Sperm or ovum. A cell that has half the number of chromosomes of a somatic cell (haploid) and can fuse with another gamete to form a zygote.

A mass of neuron cell bodies; ganglia integrate and coordinate impulses.

A polypeptide hormone secreted by certain cells of the pyloric glands, which strongly stimulates secretion of gastric acid and pepsin, and weakly stimulates secretion of pancreatic enzymes and gallbladder contraction.

The embryo in the stage following the blastula or blastocyst; the simplest type consists of two layers of cells, the ectoderm and endoderm, which have invaginated to form the archenteron and an opening, the blastopore. In human embryos the gastrula stage occurs during the third week, as the embryonic disc becomes trilaminar, establishing the ectoderm, mesoderm, and endoderm.

The biologic unit of heredity, self-reproducing and located at a definite position (locus) on a particular chromosome.

The movement of alleles into and out of a population\'s gene pool.

The collective genetic information contained within a population of sexually reproducing organisms.

The sequence of nucleotides that is the basis of heredity in the DNA molecule of a chromosome and that specifies the amino acid sequence in the synthesis of proteins.

Random fluctuations in the frequency of the appearance of a gene in a small isolated population, presumably owing to chance rather than natural selection. Also called drift.

The complete set of hereditary factors contained in the haploid set of chromosomes.

The aggregate of ordered genes received by offspring from both parents; e.g., a person with blood group AB is of genotype AB.

A taxonomic category subordinate to a tribe (or subtribe) and superior to a species (or subgenus).