The terms dominant and recessive describe the patterns of inheritance of certain traits. These are concerned with the traits expressed by an individual. Every sexually reproducing organism possess two copies of each gene in their DNA, one from the mother and one from the father. These copies are called alleles which can either be same or different from each other. If they are same they are called homozygous and if they are different they are called heterozygous . Dominant and recessive alleles are the terms which are widely used in Mendelian genetics. Alleles are different versions of a gene which determine different characters of an individual. Dominant alleles mask the expression of another allele, recessive alleles are those whose expression is masked.
What are Dominant and Recessive Alleles/Traits?
Dominant alleles are those alleles which are expressed even if the individual has only one copy of them (heterozygous). When a dominant allele pairs with a recessive allele, the dominant allele takes priority over the recessive one and determines the trait. The recessive alleles are expressed only if they in homozygous condition. The complete genome of an organism is called its genotype and physical expression of the genes is called phenotype.
Genes are shown by using a letter or letters which are descriptive of the trait in question. The first letter of the dominant trait is commonly used and is written capital. The recessive allele is shown by using the same but small letter. Take the example of brown eyes vs. blue eyes. Brown eye color is dominant over blue color. The dominant brown eye allele is expressed by ‘B’ and recessive blue eye allele is expressed by ‘b’. It means if one of your parents has brown eyes and one has blue eyes you will most likely have brown eyes. There can be two cases here;
- If the mother is homozygous for brown eyes i.e. ‘BB’ and the father has blue eyes i.e. ‘bb’ then their offsprings can only be heterozygous for the trait and will only have brown eyes.
- If one of them has blue eyes i.e ‘bb’ and one is heterozygous for the trait i.e. ‘Bb’, their offspring have equal chances of showing brown or blue eye color.
When you learn about dominant and recessive traits, it is important to learn about sex-linked traits. Some genes are found only on X chromosome while some are found only in Y chromosome. Males inherit X-linked genes from mother only but females inherit them from both parents. So, females have two alleles of X-linked genes while males have only one. Some diseases like hemophilia are caused by sex-linked genes. The allele for hemophilia is recessive so, for females to have the disease two copies of the recessive allele are needed, one from each parent. As males only have one X chromosome, they can get the disease if they inherit only one copy of the recessive allele from their mother. This is the reason why hemophilia is more common in men than in women.