Section 1

Chromosomes and Inheritance

• Genes reside on chromosomes. Sex chromosomes contain genes that determine an organism's sex. The remaining chromosomes that are not directly involved in determining the sex of an individual are called autosomes.

• In mammals, an individual carrying two X chromosomes is female. An individual carrying an X and a Y chromosome is male.

• Genes found on the X chromosome are X-linked genes. A sex-linked trait is a trait whose allele is located on a sex chromosome. Because males have only one X chromosome, a male who carries a recessive allele on the X or Y chromosome will exhibit the sex-linked condition.

Vocabulary sex chromosome (p. 236) autosome (p. 236) sex-linked trait (p. 237) linked gene (p. 238) chromosome map (p. 238)

map unit (p. 238) germ-cell mutation (p. 239) somatic-cell mutation (p. 239) lethal mutation (p. 239)

• Pairs of genes that tend to be inherited together are called linked genes. They occur close to each other on the same chromosome. The farther apart two genes are located on a chromosome, the more likely a cross-over will occur. Researchers use recombinant percentages to construct chromosome maps showing relative gene positions.

• Germ-cell mutations occur in gametes and can be passed on to offspring. Somatic-cell mutations occur in body cells and affect only the individual organism.

• Chromosome mutations are changes in the structure of a chromosome or the loss or gain of an entire chromosome. Gene mutations are changes in one or more of the nucleotides in a gene.

deletion (p. 239) inversion (p. 239) translocation (p. 239) nondisjunction (p. 239) point mutation (p. 240)

substitution (p. 240) frameshift mutation (p. 240) insertion mutation (p. 240)

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