Mendel's Laws of Inheritance

Mendel's Laws of Inheritance

Inheritance is the transmission of genetic information and characters from parents to their offspring. The study of inheritance is called genetics.

Gregor Johann Mendel, an Austrian monk and scientist, is known as the Father of Genetics. He performed systematic experiments on garden pea (Pisum sativum) and proposed the basic laws of inheritance.

1. Why Did Mendel Choose Pea Plant?

Mendel selected the garden pea (Pisum sativum) for his experiments because it had several useful characteristics.

• It has a number of clear and easily distinguishable contrasting characters.
• It has a relatively short generation time.
• It produces many offspring in a single generation.
• It is normally self-pollinating.
• Artificial cross-pollination can be easily performed.
• Pure-breeding varieties were available.

Self-pollination की वजह से pure lines maintain करना आसान था, जबकि artificial cross-pollination द्वारा Mendel अपने desired parents को cross कर सकते थे।

2. Mendel's Seven Pairs of Contrasting Characters

Basic Genetic Terms

3. Character and Trait

A character is a heritable feature, such as plant height.

A trait is a specific form of a character, such as tall or dwarf.

Example: Plant height is a character, while tallness and dwarfness are contrasting traits.

4. Gene

A gene is the functional unit of heredity. It is a segment of DNA that influences a particular characteristic or biological function.

5. Alleles

Alleles are alternative forms of a gene that occur at the same locus on homologous chromosomes.

For example, T and t can represent the alleles for tall and dwarf conditions in pea plants.

6. Dominant and Recessive Traits

A dominant trait is expressed in the phenotype even when only one copy of the corresponding dominant allele is present.

A recessive trait is expressed only when the individual has two copies of the recessive allele in a simple Mendelian case.

7. Genotype and Phenotype

Genotype refers to the genetic constitution of an organism.

Phenotype refers to the observable characteristics of an organism.

8. Homozygous and Heterozygous

An organism is homozygous when both alleles for a gene are identical.

• TT = Homozygous dominant
• tt = Homozygous recessive

An organism is heterozygous when the two alleles are different.

• Tt = Heterozygous

Monohybrid Cross

9. What is a Monohybrid Cross?

A monohybrid cross is a cross between two individuals differing in a single pair of contrasting characters.

Mendel performed a monohybrid cross using pea plants differing in height.

Pure Tall Plant × Pure Dwarf Plant

TT × tt

10. Parental Generation (P Generation)

Let:

• T = allele for tallness
• t = allele for dwarfness

The parental cross is:

TT × tt

11. First Filial Generation (F₁)

When TT is crossed with tt:

TT × tt → All Tt

All F₁ offspring are Tt and show the tall phenotype.

The dwarf trait is not expressed in F₁, but its allele is still present in the genotype. यानी recessive allele disappear नहीं हुआ; वह heterozygous condition में मौजूद है।

12. F₁ Self-Cross

Mendel allowed F₁ plants to self-pollinate.

Tt × Tt

The possible gametes from each parent are:

T and t

13. Punnett Square

14. F₂ Generation

The F₂ offspring are:

• TT = Tall
• Tt = Tall
• Tt = Tall
• tt = Dwarf

Therefore:

Genotypic Ratio = 1 TT : 2 Tt : 1 tt

Phenotypic Ratio = 3 Tall : 1 Dwarf

Law of Dominance

15. Statement of Law of Dominance

The Law of Dominance states that when two contrasting alleles are present together in a heterozygote, only one allele expresses itself phenotypically. This allele is called the dominant allele, while the other is called the recessive allele.

For example, in Tt, T expresses tallness while t remains unexpressed phenotypically.

Law of Segregation

16. Statement of Law of Segregation

The Law of Segregation states that the two alleles of a gene separate from each other during gamete formation, so that each gamete receives only one allele.

This law is also called the Law of Purity of Gametes.

For example, a heterozygous plant with genotype Tt produces two types of gametes:

Tt → T + t

Thus, the two alleles remain separate during gamete formation and come together again at fertilisation.

Dihybrid Cross

17. What is a Dihybrid Cross?

A dihybrid cross is a cross between two individuals differing in two pairs of contrasting characters.

Mendel studied seed shape and seed colour together.

For example:

• R = Round seed
• r = Wrinkled seed
• Y = Yellow seed
• y = Green seed

18. Parental Cross in Dihybrid Experiment

Pure round yellow plants are crossed with pure wrinkled green plants.

RRYY × rryy

The gametes are:

RY × ry

Therefore, all F₁ offspring are:

RrYy

All F₁ plants show round yellow seeds.

