Question

Why only tall pea plants were observed in \( F_1 \) progeny?

Answer 1

Step 1: Apply the Law of Dominance.
According to Mendel’s Law of Dominance, when two contrasting traits are present together in a hybrid, only one trait (the dominant one) is expressed, while the other (recessive) remains unexpressed in the F₁ generation.

Step 2: Identify the genotypes of parents.
The tall plant is homozygous dominant (TT), and the short plant is homozygous recessive (tt).
During gamete formation:
- The tall plant produces only T gametes.
- The short plant produces only t gametes.

Step 3: Formation of F₁ generation.
During fertilization, each offspring receives:
- One T allele from the tall parent.
- One t allele from the short parent.
Thus, all offspring have the genotype Tt.

Step 4: Phenotypic expression in F₁.
In the genotype Tt:
- T (tall) is dominant.
- t (short) is recessive.
Since the dominant allele expresses itself even in heterozygous condition, all F₁ plants show the tall phenotype.

Step 5: Conclusion.
Only tall plants appear in the F₁ generation because every offspring carries one dominant allele (T), which masks the recessive allele (t).

Final Answer:
Only tall plants were observed in the F₁ generation because tallness (T) is dominant over shortness (t), and all F₁ plants are heterozygous (Tt).

Answer 2

Step 1: Understand the objective after F₁ generation.
After observing that all F₁ plants were tall (Tt), Mendel wanted to check whether the recessive trait (shortness) had disappeared permanently or was still present in hidden form.

Step 2: Allow natural reproduction of hybrids.
Instead of crossing F₁ plants with the parent generation again, Mendel allowed the F₁ hybrid plants (Tt) to reproduce among themselves. Since pea plants are bisexual and self-fertilizing, this process occurred through self-pollination.

Step 3: Formation of gametes in F₁ plants.
Each F₁ plant has genotype Tt.
During gamete formation, the two alleles separate according to the Law of Segregation.
Thus, each plant produces two types of gametes: T and t.

Step 4: Fusion of gametes to form F₂ generation.
When these gametes fuse randomly:
- T may combine with T → TT
- T may combine with t → Tt
- t may combine with T → Tt
- t may combine with t → tt

This results in the genotypic ratio 1 TT : 2 Tt : 1 tt and phenotypic ratio 3 tall : 1 short.
The recessive trait reappears in the F₂ generation.

Final Answer:
Mendel obtained the F₂ progeny by allowing the F₁ hybrid plants (Tt) to undergo self-pollination, which led to segregation of alleles and the reappearance of the recessive trait.

Answer 3

Step 1: Meaning of dominant and recessive traits.
A dominant trait is a character that shows its effect even when only one copy of its allele is present in an organism.
A recessive trait is a character that shows its effect only when both alleles present are recessive.

Step 2: State the major difference.
The key difference is that a dominant trait can express itself in both homozygous (TT) and heterozygous (Tt) conditions, whereas a recessive trait can express itself only in the homozygous (tt) condition.

Step 3: Explanation using genotype condition.
If an organism has genotype Tt, the dominant allele (T) controls the appearance and the recessive allele (t) remains hidden.
However, for the recessive trait to appear, the genotype must be tt so that no dominant allele is present to mask it.

Step 4: Example from Mendel’s experiment.
In pea plants:
- Tallness (T) is dominant and appears in TT and Tt plants.
- Shortness (t) is recessive and appears only in tt plants.
Therefore, in the F₁ generation (Tt), only tall plants are seen because the dominant allele masks the recessive one.

Answer 4

Step 1: Consider Mendel’s monohybrid cross.
Mendel crossed two pure breeding pea plants differing in one contrasting character, such as tall (TT) and short (tt). He carefully observed the traits appearing in the F₁ generation.

Step 2: First observation – Uniformity of F₁ plants.
All the plants in the F₁ generation were identical in appearance.
They showed only one of the two contrasting parental traits.
For example, in tall × short cross, all F₁ plants were tall.
There was no mixture or intermediate character observed.

Step 3: Second observation – Disappearance of one trait.
The trait of the other parent (shortness) did not appear in the F₁ generation at all.
It seemed to be completely suppressed or masked.
This indicated that one trait was dominant and the other was recessive.

Step 4: Supporting explanation.
All F₁ plants were hybrids (Tt), carrying one allele from each parent.
However, only the dominant allele expressed itself in the phenotype.
The recessive allele remained present but unexpressed in the F₁ generation.

Final Answer:
The two main observations about F₁ progeny were:
(1) All F₁ plants were uniform and showed only the dominant trait.
(2) The recessive parental trait did not appear in the F₁ generation and was completely masked.