A tall true breeding garden pea plant is crossed with a dwarf true breeding garden pea plant. When the F1 plants were selfed the resulting genotypes were in the ratio of -
- 1 : 2 : 1 :: Tall heterozygous : Tall homozygous : Dwarf
- 3 : 1 :: Tall : Dwarf
- 3 : 1 :: Dwarf : Tall
- 1 : 2 : 1 :: Tall homozygous : Tall heterozygous : Dwarf
The Correct Option is D
Approach Solution - 1
Explanation:
- Let T = allele for tallness (dominant), and t = allele for dwarfness (recessive).
- True breeding tall plant genotype: TT
- True breeding dwarf plant genotype: tt
- Cross: \(TT \times tt\) → F₁ generation: All Tt (Tall)
- F₁ selfed: \(Tt \times Tt\)
F₂ Genotypes: \(TT : Tt : tt = 1 : 2 : 1\)
F₂ Phenotypes: \(Tall : Dwarf = 3 : 1\)
(since TT and Tt are tall, tt is dwarf)
Correct Answer:
\(\boxed{\text{Option 1: 1 : 2 : 1 :: Tall heterozygous : Tall homozygous : Dwarf}}\)
⚠️ Note: If the question is specifically about genotypes, the correct order is: \(1 \, TT : 2 \, Tt : 1 \, tt\)
But if the phenotype order was meant, that would be 3:1 (Tall: Dwarf).
Approach Solution -2
Genetic Explanation of F1 Generation
Explanation
The F1 generation that results from crossing a tall true-breeding garden pea plant (TT) with a dwarf true-breeding garden pea plant (tt) will all be heterozygous for height, meaning they are all Tt. Here, the tall trait (T) is dominant over the dwarf trait (t).
On preparing a Punnett square, the following is observed:
| T | t | |
| T | TT | Tt |
| t | Tt | tt |
Tall heterozygous (Tt): 50% of the progeny will have the tall heterozygous (Tt) genotype. Since they only carry one dominant allele for height, these plants will grow to be very tall.
Tall homozygous (TT): 25% of the progeny will carry this gene. Purebred for the tall trait, these plants will be tall.
Dwarf (tt): 25% of the progeny will carry this gene. Because they carry two recessive genes for height, these plants will be diminutive.
Consequently, the correct genotype ratio following the selfing of the F1 generation will be 1:2:1, which consists of:
- Tall homozygous (TT): 25%
- Tall heterozygous (Tt): 50%
- Dwarf (tt): 25%
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Concepts Used:
Principles of Inheritance and Variation - Mutation
A Mutation is a change in the sequence of our DNA base pairs caused by numerous environmental stimuli such as UV light or mistakes during DNA replication. Germline mutations take place in the eggs and sperm and can be passed onto offspring, whereas somatic mutations take place in body cells and are not passed on.
Types of Mutations
There are three types of mutations, which are as follows:
Silent mutation
It refers to any change in DNA sequence that has no effect on the amino acid sequence in a protein or the functions that a protein performs. There is no phenotypic indication that a mutation has occurred.
Nonsense mutation
When there is a change in the sequence of base pairs due to a point mutation, that results in a stop codon. This leads to a protein that is either shortened or non-functional.
Missense mutation
A missense mutation occurs when a point mutation causes a change in the codon, which then codes for another amino acid.
The mutation is caused by the following factors:
Internal Causes
When DNA copies incorrectly, the majority of mutations occur. Evolution occurs as a result of all of these mutations. DNA makes a copy of itself during cell division. When a copy of DNA isn't flawless, it's called a mutation since it differs somewhat from the original DNA.
External Causes
When certain chemicals or radiations are used to break down DNA, it causes the DNA to break down. The thymine dimers are broken by UV radiation, resulting in altered DNA.