19. Gametes Produced by F₁

The F₁ genotype is:

RrYy

During gamete formation, the alleles segregate and assort independently. The possible gametes are:

RY, Ry, rY, ry

20. F₁ Self-Cross

RrYy × RrYy

Each parent produces four types of gametes:

RY, Ry, rY, ry

21. F₂ Dihybrid Ratio

The classical F₂ phenotypic ratio obtained from a Mendelian dihybrid cross is:

9 : 3 : 3 : 1

Law of Independent Assortment

22. Statement of Law of Independent Assortment

The Law of Independent Assortment states that the alleles of different genes assort independently during gamete formation, provided the genes are not linked.

In a simple Mendelian dihybrid cross, the segregation of alleles for one character is independent of the segregation of alleles for another character.

For example, in RrYy, the R/r pair and Y/y pair can combine independently to produce:

RY, Ry, rY, ry

23. Monohybrid vs Dihybrid Cross

Test Cross

24. What is a Test Cross?

A test cross is a cross between an individual showing a dominant phenotype and a homozygous recessive individual to determine the genotype of the dominant individual.

For example, if a tall plant has an unknown genotype:

TT or Tt

It is crossed with a homozygous recessive dwarf plant:

tt

Case 1: TT × tt

All offspring are Tt and tall.

Case 2: Tt × tt

Offspring are:

1 Tt : 1 tt

Therefore, the phenotypic ratio is:

1 Tall : 1 Dwarf

Back Cross

25. What is a Back Cross?

A back cross is a cross between an F₁ hybrid and either of its parents.

Therefore:

F₁ × Parent = Back Cross

A test cross is a specific type of back cross when the F₁ or dominant phenotype individual is crossed with the homozygous recessive parent.

Chromosomal Basis of Mendel's Laws

26. Chromosomal Basis of Inheritance

Mendel's factors can be related to genes located on chromosomes.

• Genes occur at specific positions called loci on chromosomes.
• Alleles of a gene are located at corresponding loci on homologous chromosomes.
• Separation of homologous chromosomes during meiosis explains the Law of Segregation.
• Independent behaviour of different chromosome pairs during meiosis provides the basis for Independent Assortment.

इस तरह Mendel के laws को meiosis के chromosome behaviour से समझा जा सकता है।

Important Ratios

Probability in Mendelian Genetics

27. Basic Probability

Probability is the mathematical likelihood of occurrence of an event.

In genetics, probability helps predict the possible outcomes of genetic crosses.

For a heterozygous cross:

Tt × Tt

The probability of obtaining:

• TT = 1/4
• Tt = 1/2
• tt = 1/4

Therefore, the phenotypic probability is:

• Tall = 3/4
• Dwarf = 1/4

Common Mistakes to Avoid

• Do not confuse genotype with phenotype.
• Do not write 3 : 1 as the genotypic ratio of a monohybrid cross. It is the phenotypic ratio.
• Do not write 1 : 2 : 1 as the phenotypic ratio in a complete-dominance monohybrid cross.
• Do not confuse the Law of Dominance with the Law of Segregation.
• Do not say that a recessive allele disappears in F₁.
• Do not confuse a test cross with every type of back cross.
• Independent assortment applies to genes that assort independently; linked genes do not follow the classical independent assortment pattern.

Board-Focused Important Questions

Very Short Answer Questions

1. Who is known as the Father of Genetics?
2. What is a gene?
3. What are alleles?
4. Define genotype.
5. Define phenotype.
6. What is a homozygous condition?
7. What is a heterozygous condition?
8. What is a monohybrid cross?
9. What is a dihybrid cross?
10. State the Law of Dominance.
11. State the Law of Segregation.
12. What is the Law of Purity of Gametes?
13. State the Law of Independent Assortment.
14. What is a test cross?
15. What is a back cross?
16. What is the phenotypic ratio of a typical monohybrid cross?
17. What is the genotypic ratio of a typical monohybrid cross?
18. What is the phenotypic ratio of a typical dihybrid cross?

Short Answer Questions

1. Why did Mendel select pea plant for his experiments?
2. Explain the difference between genotype and phenotype.
3. Differentiate between homozygous and heterozygous conditions.
4. Explain a monohybrid cross with a suitable example.
5. Explain the Law of Dominance with an example.
6. Explain the Law of Segregation.
7. Explain the Law of Independent Assortment.
8. Differentiate between test cross and back cross.
9. Explain the genotypic and phenotypic ratios of a monohybrid cross.
10. Explain the F₂ phenotypic ratio of a dihybrid cross.

Long Answer Questions

1. Describe Mendel's monohybrid experiment and explain the Law of Dominance and Law of Segregation.
2. Explain Mendel's dihybrid cross and derive the 9 : 3 : 3 : 1 phenotypic ratio.
3. Describe Mendel's three laws of inheritance with suitable examples.
4. Explain the chromosomal basis of Mendel's laws of inheritance.
5. Explain test cross and back cross with suitable genetic diagrams.

