INTRODUCTION — WHY ARE CHILDREN SIMILAR TO THEIR PARENTS?
If we observe living organisms carefully, we notice that
offspring resemble their parents.
Examples:
- Humans
give birth to human babies.
- Mango
trees produce mango plants.
- Puppies
resemble dogs.
However, offspring are not completely identical to
their parents. Some differences always exist.
These similarities and differences are explained by:
Heredity
WHAT IS HEREDITY?
Heredity
is the transmission of traits and characteristics from
parents to offspring.
Traits are inherited through:
Genes
Genes are present on:
Chromosomes
which contain:
DNA
WHAT IS VARIATION?
The differences among individuals of the same species are
called:
Variations
Examples:
- Different
eye colour
- Different
height
- Different
hair texture
Variation is important because it helps organisms survive
changing environmental conditions.
8.1 ACCUMULATION OF VARIATION DURING REPRODUCTION
During reproduction:
- DNA
copies itself.
- Small
errors may occur during copying.
- These
errors create variations.
Over generations:
- Variations
accumulate.
- Diversity
increases in populations.
Example from Textbook
If one bacterium divides repeatedly:
- New
bacteria will be very similar.
- Minor
variations appear due to DNA copying errors.
But in:
Sexual reproduction
much greater variation occurs because genetic material comes
from two parents.
IMPORTANCE OF VARIATION
Not all variations help survival.
Useful variations increase chances of survival.
Textbook example:
Heat-resistant bacteria survive better during heat waves.
This natural selection forms the basis of:
Evolution
HEREDITY
The reproductive process produces organisms of similar body
design because hereditary information passes from parents to offspring.
INHERITED TRAITS
Traits passed from parents to children are called:
Inherited Traits
Examples:
- Eye
colour
- Earlobe
attachment
- Height
tendency
Activity 8.1 — Earlobe Observation
NCERT discusses two types of earlobes:
|
Type |
Description |
|
Free earlobe |
Hangs freely |
|
Attached earlobe |
Attached to side of head |
Students observe:
- Their
own earlobes
- Parents’
earlobes
Conclusion:
Traits are inherited from parents.
MENDEL AND HIS CONTRIBUTIONS
Gregor Johann Mendel
is called:
Father of Genetics
He performed experiments on:
Garden pea plants
WHY DID MENDEL CHOOSE PEA PLANTS?
Advantages:
✔ Easy to grow
✔ Short life cycle
✔ Clear contrasting traits
✔ Self-pollinating
CONTRASTING TRAITS USED BY MENDEL
|
Trait |
Contrasting Trait |
|
Tall plant |
Short plant |
|
Round seed |
Wrinkled seed |
|
Violet flower |
White flower |
MENDEL’S MONOHYBRID CROSS
Mendel crossed:
- Tall
plant (TT)
with - Short
plant (tt)
FIRST GENERATION (F1)
Result:
All plants were tall
No medium-height plants appeared.
This showed:
- Traits
do not blend.
- One
trait dominates over the other.
SECOND GENERATION (F2)
When F1 plants self-pollinated:
Result:
- Tall
plants appeared
- Short
plants appeared
Ratio:
3 Tall : 1 Short
This proved:
- Both
traits were inherited.
- One
trait remained hidden in F1 generation.
DOMINANT AND RECESSIVE TRAITS
Dominant Trait
Trait expressed even when one copy is present.
Example:
Tallness (T)
Recessive Trait
Trait expressed only when both copies are recessive.
Example:
Shortness (t)
GENOTYPE AND PHENOTYPE
|
Term |
Meaning |
|
Genotype |
Genetic makeup |
|
Phenotype |
Visible character |
Examples:
|
Genotype |
Phenotype |
|
TT |
Tall |
|
Tt |
Tall |
|
tt |
Short |
MENDEL’S DIHYBRID CROSS
Mendel also studied inheritance of:
- Seed
shape
- Seed
colour
Cross:
- Round
yellow seeds
with - Wrinkled
green seeds
F1 GENERATION
All seeds were:
Round and Yellow
Thus:
- Round
shape is dominant
- Yellow
colour is dominant
F2 GENERATION
Four combinations appeared:
|
Trait Combination |
Observation |
|
Round yellow |
Most common |
|
Round green |
Present |
|
Wrinkled yellow |
Present |
|
Wrinkled green |
Least common |
Ratio:
9 : 3 : 3 : 1
This showed:
Traits are inherited independently
LAW OF DOMINANCE
When two contrasting traits are present together:
- Only
one expresses itself in F1 generation.
That trait is:
Dominant Trait
LAW OF SEGREGATION
Traits separate during gamete formation.
Each gamete carries only one trait.
LAW OF INDEPENDENT ASSORTMENT
Different traits are inherited independently of each other.
Example:
Seed colour inheritance does not affect seed shape inheritance.
HOW DO TRAITS GET EXPRESSED?
Genes control traits by controlling:
Protein synthesis
Proteins control body structure and functions.
EXAMPLE — TALLNESS IN PLANTS
Plant growth depends on growth hormones.
If a gene produces efficient enzyme:
- More
hormone produced
- Plant
becomes tall
If enzyme is less efficient:
- Less
hormone produced
- Plant
becomes short
Thus:
Genes control characteristics through proteins.
CHROMOSOMES
Genes are located on:
Chromosomes
Important facts:
- Chromosomes
carry hereditary information.
- Humans
have chromosomes in pairs.
- One
chromosome comes from mother.
- One
chromosome comes from father.
WHY DO GAMETES HAVE HALF CHROMOSOME NUMBER?
Gametes contain:
Half the chromosome number
Reason:
During fertilisation, chromosome number restores to normal.
This maintains stability of species.
SEX DETERMINATION
Different organisms use different methods of sex
determination.
ENVIRONMENTAL SEX DETERMINATION
In some reptiles:
- Temperature
determines sex of offspring.
SEX DETERMINATION IN HUMAN BEINGS
Humans have:
23 pairs of chromosomes
Out of these:
- 22
pairs are autosomes
- 1
pair are sex chromosomes
SEX CHROMOSOMES
|
Gender |
Chromosomes |
|
Female |
XX |
|
Male |
XY |
HOW IS SEX OF CHILD DETERMINED?
Mother produces gametes carrying:
X chromosome only
Father produces gametes carrying:
X or Y chromosome
POSSIBILITIES
|
Combination |
Child |
|
XX |
Girl |
|
XY |
Boy |
Probability:
50% boy and 50% girl
Important:
Father determines the sex of child.
IMPORTANT POINT
Mother is NOT responsible for determining child’s sex.
The chromosome contributed by father decides:
- X →
Girl
- Y →
Boy
IMPORTANT DEFINITIONS
|
Term |
Definition |
|
Heredity |
Transfer of traits from parents to offspring |
|
Variation |
Differences among organisms |
|
Gene |
Unit of heredity |
|
Chromosome |
DNA-containing structure |
|
Dominant trait |
Trait expressed in F1 |
|
Recessive trait |
Hidden trait |
|
Genotype |
Genetic makeup |
|
Phenotype |
Physical appearance |
