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Name: Danielle
Status: Student
Age: 11
Location: N/A
Country: N/A
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 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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