Genetics - Some Basic Fundamentals
ICSE Class 10 Biology • Chapter 03 • Detailed Master Notes
1. What is Genetics?
Genetics: The study of transmission of body features
(both similarities and differences) from parents to offspring and the
laws relating to such transmission.
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Gregor Mendel (An Austrian Monk) is known as the
Father of Genetics. He formulated
the basic laws of heredity through his systematic experiments on pea
plants in the monastery garden in Brno.
Modern Applications of Genetics
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Genetic Engineering: The technique in which the
genetic constitution of an organism (like a bacterium) is altered by
introducing new genes into its chromosomes. E.g., the human hormone
insulin-producing gene has been successfully
introduced into certain bacteria to produce insulin in large
quantities for diabetics.
-
Genetic Counselling: Newly married couples consult
a specialist regarding possibilities of any undesirable traits or
hereditary diseases their children might inherit (e.g., haemophilia,
thalassaemia, sickle cell anaemia).
2. Heredity and Variations
Heredity: Transmission of genetically based
characteristics from parents to offspring.
Variations: Small differences among individuals
within a species or population. No two individuals (even identical
twins) are exactly alike in all aspects.
Like begets like: It means that young ones look exactly
like their parents belonging to the same species (Cats produce cats,
mango seed germinates into a mango tree).
Characters and Traits
Any inheritable feature is a character (e.g., eye
colour). The alternative forms of a character are called
traits (e.g., brown eyes or blue eyes).
| Character |
Dominant Trait |
Recessive Trait |
| Eye colour |
Brown |
Blue |
| Tongue |
Rolling |
Non-rolling |
| Hand use |
Right-handedness |
Left-handedness |
| Ear lobe |
Free |
Attached |
| Colour vision |
Normal |
Colour blind (red-green) |
Some uncommon hereditary traits: Polydactyly (Extra
fingers & toes) is Dominant. Albinism (Total absence of skin pigment
due to lack of melanin) is Recessive.
3. Chromosomes and Genes
Chromosomes are only visible when a cell nucleus is about to divide. The
chromosome number is constant for the individuals of a species. Humans
have 46 chromosomes (23 pairs).
Autosomes and Sex Chromosomes
-
Autosomes: Chromosome pairs numbered 1-22 which
determine all general body features. They are identical in both males
and females.
-
Sex Chromosomes (Allosomes): The 23rd pair that
determines the sex of the individual.
-
Females have XX (similar partners - perfectly
homologous).
-
Males have XY (dissimilar - Y is much smaller
than X and carries fewer genes).
Fig. 3.1 & 3.2: Human Karyotype showing 22 pairs of autosomes and 1
pair of sex chromosomes (XX or XY).
Sex Determination in Humans
The sex of the child depends upon the kind of sperm that fertilises the
egg. All human eggs are exactly alike (contain $22 + X$), but sperms are
of two types: X-bearing ($22 + X$) and Y-bearing ($22 + Y$).
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Egg (X) + Sperm (X) → XX (Female / Daughter)
- Egg (X) + Sperm (Y) → XY (Male / Son)
Conclusion: It is the father's sperm that determines the sex of the
child. There is always a 50% probability of having a boy or a
girl.
Genes and their Alleles
Genes: Specific parts (DNA segments) of a chromosome,
which determine the hereditary characteristics.
Genome: The full complement of DNA (including all
genes and intergenic regions) of an organism.
Alleles (Allelomorphs): Alternative forms of a gene,
occupying the same position (locus) on homologous chromosomes and
affecting the same characteristic but in different ways (e.g., Free vs
Attached earlobe).
Out of two alleles of a gene, one is
dominant (superruling) and the other is
recessive (subordinate). Dominant genes are
conventionally represented by capital letters (e.g. 'R' for tongue
rolling), recessive by small letters (e.g. 'r' for non-rolling).
Genotype and Phenotype
-
Genotype: The set of genes present in the cells of an
organism (the exact genetic constitution). E.g. RR, Rr, rr.
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Phenotype: The observable characteristic or expressed
physical trait which is genetically controlled. E.g. "Tongue roller"
or "Tall".
Types of Genotypes (The pairing of Alleles):
-
Homozygous dominant: Similar pair of dominant
alleles (RR). Will express the dominant phenotype.
