Test: Linkage and Recombination (NCERT) - NEET MCQ

Morgan observed that in his experiments with Drosophila, the F2 generation did not exhibit the expected 9:3:3:1 ratio due to linkage, with parental combinations being more frequent than recombinants. He attributed linkage to the physical association of genes on the same chromosome, not their chemical association. Thus, options (i) and (iii) are correct.

NCERT Topic: Linkage and recombination

NCERT Line: "he hybridised yellow-bodied, white-eyed females to brown-bodied, red-eyed males and intercrossed their F1 progeny. He observed that the two genes did not segregate independently of each other and the F2 ratio deviated very significantly from the 9:3:3:1 ratio (expected when the two genes are independent)." "the proportion of parental gene combinations were much higher than the non-parental type. Morgan attributed this due to the physical association or linkage of the two genes and coined the term linkage to describe this physical association of genes on a chromosome and the term recombination to describe the generation of non-parental gene combinations." "some genes were very tightly linked (showed very low recombination) while others were loosely linked (showed higher recombination)."

Morgan coined the term "recombination" to describe the generation of new, non-parental gene combinations due to crossing over during meiosis.

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NCERT Topic: Linkage and recombination

NCERT Line: "Morgan attributed this due to the physical association or linkage of the two genes and coined the term linkage to describe this physical association of genes on a chromosome and the term recombination to describe the generation of non-parental gene combinations."

Genetic maps are constructed using the frequency of recombination between gene pairs, which helps in determining the relative positions of genes on a chromosome. Options A and B are incorrect, as complete linkage results in only parental types, and T.H. Morgan did not construct the first chromosome map but contributed to the development of genetic mapping concepts.

NCERT Topic: Linkage and recombination

NCERT Line: "His student Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and 'mapped' their position on the chromosome."

The strength of the linkage is determined by the distance between the two genes. Greater the distance lower will be the linkage strength. The linkage is broken down due to the phenomenon of crossing over which occurs during meiosis.Crossing over will be relatively more frequent if the distance between the two genes is more than in a case where the distance between two genes is lesser.Depending on the distance between any two genes which is inversely proportional to the strength of linkage. The crossovers will similarly vary from 0-50% and will never exceed 50%.
So, the correct answer is '0-50%'.

Linked genes are those located near each other on the same chromosome. This proximity means that they do not assort independently and tend to segregate together during meiosis, which violates the law of independent assortment. Thus, Option A accurately describes linked genes.

NCERT Topic: Linked genes

NCERT Line: "Later, it was found that Mendel's law of independent assortment does not hold true for the genes that were located on the same chromosomes. These genes were called as ‘linked genes'. Closely located genes assorted together, and distantly located genes, due to recombination, assorted independently."

Genetic maps are extensively used as a starting point in genome sequencing projects to understand gene locations and relationships.

NCERT Topic: Genetic maps, starting

NCERT Line: "Today genetic maps are extensively used as a starting point in the sequencing of whole genomes as was done in the case of the Human Genome Sequencing Project, described later."

Morgan observed that white eye and yellow body traits in Drosophila were very tightly linked, with a low recombination frequency of only 1.3 percent, indicating very close linkage.

NCERT Topic: Linkage and recombination

NCERT Line: "he found that the genes white and yellow were very tightly linked and showed only 1.3 per cent recombination"

Genetic maps are indeed a measure of gene distance on the same chromosome, and they are used in genome sequencing projects. The incorrect statement is Option A, as Alfred Sturtevant used a dihybrid test cross to create genetic maps, not a monohybrid test cross.

NCERT Topic: Linkage and recombination

NCERT Line: "His student Alfred Sturtevant used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and 'mapped' their position on the chromosome."

In a dihybrid cross, tightly linked genes on the same chromosome show very few recombinations because they are so close together that they tend to be inherited as a unit. This is consistent with Option D, which accurately describes the nature of tightly linked genes. Options A, B, and C do not correctly describe the behavior of linked genes.

NCERT Topic: Linkage and recombination

NCERT Line: "Tightly linked genes on the same chromosome showed very low recombination."