Heredity (Class 10): Notes और MCQ Practice

आसान भाषा में

Parents से बच्चों में traits (लक्षणों) का transfer होना ही Heredity कहलाता है। Reproduction के दौरान DNA copying में हल्के variations आते हैं। Gregor Mendel (Father of Genetics) ने Garden Pea (Pisum sativum) पर experiments करके समझाया कि traits कैसे inherit और express होते हैं।

Key Concepts & Ratios

• Monohybrid Cross (Single Trait - Height):
• F1 Generation: All Tall (Tt)
• F2 Phenotypic Ratio = 3 Tall : 1 Dwarf (3:1)
• F2 Genotypic Ratio = 1 TT : 2 Tt : 1 tt (1:2:1)
• Dihybrid Cross (Two Traits - Seed Shape & Colour):
• F2 Phenotypic Ratio = 9:3:3:1 (Round-Yellow : Round-Green : Wrinkled-Yellow : Wrinkled-Green)
• Shows independent assortment of traits.
• Dominant vs Recessive: Dominant trait (e.g., 'T') single copy में भी खुद को express करता है; Recessive trait (e.g., 't') केवल homozygous state ('tt') में express होता है।
• Sex Determination:
• Female = 44 + XX, Male = 44 + XY
• Ovum हमेशा 'X' देता है। अगर Sperm 'X' दे तो Girl (XX), और अगर 'Y' दे तो Boy (XY)।

💡 2 Memory Tricks

1. P vs G Ratio: Phenotype = Physical look (दिखने में कैसा है → 3:1)। Genotype = Genetic formula (जीन्स क्या हैं → 1:2:1)।
2. Sex Determination: याद रखें Y for Youthful Boy — यानी sperm का 'Y' ही बच्चे को boy बनाता है!

⚠️ 2 Common Board Mistakes

1. Ratios का Confusion: Exam में 'Write genotypic ratio' पूछा जाता है और बच्चे 3:1 लिख आते हैं। 3:1 Phenotype है, 1:2:1 Genotype!
2. Blaming the Mother: कभी मत लिखना कि mother sex decide करती है। हमेशा स्पष्ट लिखें: "Mother produces only X gametes; sex of the child is strictly determined by the father's sperm (X or Y)."

Heredity - Class 10 NCERT Study Notes

1. Chapter in Short

This chapter explains how characteristics (traits) are passed down from parents to their children (offspring). It covers:

• How variations arise during reproduction and why they are useful.
• How Gregor Johann Mendel discovered the basic rules of inheritance using garden pea plants.
• The difference between dominant and recessive traits.
• How genes and cellular DNA make proteins that control physical traits.
• How the biological sex of a human baby is determined at fertilization.

(Note: As per the updated NCERT syllabus, the topic of Evolution has been removed from this chapter).

2. Key Concepts Explained Simply

A. Heredity and Variation

• Heredity is the transmission of physical and biological traits from parents to offspring.
• Variation means the differences shown by individuals of the same species.
• Everyday Example: Look at your family or classmates. Even though everyone has two eyes, a nose, and hair, everyone looks slightly different. Some people have free earlobes, while others have attached earlobes. This is an inherited variation.
• Why are variations useful?

If a bacterial colony lives in normal temperature water and the water suddenly heats up due to global warming, most bacteria will die. But a few bacteria with a variation that makes them resistant to heat will survive and reproduce.

B. Mendel's Experiments with Garden Peas

Gregor Mendel is called the "Father of Genetics". He chose the garden pea (Pisum sativum) because it has clear, contrasting features (like tall/short plants, round/wrinkled seeds) and is easy to grow.

1. Monohybrid Cross (Studying One Trait at a Time)

• Mendel crossed a pure tall pea plant (TT) with a pure short/dwarf pea plant (tt).
• First Generation (F1): All plants were Tall (Tt). The short trait seemed to disappear.
• Second Generation (F2): When F1 plants were self-pollinated (Tt × Tt):
• 3 out of 4 plants were Tall.
• 1 out of 4 plants was Short.
• Conclusion:
• Dominant Trait: The trait that expresses itself even when only one copy is present (represented by a capital letter, 'T').
• Recessive Trait: The trait that expresses itself only when both copies are identical (represented by a small letter, 't').

2. Dihybrid Cross (Studying Two Traits Together)

• Mendel crossed a plant having Round and Yellow seeds (RRYY) with a plant having Wrinkled and Green seeds (rryy).
• F1 Generation: All seeds were Round and Yellow (RrYy).
• F2 Generation (Self-pollination): Mendel observed four different combinations of seeds:

1. Round, Yellow (parental type) = 9
2. Round, Green (new combination) = 3
3. Wrinkled, Yellow (new combination) = 3
4. Wrinkled, Green (parental type) = 1

• Phenotypic Ratio: 9 : 3 : 3 : 1
• Conclusion: Traits are inherited independently of each other. The gene for seed shape is inherited independently from the gene for seed colour.

C. How Do Traits Get Expressed?

• Cellular DNA is the information source for making proteins in our cells.
• A specific section of DNA that provides information for one protein is called a gene.
• Example: Plant height depends on the amount of a particular plant growth hormone. The amount of hormone depends on the efficiency of an enzyme (which is a protein). If the gene works efficiently, plenty of hormone is produced, making the plant tall. If the gene has an alteration that makes the enzyme less efficient, less hormone is made, and the plant remains short.

