NCERT Class 10 Science Chapter 9: Heredity - A Comprehensive Guide

Introduction to Heredity

Have you ever wondered why you have your mother's eyes or your father's nose? Or why siblings, born to the same parents, look similar yet are uniquely different? The answers to these fascinating questions lie in the field of genetics, and the core concept we will explore in this chapter is Heredity. This chapter, from the NCERT Class 10 Science syllabus, delves into the mechanisms of inheritance, explaining how traits are passed down from one generation to the next. We will journey through the groundbreaking experiments of Gregor Mendel, understand the laws that govern inheritance, and explore the mechanism of sex determination. We will also touch upon the broader concept of evolution to see how these principles contribute to the diversity of life on Earth.

The chapter begins by differentiating between heredity and variation. Heredity refers to the transmission of characters or traits from parents to their offspring. It is the reason why a baby elephant looks like an elephant and a mango seed grows into a mango tree. However, the offspring are never an exact copy of their parents. These differences among individuals of a species are called variation. Variation is crucial for the survival of a species, as it provides the raw material for natural selection and allows populations to adapt to changing environments. Understanding heredity and variation is fundamental to understanding biology, evolution, and even medicine.

Heredity: The Principles of Inheritance

The foundation of modern genetics was laid in the 19th century by an Austrian monk named Gregor Johann Mendel. Through his meticulous experiments on garden pea plants (Pisum sativum), he uncovered the fundamental principles of heredity long before anyone knew about genes, chromosomes, or DNA. For his monumental work, he is rightfully called the 'Father of Genetics'.

Why Mendel Chose the Pea Plant

Mendel's choice of the garden pea plant was a stroke of genius. It had several advantages that made it an ideal subject for studying inheritance:

• Well-defined characters: Pea plants showed a number of clear, easily distinguishable contrasting traits (e.g., tall vs. dwarf, round vs. wrinkled seeds).
• Bisexual flowers: The flowers had both male and female reproductive parts, making self-pollination easy.
• Easy cross-pollination: The structure of the pea flower allowed for controlled cross-pollination by transferring pollen from one plant to another.
• Short life cycle: The plants grew quickly and produced a large number of offspring in a single generation, allowing Mendel to study several generations within a relatively short time.
• Pure-line varieties: It was easy to obtain pure-breeding plants (plants that consistently produce offspring with the same trait after self-pollination).

Mendel studied seven pairs of contrasting traits in the pea plant, including stem height, flower color, seed shape, and seed color.

Mendel's Experiments and Laws of Inheritance

Mendel conducted two main types of cross-breeding experiments to understand how traits are inherited: the monohybrid cross and the dihybrid cross. These experiments led to the formulation of his three famous laws of inheritance.

Monohybrid Cross: Inheritance of One Gene

A monohybrid cross is a cross between two parents that differ in only one pair of contrasting characters. For example, Mendel crossed a pure-bred tall pea plant with a pure-bred dwarf pea plant.

The Experiment:

1. Parental Generation (P): He took a pure-bred tall plant (genotype TT) and a pure-bred dwarf plant (genotype tt).
2. First Filial Generation (F1): When he cross-pollinated these two plants, he was surprised to find that all the offspring in the F1 generation were tall. The dwarf trait seemed to have disappeared.
3. Second Filial Generation (F2): Mendel then allowed the F1 generation plants to self-pollinate. In the F2 generation, he found that both tall and dwarf plants appeared. Out of 1064 F2 plants, 787 were tall and 277 were dwarf. This was approximately a 3:1 ratio of tall to dwarf plants.

