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Question : How do Mendel's experiments show that the (a) traits may be dominant or recessive, (b) traits are inherited independently? |
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Answer» Solution :(a) When Mendel cross POLLINATED pure tall pea plants with pure dwarf pea plants, only tall plants were obtained in `F_1` generation. On self pollinating the `F_1` progeny, both tall and dwarf plants appeared in `F_2` generation in the ratio 3:1. Appearance of tall character in both the `F_1` and `F_2` shows that it is a dominant character. The absence of dwarf character in `F_1` generation and its reappearance in `F_2` shows dwarfness is the recessive character. (b) When Mendel first crossed pure-breed pea plants having round-yellow seeds with pure-breed pea plants having wrinkled-GREEN seeds, he found that only round-yellow seeds were PRODUCED in the first-generation. No wrinkled-green seeds were obtained in the `F_1` generation. From this, it was concluded that round shape and yellow colour of the seeds were dominant traits over the wrinkled shape and green colour of the seeds. When the `F_1` generation pea plants having round-yellow seeds were cross-bred by self-pollination, then four types of seeds having different combinations of shape and colour were obtained in second generation `(F_2)`. These were round-yellow, round-green, wrinkled-yellow and wrinkled-green seeds. Such a cross is known as dihybrid cross as two sets of corresponding characters are considered. Mendel observed that along with round-yellow and wrinkled-green, two new combinations of CHARACTERISTICS, round-green and wrinkled-yellow, had appeared in the `F_2` generation. On the BASIS of this observation, Mendel concluded that though the two pairs of original characteristics (seed colour and shape) combine in the `F_1` generation, they get separated and behave independently in the subsequent generation. |
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