Genes are carried on chromosomes in the cell nucleus. Animals and most other multicellular organisms possess two sets of chromosomes in each cell, one set inherited from the mother, and one from the father. Such an organism is said to be
diploid. In humans, the maternal and paternal sets each include 23 chromosomes, so humans have 46 chromosomes in each cell. Analysis shows that the maternal and paternal chromosome sets are virtually identical, and they can be matched up to form 23 pairs. One pair, however, may not be a pair at all. These are the sex chromosomes, so called because they determine the sex of the organism. In humans, the female carries two identical sex chromosomes, called X chromosomes, while the male carries two dissimilar chromosomes, one X and one Y. The other 22 pairs of chromosomes are called
autosomes.
Alleles
Members of each chromosome pair (except for X and Y) carry the same set of genes, so that a diploid cell carries two copies of (almost) every gene, oneon the maternally derived chromosome, and one on the paternally derived chromosome. These two copies may be precisely identical, meaning the two genes have precisely the same sequence of nucleotides, or their sequences may be slightly different.
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