Ask A Scientist Molecular Biology Archive

name         danielle
status       educator
age          11

Question -   How do we get genes from one parent and not the other?
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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
ovum cell.

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
mitochondria.

So it is that mitochondrial genes are inherited almost exclusively from the
mother.

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
any, mitochondria.

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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Most of the time you DO get a gene for each trait from each parent.  But there are a few 
genes that only come from your mother.  Mothers give the cytoplasm of the cell to the 
baby.  The father only gives his chromosomes and no cytoplasm.  In the cytoplasm (the 
watery stuff inside the cell) are little structures called organelles.  They have jobs 
to do just like your organs have jobs to do for your body.  One of these organelles is 
called the mitochondria.  It is the "powerhouse" of the cell because food is broken down 
into energy there.  Since they are found in the cytoplasm and only the mother's egg has 
cytoplasm, all of your mitochondria in every cell came from her.  It also happens that 
mitochondria have their own DNA.  So your mitochondria have genes of their own, and since 
you only get them from your mother, these genes only come from your mother and her mother, 
and her mother, and so on.
 
vanhoeck
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As far as the DNA in the nucleus goes, you get one half from your mother, and one half 
from your father. But you also have a small amount of DNA in your mitochondrion, which 
is the organelle in your cells that plays a central role in processing energy. When the 
egg is formed, mitochondria from the mother goes into the egg. If that egg is fertilized
 by sperm from the father, the sperm does not contribute mitochondrial DNA, just the 
 father's nuclear DNA. The new embryo will have Mom's mitochondrial DNA, half of Mom's 
 nuclear DNA, and half of Dad's nuclear DNA.

So why does the mitochondrion contain DNA? There is good evidence supporting the theory 
that present day the mitochondrion is derived from what was once a free-living organism, 
sort of like a present day bacterial cell. That cell was taken up by an early eukaryotic 
cell, probably was supposed to be digested as food, but managed in some accidental way to 
become a symbiotic organism living within the early eurkaryotic cell. Over a great many 
years, the relationship changed from symbiosis, to the mitochondrion existing as an 
organelle within the eukaryotic cell. In other words, you could not take a mitochondrion 
out of a present day cell and expect it to live as an organism today. All of this probably 
took place over a great many years. At the molecular level, the evidence supporting this 
theory is very strong. We see traces of this history in present day mitochondria, such as 
the presence of DNA, ribosomes, etc. 
P.S. It looks like the present day chloroplast in higher plants probably evolved in a 
similar manner. It too has DNA in it. 

Paul Mahoney, PhD 
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