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Alternate versions of a gene.


A technique for determining genetic abnormalities in a fetus by the presence of certain chemicals or defective fetal cells in the amniotic fluid, obtained by aspiration from a needle inserted into the uterus.


In human genetics, an individual who is heterozygous at a given genetic locus, with one normal allele and one potentially harmful recessive allele. The heterozygote is phenotypically normal for the character determined by the gene but can pass on the harmful allele to offspring.


A heritable feature.

chorionic villus sampling (CVS)

A technique for diagnosing genetic and congenital defects in a fetus by removing and analyzing a small sample of the fetal portion of the placenta.


A phenotypic situation in which the two alleles affect the phenotype in separate, distinguishable ways.

complete dominance

A type of inheritance in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable.

cystic fibrosis

A genetic disorder that occurs in people with two copies of a certain recessive allele; characterized by an excessive secretion of mucus and consequent vulnerability to infection; fatal if untreated.


An organism that is heterozygous with respect to two genes of interest. A dihybrid results from a cross between parents doubly homozygous for different alleles. For example, parents of genotype AABB and aabb produce a dihybrid of genotype AaBb.

dominant allele

In a heterozygote, the allele that is fully expressed in the phenotype.


A phenomenon in which one gene alters the expression of another gene that is independently inherited.

F1 generation

The first filial, or hybrid, offspring in a genetic cross-fertilization.

F2 generation

Offspring resulting from interbreeding of the hybrid F1 generation.


The genetic makeup of an organism.


Having two different alleles for a given genetic character.


Having two identical alleles for a given trait.

Huntington's disease

A human genetic disease caused by a dominant allele; characterized by uncontrollable body movements and degeneration of the nervous system; usually fatal 10-20 years after the onset of symptoms.


The mating, or crossing, of two varieties.

incomplete dominance

A type of inheritance in which F1 hybrids have an appearance that is intermediate between the phenotypes of the parental varieties.

law of independent assortment

Mendel's second law, stating that each allele pair segregates independently during gamete formation; applies when genes for two characteristics are located on different pairs of homologous chromosomes.

law of segregation

Mendel's first law, stating that allele pairs separate during gamete formation, and then randomly re-form as pairs during the fusion of gametes at fertilization.


An organism that is heterozygous with respect to a single gene of interest. A monohybrid results from a cross between parents homozygous for different alleles. For example, parents of genotypes AA and aa produce a monohybrid genotype of Aa.


A type of phenotypic character influenced by genetic and environmental factors.

norm of reaction

The range of phenotypic possibilities for a single genotype, as influenced by the environment.

P generation

The parent individuals from which offspring are derived in studies of inheritance; P stands for parental.


A family tree describing the occurrence of heritable characters in parents and offspring across as many generations as possible.


The physical and physiological traits of an organism.


The ability of a single gene to have multiple effects.

polygenic inheritance

An additive effect of two or more gene loci on a single phenotypic character.

Punnett square

A diagram used in the study of inheritance to show the results of random fertilization.

quantitative character

A heritable feature in a population that varies continuously as a result of environmental influences and the additive effect of two or more genes (polygenic inheritance).

recessive allele

In a heterozygote, the allele that is completely masked in the phenotype.

sickle-cell disease

A human genetic disease of red blood cells caused by the substitution of a single amino acid in the hemoglobin protein; it is the most common inherited disease among African Americans.

Tay-Sachs disease

A human genetic disease caused by a dysfunctional enzyme that fails to break down brain lipids of a certain class; seizures, blindness, and degeneration of motor and mental performance usually become manifest a few months after birth.


A characteristic.


Plants that produce offspring of the same variety when they self-pollinate.




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