GENES & INHERITANCE
NUCLEUS
Nucleus is a membrane bound
structure that contains the cell’s
hereditary information and controls
the cell’s growth and reproduction.
•The shape of the nucleus is mostly
round, it may be oval, disc shaped
depending on the type of cell.
► The nucleus of every cell contains thin, thread like structures
called chromosomes.
► Chromosomes are made from a chemical substance called DNA
[deoxyribonucleic acid]. They contain heredity information and
instructions for cell growth, development, and reproduction.
► Different species of organism have different number of
chromosomes in the nuclei of their cells.
► Eg: Human cells have 46 chromosomes, tomato plant has 24
chromosomes, earthworm has 36 chromosomes.
► Each chromosome is made up of different genes. Each gene
helps to control a particular characteristic in the organism.
► Genes are short sections of DNA. A DNA molecule is twisted like
a ladder and the shape is called double helix.
FUNCTION OF NUCLEUS
►Nucleus controls the heredity
characteristics of an organism.
Stores heredity material in the form of
deoxy-ribonucleic acid (DNA)
strands. Also stores proteins and
ribonucleic acid (RNA) in the nucleolus.
•Alleles are variations of the same gene
•As we have two copies of each chromosome, we have
two copies of each gene and therefore two alleles for
each gene
•One of the alleles is inherited from the mother and the
other from the father
•For example, an individual has two copies of the gene
for eye colour but one allele could code for brown eyes
and one allele could code for blue eyes
•Alleles can be dominant or recessive
Genotype and Phenotype
Genotype
Phenotype
The hereditary information of the
The characters of an organism which
organism in the form of gene in the DNA are visible are known as phenotypes.
and remains the same throughout the
life.
Present inside the body as genetic
material.
Expression of genes as the external
appearance.
It is affected by genes.
It is affected by genotype and
environmental conditions.
For eg., Blood group, eye colour,
height, genetic diseases.
For eg., Weight, physique, beak of birds
Structure of DNA – double helix
Gametes
►A gamete is a mature reproductive sex
cell that contains 23 /half the number
of chromosomes [ 50% of the genetic
material/haploid number ]
►In humans, sperm cell [ in males ] and
egg cell [in females] are specialised
cells known as gametes.
Sperm cell [ in males]
Egg cell [ in females]
Difference between sperm cell and egg
cell
Sperm cell [ male gamete]
Egg cell [ female gamete]
Produced in the testes, in males
Produced by ovaries in females
Usually small and motile
Large and non motile
Produced in large numbers
Only a single gamete is formed at a
time
In humans, the male gamete decides
the sexuality of the offspring
Female gametes do not decide the
sexuality in humans
Contains a small cytoplasm
Contains a larger cytoplasm to nourish
the embryo
Contains a tail that helps it to swim
Does not contain a tail as it is immobile
The process by which male and female
gamete combine together to form a zygote is
called fertilisation.
Gender determination