With the help of an example justify the following statement : “A trait may be inherited, but may not be expressed”.

Q: With the help of an example justify the following statement : “A trait may be inherited, but may not be expressed”.

The statement **"A trait may be inherited, but may not be expressed"** can be justified using Mendel's monohybrid cross experiment with pea plants: 1. **The Cross**: Mendel crossed pure-breeding tall pea plants ($TT$) with pure-breeding short pea plants ($tt$). 2. **$F_1$ Generation**: In the first filial ($F_1$) generation, all the progeny were tall plants with the genotype $Tt$. 3. **Inheritance without Expression**: Even though the $F_1$ plants were phenotypically tall, they inherited one copy of the shortness allele ($t$) from the short parent alongside the tallness allele ($T$). The shortness trait ($t$) was inherited but remained unexpressed (masked) because it is a recessive trait, while tallness ($T$) is a dominant trait. 4. **Reappearance in $F_2$**: When these $F_1$ tall plants ($Tt$) were self-pollinated, the shortness trait ($tt$) reappeared in one-quarter (25%) of the second filial ($F_2$) progeny. This proves that the shortness trait was indeed present (inherited) in the $F_1$ generation, even though it was not expressed. ```diagram {"xrange": [-1.0, 1.0], "yrange": [-13.44, 1.44], "lines": [{"from": [0.0, 0.0], "to": [0.0, -3.0], "label": ""}, {"from": [0.0, -3.0], "to": [0.0, -6.0], "label": ""}, {"from": [0.0, -6.0], "to": [0.0, -9.0], "label": ""}, {"from": [0.0, -9.0], "to": [0.0, -12.0], "label": ""}], "points": [{"x": 0.0, "y": 0.0, "label": "Parental (P) Generation:\nTall (TT) x Short (tt)"}, {"x": 0.0, "y": -3.0, "label": "Gametes:\nT and t"}, {"x": 0.0, "y": -6.0, "label": "F1 Generation:\nAll Tall (Tt)\n[Shortness trait 't' is inherited but NOT expressed]"}, {"x": 0.0, "y": -9.0, "label": "Self-Pollination of F1:\nTt x Tt"}, {"x": 0.0, "y": -12.0, "label": "F2 Generation:\nTT (Tall) : Tt (Tall) : tt (Short)\nRatio = 3 Tall : 1 Short\n[Shortness trait 'tt' is expressed]"}]} ```

Heredity

Rules for the Inheritance of Traits – Mendel's Contributions

Official Answer

The statement "A trait may be inherited, but may not be expressed" can be justified using Mendel's monohybrid cross experiment with pea plants:

The Cross: Mendel crossed pure-breeding tall pea plants ( $TT$ ) with pure-breeding short pea plants ( $tt$ ).
$F_1$ Generation: In the first filial ( $F_1$ ) generation, all the progeny were tall plants with the genotype $Tt$ .
Inheritance without Expression: Even though the $F_1$ plants were phenotypically tall, they inherited one copy of the shortness allele ( $t$ ) from the short parent alongside the tallness allele ( $T$ ). The shortness trait ( $t$ ) was inherited but remained unexpressed (masked) because it is a recessive trait, while tallness ( $T$ ) is a dominant trait.
Reappearance in $F_2$ : When these $F_1$ tall plants ( $Tt$ ) were self-pollinated, the shortness trait ( $tt$ ) reappeared in one-quarter (25%) of the second filial ( $F_2$ ) progeny. This proves that the shortness trait was indeed present (inherited) in the $F_1$ generation, even though it was not expressed.

F_1

generation

F_2

generationdominant traitrecessive traittallnessshortnessalleleinheritedexpressedMendel

Marking Scheme

1{"points": 1, "description": "Describing the cross between pure tall ( $TT$ ) and short ( $tt$ ) plants resulting in all tall $F_1$ ( $Tt$ ) plants."}
2{"points": 1, "description": "Explaining that the shortness allele ( $t$ ) is inherited in $F_1$ but not expressed because it is recessive to the dominant tallness allele ( $T$ )."}
3{"points": 1, "description": "Showing that self-pollination of $F_1$ yields short plants ( $tt$ ) in $F_2$ , proving the trait was inherited but hidden in $F_1$ ."}

Hint

Recall Mendel's cross between tall ( $TT$ ) and short ( $tt$ ) plants. What was the phenotype of the $F_1$ generation? Did the short trait disappear completely, or did it reappear later?

Quick Oral Answer

In Mendel's cross, $F_1$ plants ( $Tt$ ) are tall, meaning the short trait ( $t$ ) is inherited from the parent but not expressed because it is recessive. It only expresses in the $F_2$ generation when it becomes homozygous ( $tt$ ).

Analysis & Explanation

In diploid organisms, traits are controlled by two copies of a gene (alleles). When these two alleles are different (heterozygous state, e.g., $Tt$ ), only the dominant allele ( $T$ ) expresses its phenotype (tallness), while the recessive allele ( $t$ ) remains hidden (unexpressed). However, the recessive allele is still physically present in the DNA and is passed on to the next generation, where it can express itself if it pairs with another recessive allele ( $tt$ ).

Common Mistakes

1Forgetting to write down the genotypes ( $TT$ , $Tt$ , $tt$ ) of the plants.
2Not explaining that the reappearance of the short phenotype in the $F_2$ generation is the actual proof that the trait was inherited in $F_1$ .

Interesting Facts

In humans, traits like blue eyes or red hair can skip multiple generations because they are recessive and only express when an individual inherits two copies of the recessive allele.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2016?

This question carries 3 marks in the CBSE Class 10 Science 2016 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Heredity" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Rules for the Inheritance of Traits – Mendel's Contributions" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2016 paper?

This is a Short Answer question from Section A in the CBSE Class 10 Science 2016 paper.