Free Gene Linkage and Crossing Over MCQs with Answers

1. Mendel's law of independent assortment holds only when the two genes concerned are

• A. close together on the same chromosome
• B. on different chromosomes or far apart on the same one
• C. both recessive
• D. both on the X chromosome

Explanation: Independent assortment comes from the random orientation of different homologous pairs at metaphase I, so it requires the genes to be on different pairs. Genes close together on the same chromosome are linked and travel together, which is precisely why Morgan's results departed from Mendel's ratios. Genes far apart on one chromosome are separated by crossing over often enough to behave almost independently.

2. Genes situated on the same chromosome that tend to be inherited together are said to be

• A. allelic
• B. linked
• C. homologous
• D. codominant

Explanation: All the genes on one chromosome form a linkage group, and the number of linkage groups in a species equals its haploid chromosome number. Alleles are the alternative forms of one gene occupying the same locus on homologous chromosomes, which is a different relationship altogether. Linkage reduces the proportion of recombinant offspring below the Mendelian expectation.

3. Crossing over between homologous chromosomes takes place during

• A. metaphase I
• B. anaphase I
• C. prophase I
• D. telophase II

Explanation: During the pachytene stage of prophase I the homologues are paired as bivalents, and non sister chromatids exchange equal segments at points called chiasmata. Pairing must happen before the chromosomes line up on the spindle, so no later stage can allow the exchange. The chiasmata remain visible into metaphase I and hold the bivalent together.

4. Two linked genes show a recombination frequency of 10 per cent. The distance between them on the chromosome map is

• A. 10 map units
• B. 1 map unit
• C. 90 map units
• D. 100 map units

Explanation: One map unit, or centimorgan, is defined as a recombination frequency of one per cent, so ten per cent recombinants means ten map units apart. The logic is that the further apart two genes lie, the more likely a chiasma forms between them. Frequencies above fifty per cent are never observed, because at that point the genes assort as though unlinked.

5. The biological significance of crossing over is that it

• A. keeps linkage groups permanently intact
• B. halves the chromosome number of the gametes
• C. ensures that all gametes are genetically identical
• D. produces new combinations of alleles on the same chromosome

Explanation: Exchange between non sister chromatids creates chromosomes carrying combinations of alleles that neither parent had, and this recombination is a major source of the variation natural selection works on. Halving the chromosome number is the job of the reduction division itself, not of crossing over. Crossing over does the opposite of keeping linkage groups intact, since it breaks linkage up.

6. A test cross is used to determine

• A. The genotype of a dominant phenotype individual
• B. The number of chromosomes
• C. Mutation rates
• D. Linkage groups

Explanation: A test cross between an individual showing dominant phenotype and a homozygous recessive reveals whether the dominant is homozygous or heterozygous.

7. ABO blood group inheritance demonstrates

• A. Complete dominance only
• B. Incomplete dominance
• C. Codominance and multiple alleles
• D. Sex linkage

Explanation: ABO system shows codominance (IA and IB both expressed in AB) and multiple alleles (IA, IB, i).

8. Linkage between genes

• A. Increases with distance on chromosome
• B. Decreases with distance
• C. Is unaffected by distance
• D. Only occurs on sex chromosomes

Explanation: Linkage decreases with distance; genes farther apart cross over more frequently. 1% recombination = 1 map unit.

9. The 9:3:3:1 ratio of law of independent assortment can be affected by

• A. Genetic drift
• B. Small population
• C. Gene linkage
• D. Gene duplication

Explanation: Independent assortment requires the two genes to be on different chromosomes, so genes linked on the same chromosome travel together and the dihybrid ratio departs from 9:3:3:1, with parental combinations over represented. Genetic drift and small population size shift allele frequencies across generations rather than distorting the ratio within a single cross. Crossing over partly restores the expected ratio by separating linked genes.

10. Best description of the process of crossing over during meiosis:

• A. Duplication of chromosome
• B. Exchange of genes between non-sister chromatids
• C. Movement of chromosome to opposite poles
• D. Separation of sister chromatids

Explanation: During prophase I the homologues pair as bivalents and non sister chromatids exchange equal segments at chiasmata, producing chromosomes carrying combinations neither parent had. The exchange must be between non sister chromatids, since sister chromatids are identical and swapping between them would change nothing. Separation of sister chromatids happens later, in anaphase II.