Term | Definition |
meiosis | Type of nuclear division that reduces the chromosome number from 2n to n; daughter cells receive the haploid number of chromosomes in varied combinations. |
diploid (2n) | Cell condition in which two of each type of chromosome are present. |
haploid (n) | Cell condition in which only one of each type of chromosome is present. |
gamete | Haploid sex cell; e.g., egg or sperm. |
sexual reproduction | Reproduction involving meiosis, gamete formation, and fertilization; produces offspring with chromosomes inherited from each parent with a unique combination of genes. |
zygote | Diploid cell formed by the union of two gametes; the product of fertilization. |
homologous chromosome (homologue) | Member of a pair of chromosomes that are alike and come together in synapsis during prophase of the first meiotic division; a homologue. |
allele | Alternative form of a gene; alleles occur at the same locus on homologous chromosomes. |
synaptonemal complex | Protein structure that forms between the homologous chromosomes of prophase I of meiosis; promotes the process of crossing-over. |
synapsis | Pairing of homologous chromosomes during meiosis I. |
bivalent | Homologous chromosomes, each having sister chromatids that are joined by a nucleoprotein lattice during meiosis; also called a tetrad. |
crossing-over | Exchange of segments between nonsister chromatids of a bivalent during meiosis. |
genetic recombination | Process in which chromosomes are broken and rejoined to form novel combinations; in this way, offspring receive alleles in combinations different from their parents. |
independent assortment | Alleles of unlinked genes segregate independently of each other during meiosis, so that the gametes can contain all possible combinations of alleles. |
fertilization | Fusion of sperm and egg nuclei, producing a zygote that develops into a new individual. |
interkinesis | Period of time between meiosis I and meiosis II during which no DNA replication takes place. |
life cycle | Recurring pattern of genetically programmed events by which individuals grow, develop, maintain themselves, and reproduce. |
gametophyte | Haploid generation of the alternation-of-generations life cycle of a plant; produces gametes that unite to form a diploid zygote. |
sporophyte | Diploid generation of the alternation-of-generations life cycle of a plant; produces haploid spores that develop into the haploid generation. |
gametogenesis | Development of the male and female sex gametes. |
spermatogenesis | Production of sperm in males by the process of meiosis and maturation. |
oogenesis | Production of eggs in females by the process of meiosis and maturation. |
secondary oocyte | In oogenesis, the functional product of meiosis I; becomes the egg. |
polar body | Nonfunctional product of oogenesis produced by the unequal division of cytoplasm in females during meiosis; in humans three of the four cells produced by meiosis are polar bodies. |
nondisjunction | Failure of the homologous chromosomes or sister chromatids to separate during either mitosis or meiosis; produces cells with abnormal chromosome numbers. |
euploidy | Condition in which a cell contains the correct number, and combinations, of chromosomes. |
aneuploidy | Condition in which a cell does not contain the correct number, or combinations, of chromosomes. |
monosomy | Chromosome condition in which a diploid cell has one less chromosome than normal; designated as 2n-1. |
trisomy | Chromosome condition in which a diploid cell has one more chromosome than normal; designated as 2n + 1. |
karyotype | Chromosomes arranged by pairs according to their size, shape, and general appearance in mitotic metaphase. |
Barr body | Dark-staining body in the cell nuclei of female mammals that contains a condensed, inactive X chromosome; named after its discoverer, Murray Barr. |
deletion | Change in chromosome structure in which the end of a chromosome breaks off or two simultaneous breaks lead to the loss of an internal segment; often causes abnormalities—e.g., cri du chat syndrome. |
duplication | Change in chromosome structure in which a particular segment is present more than once in the same chromosome. |
translocation | Movement of a chromosomal segment from one chromosome to another nonhomologous chromosome, leading to abnormalities—e.g., Down syndrome. |