Genetics: Some Basic Fundamentals

Q: In guinea pigs black coat (B) is dominant over white (b). A pure black guinea pig is crossed with a white one. Give the F1 genotype and the F2 phenotypic and genotypic ratios.

F1: Bb, all black. F2 phenotypic ratio black : white = 3 : 1; genotypic ratio BB : Bb : bb = 1 : 2 : 1.

ICSE Class 10 · Biology · Chapter 3

What this chapter covers

Heredity, variation and the work of Mendel
Basic genetic terms
Monohybrid cross: phenotype and genotype
Mendel's three laws of inheritance
Dihybrid cross: phenotype only
Sex determination in human beings
Sex-linked inheritance of diseases

Key definitions

Heredity is the passing of characters from parents to their offspring, generation after generation.

Variation is the difference in characters between individuals of the same species, including the differences between parents and their offspring and between brothers and sisters.

Genetics is the branch of biology that studies heredity and variation.

Alleles are the alternative forms of the same gene, found at the same locus on a pair of homologous chromosomes and controlling contrasting traits of one character.

Homozygous is the condition in which the two alleles of a gene in an organism are identical, as in TT or tt.

Heterozygous is the condition in which the two alleles of a gene in an organism are different, as in Tt.

Phenotype is the observable or measurable form of a character in an organism, such as tall or dwarf, round or wrinkled.

Genotype is the genetic make-up of an organism for a character, written as the pair of alleles it carries, such as TT, Tt or tt.

Autosomes are the chromosomes that carry genes for body characters other than sex, and a human body cell has 22 pairs of them.

Sex chromosomes are the pair of chromosomes that decide whether an individual develops as a male or a female; in humans they are called X and Y.

Sex-linked genes are genes located on the sex chromosomes, and their pattern of inheritance is called sex-linked inheritance.

X-linked inheritance is the inheritance of characters controlled by genes present on the X chromosome and having no corresponding allele on the Y chromosome.

Key points

Heredity is the passing of characters from parents to offspring; variation is the difference between individuals of a species. Genetics studies both.
Mendel worked with the garden pea, studied seven pairs of contrasting traits, began with pure-breeding plants and counted his results.
A gene is a segment of DNA at a locus that controls a character; alleles are its alternative forms, written as a capital (dominant) and small (recessive) letter.
Homozygous means two identical alleles (TT, tt); heterozygous means two different alleles (Tt).
Phenotype is the visible form of a character; genotype is the allele pair that produces it.
A mutation is a sudden heritable change in genetic material and is the source of new alleles; variation is genetic or environmental, and only genetic variation is inherited.
Monohybrid cross: F₁ all show the dominant trait; F₂ phenotypic ratio 3 : 1, genotypic ratio 1 : 2 : 1.
Dihybrid cross: F₁ show both dominant traits; F₂ phenotypic ratio 9 : 3 : 3 : 1, with two recombinant classes.
Mendel's three laws are the law of dominance, the law of segregation (purity of gametes) and the law of independent assortment.
Human cells have 22 pairs of autosomes and one pair of sex chromosomes: XX in females, XY in males.
Eggs always carry X; sperm carry X or Y in equal numbers, so the father's sperm determines the sex of the child, with a 50 per cent chance each time.
Haemophilia and colour blindness are X-linked recessive disorders; males are affected far more often because they have one X chromosome.
A carrier female (X^HX^h) passes the allele to half her sons, who are affected, and half her daughters, who become carriers; an affected father passes the allele to all his daughters and to none of his sons.

Questions answered

Define heredity and variation. Which of the two explains why siblings differ from each other?

Heredity is the passing of characters from parents to offspring; variation is the difference in characters between individuals of a species. Variation explains why siblings differ.

Give three reasons why the garden pea was a good choice for Mendel's experiments.

Any three of: it grows quickly and has a short life cycle; it produces many seeds; it naturally self-pollinates, giving pure lines; it can be cross-pollinated by hand; it shows clear pairs of contrasting traits.

Distinguish between a gene and an allele.

