| Term | Definition |
|---|
| allele | One of two or more molecular forms of a gene that arise by mutation and code for different versions of the same trait. |
| asexual reproduction | Any reproductive mode by which offspring arise from a single parent and inherit the genes of that parent only. |
| chromosome number | All chromosomes in a given type of cell. See haploidy; diploidy. |
| crossing over | At prophase I of meiosis, an interaction in which nonsister chromatids of a pair of homologous chromosomes break at corresponding sites and exchange segments; genetic recombination is the result. |
| diploid number | Total chromosome number in cells that have a pair of each chromosome type. |
| egg | Mature female gamete; an ovum. |
| fertilization | The fusion of a sperm nucleus with the nucleus of an egg, thus forming a zygote. |
| gamete | Haploid cell, formed by meiotic cell division of a germ cell; required for sexual reproduction. Eggs and sperm are examples. |
| gene | Unit of information for a heritable trait, passed from parents to offspring. |
| germ cell | Animal cell of a lineage set aside for sexual reproduction; gives rise to gametes. |
| haploid number | Total chromosome number in cells that have one of each chromosome type. |
| homologous chromosome | Of cells with a diploid chromosome number, one of a pair of chromosomes identical in size, shape, and gene sequence, and that interact at meiosis. Nonidentical sex chromosomes (e.g., X and Y) also interact as homologues during meiosis. |
| meiosis | Two-stage nuclear division process that halves the chromosome number of a parental germ cell nucleus, to the haploid number. Basis of gamete formation (and meiospore formation). |
| oocyte | Type of immature egg. |
| sexual reproduction | Production of offspring by meiosis, gamete formation, and fertilization. |
| sister chromatid | Of a duplicated chromosome, one of two DNA molecules (and associated proteins) attached at the centromere until they are separated from each other at mitosis or meiosis; each is then a separate chromosome. |
| sperm | Mature male gamete. |
| spore | A reproductive or resting structure of one or a few cells, often walled or coated; used for resisting harsh conditions, dispersal, or both. May be nonsexual or sexual, as formed by way of meiosis. Some bacteria as well as sporozoans, fungi, and plants form spores. |
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