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Sex Determination
It is the phenomena of determination of sex of an offspring.
Factors responsible for sex determination
(i) Environmental: In some animals, the temperature at which the fertilised eggs are kept decides the gender. e.g. in turtle.
(ii) Genetic: In some animals, like humans, gender of individual is determined by a pair of chromosomes called sex chromosome XX → Human Female and XY → Human Male.
Sex Determination In Humans
During gamete formation the male produce two types of gametes i.e. one having X chromosome and other having Y chromosome while both gametes produced by females are alike i.e. each having X chromosome.
When X chromosome of male fuses with X chromosome of female, girl child is born.
When Y chromosome of male fuses with X chromosome of female, boy child is born.
Two important things to be noticed here are –
(a) Birth of male and female child has equal %, because X and Y chromosomes are produced in equal number and it all depends on either sperm with X or Y chromosome fuses with egg containing X chromosome.
(b) Birth of male and female child depends on Y or X chromosomes of male, female always produce X chromosomes, so have no significant role in sex determination of offspring.
1.0Evolution
Evolution can be defined as a sequence of gradual development of complex form of life from simple form of life over the course of geological time.
Organic Evolution
The concept of organic evolution was given by Charles Robert Darwin. He defined evolution as “Descent with modification”.
Various theories have been given to explain evolution of different species. Some of them are:
Carolus Linnaeus: He said that no species is new i.e. each species originates from pre-existing species.
Jean Baptiste Lamarck: He explained in his book namely Philosophie Zoologique, the “Theory of inheritance of acquired characters or Lamarckism”. The favourable variations which remain for longer period of time results in evolution of new species. Change in environment produce new needs in animals.
This theory states that use and disuse of an organ leads to change in the organ which is inherited by the offsprings.
This theory was discarded by August Weismann as he experimentally proved that even after cutting tails of mice for 21 generations, tailless mice was never born.
2.0Darwinism
Charles Robert Darwin was born on 12th Feb 1809 in England. He travelled by H.M.S. Beagle ship along with Dr. Henslowe. He visited many islands of South America, South Africa, Australia and Galapagos Islands. Darwin was influenced by two books : “Principles of Population” by Malthus, “Principles of Geology” by Charles Lyell.
He wrote a book “Origin of Species”. The theory presented by him is called “Theory of Natural Selection or Darwinism”.
Charles Darwin explained the mechanism of origin of new species by natural selection. But he failed to explain the mechanism of source of heritable variations. This was explained by Hugo de Vries, a dutch botanist. According to him, heritable variations arise when there is a change in genes of the germplasm (protoplasm of germ cell). He called it mutation.
Note: Haeckel propounded “The theory of recapitulation” or 'Biogenetic law' which states that an individual organism in its development tends to repeat the stages passed through by its ancestors (phylogeny).