Date: March 2004
How do we get genes from one parent and not the other?
Answer: The genetic material in the chromosomes is inherited equally from
both parents. The sperm contains 23 chromosomes, one from each pair, in its
nucleus. It is a very small cell, with hardly any cytoplasm besides what it
needs to help it swim to the ovum, or egg. Like the sperm, the egg also
contributes 23 chromosomes to the fertilized zygote, which then grows and
develops into an embryo, then a fetus, and then a baby.
The ovum, or egg cell, is a much larger cell than the sperm. Since it does
not need to "travel light" as the sperm does, it contains lots of cytoplasm.
In fact, virtually all the cytoplasm in the zygote comes from the mother's
ovum, and this includes all the mitochondria present in the cytoplasm of her
Mitochondria are organelles in the cytoplasm that are important in
metabolism. They contain a small amount of DNA, and this is in addition to
the large bulk of our DNA that is present on the chromosomes in the nucleus.
As the embryo grows and divides into more cells, the new cells make new
cytoplasmic organelles as needed. Whenever they make new mitochondria, they
include mitochondrial DNA copied from the DNA template of the original
So it is that mitochondrial genes are inherited almost exclusively from the
The genes on this mitochondrial DNA, although they are few, are important to
our bodies. Occasionally harmful mutations, or genetic changes, may occur in
these genes, resulting in genetic disorders.
If a person has a harmful mutation in one of the mitochondrial genes, this
will be transmitted to any new mitochondria that are made by copying the
mutated DNA. Thus we may have a mixture of normal and mutated mitochondria
in our cells. When our cells divide, the new cells may contain somewhat more
or fewer of the mutated mitochondria, and this may differ, simply by chance,
in different parts of the body. This is called heteroplasmy.
This mode of inheritance for mitochondrial genes is quite different from
genetic disorders whose genes are on the chromosomes. Whereas every cell
needs the exactly correct number of chromosomes, the number of mitochondria
is variable, and the proportion of normal and mutated mitochondria in the
cells varies even in different cells of the mother's or the child's body.
A man with a mutation in his mitochondrial genes is unlikely to pass the
mutation on to the next generation because sperm cells contain very few, if
When a woman who has a mutation in some of her mitochondria produces an
ovum, or egg cell, of her own, it will contain an assortment of mitochondria
with normal and mutated DNA. By chance there may be fewer or more of the
mutated mitochondria, and this may cause the child to have a milder or more
severe genetic disorder.
(Note that in plants, besides the main bulk of the DNA in the chromosomes of
the nucleus and a small amount in the mitochondria, some DNA is also present
in the chloroplasts, organelles involved in photosynthesis.)
Sarina Kopinsky, MSc, H.Dip.Ed.
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Update: June 2012