Question

Bar eye in Drosophila is due to
(a)Double recessive allele
(b)Duplication of the dominant gene
(c)Deletion of a dominant gene
(d)Pseudodominance

Answer 1

Hint: It only shows its effect if two copies of the allele (also known as homozygous) are available to the organism. For instance, the blue-eye allele is recessive, so you need to have two copies of the 'blue-eye' allele to have blue eyes.

Complete answer:
Recessive alleles show their phenotype only if two identical copies of the recessive allele are borne by an individual, meaning that the recessive allele is homozygous. This implies that the genotype for the dominant allele of an organism with a dominant phenotype can be either homozygous or heterozygous.
The bar eye is one phenotype that was studied in Drosophila with regard to duplications. The fly's eye is usually an elongated oval shape, while the phenotype of the bar eye is much thinner. A duplication in region 16A of the chromosome is observed when the chromosomes of males with bar eyes are examined.

Additional Information:
This is known as duplicate dominant epistasis or duplicate gene action when there is a dominant allele masking the expression of recessive alleles at two loci. As both genes are needed in order for the correct phenotype to be present, it is also known as complementary gene action.
The unique dominant phenotypic effect of such deletions could be triggered by within a gene becoming one of the chromosome breaks, which will act as a dominant mutation when disrupted. Second, chromosomes will never return to a natural state with deletions.
Pseudodominance-The unexpected emergence, due to the deletion of a masking dominant gene, of a recessive phenotype in a pedigree. The phenomenon in which a recessive allele occurs when only one copy of the allele is present in the phenotype, as in hemizygous alleles or heterozygotes in deletion.
So, the correct answer is ‘Double recessive allele’.

Note: What makes a trait recessive has to do with the unique variation in DNA that contributes to that trait. So one way a trait can go from recessive to dominant is with a new, dominant DNA difference that causes the same characteristic.