-
Heterozygous dominant: Dissimilar pair of alleles
(Rr). Will also express the dominant phenotype because 'R'
suppresses 'r'.
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Homozygous recessive: Similar pair of recessive
alleles (rr). The only condition where the recessive phenotype is
expressed.
Fig. 3.3 & 3.4: Homologous chromosomes showing gene loci and
heterozygous/homozygous allele pairs.
4. Sex-Linked Inheritance
Sex-linked inheritance: The appearance of a trait
which is due to the presence of an allele exclusively either on the X
chromosome or on the Y chromosome.
Certain disorders like
Haemophilia (bleeder's disease, where
blood doesn't clot properly) and
Colour-blindness (inability to
distinguish between red and green colours) are caused by recessive genes
located exclusively on the X chromosome.
Why are males more affected? They are much more common
in males because a male has only one X chromosome. A single defective
gene ($X^c Y$) is enough to cause the disease. A female needs two
defective genes ($X^c X^c$) to suffer from it. A female with one
defective gene ($XX^c$) is phenotypically normal but acts as a
Carrier.
Criss-Cross Inheritance
Inheritance of X-linked recessive genes follows a criss-cross pattern.
It is passed from a Father to his Grandson through his Daughter.
Example: Colourblind Father ($X^c Y$) x Normal Mother
($XX$)
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The father passes his defective $X^c$ to ALL his daughters, making
them Carriers ($X^c X$).
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The father passes his $Y$ to ALL his sons, making them perfectly
Normal ($XY$).
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When these carrier daughters marry normal men, they have a 50%
chance of passing the colourblindness to their sons.
5. Mendel's Experiments on Inheritance
Gregor Mendel selected the garden pea (Pisum sativum) for his
breeding experiments for three critical reasons:
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Many varieties were available in well-defined alternative forms of a
character.
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Varieties were available in pure forms that bred true (produced
offspring identical to themselves generation after generation).
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Peas are normally self-pollinated but can be cross-pollinated
artificially (by removing the anthers, a process called
Emasculation).
-
They have a short life span, allowing the study of several generations
in a short time.
The 7 Pairs of Contrasting Characters Studied by Mendel
| Character |
Dominant Trait |
Recessive Trait |
| 1. Flower colour |
Purple (Red) |
White |
| 2. Seed colour |
Yellow |
Green |
| 3. Seed shape |
Round |
Wrinkled |
| 4. Pod colour |
Green |
Yellow |
| 5. Pod shape |
Inflated (Full) |
Constricted |
| 6. Flower position |
Axillary (Axial) |
Terminal |
| 7. Stem length (Height) |
Tall |
Dwarf |
Fig. 3.5: Mendel's cross-pollination technique in pea plants
(Emasculation and dusting of pollen).
A. Monohybrid Cross
A cross between two pure breeding different varieties considering the
alternative traits of one single character.
Example: Pure tall ($TT$) crossed with pure dwarf
($tt$).
-
$F_1$ generation (First Filial): All offspring are
hybrid tall ($Tt$). The dwarf trait is suppressed.
-
$F_2$ generation (Second Filial, obtained on selfing $F_1 \times
F_1$):
Gametes →
↓ |
$T$ |
$t$ |
| $T$ |
$TT$ (Tall) |
$Tt$ (Tall) |
| $t$ |
$Tt$ (Tall) |
$tt$ (Dwarf) |
Results of Monohybrid Cross ($F_2$):
-
Phenotypic ratio:
3 : 1 (3 Tall : 1 Dwarf)
-
Genotypic ratio:
1 : 2 : 1 (1 Pure Tall TT : 2
Hybrid Tall Tt : 1 Pure Dwarf tt)
B. Dihybrid Cross
A cross considering alternative traits of
two different characters simultaneously.
-
Example: Pure Round Yellow ($RRYY$) crossed with
Wrinkled Green ($rryy$).
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$F_1$ generation: All are Round and Yellow ($RrYy$).
