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-2
How do Mendel’s experiments show that traits are inherited independently?
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Mendel crossed pea plants having round green seeds (RRyy) with pea plants having
wrinkled yellow seeds (rrYY).Since the F1 plants are formed after crossing pea plants having green round seeds and
pea plants having yellow wrinkled seeds, F1 generation will have both these characters in
them. However, as we know that yellow seed colour and round seeds are dominant
characters, therefore, the F1 plants will have yellow round seeds.
Then this F1 progeny was self-pollinated and the F2 progeny was found to have yellow
round seeds, green round seeds, yellow wrinkled seeds, and green wrinkled seeds in the
ratio of 9:3:3:1. In the above cross, more than two factors are involved, and these are independently inherited.
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Mendel took two pairs of alternate expression of two traits and carried out dihybrid crosses by crossing them. The traits appeared in first generation were termed as dominant. When he used these F1 progeny to generate F2 progeny by self-pollination plants of different types were produced. In some plants both the traits were dominant, while in some plants both were recessive and some plants exhibited mixed traits. This indicates that traits are inherited independently.
Mendel crossed pure breeding tall plants having round seeds with pure breeding short plants having wrinkled seeds. The plants of F1 generation were all tall with round seeds indicating that the traits of tallness and round seeds were dominant. Self breeding of F1 yielded plants with characters of 9 tall round seeded, 3 tall wrinkled seeded , 3 short round seeded and one short wrinkled seeded. Tall wrinkled seeded and short round seeded plants are new combinations which can develop only when the traits are inherited independently.
Genes located on different chromosomes will be inherited independently of each other. Mendel observed that, when peas with more than one trait were crossed, the progeny did not always match the parents. This is because different traits are inherited independently – this is the principle of independent assortment.
During dihybrid cross by Mendel, it was observed that when two pairs of traits were considered; each trait expressed independent of the other. Thus, Mendel was able to propose the Law of Independent Assortment which says about independent inheritance of traits.
Gregor Mendel’s experiments with pea plants also led to the formulation of the Law of Independent Assortment, which suggests that the inheritance of one trait is independent of the inheritance of another trait. Here’s how Mendel’s experiments illustrate the concept of independent assortment:
1. Choice of Traits:
» Mendel selected traits that were located on different chromosomes and exhibited independent assortment during the formation of gametes. For example, he studied seed color (located on one chromosome) and seed shape (located on another chromosome).
2. Purity of Parental Lines:
» Mendel ensured the purity of his experimental plants by using true-breeding lines for each trait. This ensured that the traits were well-established and exhibited consistent expression in the parental generation.
3. Crossbreeding Experiments:
» Mendel performed controlled crosses between plants that differed in two traits simultaneously. For instance, he crossed plants with yellow, round seeds (dominant traits for both seed color and shape) with those having green, wrinkled seeds (recessive traits for both seed color and shape).
4. Observation of Offspring (First Filial Generation – F1):
» Mendel observed the traits of the F1 generation, which resulted from the cross. In this generation, he found that each individual had a combination of one dominant and one recessive trait. For example, all F1 plants had yellow, round seeds.
5. Observation of Offspring (Second Filial Generation – F2):
» Mendel allowed the F1 plants to self-fertilize or cross-fertilize. In the F2 generation, he observed the combinations of traits that resulted from the independent assortment of alleles. The traits did not seem to be linked, and the inheritance of one trait did not influence the inheritance of the other.
6. Law of Independent Assortment:
» Mendel proposed the Law of Independent Assortment, stating that genes located on different chromosomes segregate independently during the formation of gametes. This means that the inheritance of one trait does not affect the inheritance of another trait if the genes are located on different chromosomes.
The key result of Mendel’s experiments was that the traits he studied segregated independently because they were located on different chromosomes. This independent assortment is crucial in generating genetic diversity within populations, as it allows for the combination of various traits in different ways.
It’s important to note that the Law of Independent Assortment holds true for genes located on different chromosomes. Genes located on the same chromosome may be inherited together if they are physically close to each other (a phenomenon known as genetic linkage), but this was not observed in Mendel’s experiments with the traits he chose.