NCERT Solution for Class 10 Science Chapter 9
Heredity and Evolution
NCERT Books for Session 2022-2023
CBSE Board and UP Board
Intext Questions
Page No-147
Questions No-1
How do Mendel’s experiments show that traits may be dominant or recessive?
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Mendel selected true breeding tall (TT) and dwarf (tt) pea plants. Then, he crossed these
two plants. The seeds formed after fertilization were grown and these plants that were
formed represent the first filial or F1 generation. All the F1 plants obtained were tall.
Then, Mendel self-pollinated the F1 plants and observed that all plants obtained in the F2
generation were not tall. Instead, one-fourth of the F2 plants were short.
From this experiment, Mendel concluded that the F1 tall plants were not true breeding.
They were carrying traits of both short height and tall height. They appeared tall only
because the tall trait is dominant over the dwarf trait.
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Mendel took pea plants with contrasting characteristics tall plant and dwarf (or short) plant. On cross pollination, he got all tall plants in first generation (F1). But by the self¬pollination of F1 tall plants, the plants of second generation consisted of tall and short pants in the ratio of 3 : 1. On the basis of these experiments, the characteristics appeared in first generation were called dominant (i.e. tall plants) and the characteristics that did not appear were called recessive (dwarf i.e. plants).
The trait which appears in all the members of F1 generation and also in 75% numbers of F2 generation obtained by self fertilization of F1 generation is dominant character.
The trait which does not appear in F generation but after self-fertilization of F1 generation, reappears in 25% of F2 generation is known as recessive.
During monohybrid cross by Mendel, one of the pair of traits did not appear in the F1 generation. But that trait appeared in the F2 generation. Based on this observation, Mendel concluded that a trait could be dominant or recessive.
Gregor Mendel’s experiments with pea plants laid the foundation for our understanding of inheritance and the principles of genetics. Through his work, Mendel demonstrated the existence of dominant and recessive traits. Here’s a brief overview of Mendel’s experiments and how they illustrate the concept of dominance and recessiveness:
1. Choice of Traits:
» Mendel selected traits that exhibited clear and easily distinguishable variations in the pea plants, such as seed color (yellow or green), seed shape (round or wrinkled), flower color (purple or white), etc.
2. Purity of Parental Lines:
» Mendel ensured the purity of his experimental plants by using true-breeding lines. True-breeding means that when plants with a particular trait are self-fertilized or cross-fertilized, they consistently produce offspring with the same trait.
3. Crossbreeding Experiments:
» Mendel performed controlled crosses between plants with contrasting traits. For example, he crossed plants with yellow seeds (dominant trait) with those having green seeds (recessive trait).
4. Observation of Offspring (First Filial Generation – F1):
» Mendel observed that the offspring (F1 generation) of these crosses consistently displayed the dominant trait. In the case of seed color, all the F1 plants had yellow seeds.
5. Observation of Offspring (Second Filial Generation – F2):
» Mendel then allowed the F1 plants to self-fertilize or cross-fertilize. In the resulting F2 generation, he observed a 3:1 ratio of dominant to recessive traits. For example, in the case of seed color, approximately three-fourths of the F2 plants had yellow seeds, and one-fourth had green seeds.
6. Law of Segregation:
» Mendel proposed the Law of Segregation, which states that the two alleles (gene variants) for a trait segregate (separate) during the formation of gametes, and each gamete receives only one allele. This segregation explains the 3:1 ratio observed in the F2 generation.
7. Dominance and Recessiveness:
» The dominant trait, which is expressed in the phenotype of the organism, masks the expression of the recessive trait in heterozygous individuals (those carrying both dominant and recessive alleles).
Mendel’s experiments demonstrated that traits are controlled by discrete units (now known as genes) and that these units come in pairs. Dominant traits are expressed in the presence of at least one dominant allele, whereas recessive traits are only expressed when an individual carries two recessive alleles.
Mendel’s findings laid the groundwork for the understanding of inheritance patterns and genetics, and his principles continue to be fundamental in the study of genetics today.