Q: Which formula is used to calculate the recombination frequency between two genes?

$$\frac{\text{Number of recombinant offspring}}{\text{Total number of offspring}} \times 100$$

Question 1

Which of the following best explains why the phenotypic ratio of 9:3:3:1 is observed in a dihybrid cross involving genes on different chromosomes?

Accepted Answer

Because of independent assortment during metaphase I of meiosis.

Question 2

In Thomas Hunt Morgan’s experiment with Drosophila, why were recombinant phenotypes less frequent than parental phenotypes when crossing flies with linked genes?

Accepted Answer

Linked genes do not assort independently, so recombinants only arise through crossing over.

Question 3

If two genes are located close together on the same chromosome, which of the following is most likely to occur?

Accepted Answer

They will be inherited together more often than not due to linkage.

Question 4

What is the maximum recombination frequency that can be observed between two genes, and what does this value indicate?

Accepted Answer

50%; the genes assort independently and are either far apart on the same chromosome or on different chromosomes.

Question 5

A geneticist performs a three-point test cross in Drosophila and observes the following recombinant frequencies: v to ct = 20%, ct to cv = 10%, v to cv = 28%. What does this suggest about the gene order?

Accepted Answer

ct is between v and cv.

Question 6

Which formula is used to calculate the recombination frequency between two genes?

Accepted Answer

$$\frac{	ext{Number of recombinant offspring}}{	ext{Total number of offspring}} 	imes 100$$

Question 7

Why do double crossovers between two genes lead to an underestimation of the actual genetic distance between them?

Accepted Answer

Double crossovers can restore the parental allele combination, making them indistinguishable from non-recombinants.

Question 8

Which molecular marker is defined as a single base pair change in the DNA sequence?

Accepted Answer

Single nucleotide polymorphism (SNP)

Question 9

A researcher is mapping a disease gene using a microsatellite marker. What is the most likely reason for using this marker?

Accepted Answer

Microsatellite markers have a high mutation rate, making them useful for tracking inheritance.

Question 10

Which of the following best describes the relationship between genetic (recombination) maps and physical maps of chromosomes?

Accepted Answer

Genetic maps estimate the relative positions of genes based on recombination frequencies, while physical maps show the actual DNA sequence.

Question 11

If the recombination frequency between two genes is 1%, what is the genetic distance between them in map units?

Accepted Answer

$$1 	ext{ cM}$$

Question 12

During meiosis, what is the primary mechanism that generates recombinant chromosomes between linked genes?

Accepted Answer

Crossing over between non-sister chromatids of homologous chromosomes

Question 13

Which of the following scenarios would most likely result in a deviation from the expected Mendelian ratios in a dihybrid cross?

Accepted Answer

The genes are linked and located close together on the same chromosome.

Question 14

A testcross is performed between a heterozygote for two linked genes and a homozygous recessive individual. Which gamete types are expected to be most frequent?

Accepted Answer

Parental gametes

Question 15

Which of the following is NOT a type of molecular marker used in genetic mapping?

Accepted Answer

Transposon insertion site

Question 16

Why are recombination frequencies generally additive for genes that are far apart on the same chromosome?

Accepted Answer

Because the probability of crossing over between two distant genes is the sum of the probabilities for each interval.

Question 17

A disease allele is found to be linked to a molecular marker in a human pedigree. What does this imply about their chromosomal location?

Accepted Answer

They are located close together on the same chromosome.

Question 18

Which of the following best describes the role of the synaptonemal complex during meiosis?

Accepted Answer

It facilitates the alignment and crossing over between homologous chromosomes.

Question 19

In a mapping experiment, why might the observed genetic distance between two genes be less than the sum of the distances between each gene and a third gene located between them?

Accepted Answer

Because double crossovers restore the parental allele combination, leading to undercounting of recombinants.

Question 20

Which of the following is a real-world application of recombination mapping in human genetics?

Accepted Answer

Identifying the location of a disease gene by analyzing its linkage to a known molecular marker.

Recombination and Linkage: Principles and Applications in Genetics

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