Mendel’s Contribution MCQs Quiz | Class 10

Mendel’s Contribution to Genetics: Understanding Inheritance

Gregor Johann Mendel, an Austrian monk, is widely recognized as the “Father of Genetics” for his groundbreaking work on pea plants (Pisum sativum) in the mid-19th century. His meticulous experiments and mathematical analysis laid the foundation for our understanding of heredity, explaining how traits are passed from parents to offspring.

Key Contributions and Laws of Inheritance:

1. Use of Pea Plants: Mendel chose pea plants due to their easily observable contrasting traits (e.g., tall/dwarf, round/wrinkled seeds, green/yellow pods), short life cycle, and the ability to self-pollinate or cross-pollinate. This allowed him to control crosses and observe multiple generations.
2. Particulate Inheritance: Before Mendel, the prevailing theory was “blending inheritance.” Mendel demonstrated that traits are determined by discrete units (which we now call genes), which are passed on unchanged from generation to generation, rather than blending. He called these units “factors.”
3. Law of Dominance:
   • When two alternative forms of a trait (alleles) are present, one (the dominant allele) may mask the expression of the other (the recessive allele).
   • Example: In a cross between a pure tall (TT) and a pure dwarf (tt) pea plant, the F1 generation will all be tall (Tt), as tallness is dominant over dwarfness.
4. Law of Segregation (Law of Purity of Gametes):
   • During the formation of gametes (sperm and egg cells), the two alleles for a heritable character separate or “segregate” from each other, so that each gamete carries only one allele for each character.
   • This ensures that the recessive trait can reappear in subsequent generations if two recessive alleles come together.
   • Example: In the F1 generation (Tt), gametes will carry either T or t, with equal probability.
5. Law of Independent Assortment:
   • This law states that the alleles for different genes (e.g., seed color and seed shape) assort independently of one another during gamete formation, provided the genes are located on different chromosomes or are far apart on the same chromosome.
   • This leads to new combinations of traits in the offspring that were not present in the parents.
   • Example: In a dihybrid cross involving seed color (yellow/green) and seed shape (round/wrinkled), the inheritance of yellow/green is independent of the inheritance of round/wrinkled. The F2 phenotypic ratio for a dihybrid cross is typically 9:3:3:1.

Key Terms to Remember:

• Gene: A unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.
• Allele: One of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
• Dominant: An allele that expresses its phenotypic effect even when heterozygous with a recessive allele.
• Recessive: An allele that produces its characteristic phenotype only when its paired allele is identical.
• Genotype: The genetic makeup of an organism; the combination of alleles it possesses.
• Phenotype: The observable physical or biochemical characteristics of an organism, as determined by both genetic makeup and environmental influences.
• Homozygous: Having two identical alleles for a particular gene (e.g., TT or tt).
• Heterozygous: Having two different alleles for a particular gene (e.g., Tt).
• Monohybrid Cross: A cross between two organisms that are heterozygous for one gene.
• Dihybrid Cross: A cross between two organisms that are heterozygous for two different genes.

Significance of Mendel’s Work:

Mendel’s work was initially overlooked but rediscovered in 1900. It provided the first coherent explanation for the patterns of inheritance, forming the bedrock of classical genetics and paving the way for modern genetic research, including understanding genetic diseases and agricultural advancements.

Quick Revision List:

• Father of Genetics: Gregor Mendel.
• Model organism: Pea plant.
• Law of Dominance: One allele masks another.
• Law of Segregation: Alleles separate during gamete formation.
• Law of Independent Assortment: Genes for different traits assort independently.
• Key ratios: Monohybrid F2 phenotypic 3:1, genotypic 1:2:1; Dihybrid F2 phenotypic 9:3:3:1.
• Alleles, genes, dominant, recessive, genotype, phenotype.

Practice Questions:

1. In pea plants, if yellow seed color (Y) is dominant over green seed color (y), what is the phenotype of a plant with genotype Yy?
2. If two heterozygous tall pea plants (Tt) are crossed, what percentage of the offspring are expected to be dwarf (tt)?
3. According to the Law of Independent Assortment, if genes for height and seed color are on different chromosomes, how would the inheritance of one affect the other?
4. What is the main difference between genotype and phenotype?
5. Why was Mendel’s choice of pea plants crucial for the success of his experiments?