Multiple Choice Questions

1. Gregor Mendel performed his famous experiments on:

   • A. Maize
   • B. Garden pea
   • C. Drosophila
   • D. Wheat

   Answer: B. Garden pea

2. Mendel is known as the:

   • A. Father of Evolution
   • B. Father of Genetics
   • C. Father of Cytology
   • D. Father of Ecology

   Answer: B. Father of Genetics

3. The alternative forms of a gene are called:

   • A. Chromosomes
   • B. Alleles
   • C. Gametes
   • D. Loci

   Answer: B. Alleles

4. The genotype of a homozygous dominant plant is:

   • A. Tt
   • B. tt
   • C. TT
   • D. T

   Answer: C. TT

5. The phenotypic ratio of a typical monohybrid F₂ generation is:

   • A. 1 : 1
   • B. 1 : 2 : 1
   • C. 3 : 1
   • D. 9 : 3 : 3 : 1

   Answer: C. 3 : 1

6. The genotypic ratio of a typical monohybrid cross is:

   • A. 3 : 1
   • B. 1 : 2 : 1
   • C. 9 : 3 : 3 : 1
   • D. 1 : 1

   Answer: B. 1 : 2 : 1

7. The 9 : 3 : 3 : 1 ratio is obtained in:

   • A. Monohybrid cross
   • B. Dihybrid cross
   • C. Test cross only
   • D. Back cross only

   Answer: B. Dihybrid cross

8. The Law of Segregation is also called:

   • A. Law of Dominance
   • B. Law of Purity of Gametes
   • C. Law of Linkage
   • D. Law of Variation

   Answer: B. Law of Purity of Gametes

9. A cross between an individual of unknown dominant genotype and a homozygous recessive individual is called:

   • A. Monohybrid cross
   • B. Dihybrid cross
   • C. Test cross
   • D. Reciprocal cross

   Answer: C. Test cross

10. The gametes produced by RrYy are:

    • A. Rr and Yy
    • B. RY, Ry, rY, ry
    • C. RR and yy
    • D. RY and ry only

    Answer: B. RY, Ry, rY, ry

11. In a Tt × Tt cross, the probability of tt is:

    • A. 1/2
    • B. 1/4
    • C. 3/4
    • D. 1

    Answer: B. 1/4

12. Which law explains the separation of two alleles during gamete formation?

    • A. Law of Dominance
    • B. Law of Segregation
    • C. Law of Independent Assortment
    • D. Law of Linkage

    Answer: B. Law of Segregation

13. Which law explains independent distribution of alleles of different genes?

    • A. Law of Dominance
    • B. Law of Segregation
    • C. Law of Independent Assortment
    • D. Law of Mutation

    Answer: C. Law of Independent Assortment

14. In a heterozygous genotype Tt, which allele is expressed in a complete-dominance condition?

    • A. t only
    • B. T only
    • C. Both equally
    • D. Neither

    Answer: B. T only

15. A cross between F₁ hybrid and either parent is called:

    • A. Back cross
    • B. Mutation
    • C. Dihybrid cross
    • D. Reciprocal cross

    Answer: A. Back cross

Fill in the Blanks

1. Gregor Mendel is known as the Father of __________.
2. Mendel performed experiments on __________ plant.
3. Alternative forms of a gene are called __________.
4. The genetic constitution of an organism is called its __________.
5. The observable characteristics of an organism constitute its __________.
6. An organism having two identical alleles is called __________.
7. An organism having two different alleles is called __________.
8. The phenotypic ratio of a typical monohybrid F₂ generation is __________.
9. The phenotypic ratio of a typical dihybrid F₂ generation is __________.
10. The Law of Segregation is also called the Law of __________.

Answers: Genetics, Garden pea, Alleles, Genotype, Phenotype, Homozygous, Heterozygous, 3 : 1, 9 : 3 : 3 : 1, Purity of Gametes.

Match the Following

Answers: 1-c, 2-a, 3-b, 4-d, 5-e

Quick Revision

• Gregor Mendel = Father of Genetics.
• Experimental plant = Garden pea (Pisum sativum).
• Gene = Functional unit of heredity.
• Alleles = Alternative forms of a gene.
• Genotype = Genetic constitution.
• Phenotype = Observable character.
• Homozygous = Identical alleles.
• Heterozygous = Different alleles.
• Monohybrid F₂ phenotypic ratio = 3 : 1.
• Monohybrid F₂ genotypic ratio = 1 : 2 : 1.
• Dihybrid F₂ phenotypic ratio = 9 : 3 : 3 : 1.
• Law of Dominance = Dominant allele expresses in heterozygote under complete dominance.
• Law of Segregation = Alleles separate during gamete formation.
• Law of Independent Assortment = Alleles of different genes assort independently when genes are not linked.
• Test cross = Cross with homozygous recessive individual.
• Back cross = F₁ crossed with either parent.