AJF, Miller JH, Suzuki DT, et al. An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman; 2000.

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An Introduction to Genetic Analysis. 7th edition.

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Sex chromosomes and sex-linked inheritance

Most animals and many plants show sexual dimorphism; in other words, an individual can be either male or female. In most of these cases, sex is determined by special sex chromosomes. In these organisms, there are two categories of chromosomes, sex chromosomes and autosomes (the chromosomes other than the sex chromosomes). The rules of inheritance considered so far, with the use of Mendel’s analysis as an example, are the rules of autosomes. Most of the chromosomes in a genome are autosomes. The sex chromosomes are fewer in number, and, generally in diploid organisms, there is just one pair.

Let us look at the human situation as an example. Human body cells have 46 chromosomes: 22 homologous pairs of autosomes plus 2 sex chromosomes. In females, there is a pair of identical sex chromosomes called the X chromosomes. In males, there is a nonidentical pair, consisting of one X and one Y. The Y chromosome is considerably shorter than the X. At meiosis in females, the two X chromosomes pair and segregate like autosomes so that each egg receives one X chromosome. Hence the female is said to be the homogametic sex. At meiosis in males, the X and the Y pair over a short region, which ensures that the X and Y separate so that half the sperm cells receive X and the other half receive Y. Therefore the male is called the heterogametic sex.

The fruit fly Drosophila melanogaster has been one of the most important research organisms in genetics; its short, simple life cycle contributes to its usefulness in this regard (Figure 2-11 ). Fruit flies also have XX females and XY males. However, the mechanism of sex determination in Drosophila differs from that in mammals. In Drosophila, the number of X chromosomes determines sex: two X’s result in a female and one X results in a male. In mammals, the presence of the Y determines maleness and the absence of a Y determines femaleness. This difference is demonstrated by the sexes of the abnormal chromosome types XXY and XO, as shown in Table 2-3 . However, we postpone a full discussion of this topic until Chapter 23 .

Figure 2-11. Life cycle of Drosophila melanogaster, the common fruit fly.

Figure 2-11

Life cycle of Drosophila melanogaster, the common fruit fly.

Table 2-3. Chromosomal Determination of Sex in Drosophila and Humans.

Table 2-3

Chromosomal Determination of Sex in Drosophila and Humans.

Vascular plants show a variety of sexual arrangements. Dioecious species are the ones showing animal-like sexual dimorphism, with female plants bearing flowers containing only ovaries and male plants bearing flowers containing only anthers (Figure 2-12 ). Some, but not all, dioecious plants have a nonidentical pair of chromosomes associated with (and almost certainly determining) the sex of the plant. Of the species with nonidentical sex chromosomes, a large proportion have an XY system. For example, the dioecious plant Melandrium album has 22 chromosomes per cell: 20 autosomes plus 2 sex chromosomes, with XX females and XY males. Other dioecious plants have no visibly different pair of chromosomes; they may still have sex chromosomes but not visibly distinguishable types.

Figure 2-12. Two dioecious plant species: (a) Osmaronia dioica; (b) Aruncus dioicus.

Figure 2-12

Two dioecious plant species: (a) Osmaronia dioica; (b) Aruncus dioicus. (Part a, Leslie Bohm; part b, Anthony Griffiths.)

Cytogeneticists have divided the X and Y chromosomes of some species into homologous and nonhomologous regions. The latter are called differential regions (Figure 2-13 ). These differential regions contain genes that have no counterparts on the other sex chromosome. Genes in the differential regions are said to be hemizygous (“half zygous”) in males. Genes in the differential region of the X show an inheritance pattern called X linkage; those in the differential region of the Y show Y linkage. Genes in the homologous region show what might be called X-and-Y linkage. In general, genes on sex chromosomes are said to show sex linkage.

Figure 2-13. Differential and pairing regions of sex chromosomes of humans and of the plant Melandrium album.

Figure 2-13

Differential and pairing regions of sex chromosomes of humans and of the plant Melandrium album. The regions were located by observing where the chromosomes paired up in meiosis and where they did not.

The genes on the differential regions of the sex chromosomes show patterns of inheritance related to sex. The inheritance patterns of genes on the autosomes produce male and female progeny in the same phenotypic proportions, as typified by Mendel’s data (for example, both sexes might show a 3:1 ratio). However, crosses following the inheritance of genes on the sex chromosomes often show male and female progeny with different phenotypic ratios. In fact, for studies of genes of unknown chromosomal location, this pattern is a diagnostic of location on the sex chromosomes. Let’s look at an example from Drosophila. The wild-type eye color of Drosophila is dull red, but pure lines with white eyes are available (Figure 2-14 ). This phenotypic difference is determined by two alleles of a gene located on the differential region of the X chromosome. When white-eyed males are crossed with red-eyed females, all the F1 progeny have red eyes, showing that the allele for white is recessive. Crossing the red-eyed F1 males and females produces a 3:1 F2 ratio of red-eyed to white-eyed flies, but all the white-eyed flies are males. This inheritance pattern is explained by the alleles being located on the differential region of the X chromosome; in other words, by X-linkage. The genotypes are shown in Figure 2-15 . The reciprocal cross gives a different result. A reciprocal cross between white-eyed females and red-eyed males gives an F1 in which all the females are red eyed, but all the males are white eyed. The F2 consists of one-half red-eyed and one-half white-eyed flies of both sexes. Hence in sex linkage, we see examples not only of different ratios in different sexes, but also of differences between reciprocal crosses.

Figure 2-14. Red-eyed and white-eyed Drosophila.

Figure 2-14

Red-eyed and white-eyed Drosophila. (Carolina Biological Supply.)

Figure 2-15. Explanation of the different results from reciprocal crosses between red-eyed (red) and white-eyed (white) Drosophila.

Figure 2-15

Explanation of the different results from reciprocal crosses between red-eyed (red) and white-eyed (white) Drosophila. (In Drosophila and many other experimental systems, a superscript plus sign is used to designate the normal, or wild-type allele. (more...)

In Drosophila, eye color has nothing to do with sex determination, so we see that genes on the sex chromosomes are not necessarily related to sexual function. The same is true in humans, for whom pedigree analysis has revealed many X-linked genes, of which few could be construed as being connected to sexual function.

MESSAGE

Sex-linked inheritance regularly shows different phenotypic ratios in the two sexes of progeny, as well as different ratios in reciprocal crosses.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK22079

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