Question

What statement best describes the concept of alleles?  

Alleles are variations of a particular gene that will have slightly different phenotype expression  

Alleles refer to all of the genes within an organism  

Alleles refers to the precise location of a gene on a chromosome  

Alleles is the term that refers to the condensed form of DNA

Answer 1

Ans. Alleles are two or more alternate copy of a gene which share the same genetic locus.

Consider a hypothetical example-

            Gene A is present in a genetic locus 7p36.2 (chromosome 7, p-arm i.e. short arm, region 3, band 6, sub band 2) in father. Don’t worry about 7p36.2, it’s just a notation to denote the unique location of a gene in the whole genome.

            Mother has the gene A’ on the same genetic location.

Since, both the genes A (in father) and A’ (in mother) are at the same genetic locus in the genome, they are alternate copy of each other- something like two elements with same atomic number (compared with genetic locus) but different mass (compared with difference in properties of A and A’) are isotopes of the same element. Due to difference in their nucleotide sequence (say, normal hemoglobin gene and sickle-cell Hb gene differ in sequence by 1 nucleotide- share the same genetic location- so, Hb^A and Hb^S are allele of each other)- the proteins (product of gene expression) from each allele may vary from each other.   

That is,

            Gene A and A’ are alternate copy of each other.

            And, the alternate copies of a gene are called allele of each other.

            Like the two sons of same parents are called brother to each other.

Therefore, correct option is- A. Alleles are variations of a ------ expression.

Option B. Incorrect. See definition

Option C. Incorrect. The precise location of a gene on a chromosome is called its locus.

Option D. Incorrect. The condensed from of DNA is called chromosome.