Genetics

Gene

Gene

Section of DNA on a chromosome that controls a feature of one or more specific polypeptides or a functional RNA (rRNA and tRNA).

Locus

The specific linear position of a particular gene on a certain chromosome.

Allele

Alternative forms of a particular gene with different base sequences, and therefore different codes.

Homozygous

Situation or organism in which paired alleles are the same.

Heterozygous

Situation of organism in which paired alleles are different.

Dominant Allele

An allele that expressed itself in the phenotype in heterozygous organisms.

Recessive Allele

An allele that does not express itself in the phenotype in heterozygous organisms.

Co-dominance

Alleles which both contribute to the phenotype (i.e produce a blended effect) in the heterozygous condition.

Multiple alleles

if there are more than two alleles for each gene in the gene in the gene pool. NB. Only 2 chromosomes in a homologous pair so only 2 of the three or more alleles can be present in asingle organism.

Pure Breeding Strain

Group of organisms in which a certain characteristic is unaltered for generations, indicating that organisms are homozygous for that feature.

Monohybrid

Inheritance of a single pair of alleles.

Dihybrid

Inheritance of two pairs of alleles.

Haploid

Cell or nucleus containing single, unpaired chromosomes.

Diploid

Cell or nucleus containing pairs of homologous chromosomes.

Phenotype

The observable or biochemical characteristics of an organism, resulting from both its genotype and the effects of the environment.

Genotype

The combination of alleles present within the cells of an organism.

Heritable

Features determined by genes (rather than the environment) and therefore passed onto offspring.

Homologous

A pair of chromosomes, one maternal and one paternal, that have the same gene loci and therefore determine the same features. They are not necessarily identical as they may have different alleles. They are capable of pairing during meiosis.

Ratio

A measure of the relative size of two classes that is expressed as a proportion.

Sex-linked

Any gene that is carried on either the X or Y chromosome.

Autosomal linkage

The situation where two or more genes are carried on the same autosome.

Autosome

A chromosome which is not a sex chromosome.

Epistasis

This arises when the allele of one gene affects or masks the expression of another in the phenotype.