Heredity Terms

Alleles: Alternate forms of the same gene that code for different versions of an inheited trait

Amniocentesis: A fetal testing procedure, in which a small sample of amniotic fluid surrounding the developing embryo is withdrawn after the 14th week and examined for genetic diseases.

Autosomes: All of the chromosomes that are not sex chromosomes; in humans, chromosomes number 1 to 22 (the sex chromosomes are number 23).

Autosomal dominant disease: Genetic disease that develops when only one copy (or two copies) of an abnormal gene is inherited from the parents; lethal dominant genetic diseases are uncommon because they are always expressed and usually result in death before the person can reproduce.

Autosomal recessive disease: Genetic disease that develops only when two copies of an abnormal gene are inherited from the parents; most genetic diseases are recessive because a parent with only one copy the abnormal gene is a heterozygous carrier without symptoms.

Chorionic villi sampling: A fetal testing procedure, in which a small sample of the chorionic villi from the placenta are is withdrawn after the 8th week and examined for genetic diseases.

Codominance: Expression of two different alleles with an equal degree of dominance in the phenotype of the individual, such as the expression of alleles A and B of the ABO blood group.

Crossing over: A reciprocal exchange of genetic material between nonsister chromatids of a pair of homologous chromosomes during prophase I of meiosis, which increases the genetic diversity of the gametes.

Dominant allele: An allele that masks or suppresses the phenotypic effect of the recessive allele at that gene locus; homozygous dominant and heterozygous individuals will display the dominant trait.

Gene locus: A gene's chromosomal location, which includes the chromosome number and the position that the gene occupies.

Genome: The complete set of genes in an individual.

Genotype: The type of genetic makeup of an individual

Heterozygous: Having a pair of different alleles at a gene locus.

Homologous chromosomes: A pair of chromosomes identical in size, shape, and gene sequence, where one chromosome came from the mother and the other came from the father.

Homozygous: Having a pair of identical alleles at a gene locus.

Incomplete dominance inheritance: Condition where neither allele is dominant over the other; a heterozygote has a phenotype that is intermediate between that of the homozygous dominant and the homozygous recessive, such as in sickle cell anemia.

Karyotype: The diploid chromosomal complement, prepared during metaphase, that is shown as homologous chromosome pairs which are arranged from longest to shortest, with the sex chromosomes placed last.

Multiple allele inheritance: A gene that may exist as three or more different alleles in the population, such as the ABO blood groups.

Nondisjunction: Failure of either sister chromatids or a pair of homologous chromosomes to separate during meiosis, resulting in an abnormal number of chromosomes; monosomy is a genetic disease where one chromosome is missing; trisomy is a genetic disease where an additional chromosome is present.

Phenotype: The type of physical expression of a gene that produces observable traits in an individual.

Polygenic inheritance: A trait that is governed by two or more allelic genes, and each dominant allele will contribute to the phenotype in an additive manner; the phenotypes of the offspring may be more varied than that of the parents.

Punnett-square method: Construction of a simple diagram to predict the possible gene combinations resulting from the mating of parents of known genotypes

Recessive allele: An allele that will be masked or suppressed if its homologous chromosome contains the dominant allele at that gene locus; only homozygous recessive individuals will display the recessive trait.

Sex chromosomes: The chromosome pair that determines sex; in humans, the sex chromosomes constitute chromosome number 23; and males have an XY pair, while females have an XX pair.

SRY Gene: The master gene for male sex determination.

X-linked recessive disease: Genetic disease that develops in males when only one copy of the abnormal gene is inherited, but females must inherit two copies of the abnormal gene for the disease to develop, andfemales who inherit only one copy are heterozygous carriers without symptoms.