Genetics Class 10 ICSE — Selina Chapter 3
Genetics class 10 ICSE starts here, in Chapter 3 of the Selina Biology book. The chapter, Genetics Some Basic Fundamentals, explains how body features pass from parents to offspring. It covers chromosomes, genes and alleles, sex determination, Mendel’s experiments and laws, and mutation. Each section builds vocabulary you need for exams, including variation, genotype, phenotype, and the monohybrid and dihybrid ratios. Read the sections in order with the chapter open beside you.
ICSE Class 10 Biology Selina Concise Chapter 3 Genetics Some Basic Fundamentals PDF
What is inside Chapter 3
14 pages · 13 sections · 7 exercise questions
| Section | Title |
|---|---|
| 3.1 | WHAT IS GENETICS ? |
| 3.2 | VARIATIONS IN POPULATION |
| 3.3 | CHROMOSOMES—THE CARRIERS OF HEREDITY |
| 3.4 | THE TWO MAIN CATEGORIES — AUTOSOMES AND SEX CHROMOSOMES |
| 3.5 | SEX DETERMINATION — SON OR DAUGHTER |
| 3.6 | CHROMOSOMES — CARRIERS OF GENES |
| 3.7 | GENES AND THEIR ALLELES |
| 3.8 | GENOTYPE AND PHENOTYPE |
| 3.9 | FROM PARENTS TO CHILDREN – TONGUE ROLLING – AN EXAMPLE OF INHERITANCE |
| 3.10 | SEX-LINKED INHERITANCE |
| 3.11 | MENDEL'S EXPERIMENTS ON INHERITANCE |
| 3.12 | MENDEL'S LAWS OF INHERITANCE |
| 3.13 | MUTATION |
- PROGRESS CHECK: 2 questions
- PROGRESS CHECK: 2 questions
- PROGRESS CHECK: 3 questions
3.1 WHAT IS GENETICS ?
Genetics is the study of how body features pass from parents to offspring, and the laws behind that transmission. The term covers both similarities and differences between generations. This definition frames the whole chapter.
Gregor Mendel (1822–84) is the central figure. He was an Austrian monk who studied science and mathematics in Vienna. He failed his teaching certificate examination and returned to his monastery. There he used the garden to study inheritance in pea plants, and his findings became Mendel’s laws of inheritance.
Heredity means the transmission of genetically based characteristics from parents to offspring. The book calls this “like begets like”: cats produce cats, mango seeds grow mango trees, humans give birth to humans. Yet offspring are never identical to parents. Small differences always appear.
The chapter also names two modern applications. Genetic engineering alters an organism’s genes, and the first product was insulin made by bacteria. Genetic counselling helps couples assess the risk of inherited diseases such as haemophilia, thalassaemia and sickle cell anaemia. For syllabus and exam updates, check the official CISCE website.
3.2 VARIATIONS IN POPULATION
This section defines variation. Small differences among individuals of the same species, race, tribe or family are called variations. Humans share traits that identify the species Homo sapiens, yet family members still differ in body features.
The section also separates a character from its traits. Any inheritable feature is a character. Its alternative forms are traits. For example, eye colour is a character, while brown and blue are its traits.
The book lists human characters you can observe:
- Eye colour — brown or blue
- Hair shape — curly or straight
- Tongue rolling — rolling into a U-shape or no rolling
- Ear lobe — free or attached
- Rh blood group — positive or negative
- Colour vision — normal or red-green colour blind
The first PROGRESS CHECK has two questions. One asks you to mark statements true or false, such as whether genetics and heredity are the same thing. The other asks which listed items are established genetic traits, such as curly hair and left-handedness.
3.3 CHROMOSOMES—THE CARRIERS OF HEREDITY
Chromosomes become distinctly visible only when the nucleus is about to divide. Photographs taken then can be arranged by size and shape on a chart called a karyotype. This is how doctors study a person’s full chromosome set.
