Question

Homologous chromosomes can be defined as
A) Chromatids of same chromosome
B) Same chromosome, same gene, different allele in different order
C) Same chromosome, different gene, same allele
D) Same chromosome, same gene, different allele in same order

Answer 1

Homologous chromosomes are pairs of chromosomes that have the same structure including centromere position and staining, and length, and have alleles for the same genes at the same loci on each chromosome.

Complete answer:
When a chromosome undergoes duplication, each chromosome in the pair is called a sister chromatid, and they are joined together at the centromere. Each chromatid in the pair separates during cell division one to each daughter cell. Sister chromatids of each homolog form a new pair of homologous chromosomes in the daughter cells. Option A is not correct.

Homologous chromosomes come one from each parent. They are the same chromosome, for example one copy of chromosome 3 from the father and one copy of chromosome 3 from the mother. They will contain alleles of the same gene, arranged in the same order. Option B is incorrect.

Homologous chromosomes have different alleles of the same gene. Option C is therefore not correct. If they have different genes, it means some genetic disorder is involved, or crossing over was not complete.

To be homologues, the chromosomes have to have the same genes, with their different allelic forms in the same order on each chromosome.The correct Answer is option D.

Hence the correct answer is option ‘D’.

Note: Homologous chromosomes are found in diploid organisms. Haploid cells like bacteria or sperm have one copy of their genome. Therefore they do not have homologues. Sometimes these chromosomes undergo nondisjunction. That is, sister chromatids do not separate correctly during cell division leading to changes in chromosome number or ploidy.