Biology - Inheritance at one gene locus - monohybrid crosses
Inheritance at one gene locus - monohybrid crosses
Somatic cells [1] have two copies of each chromosome (the homologous [2]
pairs), so each gene is represented twice in each cell. Normal homologous
chromosomes always have the same gene at the same position or locus (plural
loci). Genes may have two or more different forms termed alleles. [3] Over
many generations, mutations [4] may have caused slight differences in the
base [5] sequences of the genes in different individuals so that slightly
different forms of the gene exist. These are the alleles [6] for that gene.
A gene with two alleles could be labelled A and a. The individual is
homozygous for that characteristic if it has two alleles the same, either
genotype AA or aa. An individual with different alleles has genotype AA and
is heterozygous. [7]
* A character or trait [8] is dominant [9] if it is expressed in the
heterozygote, that is _AA_
* A character is described as recessive [10] if it is expressed only when
the genotype for that gene is homozygous, [11] for example, _AA_
If it is not known which characteristic (and therefore the allele) is
dominant, then the alleles are denoted, for example, as A1 and A2.
