Morgan, in his dihybrid crosses with Drosophila observed deviations in the phenotypic ratio of F 2 progeny in comparison to that of Mendel. With the help of a suitable example, explain how his results deviated from that of Mendel.

Morgan carried out several dihybrid crosses in Drosophila melanogaster (fruit fly) to study genes that were sex-linked. The crosses were similar to the dihybrid crosses carried out by Mendel in peas. E.g. Morgan hybridised yellow bodied white-eyed female to brown bodied red-eyed male and intercrossed their F1 progeny. He observed that the two genes did not segregate independently of each other and F2 ratio deviated vary significantly from the 9:3:3: 1 ratio. Based on the cross Morgan stated that: (i) when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combination was much higher than the non-parental type. It is due to the linkage of two genes and the nonparental gene combination is described as recombination. (ii)when genes are grouped on the same chromosome, some genes were very tightly linked (low recombination) while others are loosely linked (high recombination). For eg. The genes white(w)and yellow(y) were tightly linked and showed 1.3% recombination while white(w) and miniature wing(m) showed 37.2% recombination.

E M B I B E

Xam Idea Biology>Chapter 5 - Principles of Inheritance and Variation>SELF-ASSESSMENT TEST>Q 5

MEDIUM

12th CBSE

IMPORTANT

Earn 100

A pea plant homozygous for axial flowers and constricted pods is crossed with the pea plant homozygous for terminal flowers having inflated pods. Work out the cross up to F₁ generation. Show the genotypes of the parents and phenotype and genotype of the progeny.

Important Questions on Principles of Inheritance and Variation

MEDIUM

12th CBSE

IMPORTANT

Xam Idea Biology>Chapter 5 - Principles of Inheritance and Variation>SELF-ASSESSMENT TEST>Q 6

Colour blindness in humans is a sex-linked trait. Explain with the help of a cross.

MEDIUM

12th CBSE

IMPORTANT

Xam Idea Biology>Chapter 5 - Principles of Inheritance and Variation>SELF-ASSESSMENT TEST>Q 7

How can you say that the sex of a child is determined by the father and not by the mother?

MEDIUM

12th CBSE

IMPORTANT

Xam Idea Biology>Chapter 5 - Principles of Inheritance and Variation>SELF-ASSESSMENT TEST>Q 8

Why is haemophilia generally observed in men? Under what condition can women also suffer from this disorder?

MEDIUM

12th CBSE

IMPORTANT

Xam Idea Biology>Chapter 5 - Principles of Inheritance and Variation>SELF-ASSESSMENT TEST>Q 9

How does a test cross help in identifying the genotype of the organism? Explain.

MEDIUM

12th CBSE

IMPORTANT

Xam Idea Biology>Chapter 5 - Principles of Inheritance and Variation>SELF-ASSESSMENT TEST>Q 10

When tall pea plants were selfed some of the offsprings were dwarf. Explain with the help of a Punnett square.

HARD

12th CBSE

IMPORTANT

Xam Idea Biology>Chapter 5 - Principles of Inheritance and Variation>SELF-ASSESSMENT TEST>Q 11

What is heterogamety? Explain the mechanism of sex determination in Drosophila.

MEDIUM

12th CBSE

IMPORTANT

Xam Idea Biology>Chapter 5 - Principles of Inheritance and Variation>SELF-ASSESSMENT TEST>Q 12

Why did T.H. Morgan select Drosophila melanogaster as a specimen for his experiments?

HARD

12th CBSE

IMPORTANT

Xam Idea Biology>Chapter 5 - Principles of Inheritance and Variation>SELF-ASSESSMENT TEST>Q 12

Morgan, in his dihybrid crosses with Drosophila observed deviations in the phenotypic ratio of F₂ progeny in comparison to that of Mendel. With the help of a suitable example, explain how his results deviated from that of Mendel.