Linkage and Recombination Practice Questions

Correct answer: Gene linkage

Linkage occurs when genes are located close together on the same chromosome and tend to be inherited together. This physical proximity prevents them from assorting independently during meiosis.

Correct answer: Thomas Morgan

Thomas Hunt Morgan provided the experimental evidence for linkage through his work with fruit flies. He observed that certain traits did not follow Mendel's 9:3:3:1 dihybrid ratio because the genes were on the same chromosome.

Correct answer: Pachytene

Crossing over, or the exchange of genetic material between non-sister chromatids of homologous chromosomes, occurs during the pachytene stage of Prophase I. This process is facilitated by the formation of the synaptonemal complex.

Correct answer: Genes are unlinked or very far apart

A recombination frequency of 50% is the maximum possible value and is indistinguishable from independent assortment. This happens when genes are on separate chromosomes or so far apart on the same chromosome that crossing over occurs in every meiosis.

Correct answer: Chiasma

Chiasmata (plural) are the X-shaped regions where non-sister chromatids remain connected after crossing over has taken place. They are visible during the diplotene stage of prophase I.

Correct answer: Parental types

Because linked genes tend to stay together, the gametes carrying the original parental combinations are produced more frequently than recombinant gametes. Consequently, parental phenotypes dominate the offspring population.

Correct answer: RF = (Recombinant offspring / Total offspring) x 100

Recombination frequency is calculated by dividing the number of recombinant individuals by the total number of offspring and multiplying by 100. This value represents the percentage of crossovers that occurred.

Correct answer: 15 cM

Genetic map distances are additive for short intervals. If gene B is between A and C, the total distance A-C is the sum of A-B (10) and B-C (5), resulting in 15 centimorgans.

Correct answer: Linkage becomes much weaker

The strength of linkage is inversely proportional to the distance between genes. As genes get further apart, the probability of a crossover event occurring between them increases, thereby weakening the linkage.

Correct answer: Male Drosophila

In male Drosophila melanogaster, crossing over does not occur at all during meiosis. This leads to complete linkage where genes on the same chromosome are always inherited in their parental combinations.

Correct answer: One map unit

One map unit (m.u.) is defined as the distance between genes for which one product of meiosis out of one hundred is recombinant. It is synonymous with one centimorgan.

Correct answer: Interference

Interference is the phenomenon where a crossover in one region of a chromosome decreases the probability of a second crossover nearby. It is quantified using the Coefficient of Coincidence.

Correct answer: Homozygous recessive

A test cross involves crossing an F1 dihybrid with a double homozygous recessive parent. The recessive parent only contributes recessive alleles, allowing the alleles from the F1 parent to be expressed and identified in the offspring.

Correct answer: Double crossovers

Double crossovers require two independent breakage and reunion events to occur simultaneously between the three genes. Since each crossover is a relatively rare event, the probability of both occurring together is very low.

Correct answer: Genetic linkage mapping

Genetic or linkage mapping uses the frequency of recombination to determine the relative order and distance between genes. This differs from physical mapping, which measures the actual number of nucleotide base pairs.

Correct answer: Dominant alleles of both genes are on the same chromosome

In the cis or coupling configuration, the dominant alleles (e.g., A and B) are located on one homolog, while the recessive alleles (a and b) are on the other. In repulsion (trans), each chromosome carries one dominant and one recessive allele.

Correct answer: Distance between genes

Recombination frequency is primarily determined by the physical distance between two genes on a chromosome — genes farther apart cross over more often, which is exactly the principle used to build genetic maps (1 map unit equals 1% recombination). Temperature and age can cause minor fluctuations in crossover rates in some organisms, but neither is a reliable predictor of recombination frequency the way physical distance is. The number of offspring an organism produces has no bearing on how often crossing over occurs.

Correct answer: 6%

The probability of a double crossover is the product of the individual recombination frequencies. Thus, 0.20 (for X-Y) multiplied by 0.30 (for Y-Z) equals 0.06, or 6%.

Correct answer: Alfred Sturtevant

Alfred Sturtevant, a student of T.H. Morgan, realized that variations in recombination frequencies could be used to map gene positions. He successfully created the first chromosome map for Drosophila.