Sex determination in human beings
Human beings have 23 pairs of chromosomes in the nucleus of the cell. Out of this two chromosomes are sex chromosomes X and Y. The female has two X chromosomes (XX) and male has one X and one Y chromosome (XY). The sperms and eggs have one set of sex chromosomes. Some sperms have X chromosome and some have Y chromosome. All eggs have X chromosome.
If a sperm having X chromosome fuses with an egg having X chromosome the child will be a girl. If a sperm having Y chromosome fuses with an egg having X chromosome the child will be a boy.
Variations may or may not help organisms to survive :-
a) Some variations help organisms to survive :-
Eg :- There are some beetles living in green bushes. They increase their numbers by reproduction. Crows can easily see the red beetles and they are eaten by the crows. During reproduction due to some variation some green beetles are produced instead of red beetles. The green beetles are not visible to crows and are not eaten by them. Then gradually the population of the red beetles decreases and the population of the green beetles increases. This variation has helped the organisms to survive.
b) Some variations do not help organisms to survive :-
Eg :- During sexual reproduction a colour variation occurs in red beetles and some blue beetles are produced instead of red beetles. Both the red and blue beetles are visible to crows and are eaten by them. Then the population of both red and blue beetles decreases. This variation has not helped the organisms to survive
c) Aquired traits cannot be passed from one generation to the next :-
Eg :- If the population of beetles increases and plants are affected by diseases, then the food available for the beetles decreases and their body weight also decreases. If after a few years the availability of food increases then the body weight of the beetles also increases. This acquired trait cannot be passed from one generation to the next
Speciation :- ( Formation of new species from existing species )
The formation of new species from existing species is mainly due to one or more of the following factors. They are :- Accumulation of variations, Physical barriers, Genetic drift, Natural selection and migration.
i) Accumulation of variations :- The differences between the individuals of the same species is called variations. The accumulation of variations over several generations produce new species.
ii) Physical barriers :- Populations may get separated by physical barriers like mountains, rivers, lakes etc. These isolated groups produce variations which can produce new species.
iii) Genetic drift :- Natural calamities or introduction of new members of the same species in an area can produce changes in the gene pool of the population and new variations are produced which can produce new species.
iv) Natural selection :- Only those individuals of a species which have useful variations and can adapt to the changes in the environment survive and the others die. These organisms can produce variations and new species.
v) Migration :- Some individuals of a species may migrate to a new geographical area and adjust to the changes in the environment there
Evolution is the sequence of gradual changes which takes place in the primi tive organisms, over millions of years, in which new species are produced.
ACQUIRED AND INHERITED TRAITS
The gradual changes taking place in living organisms giving rise to
new organisms due to changes in their genetic composition is called
8a. Inherited traits :-
Inherited traits are traits in an organism due to changes in the
genetic composition and it can be passed from one generation to the
next and it results in evolution..
b. Acquired traits :-
Acquired traits are traits which are acquired by an organism
during its lifetime and it cannot be passed from one generation to the
next and it does not result in evolution.
9. Evidences of evolution :-
There are a number of common features in different organisms
which provide evidence to show evolutionary relationship. The main
evidences of evolution are from the study of :-
Homologous organs, Analogous organs and Fossils.