An organism's characteristics are controlled by the genes it inherits from its parents.
A gene is a location on a chromosome where there is a DNA base sequence that codes for a protein that controls a particular characteristic.
An individual usually receives two copies of the gene for each characteristic. One copy is carried on a chromosome passed on by the male gamete and one on the matching chromosome passed on by the female gamete.
Different versions of a gene contain different sequences of DNA bases, that code for different proteins and produce different phenotypes. The different versions of a particular gene are called alleles.
There are two alleles of the gene that controls seed shape in a pea plant:
The genotype of an organism is a statement of the alleles it has for a characteristic.
If the two alleles of a particular gene are identical, the individual is described as being homozygous for that characteristic.
If the two alleles of a particular gene are different, the individual is described as being heterozygous for that characteristic.
A monohybrid cross is a genetics cross that shows the inheritance of one characteristic, such as pea seed shape.
The pea seed shape phenotypes of three plants with different genotypes are shown in the table below.
|1 Homozygous for allele r||rr||Wrinkled|
|2 Homozygous for allele R||RR||Round|
Plants 2 and 3 both have the same phenotype even though they have different genotypes. This is because the effect of allele R masks the effect of allele r.
The round pea seed shape is the dominant characteristic and the wrinkled pea seed shape is the recessive characteristic.