Question

Mendel worked out the rules of heredity by working on garden pea using a number of visible contrasting characters. He conducted several experiments by making a cross with one or two pairs of contrasting characters of pea plant. On the basis of his observations, he gave some interpretations which helped to study the mechanism of inheritance.

1. When Mendel crossed pea plants with pure tall and pure short characteristics to produce F₁ progeny, which two observations were made by him in F₁ plants?       1
2. Write one difference between dominant and recessive trail.       1
3. 1. In a cross with two pairs of contrasting characters
      

      RRYY	×	rryy
      (Round Yellow)		(Wrinkled Green)

      Mendel observed 4 types of combinations in F₂ generation. By which method did he obtain F₂ generation? Write the ratio of the parental combinations obtained and what conclusions were drawn from this experiment?         2
      OR
   2. Justify the statement:
      "It is possible that a trait is inherited but may not be expressed."         2

Answer 1

1. When Mendel crossed pea plants with pure tall (dominant) and pure short (recessive) characteristics to produce F₁ progeny, two observations were made:
   1. All F₁ plants were tall: In the first generation (F₁), all plants exhibited the tall characteristic, whereas the short feature seemed to disappear.
   2. Short trait reappeared in F₂: In the second generation (F₂), the short characteristic reappeared, resulting in a 3 : 1 ratio of tall to short plants.
2. Difference between Dominant and Recessive Traits:
   • Dominant Trait: In a heterozygous individual, the dominant trait overpowers the recessive trait. Capital letters, such as T, signify dominant features.
   • Recessive Trait: This trait is only expressed in homozygous individuals and remains hidden in the presence of dominant alleles. Recessive features are denoted with lowercase letters (e.g., t).
3. 1. Mendel achieved the F₁ generation by a dihybrid cross. In this cross, he evaluated two contrasting characters: seed form (round or wrinkled) and seed colour (green or yellow).
      The parental genotypes were:
      
      • RRYY (Round Yellow) for one parent
      • rryy (Wrinkled Green) for the other parent
        We can explain it by taking an example
        Cross between → RrYy × RrYy
        

        RR-round	yy-green
        Rr-round	Yy-green
        rr-Wrinkled	yy-yellow

        Cross →
        

        	RY	Ry	rY	ry
        RY	RRYY	RRYy	RrYY	RrYy
        Ry	RRYy	RRyy	RrYy	Rryy
        rY	RrYY	RrYy	rrYY	rrYy
        ry	RrYy	Rryy	rrYy	rryy

        (Phenotypic ratio) → 9 : 3 : 3 : 1
        Genotypic ratio → 1 : 2 : 2 : 1 : 4 : 2 : 1 : 2 : 1
        Conclusions drawn from this experiment: 
      • Law of Independent Assortment: Mendel found that genes for various characteristics segregate independently during gamete development. Seed shape (round or wrinkled) and colour (yellow or green) are not inherited.
      • Dihybrid Ratio: The F₂ generation exhibits a phenotypic ratio of 9 : 3 : 3 : 1, supporting the Law of Independent Assortment. The dihybrid ratio shows the distribution of attributes between two opposing pairs in a genetic cross.
   2. The concept of dominant and recessive alleles in genetics supports the statement that a trait might be inherited but not necessarily displayed. Individuals often inherit two alleles for a specific trait, one from each parent. These alleles can be homozygous (identical) or heterozygous (different). The interaction of these alleles affects whether the trait manifests in the phenotype.
      1. Dominant Allele: 
         • If a person inherits at least one dominant allele for a characteristic, it will be expressed in the phenotype.
         • Dominant alleles are typically represented by uppercase letters (e.g., T).
      2. Recessive Allele:
         • Individuals must inherit both recessive alleles (homozygous recessive) to express a recessive characteristic.
         • Recessive alleles are typically represented by lowercase letters (e.g., t).
      3. Heterozygous Individuals:
         • When an individual is heterozygous for a characteristic, the dominant allele masks the expression of the recessive allele, resulting in the absence of the trait associated with the recessive allele in the phenotypic.
         • The individual carries the recessive trait but does not show it.
           The presence of an allele in an individual's genotype does not always lead to trait expression. Trait expression is determined by the interaction of inherited alleles based on Mendelian genetic principles.