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At the outset, we should note that the sex of an organism usually depends on a very complicated series of developmental changes under genetic and hormonal control. However, often one or a few genes can determine which pathway of development an organism takes. Those switch genes are located on the sex chromosomes, a heteromorphic pair of chromosomes, when those chromosomes exist. However, sex chromosomes are not the only determinants of an organism s sex. The ploidy of an individual, as in many hymenoptera ( bees, ants, wasps), can determine sex; males are haploid and females are diploid. Allelic mechanisms may determine sex by a single allele or multiple alleles not associated with heteromorphic chromosomes; even environmental factors may control sex. For example, temperature determines the sex of some geckos, and the sex of some marine worms and gastropods depends on the substrate on which they land. In this chapter, however, we concentrate on chromosomal sex-determining mechanisms.
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Sex Chromosomes
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Basically, four types of chromosomal sex-determining mechanisms exist: the XY, ZW, X0, and compound chromosomal mechanisms. In the XY case, as in human beings or fruit ies, the females have a homomorphic pair of chromosomes (XX) and males are heteromorphic (XY). In the ZW case, males are homomorphic (ZZ), and females are heteromorphic (ZW). (XY and ZW are chromosome notations and imply nothing about the sizes or shapes of these chromosomes.) In the X0 case, the organism has only one
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Tamarin: Principles of Genetics, Seventh Edition
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II. Mendelism and the Chromosomal Theory
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5. Sex Determination, Sex Linkage, and Pedigree Analysis
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Sex Determination, Sex Linkage, and Pedigree Analysis
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Human male karyotype. Note the X and Y chromosomes. A female would have a second X chromosome in place of the Y.
(Reproduced courtesy of Dr. Thomas G. Brewster, Foundation for Blood Research, Scarborough, Maine.)
Genic Balance in Drosophila
When geneticist Calvin Bridges, working with Drosophila, crossed a triploid (3n) female with a normal male, he observed many combinations of autosomes and sex chromosomes in the offspring. From his results, Bridges suggested in 1921 that sex in Drosophila is determined by the balance between (ratio of ) autosomal alleles that favor maleness and alleles on the X chromosomes that favor femaleness. He calculated a ratio of X chromosomes to autosomal sets to see if this ratio would predict the sex of a y. An autosomal set (A) in Drosophila consists of one chromosome from each autosomal pair, or three chromosomes. (An autosomal set in human beings consists of twenty-two chromosomes.) Table 5.1, which presents his results, shows that Bridges s genic balance
Calvin B. Bridges (1889 1938).
(From Genetics 25 (1940): frontispiece. Courtesy of the Genetics Society of America.)
Sperm One autosomal set plus Ovum One autosomal set plus X X Y
Two autosomal Two autosomal sets plus sets plus XY XX Son Daughter X X X Y
Segregation of human sex chromosomes during meiosis, with subsequent zygote formation.
Chromosomes of Drosophila melanogaster.
Tamarin: Principles of Genetics, Seventh Edition
II. Mendelism and the Chromosomal Theory
5. Sex Determination, Sex Linkage, and Pedigree Analysis
The McGraw Hill Companies, 2001
Sex Determination
Table 5.1 Data Supporting Bridges s Theory of Sex Determination by Genic Balance in Drosophila
Number of X Chromosomes 3 4 4 3 2 1 2 1 1 Number of Autosomal Sets (A) 2 3 4 3 2 1 3 2 3 Total Number of Chromosomes 9 13 16 12 8 4 11 7 10 X Ratio A 1.50 1.33 1.00 1.00 1.00 1.00 0.67 0.50 0.33 Sex Metafemale Female Female Female Female Female Intersex Male Metamale
theory of sex determination was essentially correct. When the X:A ratio is 1.00, as in a normal female, or greater than 1.00, the organism is a female. When this ratio is 0.50, as in a normal male, or less than 0.50, the organism is a male. At 0.67, the organism is an intersex. Metamales ( X/A = 0.33) and metafemales ( X/A = 1.50) are usually very weak and sterile.The metafemales usually do not even emerge from their pupal cases. A sex-switch gene has been discovered that directs female development. This gene, Sex-lethal (Sxl ), is located on the X chromosome. ( It was originally called femalelethal because mutations of this gene killed female embryos.) Apparently, Sxl has two states of activity. When it is on, it directs female development; when it is off, maleness ensues. Other genes located on the X chromosome and the autosomes regulate this sex-switch gene. Genes on the X chromosome that act to regulate Sxl into the on state (female development) are called numerator elements because they act on the numerator of the X/A genic balance equation. Genes on the autosomes that act to regulate Sxl into the off state (male development) are called denominator elements. Geneticists have discovered four numerator elements genes named sisterless-a, sisterless-b, sisterless-c, and runt. Sxl counts the number of X chromosomes; it turns on when two are present. It counts by measuring the level of the numerator genes protein product. If the level is high, Sxl turns on, and the organism develops as a female. If the level is relatively low, Sxl does not turn on, and development proceeds as a male.
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