Chromosome number is constant for a species. Humans have 46 chromosomes. The book gives counts for other organisms:
- Ascaris 2, onion 16, maize 20
- Lion, tiger and domestic cat 38, mouse 40
- Humans 46, gorilla and potato 48, monkey 54
- Dog and chicken 78, crayfish 200, some insects over 1000
Chromosomes occur in even numbers because they come in pairs. A pair of corresponding chromosomes of the same shape and size, one from each parent, is called a homologous pair. This idea connects directly to what you studied in the cell cycle and cell division chapter in Class 10 Biology.
3.4 THE TWO MAIN CATEGORIES — AUTOSOMES AND SEX CHROMOSOMES
In the human karyotype, pairs numbered 1 to 22 have identical partners in both males and females. These are autosomes. The 23rd pair is different and holds the sex chromosomes, labelled X and Y.
Females carry an XX pair, with similar partners. Males carry an XY pair, with dissimilar partners. The Y chromosome is much smaller than the X chromosome. In a karyotype, the sex chromosomes are the only pair that may not match in size.
Because chromosomes always come in pairs, the 46 human chromosomes form 23 pairs. Of these, 22 are autosome pairs and one is the sex-chromosome pair. This distinction matters for everything that follows, because the sex chromosomes decide the sex of the child. Section 3.5 explains how that decision works.
3.5 SEX DETERMINATION — SON OR DAUGHTER
The sex of the child depends on which kind of sperm fertilises the egg. The egg always carries one X chromosome. Half the sperms carry an X and half carry a Y.
If an X-bearing sperm fertilises the egg, the combination is XX and the child is a daughter. If a Y-bearing sperm fertilises the egg, the combination is XY and the child is a son. It is a matter of chance which sperm fuses with the ovum.
The PROGRESS CHECK here asks about a couple with four daughters and no son. The father still produces Y-bearing sperms. Each conception is an independent 50/50 event, so four daughters do not change the odds for the next child.
The book notes that trials have been reported which isolate the two kinds of sperms to obtain a desired sex.
3.6 CHROMOSOMES — CARRIERS OF GENES
The chromosome number alone does not decide an organism’s features. The lion and the cat both have 38 chromosomes, yet they differ in size, appearance, colour and behaviour. Those differences come from genes located on the chromosomes.
A gene is a specific part of a chromosome, a DNA segment, that determines hereditary characteristics. Humans have nearly 30,000 genes. Chromosome 1 carries the most, with 2968 genes. The Y chromosome carries the fewest, with 231.
The book notes that characteristics cover physical appearance, body functions and behaviour. The genome is the full complement of DNA of an organism, including all genes and the intergenic regions. So the chromosome count stays fixed for a species, while the genes on those chromosomes carry the information that shapes each organism.
3.7 GENES AND THEIR ALLELES
Most genes have two alternative forms for a character, called alleles. The two alleles occupy the same position, or locus, on homologous chromosomes. They affect the same characteristic but in different ways.
For tongue rolling, the gene has two alleles: one for rolling and one for non-rolling. Out of the two alleles, one is dominant and the other is recessive. By convention, the dominant allele is written with a capital letter and the recessive allele with the same lowercase letter. So R means rolling and r means non-rolling.
A recessive gene is one that is not expressed in the presence of the contrasting dominant gene. The chromosome as a whole is neither dominant nor recessive. Only the individual genes carry that label.
The term gene was coined by geneticist Wilhelm Johanssen in 1909. It simply described what parents passed to offspring, long before the detailed DNA structure was known.
3.8 GENOTYPE AND PHENOTYPE
For tongue rolling, three gene-pair situations exist. RR with both dominant genes gives a tongue roller. Rr with one dominant and one recessive gene also gives a tongue roller. rr with both recessive genes gives a non-roller.
The set of genes present in the cells is the genotype. The observable, genetically controlled characteristic is the phenotype. So the roller phenotype has two genotypes: homozygous dominant (RR) and heterozygous dominant (Rr). The non-roller has only one genotype, homozygous recessive (rr).
