Connecting Concepts Across Chapters S

In this chapter, we have examined sex determination and the inheritance of traits encoded by genes located on the sex chromosomes. An important theme has been that sex is determined in a variety of different ways — not all organisms have the familiar XX-XY system seen in humans. Even among organisms with XX-XY sex determination, the sexual phenotype of an individual can be shaped by very different mechanisms.

The discussion of sex determination lays the foundation for an understanding of sex-linked inheritance, covered in the last part of the chapter. Because males and females differ in sex chromosomes, which are not homologous, they do not possess the same number of alle-les at sex-linked loci, and the patterns of inheritance for sex-linked characteristics are different from those for autosomal characteristics. This material augments the principles of inheritance presented in Chapter 3. The chromosome theory of inheritance, which states that genes are located on chromosomes, was first elucidated through the study of sex-linked traits. This theory provided the first clues about the physical basis of heredity, which we will explore in more detail in Chapters 10 and 11.

The ways in which sex and heredity interact are explored further in Chapter 5, where we consider additional exceptions to Mendel's principles, including sex-limited and sex-influenced traits, cytoplasmic inheritance, genetic maternal effect, and genomic imprinting. The inheritance of human sex-linked characteristics will be discussed in Chapter 6, and we will take a more detailed look at chromosome abnormalities, including abnormal sex chromosomes, in Chapter 9.

CONCEPTS SUMMARY]_

• Sexual reproduction is the production of offspring that are genetically distinct from the parents. Among diploid eukaryotes, sexual reproduction consists of two processes: meiosis, which produces haploid gametes, and fertilization, in which gametes unite to produce diploid zygotes.

• Most organisms have two sexual phenotypes — males and females. Males produce small gametes; females produce large gametes. The sex of an individual normally refers to the individual's sexual phenotype, not its genetic makeup.

• The mechanism by which sex is specified is termed sex determination. Sex may be determined by differences in specific chromosomes, ploidy level, genotypes, or environment.

• Sex chromosomes differ in number and appearance between males and females; other, nonsex chromosomes are termed autosomes. In organisms with chromosomal sex-determining systems, the homogametic sex produces gametes that are all identical with regard to sex chromosomes; the heterogametic sex produces two types of gametes, which differ in their sex-chromosome composition.

• In the XX-XO system, females possess two X chromosomes, and males possess a single X chromosome.

• In the XX-XY system, females possess two X chromosomes, and males possess a single X and a single Y chromosome. The X and Y chromosomes are not homologous, except at the pseudoautosomal region, which is essential to pairing in meiosis in males.

• In the ZZ-ZW system of sex determination, males possess two Z chromosomes and females possess an LZ and a LW chromosome.

• In some organisms, ploidy level determines sex; males develop from unfertilized eggs (and are haploid) and females develop from fertilized eggs (and are diploid). Other organisms have genic sex determination, in which genotypes at one or more loci determine the sex of an individual. Still others have environmental sex determination.

• In Drosophila melanogaster, sex is determined by a balance between genes on the X chromosomes and genes on the autosomes, the X:A ratio. An X:A ratio of 1.0 produces a female; an X:A ratio of 0.5 produces a male; and an X:A ratio between 1.0 and 0.5 produces an intersex.

• In humans, sex is ultimately determined by the presence or absence of the SRYgene located on the Y chromosome.

• Sex-linked characteristics are determined by genes on the sex chromosomes; X-linked characteristics are encoded by genes on the X chromosome, and Y-linked characteristics are encoded by genes on the Y chromosome.

• A female inherits X-linked alleles from both parents; a male inherits X-linked alleles from his female parent only.

(important terms sex (p. 78)

sex determination (p. 78) hermaphroditism (p. 78) monoecious (p. 78) dioecious (p. 78) sex chromosomes (p. 79) autosomes (p. 79) heterogametic sex (p. 79)

homogametic sex (p. 79) pseudoautosomal region (p. 80) genic sex determination (p. 81)

sequential hermaphroditism (p. 81)

genic balance system (p. 81)

X:A ratio (p. 81) Turner syndrome (p. 82) Klinefelter syndrome (p. 83) triplo-X syndrome (p. 83) sex-determining region Y

(SRY) gene (p. 84) sex-linked characteristic (p. 85)

X-linked characteristic (p. 85)

Y-linked characteristic (p. 85) hemizygous (p. 86) nondisjunction (p. 86) dosage compensation (p. 90) Barr body (p. 90) Lyon hypothesis (p. 90)

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