D. Sex Determination in Humans

• Human body cells have 23 pairs of chromosomes (46 chromosomes in total).
• 22 pairs are called autosomes (same in both males and females).
• 1 pair is called sex chromosomes:
• Females have two matching sex chromosomes: XX.
• Males have two mismatched sex chromosomes: XY (X is normal-sized, Y is shorter).
• How is sex determined?
• All egg cells produced by the mother carry an X chromosome.
• Sperm cells produced by the father carry either an X chromosome (50%) or a Y chromosome (50%).
• If an X-carrying sperm fertilizes the egg: XX = Female child (Girl).
• If a Y-carrying sperm fertilizes the egg: XY = Male child (Boy).
• Key Fact: The father determines the sex of the child, not the mother.

3. Important Definitions, Facts, and Ratios

• Heredity: The transmission of characters from parents to offspring.
• Gene: The basic unit of heredity present on chromosomes, made up of DNA.
• Allele: An alternate form of a gene (e.g., 'T' and 't' are alleles for plant height).
• Genotype: The genetic makeup of an organism (e.g., TT, Tt, or tt).
• Phenotype: The observable physical appearance of an organism (e.g., Tall or Short).
• Dominant Trait: A trait that shows its effect even in the presence of a contrasting trait (e.g., 'T' in Tt).
• Recessive Trait: A trait that shows its effect only when both copies are identical (e.g., 'tt').
• Monohybrid F2 Phenotypic Ratio: 3 : 1 (3 Tall : 1 Short)
• Monohybrid F2 Genotypic Ratio: 1 : 2 : 1 (1 TT : 2 Tt : 1 tt)
• Dihybrid F2 Phenotypic Ratio: 9 : 3 : 3 : 1
• Human Chromosome Count: 46 total = 22 pairs of autosomes + 1 pair of sex chromosomes.

4. Solved Board-Style Questions

Question 1

A cross is made between a pure tall pea plant (TT) and a pure short pea plant (tt). What will be the percentage of tall plants in the F1 and F2 generations?

Answer:

1. F1 Generation:

• Parents: TT × tt
• Gametes: T from one parent, t from the other
• Offspring: All Tt (heterozygous tall)
• Percentage of tall plants in F1 = 100%

1. F2 Generation (Selfing of F1: Tt × Tt):

• Possible combinations: TT, Tt, Tt, tt
• Phenotypes: 3 Tall (TT, Tt, Tt) and 1 Short (tt)
• Percentage of tall plants = (3 ÷ 4) × 100 = 75%

Question 2

"A child inherits traits from both parents, yet the amount of DNA does not double in every generation." Explain how this is achieved.

Answer:

• Gametes (germ cells or sperms and eggs) are formed by a special type of cell division (meiosis) which cuts the chromosome number to half.
• Therefore, a gamete contains only one set of chromosomes (haploid).
• When the male gamete (with 23 chromosomes) fuses with the female gamete (with 23 chromosomes) during fertilization, the resulting zygote gets the original number restored (46 chromosomes or 23 pairs).
• This maintains a constant amount of DNA from generation to generation.

Question 3

In a family, three daughters were born continuously. The mother was blamed by her relatives. As a biology student, explain why this blame is scientifically incorrect.

Answer:

• Women have XX sex chromosomes and produce only one type of gamete (eggs), each carrying an X chromosome.
• Men have XY sex chromosomes and produce two types of sperms: 50% carrying X and 50% carrying Y.
• If a sperm carrying an X chromosome fertilizes the egg, the baby will be a girl (XX).
• If a sperm carrying a Y chromosome fertilizes the egg, the baby will be a boy (XY).
• Thus, the sex of the child is decided solely by which type of sperm from the father fertilizes the egg. Blaming the mother is scientifically incorrect.

Question 4

What are the new combinations of traits observed in the F2 generation of a dihybrid cross between a plant with round yellow seeds and a plant with wrinkled green seeds?

Answer:

• The original parental types are:

1. Round, Yellow
2. Wrinkled, Green

• The two new combinations (recombinant phenotypes) observed in the F2 generation are:

1. Round, Green seeds
2. Wrinkled, Yellow seeds

• This occurs because the alleles for seed shape and seed colour assort independently during gamete formation.

5. Common Mistakes and Quick Revision Points

Common Mistakes to Avoid:

• Confusing Genotype and Phenotype: Phenotype is what you see (e.g., Tall), while genotype is the genetic code (e.g., TT or Tt).
• Mixing up F2 Ratios: Remember that 3 : 1 is the phenotypic ratio of a monohybrid cross, whereas 1 : 2 : 1 is the genotypic ratio.
• Capital vs Lowercase letters: Always use the same letter family for a trait. For tall and dwarf, use T and t (do not use 'T' for tall and 'd' for dwarf).
• Misunderstanding Sex Chromosomes: Do not write that mothers carry Y chromosomes. Mothers are always XX; fathers are XY.

Quick Revision Points:

• Monohybrid Cross: Studies 1 trait → F2 phenotype = 3 : 1; F2 genotype = 1 : 2 : 1.
• Dihybrid Cross: Studies 2 traits → F2 phenotype = 9 : 3 : 3 : 1.
• Probability of having a boy or girl: Exactly 50% (1 : 1) for every pregnancy.
• Gene function: Gene → mRNA → Protein/Enzyme → Trait.