Principles of Inheritance & Variation till Inheritance of One Gene - Biology

Mendel’s Laws of Inheritance

Mendel's work established the basic principles that explain how biological traits are transmitted from one generation to the next. By using carefully designed hybridisation experiments on garden pea plants, Gregor Mendel discovered predictable patterns of inheritance for simple, contrasting traits and introduced the concept of discrete hereditary units called factors (now called genes).

Seven pairs of contrasting traits in pea plant studied by Mendel

• During the mid-19th century, progress in understanding heredity advanced through careful experimental study and quantitative analysis.
• Gregor Mendel carried out hybridisation experiments on garden pea plants from 1856 to 1863, and formulated fundamental rules of inheritance from his observations.
• Mendel introduced statistical and mathematical analysis into biology and worked with large sample sizes to ensure reliability of his results.
• His conclusions were confirmed by successive generations and are considered general rules of inheritance for many traits, although there are notable exceptions.

True‑Breeding Lines

• A true-breeding line is a group of organisms that, over many generations of self-pollination, consistently produce offspring with the same stable traits.
• Mendel selected 14 pairs of true-breeding pea plant varieties that were similar except for one contrasting trait. For instance, he might choose one variety with smooth seeds and another with wrinkled seeds.

Contrasting Traits Studied by Mendel

Some of the contrasting characters Mendel studied in pea plants include:

• Plant height: tall vs dwarf
• Pod shape: inflated vs constricted
• Seed texture: round (smooth) vs wrinkled
• Seed colour: yellow vs green
• Flower colour: purple vs white
• Pod colour: green vs yellow
• Flower position: axial vs terminal

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Basic Terms and Definitions

• Trait: A feature or characteristic of an organism (e.g., seed colour, height).
• Gene: A unit of heredity — a segment of DNA that influences a specific trait.
• Allele: An alternative form of a gene (for example, the allele for tallness T and the allele for dwarfness t).
• Homozygous: Having two identical alleles for a gene (e.g., TT or tt).
• Heterozygous: Having two different alleles for a gene (e.g., Tt).
• Genotype: The genetic constitution of an organism (e.g., TT, Tt, tt).
• Phenotype: The observable appearance or trait (e.g., tall or dwarf plant).
• Dominant allele: An allele whose effect is seen in the phenotype when present in a single copy (e.g., T).
• Recessive allele: An allele whose effect is masked in the presence of a dominant allele and only appears in the homozygous recessive condition (e.g., t).
• Gametes: Sex cells (pollen and egg in plants) that carry one allele for each gene.

Introduction to Mendel's Monohybrid Experiment

• Mendel crossed true‑breeding tall (TT) and dwarf (tt) pea plants and observed the progeny.
• Seeds produced from this cross were sown to obtain the Filial 1 (F1) generation.

Steps in making a cross in pea

Observations in the F1 Generation

• All F1 plants were tall and resembled only one parent (the tall parent).
• The dwarf trait was not expressed in F1 but the information for it was not lost.

Self‑Pollination of F1 and the F2 Generation

• When F1 tall plants (Tt) were self‑pollinated, the F2 generation contained both tall and dwarf plants.
• Phenotypic ratio observed in F2: 3 tall : 1 dwarf.
• Genotypic ratio in F2: 1 TT : 2 Tt : 1 tt.

Conceptual Interpretation by Mendel

• Mendel proposed that hereditary characters are controlled by discrete units called factors (genes) that retain their identity across generations.
• Each organism carries two factors for a character, one inherited from each parent.

Alleles and Genotypes (Detailed)

• Pairs of alternative forms of a gene are called alleles.
• Notation example for height: T = tall allele, t = dwarf allele.
• Possible genotypes and their meanings:
  • TT — homozygous dominant (tall)
  • Tt — heterozygous (tall phenotype, carries dwarf allele)
  • tt — homozygous recessive (dwarf)

Dominance and Recessiveness

• In a heterozygote, one allele may mask the expression of the other; this is called dominance.
• In height example, T (tall) is dominant over t (dwarf).

Monohybrid Cross and the Principle of Segregation

• A cross between two parents differing in one character (e.g., TT × tt) is called a monohybrid cross.
• Mendel’s observations led to the Principle of Segregation — during gamete formation, the two alleles of a gene separate (segregate) so that each gamete receives only one allele.

Punnett Square (Predicting Offspring Genotypes)

• The Punnett square (by Reginald C. Punnett) is a simple grid method to predict genotypic and phenotypic ratios of offspring given parental gametes.
• Example: Cross TT × tt.
  • Parental gametes: TT → T, T; tt → t, t.
  • F1 genotypes (from Punnett square): all Tt (heterozygous) → F1 phenotype: all tall.