The word roots help you remember. Homo means similar, hetero means different, and zygos means pair.
Two common mistakes appear here. Confusing genotype with phenotype mixes the gene combination with what actually shows. Treating the recessive allele as the one that always shows is also wrong. The recessive allele shows only when both alleles are recessive, as in rr.
3.9 FROM PARENTS TO CHILDREN – TONGUE ROLLING – AN EXAMPLE OF INHERITANCE
This section teaches two tools: the pedigree chart and the Punnett square. In a pedigree chart, squares show males and circles show females. A solid symbol marks the character being traced.
The book’s family chart has two roller parents and three children: two rollers and one non-roller. The non-roller child must be rr and received one r from each parent. So both parents are heterozygous Rr.
A Punnett square places one parent’s gametes along one side and the other parent’s gametes along the other side. The sub-squares show the possible genotypes. Here is the pattern for two heterozygous rollers, Rr × Rr:
| R | r | |
|---|---|---|
| R | RR | Rr |
| r | Rr | rr |
The offspring genotypes are one RR, two Rr and one rr. The roller phenotype appears three times and the non-roller once. The phenotypic ratio is 3:1, and the genotypic ratio is 1:2:1.
The book also lists dominant and recessive human traits. Brown eyes dominate blue, tongue rolling dominates non-rolling, right-handedness dominates left-handedness, free ear lobe dominates attached, Rh positive dominates negative, and normal colour vision dominates colour blindness.
3.10 SEX-LINKED INHERITANCE
Sex-linked inheritance appears when a trait is due to an allele present exclusively on the X or the Y chromosome. X-linked disorders such as colour blindness and haemophilia are more common in males, because a male has only one X chromosome.
Haemophilia is a genetic disorder in which the blood fails to clot, putting sufferers at risk of bleeding to death. In colour blindness, a normal homozygous mother (XX) and a colour-blind father (X^\circY) produce daughters who are carriers (XX^\circ) and sons who are normal. The daughter carries the defective allele but does not show the disorder.
This pattern is called criss-cross inheritance. A mother passes the trait to her son, and a father passes it to his daughter.
Y-linked traits occur only in males. Hypertrichosis of the ears and pattern baldness are examples, because their dominant genes sit on the Y chromosome, which determines male sex.
3.11 MENDEL’S EXPERIMENTS ON INHERITANCE
Mendel chose the garden pea (Pisum sativum) for three reasons. Many varieties showed alternative forms of a character. Pure-breeding varieties bred true, producing the same type generation after generation. And the flowers could be self-pollinated or cross-pollinated by removing the stamens and carpels.
The symbols \mathbf{F}_1, \mathbf{F}_2 and \mathbf{F}_3 mean the first, second and third filial generations. Filial means a cross between offspring.
In a monohybrid cross, Mendel took one character at a time. The book’s example crosses terminal flowers with axillary flowers. All \mathbf{F}_1 plants had axillary flowers. In \mathbf{F}_2, axillary and terminal plants appeared in a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio. The same ratios appeared for tall versus dwarf and round versus wrinkled seeds.
In a dihybrid cross, Mendel crossed round yellow seeds with wrinkled green seeds. The \mathbf{F}_2 phenotypic ratio was 9:3:3:1. The two kinds of ratios sit side by side:
| Cross | \mathbf{F}_2 phenotypic ratio | \mathbf{F}_2 genotypic ratio |
|---|---|---|
| Monohybrid | 3:1 | 1:2:1 |
| Dihybrid | 9:3:3:1 | Very complex |
Mendel drew five generalised principles from these results. Each contrasting character depends on a pair of genes. Each individual carries such genes in duplicate. Gametes carry one member of a pair. Fertilisation restores the duplicate condition. And sex cells fuse at random.
3.12 MENDEL’S LAWS OF INHERITANCE
Mendel summarised his breeding results in three laws.
