Chromosome Numbers

Each species has a characteristic number of chromosomes in each cell. Table 8-1 lists the number of chromosomes found in some organisms. Some species of organisms have the same number of chromosomes. For example, potatoes, plums, and chimpanzees all have 48 chromosomes in each cell.

Sex Chromosomes and Autosomes

Human and animal chromosomes are categorized as either sex chromosomes or autosomes. Sex chromosomes are chromosomes that determine the sex of an organism, and they may also carry genes for other characteristics. In humans, sex chromosomes are either X or Y. Females normally have two X chromosomes, and males normally have an X and a Y chromosome. All of the other chromosomes in an organism are called autosomes. Two of the 46 human chromosomes are sex chromosomes, and the remaining 44 chromosomes are autosomes.

Every cell of an organism produced by sexual reproduction has two copies of each autosome. The organism receives one copy of each autosome from each parent. The two copies of each auto-some are called homologous chromosomes, or homologues. Homologous chromosomes are the same size and shape and carry genes for the same traits. For example, if one chromosome in a pair of homologues contains a gene for eye color, so will the other chromosome in the homologous pair.

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