Key Genetic Terms:

• Gene: A segment of DNA that is the basic unit of heredity. Mendel called them 'factors'.
• Alleles: Different forms of the same gene. For height, the alleles are 'T' (for tallness) and 't' (for dwarfness).
• Dominant Trait: The trait that expresses itself in the F1 generation (e.g., Tallness). It is represented by a capital letter.
• Recessive Trait: The trait that is suppressed in the F1 generation but reappears in the F2 generation (e.g., Dwarfness). It is represented by a small letter.
• Genotype: The genetic makeup of an organism (e.g., TT, Tt, tt).
• Phenotype: The observable physical characteristic of an organism (e.g., tall, dwarf).
• Homozygous: Having two identical alleles for a trait (e.g., TT or tt).
• Heterozygous: Having two different alleles for a trait (e.g., Tt).

A Punnett square can be used to visualize this cross:

From the cross, we get:

• Phenotypic Ratio: 3 Tall : 1 Dwarf
• Genotypic Ratio: 1 TT (Homozygous Tall) : 2 Tt (Heterozygous Tall) : 1 tt (Homozygous Dwarf)

This experiment led Mendel to formulate his first two laws.

Law of Dominance: When parents with pure, contrasting traits are crossed, only one form of the trait appears in the next generation. The allele that expresses itself is the dominant allele, and the one that is masked is the recessive allele.

Law of Segregation: During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then 'recombined' at fertilization, producing the genotype for the traits of the offspring.

Dihybrid Cross: Inheritance of Two Genes

A dihybrid cross involves a study of inheritance of two pairs of contrasting traits. Mendel crossed a pea plant having round, yellow seeds with one having wrinkled, green seeds.

The Experiment:

1. Parental Generation (P): He crossed a pure-bred plant with round and yellow seeds (RRYY) with a pure-bred plant with wrinkled and green seeds (rryy). Here, round (R) is dominant over wrinkled (r), and yellow (Y) is dominant over green (y).
2. First Filial Generation (F1): All the plants in the F1 generation had round and yellow seeds (genotype RrYy), as expected from the law of dominance.
3. Second Filial Generation (F2): When the F1 plants were self-pollinated, the F2 generation showed four different phenotypes in a specific ratio: Round Yellow, Round Green, Wrinkled Yellow, and Wrinkled Green. The ratio was approximately 9:3:3:1.

This result showed that the traits for seed shape and seed color were inherited independently of each other. This led to Mendel's third law.

Law of Independent Assortment: This law states that the alleles of two (or more) different genes get sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene.

How are Traits Expressed? (Molecular Basis of Inheritance)

Mendel described the 'factors' that control traits, but what are these factors physically? Today, we know them as genes. Genes are specific segments of DNA (Deoxyribonucleic Acid) located on chromosomes inside the nucleus of a cell.

The process works like this:

1. Information Storage: A gene contains the information to produce a specific protein.
2. Protein Synthesis: The cell 'reads' the gene and synthesizes a protein based on that information.
3. Function: This protein then performs a specific function. For example, it could be an enzyme that catalyzes a biochemical reaction.

Let's take the example of plant height. The gene for tallness (T) produces an enzyme that promotes the synthesis of a growth hormone. This leads to efficient growth, and the plant becomes tall. The allele for dwarfness (t) is a slightly altered version of this gene. This altered gene produces a less efficient enzyme, or no enzyme at all. As a result, less growth hormone is produced, and the plant remains dwarf. In a heterozygous plant (Tt), the 'T' allele produces enough enzyme for the plant to grow tall, which is why tallness is dominant over dwarfness.

Sex Determination in Human Beings

One of the most fundamental inherited traits is an individual's sex. How is this determined? In human beings, the sex of an individual is determined genetically.

Human cells have 23 pairs of chromosomes. 22 of these pairs are called autosomes, which control the general body characteristics. The 23rd pair is the sex chromosomes, which determine the sex of the individual.

• Females have two identical sex chromosomes, designated as XX.
• Males have two different sex chromosomes, designated as XY.

During gamete formation (meiosis), a female produces eggs that all contain a single X chromosome. A male, however, produces two types of sperm in equal proportions: half of the sperm carry an X chromosome, and the other half carry a Y chromosome.

