Chapter 8 heredity- Science class 10th

Chapter 8: Heredity (Class 10 Science)

Introduction

Heredity is the process by which traits and characteristics are passed from one generation to the next through genes. It explains why children resemble their parents and how variations occur among individuals. The study of heredity helps us understand genetics, including how dominant and recessive traits work. Gregor Mendel, the father of genetics, conducted experiments on pea plants that led to the discovery of fundamental laws of inheritance. This chapter explores concepts like phenotype, genotype, sex determination, and genetic crosses such as monohybrid and dihybrid crosses.

Key Terms of the Chapter

1. Heredity – The transmission of genetic characteristics from parents to offspring.
2. Genes – The basic unit of heredity present on chromosomes that control traits.
3. Alleles – Different forms of a gene (dominant or recessive).
4. Dominant Trait – A trait that expresses itself even when present with a recessive allele.
5. Recessive Trait – A trait that appears only when two copies of the recessive allele are present.
6. Genotype – The genetic makeup of an organism (e.g., TT, Tt, or tt).
7. Phenotype – The observable characteristics of an organism.
8. Homozygous – An organism with two identical alleles (TT or tt).
9. Heterozygous – An organism with different alleles for a trait (Tt).
10. Monohybrid Cross – A genetic cross involving one trait.
11. Dihybrid Cross – A genetic cross involving two traits.
12. Mendel’s Laws – Laws that describe inheritance patterns (Law of Dominance, Law of Segregation, and Law of Independent Assortment).
13. Sex Chromosomes – Chromosomes (X and Y) that determine the sex of an organism.
14. Mutation – A sudden change in the genetic material of an organism.
15. Variation – Differences in characteristics among individuals of the same species.

Mendel’s Laws of Inheritance

Gregor Mendel conducted experiments on pea plants and formulated three fundamental laws of inheritance:

1. Law of Dominance:

1. • When two different alleles of a trait are present, only the dominant trait expresses itself.
   • Example: In pea plants, tallness (T) is dominant over dwarfness (t). A plant with genotype Tt will be tall.

2. Law of Segregation:

1. • Each individual has two alleles for a trait, and these alleles separate during gamete formation.
   • Each gamete receives only one allele.

3. Law of Independent Assortment:

1. • Alleles of different genes are inherited independently of one another.
   • Example: The inheritance of seed color does not affect seed shape in pea plants.

Table: Mendel’s Experiment on Pea Plants

Phenotype and Genotype

• Genotype: It refers to the genetic composition of an organism. It represents the combination of alleles inherited from parents.
  • Example: TT (homozygous dominant), Tt (heterozygous), tt (homozygous recessive).
• Phenotype: It refers to the observable characteristics or traits of an organism.
  • Example: A plant with TT or Tt genotype will be tall, whereas a plant with tt genotype will be short.

Example Table: Genotype vs. Phenotype in Pea Plants

Monohybrid Cross

A monohybrid cross is a genetic cross between two individuals involving one trait.

Example: Cross Between Tall (TT) and Dwarf (tt) Pea Plants

Step 1: Parent Generation (P)

• Tall plant (TT) × Dwarf plant (tt)

Step 2: Gametes Formation

• Tall parent (T) and Dwarf parent (t)

Step 3: F1 Generation (First Filial Generation)

• Offspring: Tt (All tall)

Step 4: Selfing of F1 Generation (Tt × Tt)

• F2 Generation (Second Filial Generation):
  • TT (Tall)
  • Tt (Tall)
  • tt (Dwarf)
  • Ratio: 3 Tall : 1 Dwarf

Punnett Square for Monohybrid Cross

Phenotypic Ratio: 3 Tall : 1 Dwarf
Genotypic Ratio: 1 TT : 2 Tt : 1 tt

Dihybrid Cross

A dihybrid cross studies the inheritance of two traits simultaneously.

Example: Cross Between Pea Plants with Yellow Round Seeds (YYRR) and Green Wrinkled Seeds (yyrr)

Step 1: Parent Generation (P)

• Yellow Round (YYRR) × Green Wrinkled (yyrr)

Step 2: Gametes Formation

• Parent 1: YR, Parent 2: yr

Step 3: F1 Generation

• All plants: YyRr (Yellow Round)

Step 4: Selfing of F1 Generation (YyRr × YyRr)

• F2 Generation: Phenotypic Ratio: 9:3:3:1
  • 9 Yellow Round
  • 3 Yellow Wrinkled
  • 3 Green Round
  • 1 Green Wrinkled

Punnett Square for Dihybrid Cross

Sex Determination in Humans

Sex determination is the process by which the biological sex of an organism is established.

• Humans have 23 pairs of chromosomes (46 in total).
• 22 pairs are autosomes, and one pair (XX or XY) is the sex chromosome.
• Females have XX chromosomes.
• Males have XY chromosomes.
• The sex of a child is determined by the sperm:
  • If the sperm carries an X chromosome, the child will be female (XX).
  • If the sperm carries a Y chromosome, the child will be male (XY).

Table: Sex Determination in Humans

Additional Concepts in Heredity

1. Variations in Inheritance

Variations refer to the differences in traits among individuals of a species. These variations arise due to:

• Genetic factors (crossing over during meiosis, mutations).
• Environmental factors (nutrition, climate).
  Variations are essential for evolution as they provide diversity, allowing species to adapt to changing environments.

2. Importance of Heredity and Variation

• Helps in Evolution: Beneficial traits accumulate over generations, leading to adaptation.
• Ensures Diversity: Genetic differences lead to biodiversity.
• Survival of Species: Variations can provide advantages in changing environments.

3. Mutation: A Source of Variation

A mutation is a sudden change in the DNA sequence that can result in new traits.

• Beneficial Mutation: Helps in adaptation (e.g., antibiotic resistance in bacteria).
• Harmful Mutation: Causes genetic disorders (e.g., sickle cell anemia).

4. Sex-linked Inheritance

Some traits are inherited through the sex chromosomes (X and Y).

• X-linked Traits: Hemophilia, color blindness (more common in males).
• Y-linked Traits: Traits passed only from father to son (e.g., hairy ears).

Table: Common Sex-linked Disorders

5. Evolution and Heredity

Heredity and variations play a major role in evolution.

• Natural Selection: Charles Darwin’s theory states that favorable traits survive while unfavorable ones disappear.
• Genetic Drift: A random change in gene frequency.

6. Heredity in Humans

Some inherited traits in humans include:

• Earlobe Attachment: Free or attached.
• Tongue Rolling: Some can roll their tongue; others cannot.
• Dimple Formation: Presence or absence of dimples is inherited.

Conclusion

Heredity and genetics explain how traits are passed from parents to offspring. Mendel’s experiments provide the basis for modern genetics, and concepts like sex determination and genetic variations help us understand the complexity of inheritance. These principles are essential in fields like medicine, agriculture, and evolutionary biology.