1.0 Genetics: Some Basic Fundamentals
Genetics is the branch of biology that deals with Heredity (the transmission of characters from parents to offspring) and Variation (the differences between individuals of the same species). In this chapter, we explore why children look like their parents yet remain unique individuals.
Heredity: The process of passing on physical or mental characteristics genetically from one generation to another.
Common Genetic Terms
To master Genetics, you must speak the language. Here are the core terms used in ICSE examinations:
Homozygous vs. Heterozygous
- Homozygous (Pure): Having identical alleles for a particular trait (e.g., TT or tt).
- Heterozygous (Hybrid): Having different alleles for a particular trait (e.g., Tt).
Gregor Johann Mendel is known as the "Father of Genetics." He conducted his experiments on garden pea plants (Pisum sativum) for seven long years!
Always write the Dominant allele with a Capital letter (T) and the Recessive allele with the same letter in Small case (t). Never mix letters like T and d for Tall and Dwarf!
2.0 Why Mendel Chose the Garden Pea?
Mendel's success was largely due to his careful choice of the Garden Pea (Pisum sativum). This plant provided several advantages that allowed him to observe clear, mathematical patterns in inheritance.
- Short Life Cycle: The plant grows quickly, allowing the study of many generations in a short time.
- Contrasting Traits: It showed clear "either-or" characters (e.g., either Purple or White flowers, no pink).
- Self-Pollination: Naturally self-pollinating, making it easy to obtain "pure lines."
- Easy Cross-Pollination: Artificial pollination could be performed easily by removing anthers (Emasculation).
- Large Number of Seeds: Each cross produced many offspring for statistical analysis.
The 7 Pairs of Contrasting Traits
Mendel studied seven specific characters. For each character, one trait was Dominant and the other was Recessive.
Be careful with Seed Color vs. Pod Color. For Seeds, Yellow is dominant. For Pods, Green is dominant. This is a very common point of confusion in ICSE papers!
Mendel presented his work in 1865, but it was ignored for 35 years! It was only in 1900 that three other scientists independently rediscovered his laws.
3.0 The Monohybrid Cross
A Monohybrid Cross is a genetic cross between two individuals involving only one pair of contrasting characters (e.g., crossing a pure tall plant with a pure dwarf plant).
Punnett Square: A graphical representation (checkerboard) used to calculate the probability of all possible genotypes of offspring in a cross.
Mendel's Laws (Based on Monohybrid Cross)
1. Law of Dominance: Out of a pair of contrasting characters, only one (the dominant) is able to express itself in the F1 generation, while the other (the recessive) remains hidden.
2. Law of Segregation (Law of Purity of Gametes): During gamete formation, the two alleles of a pair separate (segregate) such that each gamete receives only one allele. This is why gametes are always "pure" for a trait.
F2 Generation Ratios (Monohybrid)
In the F1 generation of a monohybrid cross, 100% of plants are Phenotypically Tall but Genotypically Heterozygous (Tt). Students often lose marks by not specifying both ratios clearly.
The Law of Segregation is the only law of Mendel that has no exceptions. It applies to all sexually reproducing organisms!
4.0 The Dihybrid Cross
A Dihybrid Cross involves the study of two pairs of contrasting characters simultaneously. Mendel crossed a plant with Round and Yellow seeds (RRYY) with a plant having Wrinkled and Green seeds (rryy).
Law of Independent Assortment: When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.
The Dihybrid Ratio
In the F2 generation of a dihybrid cross, we see four different phenotypes in a very specific ratio. This ratio is a staple for ICSE long-answer questions.
The dihybrid ratio 9:3:3:1 is actually just the product of two monohybrid ratios: (3:1) x (3:1). Understanding this math helps you double-check your Punnett square quickly during exams!
While Mendel's laws work for most traits, we now know about Linkage—where genes located very close together on the same chromosome do not assort independently!
5.0 Sex Determination in Humans
In humans, the sex of an individual is determined genetically at the time of fertilization. Out of the 23 pairs of chromosomes, 22 pairs are Autosomes (identical in both sexes), and the 23rd pair is the Sex Chromosome.
Allosomes: Also known as sex chromosomes, these are the chromosomes (X and Y) that carry the genes determining the sex of an organism.
- 👩 Females: Have a Homomorphic pair of sex chromosomes (XX). All ova produced carry an X chromosome.
- 👨 Males: Have a Heteromorphic pair (XY). Half the sperm carry X, and half carry Y.
6.0 Sex-Linked Inheritance
Certain disorders are caused by recessive genes located on the X-chromosome. These are called sex-linked diseases. Examples include Haemophilia and Color Blindness.
Sex-linked traits are usually passed from a father to his grandson through his daughter. This staggered pattern is a hallmark of X-linked recessive traits.
In sex-linked inheritance questions, remember that males can never be 'carriers'. Since they have only one X chromosome, they are either affected or normal. Only females can be carriers (XXc).
Haemophilia is often called the "Royal Disease" because it was passed through the royal families of Europe, starting with Queen Victoria, who was a carrier.
7.0 Variation and Mutation
While heredity ensures we look like our parents, Variation explains why no two individuals (except identical twins) are exactly alike. These differences are often caused by sudden changes in the genetic code.
Mutation: A sudden, permanent heritable change in the DNA sequence or the number/structure of chromosomes that results in a change in the phenotype.
Types of Variations
Variations can be broadly classified into two categories based on how they are passed down:
- 🧬 Somatic Variations: These occur in body cells and are not inherited (e.g., muscles built by exercise).
- 🧬 Germinal Variations: These occur in reproductive cells and are inherited by the offspring (e.g., eye color).
Agents that cause mutations are called Mutagens. These can be physical (X-rays, UV radiation) or chemical (certain drugs, pollutants).
Genetic Disorders in Humans
Sometimes, mutations lead to serious health conditions. Here are two examples often mentioned in the syllabus:
- Sickle Cell Anemia: A mutation in the gene for hemoglobin causing RBCs to become sickle-shaped.
- Down Syndrome: Caused by an extra copy of chromosome number 21 (Trisomy 21).
While mutations are usually harmful or neutral, they are the ultimate source of Evolution. Without mutations, there would be no new traits for natural selection to act upon!
Natural mutations occur very slowly. However, humans have used artificial mutagens to create new varieties of crops, a process called Mutation Breeding!