Inheritance and variation
Read the following passage carefully and answer the given questions.
Mendel’s approach of using mathematics to explain biological phenomena was totally new and unacceptable to many of the biologists of his time. Finally, though Mendel’s work suggested that factors (genes) were discrete units, he could not provide any physical proof for the existence of factors or say what they were made of. In the dihybrid cross, the phenotypes round, yellow; wrinkled, yellow; round, green and wrinkled, green appeared in the ratio 9:3:3:1. Such a ratio was observed for several pairs of characters that Mendel studied. The important things to remember are that chromosomes as well as genes occur in pairs. Morgan and his group also found that even when genes were grouped on the same chromosome, some genes were very tightly linked while others were loosely linked. In a number of other insects and mammals including man, XY type of sex determination is seen where both male and female have same number of chromosomes. Grasshopper is an example of XO type of sex determination in which the males have only one X-chromosome besides the autosomes, whereas females have a pair of X-chromosomes.
Morgan called the physical association of the two genes on a chromosome as:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Linkage
Morgan made dihybrid crosses in Drosophila with genes on the X chromosome. The two genes did not sort out independently, and the parental combinations appeared far more often than the new ones. Morgan said this was due to the physical association of the two genes on one chromosome and called it linkage.
(1) Recombination — Incorrect. Morgan used this term for the making of non-parental gene combinations. It is the opposite effect of linkage.
(2) Crossing over — Incorrect. Crossing over is the exchange of parts between paired chromosomes in meiosis. It is the event that separates linked genes and gives recombinants.
(3) Linkage — Correct. Linkage is the physical association of genes on a chromosome. Tightly linked genes show very little recombination; loosely linked genes show more.
(4) Mutation — Incorrect. A mutation is a change in the DNA sequence. It has nothing to do with two genes staying together.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
Linkage and Recombination – …
Figure 4.11 Linkage: Results of two dihybrid crosses conducted by Morgan. Cross A shows crossing between gene y and w; Cross B shows crossing between genes w and m. Here dominant wild type alleles are represented with (+) sign in superscript