Download HEREDITY AND EVOLUTION-3 Q1. What factors could lead to the

HEREDITY AND EVOLUTION-3
Q1. What factors could lead to the rise of new species?
Ans. The factors are1. Geographic isolation – It leads to reproductive isolation. So, there is no gene flow between the
separated groups of population.
2. Genetic drift – Caused by drastic change in frequency of certain genes.
3. Variation and natural selection.
Q2. Why are the small numbers of surviving tigers a cause for worry from the point of view of genetics?
Ans. It is a problem because their genes will be lost forever if they all die out and become extinct.
Q3. In humans, if gene B is for brown eyes and b is for blue eyes, then what will be the colour of the eyes
of a person with –
1. Bb
2. Bb
3. BB
Ans.
BB X bb
F1 ---------- Bb
B
b
F2 -------BB, Bb,
Brown
B
BB
Bb
Bb,
b
Bb
bb
bb
blue
( brown: blue = 3:1)
Therefore, Bb = brown
Bb = blue
BB = brown
Q4. How is the equal genetic contribution of male and female parents ensured in the progeny?
Ans. The male and female gametes carry genes for each characteristic from the gene pair of parents.
When they fuse, a new cell with the full set of genes ( or a full set of chromosomes) develops having
characteristics from both the parents.
Q5. What are fossils? What do they tell us about the process of evolution?
Ans.
1. Fossils are impressions /traces/remains of organisms that lived in the past.
2. They show phylogeny i.e. evolutionary history of organisms. They provide evidence that the
present plants and animals have originated from previously existing ones through the process
of continuous evolution.
Q6. Explain how sexual reproduction gives rise to more viable variations than asexual reproduction. How
does this affect the evolution of those organisms that reproduce sexually?
Ans.
1. In asexual reproduction, variation arises due to small errors or exchanges during DNA copying.
2. In sexual reproduction, two parents contribute equally to the DNA of the child.
3. During formation of gametes, there is mixing up of parental characters. Then, one chromosome
of each pair is taken up by the gamete.
4. When 2 such gametes fuse, the zygote gets a number of new characters.
Q7. Explain the mechanism of sex determination in the zygote.
1. Draw the flow diagram.
2. In human beings, sex is determined by chromosomes.
3. Out of the 23 pairs of chromosomes, 22 pairs are autosomes and the 23rd pair is the sex
chromosome, in both male and female.
4. The 23rd pair in male is composed of XY. Thus, 50% of the sperms produced have X
chromosome and 50% have Y chromosome.
5. The 23rd pair in female is composed of XX. Thus, all the ova (eggs) produced, have only X
chromosome.
6. If the ovum is fertilized by a sperm carrying X-chromosome, the zygote develops into a
female individual.
7. If the ovum is fertilized by a sperm carrying Y-chromosome, the zygote develops into a male
individual.
8. Thus, the sex of an individual is determined at the time of fertilization by the type of sperm
fertilizing the ovum.