A gene is a segment of DNA at a particular locus that controls one character. Alleles are the alternative forms of that gene, controlling contrasting traits of the same character, such as T and t.

Distinguish between genotype and phenotype, giving one example of two plants with the same phenotype but different genotypes.

Genotype is the pair of alleles an organism carries; phenotype is the visible form of the character. TT and Tt pea plants are both tall but have different genotypes.

What is meant by homozygous and heterozygous? How many kinds of gametes does each form for one gene?

Homozygous means both alleles are identical (TT or tt) and only one kind of gamete is formed. Heterozygous means the two alleles differ (Tt) and two kinds of gamete are formed in equal numbers.

What is a mutation? Why is it important for variation?

A mutation is a sudden heritable change in a gene or a chromosome. It is the original source of new alleles, without which there would be no new heritable variation.

In guinea pigs black coat (B) is dominant over white (b). A pure black guinea pig is crossed with a white one. Give the F₁ genotype and the F₂ phenotypic and genotypic ratios.

F₁: Bb, all black. F₂ phenotypic ratio black : white = 3 : 1; genotypic ratio BB : Bb : bb = 1 : 2 : 1.

State Mendel's law of segregation. Why is it also called the law of purity of gametes?

The two alleles of a gene separate during gamete formation so that each gamete receives only one allele. Each gamete is "pure" because it carries one allele, never both and never a blend.

Past board exam questions

ICSE 2027 sample paper · 1 mark

Tongue rolling is a dominant trait. Arun who cannot roll his tongue, marries a woman who is a carrier of the trait. What percentage of their children can roll their tongue?

25%
50%
75%
100 %

Show answer

50%

ICSE 2027 sample paper · 2 marks

State Mendel’s Law of Segregation of gametes.

Show answer

The two alleles (factors) of a character present in an individual separate (segregate) from each other during gamete formation, so that each gamete receives only one allele of each pair.

ICSE 2027 sample paper · 2 marks

Why is haemophilia uncommon in human females?

Show answer

Haemophilia is caused by a recessive gene on the X chromosome. Females have two X chromosomes, so if one carries the recessive gene the normal dominant allele on the other X suppresses it (they are only carriers); a female suffers only if both X chromosomes carry the gene, which is rare. Males have only one X chromosome, so a single recessive gene causes the disease.

ICSE 2026 board paper · 1 mark

A family has a history of colour blindness. During a genetic testing, it was found that the mother is a carrier of colour blindness (X^C X) and the father has normal vision (XY). What is the probability of their sons being colour blind?

Show answer

50%

ICSE 2026 sample paper · 1 mark

The number of autosomes and allosomes present in a human cell:

22 allosomes, 1 autosome
22 pairs of autosomes, 1 pair of allosome
22 autosomes, 1 allosome
22 pairs of allosomes, 1pair of autosome

Show answer

22 pairs of autosomes, 1 pair of allosome

ICSE 2026 board paper · 2 marks

Mention the number of Autosomes and Allosomes in a human body cell.

Show answer

Autosomes: 44 (22 pairs). Allosomes (sex chromosomes): 2 (1 pair — XX in females, XY in males).

ICSE 2026 sample paper · 2 marks

Give the common name and the scientific name of the plant on which Gregor Mendel conducted his breeding experiments.

Show answer

Common name – Garden pea Scientific name – Pisum sativum

ICSE 2026 sample paper · 3 marks

Santosh loved his garden, especially his pea plants. One year, he noticed that all the pea plants he grew from one batch of seeds had round seeds. Curious, he took the seeds from one of those plants and grew a new generation. To his surprise, 75% of the new plants had round seeds, but 25% had wrinkled seeds. (a) What were the genotypes of the parent plants that Santosh originally planted? (b) Which trait is dominant – round or wrinkled seeds? (c) What is the genotypic ratio of the second generation (F2) plants?

Show answer

(a) Heterozygous round – Rr (selfing Rr gives 3 round : 1 wrinkled). (b) Round seed is dominant. (c) 1 RR : 2 Rr : 1 rr, i.e. 1 : 2 : 1