-
$F_2$ generation (Phenotypic ratio):
9 : 3 : 3 : 1
- 9 = Round Yellow (Both dominant)
- 3 = Round Green (One dominant, one recessive)
- 3 = Wrinkled Yellow (One recessive, one dominant)
- 1 = Wrinkled Green (Both recessive)
6. Mendel's Laws of Inheritance
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Law of Dominance: Out of a pair of contrasting
characters present together in a heterozygous condition, only one is
able to express itself (dominant) while the other remains suppressed
(recessive).
-
Law of Segregation (Law of Purity of Gametes): The
two members of a pair of factors separate (segregate) during the
formation of gametes. They do not blend. A gamete contains only one
factor (allele) of a trait, making it "pure" for that trait.
(This law has no exceptions).
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Law of Independent Assortment: When there are two
pairs of contrasting characters, the distribution of the alleles of
one character into the gametes is totally independent of the
distribution of the alleles of the other character. (Proven by the
Dihybrid cross).
7. Mutation
Mutation: A sudden, spontaneous change in one or more
genes, or in the number or structure of chromosomes.
Mutation permanently alters the hereditary material. Causes of mutation
are called Mutagens.
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Sickle cell anaemia: A genetic blood disease caused
by a gene mutation resulting in defective haemoglobin, making RBCs
sickle-shaped.
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Radioactive radiations: X-rays, UV rays, and
radioactive fallout can alter gene structure. (e.g., The atomic
explosions in Hiroshima and Nagasaki during WWII led to severe genetic
deformities that persisted for generations).
8. ICSE Board Exam Practice Questions (PYQ Trends)
Genetics is highly scoring if you understand the terminology and
crosses perfectly. Practice these standard ICSE patterns.
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NAME THE FOLLOWING The alternative forms
of a gene occupying the same position on homologous chromosomes.
Ans: Alleles (Allelomorphs)
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NAME THE FOLLOWING The genetic
constitution of an organism.
Ans: Genotype
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NAME THE FOLLOWING The sudden change in
one or more genes.
Ans: Mutation
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NAME THE FOLLOWING Chromosomes that
determine general body features.
Ans:
Autosomes
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NAME THE FOLLOWING The father of
genetics.
Ans: Gregor Mendel
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REASONING Why did Mendel select the
pea plant for his experiments?
Ans: Because pea plants possess many pairs of
contrasting characters, they naturally self-pollinate but can be
cross-pollinated, and they have a short life span allowing rapid
study of multiple generations.
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REASONING Why are males more prone to
colour blindness than females?
Ans: Colour blindness is an X-linked recessive
disorder. Males ($XY$) have only one X chromosome, so a single
defective gene causes the disease. Females ($XX$) have two X
chromosomes and need two defective genes to express the disorder.
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REASONING Law of segregation is also
called the Law of purity of gametes. Why?
Ans: Because during gamete formation, the two
alleles of a trait segregate and a single gamete receives only one
allele. Thus, every gamete is absolutely pure for that specific
trait.
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CONCEPT CHECK State the phenotypic and
genotypic ratios of a monohybrid cross in the $F_2$
generation.
Ans: Phenotypic ratio = 3 : 1 (Dominant :
Recessive). Genotypic ratio = 1 : 2 : 1 (Homozygous Dominant :
Heterozygous Dominant : Homozygous Recessive).
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CROSS QUESTION A homozygous tall plant
(TT) is crossed with a homozygous dwarf plant (tt). What will be
the phenotype of the $F_1$ generation?
Ans: All plants in the $F_1$ generation will be
phenotypically Tall.
-
DIHYBRID RATIO What is the phenotypic
ratio of a Dihybrid cross in the $F_2$ generation?
Ans: 9 : 3 : 3 : 1.
ICSE FAVOURITE Differentiate between the
following:
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Genotype and Phenotype: Genotype is the genetic
makeup of an organism (e.g., Tt), whereas Phenotype is the
observable physical characteristic (e.g., Tall).
-
Autosomes and Sex Chromosomes: Autosomes (22 pairs)
determine general body traits and are identical in both sexes. Sex
chromosomes (1 pair, XX or XY) determine the sex of the individual.
-
Homozygous and Heterozygous: Homozygous is a
condition where both alleles of a gene pair are identical (e.g., TT
or tt). Heterozygous is a condition where the two alleles are
different (e.g., Tt).
