Heredity & Evolution
NCERT Exemplar Problems
Question 26: How is the sex of a newborn determined in humans?
Answer: The sex of a newborn in humans is determined by the 23rd chromosome in the sperm which ends up fertilizing the egg. If a sperm with X chromosome fertilizes the egg; it results in development of a girl child. On the other hand, if a sperm with Y chromosome fertilizes the egg; it results in development of a male child.
Question 27: Does genetic combination of mothers play a significant role in determining the sex of a new born?
Answer: Sex of a newborn is determined by the 23rd chromosome in the sperm which ends up fertilizing the egg. The genetic combination of mother has no role in determining the gender of a newborn.
Question 28: Mention three important features of fossils which help in the study of evolution.
Answer: Three important features of fossils which help in the study of evolution are as follows:
- Age of fossil: This helps in determining the period in which a particular species was found.
- Characters of organism as shown by fossil: This helps in determining the sequence of appearance of a particular trait in a particular group of animals.
- Linking characters of fossils: This helps in determining the linkage between two separate groups of animals, e.g. presence of feathers in the fossil of Archaeopteryx.
Question 29: Why do all the gametes formed in human females have an X chromosome?
Answer: The 23rd pair in a somatic cell of human females contains XX chromosomes. During gametogenesis, each gamete gets one X chromosome as the 23rd chromosome. Hence, all the female gametes in humans have X chromosome.
Question 30: In human beings, the statistical probability of getting either a male or female child is 50 : 50. Give a suitable explanation.
Answer: The somatic cell in a male human has XY chromosomes in the 23rd pair. If one gametocyte undergoes meiosis to produce two sperms then one sperm gets X chromosome and another gets Y chromosome. Thus, 50% of the sperms get X chromosome while remaining 50% get Y chromosome. Due to this, the statistical probability of getting either a male or female child is 50:50.
Question 31: A very small population of a species faces a greater threat of extinction than a larger population. Provide a suitable genetic explanation.
Answer: A small population of a species faces a greater threat of extinction than a larger population. This happens because of following reasons:
- Inbreeding: Inbreeding is the biggest problem in a small population because of limited number of individuals. Inbreeding results in negligible variations through successive generations.
- Genetic drift: Since number of individuals is small, the population is always exposed to sudden changes in the environment. This can lead to genetic drift which may eventually wipe off the entire population.
- Inbreeding depression: This results in negative growth of population. This would further reduce the population.
Question 32: What are homologous structures? Give an example. Is it necessary that homologous structures always have a common ancestor?
Answer: Structures with similar basic design but serving different functions in different organisms are called homologous structures. Forelimbs of frog and dog are examples of homologous structures. In both the organisms; the forelimb is composed of similar bones which are humerus, radio-ulna, carpals and metacarpals. The forelimbs of frog are adapted for jumping movement, while the forelimbs of dog are adapted for walking and running. It is necessary that homologous structures always have a common ancestor.
Question 33: Does the occurrence of diversity of animals on earth suggest their diverse ancestry also? Discuss this point in the light of evolution.
Answer: Diversity of animals on earth does not suggest their diverse ancestry. At present, there is a huge diversity of animals on this earth. But we can group the animals into different groups and subgroups; on the basis of certain common characters. For example; all the animals with four limbs are grouped under the superclass tetrapoda. Similarly, all the animals without notochord are grouped under invertebrates. Because of common characters, we draw a conclusion about common ancestry.
Question 34: Give the pair of contrasting traits of the following characters in pea plant and mention which is dominant and recessive (i) yellow seed (ii) round seed.
Answer: Green seed is the contrasting character for yellow seed. Wrinkled seed is the contrasting character for round seed. Either of the characters can be dominant or recessive. This means either yellow or green colour of seed can be dominant or recessive. Similarly, either smooth or rough texture of seed can be dominant or recessive.
Question 35: Why did Mendel choose pea plant for his experiments?
Answer: Mendel chose pea plant for his experiments due to following reasons:
- Pea is an annual plant. So, many generations of a pea plant can be observed in a scientist’s lifetime.
- There are many easily recognizable sets of contrasting traits in a pea plant.
- Carrying out cross pollination is quite easy in a pea plant.
Question 36: A woman has only daughters. Analyse the situation genetically and provide a suitable explanation.
Answer: The birth of a male or a female child is a matter of chance. The statistical probability of birth of a boy or girl is 50:50. This depends on the presence of X or Y chromosome in the sperm which ends up fertilizing the egg. So, it is the father who genetically contributes towards the birth of a girl child or a male child. The woman has no role to play in this.