NCERT Class X Science Class: Chapter –9. Heredity and Evolution – Part-6

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Short Answer Questions

Question 26:

How is the sex of a newborn determined in humans?


The sex of the individual is genetically determined i.e., genes inherited from parents decide whether the new born will be a boy or a girl. A new born who inherits an ‘X’ chromosome from father will be a girl and one who inherits a ‘Y’ chromosome will be a boy.

Image sex of a newborn determined in humans

Image Sex of a Newborn Determined in Humans

Question 27:

Do genetic combination of mothers play a significant role in? Determining the sex of a new born?


No, because mothers have a pair of X-chromosomes. All children will inherit an ‘X’ chromosome from their mother regardless of whether they are boys or girls.

Image mothers have a pair of X-chromosomes

Image Mothers Have a Pair of X-Chromosomes

Question 28:

Mention three important features of fossils which help in the study of evolution.


(a) Fossils represent modes of preservation of ancient species.

(b) Fossils help in establishing evolutionary traits among organisms and their ancestors.

(c) Fossils help in establishing the time period in which organisms lived.

Question 29:

Why do all the gametes formed in human females have an X chromosome?


Human females have two X chromosomes called sex chromosomes. During meiosis at the time of gamete formation, one X chromosome enters each gamete. Hence all the gametes possess an X chromosome.

Question 30:

In human beings, the statistical probability of getting either a male or female child is Give a suitable explanation.


The sex of an infant is determined by the type of sex chromosome contributed by the male gamete. Since the ratio of male gametes containing X chromosome and those containing Y chromosome is, the statistical probability of male or a female infant is also.

Image chromosomes

Image Chromosomes

Question 31:

A very small population of a species faces a greater threat of extinction than a larger population. Provide a suitable genetic explanation.


Fewer individuals in a species impose extensive inbreeding among them. This limits the appearance of variations and puts the species at a disadvantage if there are changes in the environment. Since the individuals fail to cope up with the environmental changes, they may become extinct.

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