How does the fertilized ovum obtain its particular set of chromosomes in
the first place?
Each adult possesses gonads in which _sex cells_ are formed. In the
male, sperm cells are formed in the testes; in the female, egg cells are
formed in the ovaries.
In the formation of the sperm cells and egg cells there is a key
step—_meiosis_—a cell division in which the chromosomes group into pairs
and are then apportioned between the daughter cells, one of each pair to
each cell. Such a division, unaccompanied by replication, means that in
place of the usual 23 pairs of chromosomes in each other cell, each sex
cell has 23 individual chromosomes, a “half-set”, so to speak.
In the process of fertilization, a sperm cell from the father enters and
merges with an egg cell from the mother. The fertilized ovum that
results now has a full set of 23 pairs of chromosomes, but of each pair,
one comes from the father and one from the mother.
In this way, each newborn child is a true individual, with its
characteristics based on a random reshuffling of chromosomes. In forming
the sex cells, the chromosome pairs can separate in either fashion (_a_
into cell 1 and _b_ into cell 2, or vice versa). If each of 23 pairs
does this randomly, nearly 10 million different combinations of
chromosomes are possible in the sex cells of a single individual.
Furthermore, one can’t predict which chromosome combination in the sperm
cell will end up in combination with which in the egg cell, so that by
this reasoning, a single married couple could produce children with any
of 100 trillion (100,000,000,000,000) possible chromosome combinations.
It is this that begins to explain the endless variety among living
beings, even within a particular species.
It only begins to explain it, because there are other sources of
difference, too. A chromosome is capable of exchanging pieces with its
pair, producing chromosomes with a brand new pattern of gene varieties.
Before such a _crossover_, one chromosome may have carried a gene for
blue eyes and one for wavy hair, while the other chromosome may have
carried a gene for brown eyes and one for straight hair. After the
crossover, one would carry genes for blue eyes and straight hair, the
other for brown eyes and wavy hair.
[Illustration: Meiosis]
Interphase
Prophase
Metaphase
Anaphase
Interphase
Metaphase
Interphase