NCERT 10th Class Science Chapter Heredity and Evolution
Heredity and variations
It is commonly seen that members of a species are largely alike. An elephant resembles other elephants, a rose plant looks alike other rose plants, and children resemble their parents, even grandparents or great grandparents. This resemblance among the individuals of a species has given rise to a general truth ‘like begets like’ which implies continuity of life. It is, however, not absolutely true as the members of a species are seldom exactly alike. For instance, in human beings, the children often have some individual characters in which they differ from one another, and also from their parents. In fact, their differences are as marked as their resemblances. This is true about other species as well.
The similarities and differences among the members of a species are not coincidental. They are received by the young ones from their parents. The hereditary information, in fact, is present in the gametes (egg and sperm) which fuse to form the fertilized egg or zygote during sexual reproduction. The zygote then develops into an organism of a particular type. For instance, fertilized eggs of sparrows hatch into sparrows only and the fertilized eggs of pigeons hatch into pigeons only. Similarly, a cow gives birth to calves only. The wheat plant gives rise to seeds which, in turn, develop into wheat plants.
Heredity :- The transmission of characters [or traits] from one generation to another generation.
The transmission of characters from the parents to their offsprings.
Variations :- The differences in the characters [or traits] among the individuals of a species are called variations. e.g. Plant height - Tall, dwarf & middle.
Ear lobe in human being :- The lowest part of our ear is called earlobe.
u In most of the people, the ear lobe is hanging and it is called free earlobe.
u In some people, the earlobe is closely attached to the side of the head and it is called attached ear lobe.
Accumulation of Variations During Reproduction
Heredity involves inheritance of basic body design (similarities) as well as subtle changes (variations) in it from one generation to the next generation, i.e., from parents to the offspring. When individuals of this new generation reproduce, the offsprings of second generation will have the basic body design, the differences that they inherit from first generation as well as newly developed differences.
Asexual reproduction involves single parent. When a single individual reproduces asexually, the resultant two individuals again after sometime reproduce to form four individuals. All these individuals would be similar. However, there would be only very minor differences between them. These very minor differences arise due to small inaccuracies in DNA copying.
Sexual reproduction, on the other hand, generates even greater diversity. This is so because sexual reproduction involves t