A change in the allele results in a different phenotypic expression of the gene product. The similarity or dissimilarity of these alleles determines the zygosity of that particular gene, i.e., whether the gene is heterozygous or homozygous.
According to the inheritance pattern of genes, homozygosity also displays a dominant and a recessive allele. In a homozygous-dominant case, the organism possesses two copies (both alleles) of the dominant allele. If the dominant allele for a gene is represented by “Q”, and the recessive allele is represented by “q”, a homozygous-dominant organism for the gene will have a genotype of “QQ”. For example, in case of eye color in humans, the allele for brown color (B) is dominant over the allele for blue color (b). Hence, an individual, who is homozygous dominant for eye color, will have brown eyes with the genotype “BB”.
Similarly, in a homozygous-recessive case, the organism possesses two copies of the recessive allele, and their genotype would be “qq” for a particular gene. For example, in case of the recessive blood disorder of hemophilia (h), the condition occurs only when the individual is homozygous recessive; hence, the affected individual will have the genotype “hh”.
In case of a given trait that follows simple dominance, the phenotype displayed is the dominant phenotype despite the heterozygous nature of the alleles (the recessive allele is overridden by the dominant allele). This state of zygosity displays more complex forms of dominance as well, including co-dominance and incomplete dominance.
In case of co-dominance, the phenotypic traits of both alleles can be observed in the individual, e.g., human blood groups, and in case of incomplete dominance, the heterozygous state displays a phenotype that is an intermediate between that exhibited by the homozygous-dominant and homozygous-recessive phenotype, e.g., the 4 o’clock plant produces a pink flower in heterozygous conditions which is an intermediate between the red flower of the homozygous-dominant state and the white flower of the homozygous-recessive state.
|Identical||Distinct and dissimilar|
|Breed of Offspring|
|Purebred and homozygous||Not purebred and have diverse genotypes|
|Inheritance of Alleles|
|Individuals – possess dominant or recessive alleles, but not both||Individuals – have both a dominant and a recessive allele|
|One type||Two types|