The sex of the child depends on which type of sperm fertilizes the egg:

• If a sperm carrying an X chromosome fertilizes the egg (X), the resulting zygote will have the genotype XX and will develop into a female.
• If a sperm carrying a Y chromosome fertilizes the egg (X), the resulting zygote will have the genotype XY and will develop into a male.

This can be shown with a simple cross:

Clearly, there is a 50% probability of having a male child and a 50% probability of having a female child. It is the genetic contribution from the father (the type of sperm) that determines the sex of the baby. Therefore, the outdated and unscientific practice of blaming the mother for giving birth to a girl child has no biological basis whatsoever.

A Brief Overview of Evolution

(Note: As per the latest NCERT syllabus rationalization, the detailed study of Evolution is not included for final examination purposes, but the concepts are useful for a complete understanding of biology.)

Evolution is the gradual change in the inherited characteristics of biological populations over successive generations. The variations we studied in heredity are the raw material for evolution.

An Illustration: The Beetle Population

Imagine a population of red beetles living in green bushes. Crows eat these beetles. Two scenarios can lead to evolution:

1. Natural Selection: One day, a green beetle is born due to a random color variation (mutation). Crows can easily spot and eat red beetles against the green leaves, but they often miss the green ones. The green beetles survive better, reproduce more, and pass on their green color trait to their offspring. Over generations, the beetle population shifts from being predominantly red to predominantly green. This is survival of the fittest, or natural selection.
2. Genetic Drift: Now imagine the same initial population of red beetles, but a few blue beetles also arise due to mutation. One day, an elephant walks by and accidentally stamps on the bush, killing most of the red beetles. By pure chance, most of the surviving beetles are blue. This blue population now reproduces, and their offspring are also blue. The change in the frequency of a gene variant (allele) in a population due to random chance is called genetic drift. It is more pronounced in small populations.

Acquired and Inherited Traits

It is crucial to distinguish between two types of traits:

• Inherited Traits: These are characteristics passed on from parents to offspring because they are determined by genes (e.g., eye color, hair type, blood group).
• Acquired Traits: These are characteristics developed by an individual during their lifetime due to environmental influences or their own efforts. They are not coded in the DNA of germ cells (sperm or egg). Examples include learning to ride a bicycle, building muscle through exercise, or having a scar from an injury.

Acquired traits cannot be inherited because they do not cause any change in the DNA of the reproductive cells. August Weismann proved this by cutting the tails of mice for over 20 generations; each new generation was still born with a full tail.

Speciation

Speciation is the evolutionary process by which new biological species arise. It occurs when a population becomes reproductively isolated from other populations of the same species. Key factors include:

• Geographical Isolation: A population gets split into two or more groups by a physical barrier like a river, mountain range, or ocean.
• Genetic Drift & Natural Selection: The isolated populations experience different random changes (genetic drift) and face different environmental pressures (natural selection).
• Reproductive Isolation: Over many generations, the separated groups become so genetically different that even if the barrier is removed, they can no longer interbreed to produce fertile offspring. At this point, they are considered separate species.

Evolution and Classification

Tracing Evolutionary Relationships

Scientists classify organisms based on shared characteristics. The more characteristics two species have in common, the more closely they are related, suggesting they shared a more recent common ancestor. We can trace these relationships using evidence from:

Homologous Organs

These are organs that have a similar basic structure and origin but have been modified to perform different functions in different organisms. For example, the forelimbs of a human, a cheetah, a whale, and a bat all share a similar bone structure (humerus, radius, ulna, carpals, etc.). However, they are used for grasping, running, swimming, and flying, respectively. Homologous organs point towards a common ancestor.

Analogous Organs

These are organs that have different structures and origins but perform a similar function. For example, the wings of a bird and the wings of an insect are both used for flying. However, a bird's wing is a modified forelimb with bones, flesh, and feathers, while an insect's wing is a thin membrane of chitin. Analogous organs do not suggest a common ancestry but rather convergent evolution, where different organisms independently evolve similar traits as a result of having to adapt to similar environments.