NEET PYQ from this Topic:

Q1: The production of gametes by the parents, formation of zygotes, the F₁ and F₂ plants, can be understood from a diagram called: (NEET 2021)
(a) Punnett square
(b) Net square
(c) Bullet square
(d) Punch square

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Ans: (a)
Punnett square is a tool that helps to show all possible allelic combinations of gametes in a cross of parents with known genotypes in order to predict the probability of their offspring possessing certain sets of alleles. For a cross involving two genes, a Punnett square is still a good strategy.

F2 Generation Ratios — Mathematical Representation

• When heterozygous F1 (Tt) self‑pollinate, gametes produced by each parent are in the proportion 1/2 T : 1/2 t.
• Using the binomial expression, probability of offspring genotypes is obtained by squaring the gamete distribution: (1/2 T + 1/2 t)² = 1/4 TT : 1/2 Tt : 1/4 tt.
• Hence phenotypic ratio of tall to dwarf = 3 : 1.

Test Cross

• A test cross is used to determine the genotype of an individual showing a dominant phenotype but unknown genotype (homozygous dominant or heterozygous).
• Procedure: Cross the unknown (phenotypically dominant) individual with a homozygous recessive (tt) individual.
• Interpretation:
  • If all offspring show dominant phenotype → unknown parent was likely homozygous dominant (TT).
  • If offspring show 1:1 ratio of dominant:recessive → unknown parent was heterozygous (Tt).

Conclusion from Monohybrid Studies

Mendel’s monohybrid crosses established that inheritance is particulate (not blending), alleles segregate during gamete formation, and dominance relationships determine which phenotype appears in heterozygotes.

NEET PYQ from this Topic:
In a plant, black seed color (BB/Bb) is dominant over white seed color (bb). In order to find out the genotype of the black seed plant, with which of the following genotype will you cross it? (NEET 2024)
(a) BB
(b) bb
(c) Bb
(d) BB/Bb

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Ans: (b)
To determine the genotype of a black seed plant that could either be homozygous dominant (BB) or heterozygous (Bb), you need to perform a test cross. A test cross involves crossing the individual in question with a homozygous recessive individual. In this scenario, that would be a plant with white seed color, or genotype bb.
A test cross is used because it can reveal whether the black seed plant carries the recessive b allele. When crossed with a homozygous recessive (bb) plant:
By observing the seed colors of the offspring, you can determine whether the black seed plant was homozygous dominant or heterozygous. If any white seeds appear among the offspring, the black seed plant must be heterozygous (Bb). If no white seeds appear, the black seed plant is likely homozygous dominant (BB).
Therefore, the correct option for crossing to determine the genotype of the black seed plant is: Option B bb

From monohybrid studies, Mendel formulated foundational rules now stated as laws of inheritance. These explain how alleles behave during sexual reproduction.

Law of Dominance (First Law)

• When two different alleles are present in a pair, one may mask the expression of the other in the heterozygote — the expressed allele is dominant and the masked allele is recessive.
• In a cross of contrasting true‑breeding parents, the F1 resembles only the parent with the dominant character.
• The recessive character can reappear in the F2 when alleles segregate.

NEET PYQ from this Topic:
Q3: Which one of the following can be explained on the basis of Mendel's Law of Dominance?     (NEET 2024)
A. Out of one pair of factors one is dominant and the other is recessive.
B. Alleles do not show any expression and both the characters appear as such in F2 generation.
C. Factors occur in pairs in normal diploid plants.
D. The discrete unit controlling a particular character is called factor.
E. The expression of only one of the parental characters is found in a monohybrid cross.
Choose the correct answer from the options given below:
(a) A, B and C only
(b) A, C, D and E only
(c) B, C and D only
(d) A, B, C, D and E

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Ans: (b)
The correct answer is: Option B: A, C, D and E only Explanation:
Mendel's Law of Dominance states that in a heterozygote, the dominant allele will mask the expression of the recessive allele. Let's analyze the options:
A. Out of one pair of factors one is dominant and the other is recessive: This is the core principle of Mendel's Law of Dominance.
C. Factors occur in pairs in normal diploid plants: This is a fundamental concept in genetics, as diploid organisms have two copies of each chromosome, thus two copies of each gene (factors).
D. The discrete unit controlling a particular character is called factor: Mendel used the term "factor" to describe what we now know as genes.
E. The expression of only one of the parental characters is found in a monohybrid cross: This is a direct consequence of the law of dominance, where the dominant trait masks the expression of the recessive trait in the F1 generation of a monohybrid cross.

Option B is incorrect:
B. Alleles do not show any expression and both the characters appear as such in F2 generation: This statement is incorrect. While the recessive allele is not expressed in the F1 generation, it reappears in the F2 generation in a 3:1 ratio (dominant:recessive). This is due to the Law of Segregation, not the Law of Dominance.

