In diploid (2n) organisms, the genome is composed of homologous chromosomes. One chromosome of each homologous pair comes from the mother (called a maternal chromosome) and one comes from the father (paternal chromsosome).

Homologous chromosomes are similiar but not identical. Each carries the same genes in the same order, but the alleles for each trait may not be the same. In garden peas, for example, the gene for pod color on the maternal chromosome might be the yellow allele; the gene on the homologous paternal chromosome might be the green allele.

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