Fossils: Evidence of Evolution

Fossils are the preserved remains or impressions of organisms that lived in the distant past. They provide direct evidence of evolution.

• How they form: When an organism dies, its soft parts usually decompose. The hard parts (bones, shells, teeth) may be buried in sediment. Over millions of years, the sediment hardens into rock, preserving the remains.
• What they tell us: Fossils show us what ancient organisms looked like. By arranging fossils in chronological order, we can trace the evolutionary history of a species. For example, fossils like Archaeopteryx, which had features of both reptiles (teeth, long tail) and birds (feathers, wings), serve as a 'missing link' showing that birds evolved from reptiles.
• Dating Fossils: Scientists can determine the age of fossils using two main methods: Relative dating (fossils found in deeper rock layers are older than those in upper layers) and Absolute dating (using the decay rate of radioactive isotopes, like Carbon-14, found in the fossil).

Important Questions and Answers

Question 1: In a monohybrid cross between a tall pea plant (TT) and a short pea plant (tt), what will be the phenotype and genotype of the F1 and F2 generations?

Answer:

• Parental Cross: Tall (TT) x Short (tt)
• F1 Generation: All offspring will have the genotype Tt. Since 'T' (tall) is dominant over 't' (short), the phenotype of all F1 plants will be Tall.
• F2 Generation (from self-crossing F1, i.e., Tt x Tt):
• Genotypic Ratio: 1 (TT) : 2 (Tt) : 1 (tt). This means 25% are homozygous tall, 50% are heterozygous tall, and 25% are homozygous short.
• Phenotypic Ratio: 3 (Tall) : 1 (Short). This means 75% of the plants will be tall (both TT and Tt genotypes result in a tall phenotype) and 25% will be short (tt).

Question 2: How is the sex of a child determined in human beings?

Answer: The sex of a child in humans is determined by the sex chromosomes inherited from the parents. Females have two X chromosomes (XX) and produce eggs that all contain one X chromosome. Males have one X and one Y chromosome (XY) and produce two types of sperm: 50% carry an X chromosome and 50% carry a Y chromosome. If a sperm with an X chromosome fertilizes the egg, the child will be a female (XX). If a sperm with a Y chromosome fertilizes the egg, the child will be a male (XY). Therefore, it is the father's genetic contribution that determines the sex of the child.

Question 3: Differentiate between homologous organs and analogous organs with examples.

Answer:

Question 4: Why are traits acquired during the life-time of an individual not inherited?

Answer: Traits acquired during a lifetime, such as building muscle, learning a skill, or getting a scar, are not inherited because they do not cause any change in the genetic material (DNA) of the germ cells (sperm and eggs). Inheritance is based on the passing of genes from parents to offspring. Since acquired traits do not alter the genes in the reproductive cells, they cannot be passed on to the next generation.

Chapter Summary

Here are the key takeaways from our exploration of Heredity:

• Heredity is the passing of genetic traits from parents to offspring.
• Variation refers to the differences among individuals of a species, which is crucial for evolution.
• Gregor Mendel, the 'Father of Genetics', formulated the basic laws of inheritance through his experiments on pea plants.
• Mendel's Laws:
  • Law of Dominance: One factor (allele) in a pair can mask the effect of the other.
  • Law of Segregation: Alleles for a trait separate during gamete formation.
  • Law of Independent Assortment: Alleles for different traits are inherited independently of each other.
• Genes, which are segments of DNA, control traits by providing instructions for making proteins.
• In humans, sex is determined by the X and Y chromosomes. Females are XX and males are XY. The father's sperm determines the sex of the child.
• Evolution is the gradual change in the inherited traits of a population over generations, driven by mechanisms like natural selection and genetic drift.
• Evidence for evolution comes from the study of homologous organs, analogous organs, and fossils.