State and explain Mendel’s law of independent assortment.

Mendel’s Law of Independent Assortment is another fundamental principle of genetics proposed by Gregor Mendel. This law describes how genes located on different chromosomes are inherited independently of each other.

Mendel’s Law of Independent Assortment

State:
The Law of Independent Assortment states that alleles for different traits assort independently of one another during the formation of gametes. This means that the inheritance of an allele for one trait does not influence the inheritance of an allele for another trait, as long as the genes controlling those traits are located on different chromosomes or are far apart on the same chromosome.

Explanation

  1. Genes and Alleles:

    • Every organism has two alleles for each gene, one inherited from each parent. These alleles determine specific traits, such as flower color or seed shape in pea plants.
    • Mendel studied the inheritance of two traits simultaneously, such as seed color (yellow or green) and seed shape (round or wrinkled).

  2. Independent Assortment During Gamete Formation:

    • During meiosis (the process of gamete formation), chromosomes are randomly distributed into gametes. The Law of Independent Assortment states that genes on different chromosomes are inherited independently of each other.
    • For example, if one gene controls seed color (with alleles for yellow Y and green y), and another gene controls seed shape (with alleles for round R and wrinkled r), the inheritance of the Y or y allele (seed color) will not affect whether the R or r allele (seed shape) is inherited.

  3. Result of Independent Assortment:

    • The alleles of different genes segregate into gametes independently. As a result, new combinations of traits can appear in the offspring that were not present in either parent.
    • For example, when Mendel crossed YYRR (homozygous yellow, round seeds) with yyrr (homozygous green, wrinkled seeds), the offspring would initially be YyRr (heterozygous yellow, round seeds). During the next generation, these offspring can produce gametes with combinations of alleles such as YR, Yr, yR, and yr, which will assort independently into the next generation.

  4. Genetic Diversity:

    • This independent assortment contributes to genetic diversity because it allows different combinations of alleles to come together in offspring, leading to variations in the traits they exhibit.
    • It is important to note that independent assortment applies only to genes located on different chromosomes or genes that are far apart on the same chromosome. Genes that are closer together on the same chromosome tend to be inherited together, a phenomenon called genetic linkage.

Mendel’s Experiment and Evidence

Mendel tested the Law of Independent Assortment by performing dihybrid crosses, where he tracked the inheritance of two traits at the same time. For example:

  • He crossed plants that were homozygous for both traits: YYRR (yellow, round seeds) and yyrr (green, wrinkled seeds).
  • The F1 generation was YyRr (heterozygous for both traits).
  • Mendel then crossed the F1 generation plants with each other (i.e., YyRr x YyRr).
  • The resulting F2 generation showed a phenotypic ratio of 9:3:3:1, which indicated that the traits segregated independently, as predicted by the law.

This ratio is characteristic of a dihybrid cross when two genes are inherited independently.

Example of Independent Assortment

Let’s look at the inheritance of two traits in pea plants: seed color (yellow, Y vs. green, y) and seed shape (round, R vs. wrinkled, r).

  1. Parental Generation:

    • YYRR (homozygous yellow, round) × yyrr (homozygous green, wrinkled)
    • All F1 offspring are YyRr (heterozygous yellow, round).

  2. F1 Cross:

    • F1 plants (YyRr × YyRr) produce four types of gametes: YR, Yr, yR, and yr.

    • The F2 generation will show a 9:3:3:1 ratio of phenotypes:

      • 9 Yellow, Round (dominant traits, both alleles)
      • 3 Yellow, Wrinkled (only seed shape is recessive)
      • 3 Green, Round (only seed color is recessive)
      • 1 Green, Wrinkled (both alleles are recessive)

This demonstrates that seed color and seed shape segregate independently, leading to a variety of genetic combinations in the offspring.

Key Points of Mendel’s Law of Independent Assortment:

  1. Genes on different chromosomes assort independently during gamete formation, meaning that the inheritance of one gene does not affect the inheritance of another.
  2. The law applies only to genes that are on different chromosomes or are far apart on the same chromosome.
  3. The law contributes to genetic diversity by producing new combinations of alleles in offspring.

Related Questions:

  1. How do genes control inheritance of characters?
  2. Explain the composition of chromatin material.
  3. Give an account of genes and alleles.
  4. Describe the role of genes in protein synthesis.
  5. State and explain Mendel’s law of segregation.
  6. Write the differences between:
  7. Name sources of variation.
  8. Give examples of artificial selection.
  9. What kind of gametes would be produced by organisms having the following genotypes?
  10. Two white sheep produce a black offspring. What must the parents’ genotype for colour be? What is the probability that the next offspring will be black?
  11. The diagram shows two varieties of moths in England.
  12. Among the following genotypes, which ones are heterozygous and which are homozygous? Which of the genotypes have the same phenotype?
  13. Name the structure through which the exchange of gases can take place in plants.
  14. Name the ways through which a plant can obtain oxygen.
  15. Why should one not sleep under a tree at night?
  16. Why would very wet soil be dangerous to plants
  17. Why do plants not need a specialized respiratory system?
  18. How does the exchange of gases take place in alveoli?
  19. What is the effect of exercise on the rate of breathing?
  20. Compare inspired air and expired air.