FUNDAMENTALS OF BIOLOGY I LECTURE
GENETICS VOCABULARY
Compiled by Dr. Susan Petro

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Alleles - Genes governing variation of the same character that occupy corresponding positions (loci) on homologous chromosomes; alternative forms of genes
 

Autosome - A chromosome other then the sex chromosomes 
 

Barr body - A condensed and inactivated X-chromosome appearing as a distinctive dense spot in the nucleus of certain cells of female mammals.
 

Codominance - Condition in which two alleles of a locus are expressed in a heterozygote
 

Dihybrid cross - A genetic cross that takes into account the behavior of alleles of two loci. 
 

Dominance, principle of - Genetic principle that states that in an F₁ hybrid the allele contributed by one parent (the dominant allele) masks expression of the allele contributed by the other parent (the recessive allele). First noted by Gregor Mendel, the principle of dominance has many exceptions.

Dominant allele - An allele that is always expressed when it is present, regardless of whether it is homozygous or heterozygous.
 
 

Dosage compensation - Genetic mechanism by which the expression of X-linked genes is made equivalent in XX females and XY males.
 
 

Epistasis - Condition in which certain alleles of one locus can alter the expression of alleles of a different locus.
 
 

F₁ generation (first filial generation) - The first generation of hybrid offspring resulting from a cross between parents from two different true-breeding lines.
 
 

F₂ generation (second filial generation) -The offspring of the F₁generation.
 
 

Genes - A segment of DNA that serves as a unit of hereditary information. Most eukaryotic genes are in chromosomes.
 
 

Genotype - The genetic makeup of an individual.
 
 

Hemizygous - Possessing only one allele for a particular locus; a human male is hemizygous for all X-linked genes.
 
 

Hermaphrodite - An organism that possesses both male and female sex organs.
 
 

Heterogametic - Term describing an individual that produces two classes of gametes with respect to their sex chromosome constitutions. Human males (XY) are heterogametic, producing X and Y sperm.
 
 

Heterozygous - Possessing a pair of unlike alleles for a particular locus.
 
 

Homogametic - Term referring to an individual that produces gametes with identical sex chromosome constitutions. Human females (XX) are homogametic, producing all X eggs.
 
 

Homologous chromosomes - Chromosomes that are similar in morphology and genetic constitution. In humans there are 23 pairs of homologous chromosomes, one member of each pair is inherited from the mother and the other from the father
 
 

Homozygous - Possessing a pair of identical alleles for a particular locus.
 
 

Hybrid - Offspring of two genetically dissimilar parents.
 
 

Incomplete dominance - Condition in which neither member of a pair of contrasting alleles is completely expressed when the other is present.
 
 

Independent assortment, principle of - Genetic principle, first noted by Gregor Mendel, that states that the alleles of unlinked loci are randomly distributed to gametes.
 
 

Linkage - The tendency for a group of genes carried by the same chromosome to be inherited together in successive generations.
 
 

Locus (gene locus) - The place on the chromosome at which the gene for a given trait occurs.
 
 

Monohybrid cross - A genetic cross that takes into account the behavior of alleles of a single locus.
 
 

Multiple alleles - Three or more alleles of a single locus (in a population) e.g. the alleles governing the ABO series of blood types.
 
 

P generation (Parental generation) - Members of two different true breeding lines that are crossed to produce the F₁ generation
 
 

Phenotype - The physical or chemical expression of an organism’s genes.
 
 

Pleiotropic - Term referring to an allele that affects a number of characteristics of an individual.
 
 

Polygenic inheritance - Inheritance in which several independently assorting or loosely linked non-allelic genes modify the intensity of a trait or contribute to the phenotype in additive fashion.
 
 

Punnett square - Grid structure, first developed by Reginald Punnett, that allows direct calculation of the probabilities of occurrence of all possible offspring of a genetic cross.
 
 

Recessive allele - Allele that is not expressed in the heterozygous state.
 
 

Segregation, principle of - Genetic principle, first formulated by Gregor Mendel that states that two alleles of a locus become separated into different gametes.
 
 

Sex-influenced trait - Genetic trait that is expressed differently in males and females.
 
 

Sex-linked gene - Gene carried on a sex chromosome. In mammals almost all sex-linked genes are borne on the X chromosome (X-linked).
 
 

Test cross - Genetic cross in which either an F₁ individual, or an individual of unknown genotype, is mated to a homozygous recessive individual.
 
 

True breeding line - A genetically pure strain of organism i.e. one in which all individuals are homozygous for the traits under consideration.
 
 

Wild type - The phenotypically normal (naturally occurring) form of a gene or organism
 
 

X-linked gene - Gene carried on an X chromosome