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 How do germ-cells make a single set of genes from the normal two copies that all
other cells in the body have? If progeny plants inherited a single whole gene set
from each parent, then the experiment in the above Figure cannot work. This is
because the two characteristics „R‟ and „y‟ would then be linked to each other
and cannot be independently inherited. This is explained by the fact that each
gene set is present, not as a single long thread of DNA, but as separate
independent pieces, each called a chromosome.
 Thus, each cell will have two copies of each chromosome, one each from the
male and female parents.
 Every germ- cell will take one chromosome from each pair and these may be of
either maternal or paternal origin.
 When two germ cells combine, they will restore the normal number of
chromosomes in the progeny, ensuring the stability of the DNA of the species.
 Such a mechanism of inheritance explains the results of the Mendel experiments,
and is used by all sexually reproducing organisms. But asexually reproducing
organisms also follow similar rules of inheritance.

SEX DETERMINATION
The process of determining the sex of an individual based on the composition of the
genetic material is called sex determination.
Environmental Sex Determination:
 Different species use very different strategies for this. Some rely entirely on
environmental cues.
 Thus, in some animals, the temperature at which fertilised eggs (e.g- Turtles) are
kept determines whether the animals developing in the eggs will be male or
female.
 In other animals, such as snails, individuals can change sex, indicating that sex
is not genetically determined.

Genetic Sex Determination: e.g- Human beings
 In different animals, sex of an embryo is determined by different factors.
 In humans, sex determination happens on the basis of the presence or absence
of Y chromosome.
 XX is female and XY is male
 Female Ovum: 23 Chromosomes = 22 Autosomes + XX sex chromosomes
 Male Sperm : 23 Chromosomes = 22 Autosomes + X or 22 Autosomes +Y
 An ovum always contains X chromosome.
 An Ovum, upon fusion with Y containing sperm, gives rise to a male child and
upon fusion with X containing sperm gives rise to a girl child.

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