Question

Validity of Mendel's law of segregation is established only when

• A. One parent is crossed with F1 hybrid
• B. Two pure breeding contrasting characters are crossed
• C. Two F1 hybrids are crossed
• D. None of the above

Answer 1

The correct option is C

Two F1 hybrids are crossed

The correct option is C.

Explanation of the correct option:

1. Each gamete produced by an organism has a single gene copy that is chosen at random.
2. This is referred to as the law of segregation.
3. The genotypes (allele combinations) and phenotypes (observable traits) of progeny from genetic crossings can be predicted using a Punnett square.
4. Dominant and recessive alleles separate in the gametes and are expressed in the F2 generation when two F1 hybrids are crossed.
5. Every allele in its pair is separated during fertilisation in every organism and enters a gamete.
6. So, each gamete only possesses one of the pair's alleles.
7. Each gamete is hence pure for that feature.
8. This explains why a characteristic that is not manifested in one generation may express itself in the following.
9. Following the crossing of an F1 hybrid (Tt) with another F1 hybrid, G.J. Mendel proposed this theory (Tt).
10. The second law of Mendel, sometimes known as the law of gamete purity, expresses this.
11. Therefore, "Two hybrids are crossed" is the right answer.

Explanation of the incorrect option:

Option A:

1. A Test Cross is defined as a cross between an F1 hybrid and the homozygous recessive parent.
2. To identify the genotype of an F2 generation plant displaying dominant traits, test crosses are performed.
3. An F2 plant with the dominant trait is crossed with a homozygous recessive in the test cross.

Option B:

1. Test cross is a cross between an F1 hybrid and a recessive parent.
2. In the following generation, plants are produced in a 1:1 ratio with 50% of the progeny being heterozygous dominant and 50% of the progeny being homozygous recessive.
3. Mendel examined the genetics of pea plants and determined how traits like flower colour, flower location, seed colour, and seed shape are passed down through the generations.
4. To do this, he began by mating pure-breed parent plants with various variations of a trait, such as violet and white flowers.
5. Pure breeding simply indicates that a plant will produce more of its own kind over the course of several generations of self-fertilization.
6. He referred to the trait present in the F1 generation as the dominant trait (violet flowers), and the trait absent or lost (white flowers) as the recessive trait.
   

Final answer: The validity of Mendel's law of segregation is established only when two F1 hybrids are crossed.