Law of Segregation (Second Law)

• Each individual contains a pair of alleles for a trait and these alleles segregate during gamete formation so that each gamete carries only one allele for each gene.
• Thus, offspring receive one allele from each parent, restoring the pair in zygotes.
• This law explains the reappearance of recessive traits in the F2 generation and the 1:2:1 genotypic ratio of a monohybrid cross.

While Mendel’s laws explain many inheritance patterns, biological reality also shows other modes that modify or extend Mendelian expectations.

Incomplete Dominance

In incomplete dominance, the heterozygote displays a phenotype intermediate between the two homozygotes (a blend). The heterozygous phenotype is distinct and not identical to either homozygote.

Example — Snapdragons (Antirrhinum)

• Cross: RR (red) × rr (white) → F1: Rr (pink).
• Selfing F1 (Rr) → F2 phenotypic ratio: 1 red : 2 pink : 1 white.
• Genotypic ratio remains 1 RR : 2 Rr : 1 rr, but phenotype distribution differs from a simple dominant/recessive pattern.

Key Points About Dominance

• Dominance is a relationship between alleles and concerns phenotypic expression, not the physical absence of an allele.
• Not all traits show simple dominance — incomplete dominance and co‑dominance are common exceptions.
• Molecularly, dominance often depends on whether an allele produces a functional product (for example, an enzyme) and whether one functional copy is sufficient for the trait.

If one allele produces a non‑functional enzyme and the other allele produces a functional enzyme, the phenotype will often reflect the functioning allele. In such cases, the functional allele is phenotypically dominant and the non‑functional allele is recessive.

NEET PYQ from this Topic:
A pink flowered Snapdragon plant was crossed with a red flowered Snapdragon plant. What type of phenotype/s is/are expected in the progeny? (NEET 2024)
(a) Only red flowered plants
(b) Red flowered as well as pink flowered plants
(c) Only pink flowered plants
(d) Red, Pink as well as white flowered plants

View Answer  

Ans: (b)
Pink colour flower in snapdragon have genotype Rr
Red flowered snapdragon have genotype RR when they both are crossed
So the progeny that we get are red and pink flowered plants only.

In co‑dominance both alleles in a heterozygote are fully expressed and both phenotypes are visible simultaneously (no blending).

Example — ABO Blood Group System in Humans

• The ABO blood group gene (I) has three common alleles: I_A, I_B, and i.
• I_A and I_B are co‑dominant to each other and both are dominant over i.
• When an individual has genotype I_AI_B, both A and B antigens are produced, giving blood type AB.
• Because humans are diploid, any two of the three alleles can be present in an individual, producing multiple genotype–phenotype combinations.

Multiple Alleles

The ABO blood grouping is a clear example of multiple alleles at play. In this case, there are three alleles responsible for the same trait. While an individual can only possess two alleles, multiple alleles can be observed when studying a population. 

(a) Starch Synthesis in Pea Seeds

• Gene and Alleles: Starch synthesis in pea seeds is controlled by a single gene with two alleles: B (dominant) and b (recessive). 
• BB Homozygotes: Pea seeds with the BB genotype synthesize starch effectively, resulting in large starch grains. These seeds are round when mature. 
• bb Homozygotes: Seeds with the bb genotype have lower starch synthesis efficiency, leading to smaller starch grains. When mature, these seeds are wrinkled. 
• Heterozygotes (Bb): Seeds with the Bb genotype produce round seeds, but the starch grains are of intermediate size between those of BB and bb seeds. 

Starch synthesis in pea seeds is controlled by one gene with two alleles (B and b)

(b) Phenotypic Expression:

• Dominance: In terms of seed shape, the B allele appears dominant because BB and Bb seeds are round. However, when considering starch grain size, the B allele shows incomplete dominance because Bb seeds have intermediate-sized starch grains. 
• Incomplete Dominance: The concept of incomplete dominance becomes evident when focusing on starch grain size as the phenotype. In this case, the Bb genotype does not produce the same phenotype as either homozygote but an intermediate phenotype. 

(c) Conclusion:

• Relationship between Dominance and Phenotype: Dominance is not an inherent characteristic of a gene or its product. It depends on the specific phenotype being examined and how the gene product influences the expression of multiple phenotypes. 
• Example of Gene with Multiple Effects: The example of starch synthesis in pea seeds illustrates how a single gene can have multiple effects depending on the alleles present and the phenotype being considered. 

Question for Principles of Inheritance & Variation till Inheritance of One Gene

Try yourself:

What is the Law of Dominance in genetics?

• A. Factors exist in pairs, and the dominant factor is expressed over the recessive factor.
• B. Alleles blend together to form a new phenotype in the offspring.
• C. Both parental traits are expressed equally in the offspring.
• D. Factors do not exist in pairs, and all traits are expressed equally.

Explanation

- The Law of Dominance states that factors exist in pairs, and when a pair of factors is dissimilar, one factor dominates over the other.
- The dominant factor is expressed, while the recessive factor is masked, resulting in the expression of only one parental trait in the offspring.

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