Question

Read the following statements:
(A). Alleles are alternative forms of a gene.
(B). Genes located in the same chromosomes never show independent segregation.
(C). Calvin Bridges proposed the genic balance theory of sex determination in Drosophila.
(D). In chickens, the character "creeper" is recessive for its effect on phenotype but dominant for lethality.
Choose the correct answer from the options given below:

• A. (A), (B) and (D) only.
• B. (A) only.
• C. (A) and (C) only.
• D. (B), (C) and (D) only.

Hint:

For lethal alleles like the creeper gene in chickens or yellow coat in mice, the allele is dominant for its phenotypic effect but recessive for its lethal effect (homozygous lethality).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
This question evaluates core concepts of genetics, linkage, sex determination, and lethal alleles.

Step 3: Detailed Explanation:
Let us analyze the validity of each statement:

• Statement (A): Alleles are alternative versions of a gene that sit at the same locus on homologous chromosomes. This is True.

• Statement (B): Linked genes on the same chromosome can show independent segregation (recombination frequency of \(50\%\)) if they are located very far apart on the chromosome. Thus, the word "never" makes this statement False.

• Statement (C): Calvin Bridges (1921) proposed the Genic Balance Theory of sex determination in Drosophila, which states that sex is determined by the ratio of X-chromosomes to the autosomes (X:A ratio). This is True.

• Statement (D): In chickens, the "creeper" phenotype (extremely short legs) is dominant. The allele (\(Cp\)) is dominant for its phenotype (heterozygotes \(Cp/cp\) are creepers) but recessive for its lethal effect (homozygotes \(Cp/Cp\) die). The statement claims it is recessive for phenotype and dominant for lethality, which is inverted. This is False.
Thus, only (A) and (C) are correct.

Step 4: Final Answer
The correct choice is (A) and (C) only.