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.
   OR
    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 si
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