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Heredity and Evolution

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Class 10 Class 12

What is DNA copying? State its importance.

The process of production of a new DNA molecule from an already existing DNA molecule through chemical reactions is known as DNA copying. It is accompanied by the creation of new cellular apparatus so that the daughter DNA and parent DNA can be separated.

Importance of DNA copying:

1. It helps in the transmission of characteristics from parents to offspring.
2. It helps in the maintenance of the chromosomal number in the new cell.


How many pairs of chromosomes are present in human beings? Out of these how many are sex chromosomes? How many types of sex chromosomes are found in human beings?

"The sex of a newborn child is a matter of chance and none of the parents may be considered responsible for it". Draw a flow chart showing the determination of sex of a newborn to justify this statement.

Twenty-three pairs of chromosomes are present in human beings. Out of these, one pair is the  sex chromosomes.

Two types of sex chromosomes are found in human beings: X and Y. Males contain one X chromosome and one Y chromosome (XY), while females contain two copies of X chromosomes (XX).
The sperm has either X or Y chromosome, while the egg has only X chromosome. So, if the sperm carrying Y chromosome fuses with the egg, it results in the formation of a male child; and if the sperm carrying X chromosome fuses with the egg, it results in the formation of a female child.

                                                          Male             Female

Sex  Chromosomes                          XY         ×        XX

                                                       ↓                     ↓

Gamete produced                           X and Y            X and X

Fusion of gamete                            XX Female or XY Male

Thus, there is an equal chance of fusion of either X or Y chromosome with the egg. Therefore, we can say that the sex of a newborn child is a matter of chance and none of the parents is responsible for it.


21.How do Mendel’s experiments show that the
(a) traits may be dominant or recessive
(b) traits are inherited independently

(a) Mendel carried out a Monohybrid cross. He crossed pure bred (homozygous) tall pea plants (TT) with pure bred , Homozyzous dwarf pea plants (tt) and found that only tall pea plants were produced in the first generation and there were no dwarf pea plants.

He concluded that the first generation showed the traits of only one of the parent plant with the trait for tallness. The trait of the other parent plant—dwarfness— did not show up in the progeny of the first generation.

He then crossed the tall pea plants obtained in the first generation (F1 generation) and found that both tall plants and dwarf plants were obtained in the second generation (F2 generation) in the ratio of 3:1. Mendel noted that the dwarf trait of the parent pea plant which disappeared in the first generation progeny reappeared in the second generation.

Thus Mendel concluded that the trait for Tallness (T) was dominant over the trait for dwarfness (t). The recessive trait was not able to express itself in the heterozygous condition like in the F1 generation. However, it was able to express itself in the homozygous condition like that in the f2 generation.

In this way, Mendel’s experiments with tall and dwarf pea plants showed that the traits may be dominant and recessive.

(b) Mendel performed an experiments in which he took a tall plant with round seeds and a short plant with wrinkled-seeds. In F1, all plants were tall and had round seeds. Tallness and round seeds were thus dominant traits. When these these F1 progeny  were crossed to generate F2 progeny it was found that some F2 progeny were tall plants with round seeds, and some were short plants with wrinkled seeds. At the same time there were new combinations besides the parental ones which were tall but had wrinkled seeds, while others were short, but had round seeds. Thus, Mendel’s experiments showed that the tall/short trait and the round seed/wrinkled seed trait are independently inherited.


Define evolution. How does it occur? Describe how fossils provide us evidences in support of evolution.

Biological evolution is any change in the frequency of alleles in a population of organisms.   Evolution is a slow change in the heritable characteristics of biological populations over successive generations. Evolutionary processes give rise to biodiversity at every level of biological organisation, including the levels of species, individual organisms, and molecules.

Evolution occurs due to the following 

i. Accumulation of variations over generations - The 
variations arise due to crossing over, recombination, fertilisation or  errors during DNA replication, mutation, recombination during sexual reproduction, etc . Geographical barrier  also plays a great role in the accumulation of these variations.

ii. Natural Selection - The organisms with the favourable mutations are selected by nature and have better chances of survival than those who do not have favourable variations.

iii. Genetic Drift refers to the fact that variations in gene frequencies within populations can occur by chance rather than by natural selection. Random genetic drift may be an important mechanism in evolutionary change in small population.

All this leads to changes in the heritable characters and finally results in evolution of the organism. 

Fossils − They represent plants and animals that lived millions of years ago and are now extinct. Different aged rock sediments contain fossils of different life-forms, which probably died during the formation of the particular sediment. 

It is observed that although fossils appeared different from the existing species they may show certain features similar to the existing species thus providing linkages between pre-existing and existing forms Provide information about the extinct species which were different from the existing species

(a) Draw a sectional view of human female reproductive system and label the part where
(i) eggs develop.
(ii) fertilisation takes place.
(iii) fertilised egg gets implanted.
(b) Describe, in brief, the changes the uterus undergoes
(i) to receive the zygote.
(ii) if zygote is not formed.

i. Eggs develop in - Ovary
ii. Fertilisation takes place in - Fallopian tube
iii. Fertilised egg gets implanted in the - Uterus


(i) On recieving the zygote the uterine walls becomes thicker and if the zygote gets implanted in the into the uterus then the female becomes pregnant. The
embryonic development of the zygote starts immediately. The embryo moves
down into the uterus forming a thick and soft lining of blood vessels around
itself. After implantation, placenta which is a special tissue, develops between the uterine wall and the embryo.

(ii) If the egg released by the ovary is not fertilized and the zygote is not
formed, then the thick lining of the uterus breaks down and comes out
through the vagina in the form of blood and mucous. This is called


What is meant by speciation? List four factors that could lead to speciation. Which of them cannot be a major factor in the speciation of a self pollinating plant species. Give reason to justify your answer. 

Speciation is the the formation of new and distinct species in the course of evolution.

Reasons for speciations are:

i. Reproductive isolation
ii. Genetic drift. 
iii. Geographical isolation
iv. Natural selection

In the case of plant species with self pollinating flowers the factor reproductive isolation will not have much effect because the plant is not dependent on any other species for reproduction but has male and female parts in the same flower or the same plant. Thus there is no effect of reproductive isolation.