Law of Dominance. Of a pair of contrasting characters present together, only one expresses itself while the other stays suppressed. The recessive character can express only when the pair is homozygous recessive.
Law of Segregation, also called the law of purity of gametes. The two members of a pair of factors separate during gamete formation and do not blend. They segregate into different gametes and combine by random fusion at zygote formation.
Law of Independent Assortment. With two pairs of contrasting characters, the distribution of one pair into gametes is independent of the distribution of the other pair. This is seen in the gametes of the \mathbf{F}_1 hybrid in a dihybrid cross.
The laws have three applications. They help predict new combinations and their frequency in hybrid progeny. They help plant and animal breeders produce better breeds. And they allow new plant types with useful combinations of characters to be produced by hybridisation.
3.13 MUTATION
A mutation is a sudden change in one or more genes, or in the number or structure of chromosomes. It alters the hereditary material of an organism’s cells and changes certain characters or traits.
Sickle cell anaemia is the book’s example of a gene mutation. The mutation changes the DNA, producing sickle-shaped red blood cells.
Radioactive radiations also alter gene structure, and their effects pass down generations. The atomic explosions at Hiroshima and Nagasaki in 1945, during World War II, led to deformities in plants and animals that still persist. Mutations can be inherited, which is why the effects of radiation appear generation after generation.
The final PROGRESS CHECK has three questions. It asks who discovered the basic principles of genetics, the common and scientific names of Mendel’s organism, and definitions of monohybrid, dihybrid and filial generation.
What to practise from this chapter
The chapter gives three PROGRESS CHECK sets to test yourself as you read, then five lettered exercise blocks at the end.
- PROGRESS CHECK 1 (2 questions) — true or false statements about genetics and heredity, and identifying established genetic traits. It follows Section 3.2.
- PROGRESS CHECK 2 (2 questions) — chromosome pair counts and the four-daughters question about sex determination. It follows Section 3.5.
- PROGRESS CHECK 3 (3 questions) — the discoverer of genetics principles, Mendel’s organism, and definitions of monohybrid, dihybrid and filial generation. It appears in Section 3.13.
- Exercise A. Multiple Choice Type
- Exercise B. Very Short Answer Type
- Exercise C. Short Answer Type
- Exercise D. Long Answer Type
- Exercise E. Structured/Application and Skill Type
Work the progress checks after reading each section. Attempt the lettered blocks once you finish the chapter. These blocks follow the exercise pattern used across other chapters in Class 10 Biology.
Frequently asked questions
Why is colour blindness more common in boys than in girls?
Colour blindness is caused by a recessive gene on the X chromosome. A boy has only one X chromosome, so one defective allele makes him colour blind. A girl has two X chromosomes and needs the defective allele on both to show the disorder, which is rare.
What is the difference between genotype and phenotype?
Genotype is the set of genes present in the cells, such as RR, Rr or rr. Phenotype is the observable characteristic those genes produce, such as being a tongue roller or a non-roller. Two different genotypes, RR and Rr, can produce the same phenotype.
Does the sex of the child depend on the father?
Yes. The egg always carries one X chromosome, while sperms are half X-bearing and half Y-bearing. If a Y-bearing sperm fertilises the egg, the child is a son; if an X-bearing sperm does, the child is a daughter. It is a matter of chance which sperm fuses with the ovum.
What does the 3:1 ratio in the F₂ generation mean?
In a monohybrid cross, three offspring show the dominant trait for every one that shows the recessive trait. For example, in F₂, three axillary flower plants appear for every one terminal flower plant. The underlying genotypic ratio is 1:2:1.
What is criss-cross inheritance?
Criss-cross inheritance is the pattern where a mother passes an X-linked trait to her son and a father passes it to his daughter. It appears in colour blindness and haemophilia. A carrier mother can pass the defective allele to her son, making him affected.
Reference: ICSE Class 10 Biology textbook material, chapter 3: Genetics Some Basic Fundamentals — CISCE-aligned.