Question

The allele which is unable to express its effect in presence of another is called
A. Codominant
B. Supplementary
C. Complementary
D. Recessive

Answer 1

Alleles are variations or forms of the same gene. They are located at a specific chromosomal site. Alleles are known to be present in pairs and control the same character or trait. Out of the pair, one allele can dominate the effects of the other or both the alleles can be expressed.

Complete answer:
Option (a) is incorrect. Codominance is the condition when both alleles are expressed. That is, none of the alleles’ expressions is masked by the other. A classic example of codominance is seen in blood grouping. Blood group A and B are codominant alleles. Thus, a child with parents having blood group A and blood group B will always have blood group AB.
Option (b) is incorrect. Genes that produce a particular phenotype by interacting with one another are known as Supplementary genes. Supplementary genes are typically non-allelic genes. When expressed alone, these genes produce their own trait. Both the genes in a supplement pair are dominant.
Option (c) is incorrect. Alleles are said to be complementary, if they can produce wild type phenotypes, that is the phenotypes that occur naturally without mutations. For example, consider two parent alleles Aa and Bb that express mutant phenotypes. If these alleles are able to produce the wild type AB phenotype when crossed, then the parent alleles are said to be complementary.
Option (d) is correct. The particular allele whose expression is masked by the presence of another allele is known as a Recessive allele. The other allele of the pair, who masks the expression is known as the Dominant allele. For example, a brown coloured mouse has a genotype BB. It is crossed with an albino mouse with the genotype bb. The offsprings will all have the genotype Bb and will be brown in colour. This is because, B is the dominant allele that codes for the brown colour which masks the expression of the recessive gene b, which codes for the white colour of the mice. hence, all the progeny mice are brown in colour.

Hence, the correct answer is option (D).

Note: Alleles are the different forms of a gene that occupy the same position on homologous chromosomes. Alleles are present in pairs and they are known to influence the same character. Alleles can be dominant or recessive. Dominant alleles obscure the expression of recessive alleles. Hair texture, eye colour, blood groups are all examples of alleles present in the population.