Glossary of genetic terms

Gene

The unit of heredity that is located on and transmitted by a chromosome from one generation to the next. It is responsible for determining the characteristics of the individual possessing that gene. A more general definition would be: The sequence of nucleotides in DNA that will code for a protein.

Heredity

Is the transmission of genes to decedents.

Genetics

The science or study of the transmission of genes.

Allele

An alternative form of a gene and consists of a particular sequence of nucleotides that occupies the same locus on a chromosome. In diploid organisms there are two alleles present for each gene, occupying the same loci on homologous chromosomes. These alleles may code for the same or different versions of the characteristic controlled by the gene. This can result in homozygous or heterozygous genotypes and may or may not be dominant or recessive in the way they are expressed.

Genotype

The genotype of an organism is its genetic constitution with respect to the alleles under consideration. Homologous chromosomes

The chromosomes in a diploid nucleus that occur in pairs, one of the pair being the maternal chromosomes and the other the paternal chromosomes.

Homozygous

A homozygous genotype is one in which the alleles occupying a given locus on homologous chromosomes are identical. An organism that is homozygous is pure breeding.

Heterozygous

A heterozygous genotype is one in which the alleles occupying a given locus on homologous chromosomes are different.

Dominant

The characteristic determined by a dominant allele is always expressed in the phenotype even in the presence of an alternative allele.

Recessive

Recessive alleles have no apparent effect on the phenotype unless in the homologous, double recessive state.

Locus

Particular position on a chromosome occupied by a particular gene. The corresponding locus on the homologous chromosome carries the same gene, though the occupying alleles need not be the same.

Inheritance

The acquisition of characteristics by the transfer of genetic material from one ancestor to decedent.

Monohybrid inheritance

The inheritance of one pair of contrasting characteristics. This involves the inheritance of one gene and hence two alleles.

Dihybrid inheritance

The inheritance of two pairs of contrasting characteristics. The involves the inheritance of two genes and hence 4 alleles. The genes will be at different loci.

Codominant

Codominant alleles at a given locus will be both expressed in the phenotype of a heterozygous.

Sex linkage

All the genes on a particular chromosome form a linkage group. Those genes present on the sex chromosomes are sex linked.

Multiple alleles

Multiple alleles occur when genes have more than to alleles. For example the

ABO blood group system is controlled by a single gene which has three alleles.