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Chapter 2 - Inheritance and Mutations

TRANSCRIPT

You’re watching Inheritance and Mutations, chapter two of the genetics and fertility video series from the American Society for Reproductive Medicine. 

Diseases can be passed down in different ways, depending on the disease and how many copies of the gene a child inherits. When illustrating inheritance it helps to use a Punnett square. The square is named for Reginald C. Punnett, who devised the following approach.

Draw a large square divided into four smaller squares, you should have two rows and two columns. One parent is drawn on the top and one parent is drawn on the side, shown here in gray.


The diagram is used to predict the genetic makeup of the children, also shown here in gray. In our example each square represents 25% of the genes a child will inherit for a particular trait for any child born from the two parents. Each parent can pass down one of two potential chromosomes. For instance the mother in this example scenario has one chromosome with a mutation in purple, and one chromosome without a mutation in orange. The mother passes one copy of each of her chromosomes down each column from top to bottom. Meanwhile the father who has one chromosome with a mutation in purple and one chromosome without a mutation in orange passes each of his chromosomes across each row from left to right.

 

Each child has a 50% chance to receive one of two chromosomes from their father and a 50% chance to receive one of two chromosomes from their mother. Typically a child will not receive two chromosomes from one parent, and typically a child will not have more than two of each chromosome.

So the outcome for this scenario means that there is a 25% chance that one child will be born with two chromosomes that carry the mutation, and a 25% chance that the child will be born without the mutation on either chromosome. The remaining 50% means that the child will be born with one copy of the mutation and one copy of the chromosome without the mutation like their parents.

The Punnett square determines the likelihood of having an affected child with each pregnancy. This likelihood resets for each pregnancy, so even if your child is affected, their siblings will have the same chance of having a mutation or not having a mutation. So while it is rare for two parents with one mutation each to have four children that all have two copies of the mutation it is still possible. Now that we understand the basics of inheritance let’s look as some of the different types of genetic mutations.

Genetic disorders caused by mutations in a single gene are called monogenic. These disorders can be autosomal—meaning the mutation is on any of the autosomes shown here in orange—or sex-linked—meaning the mutation is on the X or Y chromosome, shown here in pink and blue.

Autosomal mutations can be either recessive or dominant.

Autosomal recessive means that you need 2 copies of the mutated gene to have the disease. Examples are sickle cell anemia and cystic fibrosis.

If you only have one copy of a gene with an autosomal recessive disease mutation, and your other copy is normal, you are called a carrier. This means you won’t have the disease yourself but could pass it on to your children if the child’s other parent is a carrier and has a recessive mutation in the same gene. 

The chance of each outcome breaks down like this:
  • Affected person plus a non-carrier = All children are carriers, no children have the disease, and no children are without the mutation.
  • Affected person plus a carrier = Statistically, half the children will have the disease and half will be carriers.
  • Carrier plus a carrier = Statistically, half the children will be carriers, 1/4 will have the disease, and 1/4 will not have the mutation.
  • Carrier plus a non-carrier = Statistically, half of children are carriers and half are non-carriers. 
On the other hand, autosomal dominant diseases only need one copy of the mutation to manifest as disease, for example, achondroplastic dwarfism. With autosomal dominant diseases, the chances of having an affected child are 50%, since a child can be affected by a disease with only one copy of the mutated gene from either parent.

It breaks down like this:
  • If one parent has one mutated copy and the other parent is healthy, there‘s a 50% chance of having a healthy child and a 50% chance of having an affected child with one mutated copy.
  • If at least one parent has both copies of an affected gene, all children will be affected- this is rare.
  • If both parents have one mutated copy of the gene, there is a 1 in 4 chance of a healthy child, a 2 in 4 chance of getting a child with one mutated copy (and thus affected), and a 1 in 4 chance of having a child with 2 mutated copies. Because this is autosomal dominant, the overall chance that a child will be affected by the disease is 3 in 4.

Mutations can also occur in the sex chromosomes
X and Y.

These mutations can be trickier because, depending on the sex, the chromosome pairs are not the same. Men are XY, women are XX.

Some Y-chromosome mutations can cause absent or low sperm counts. These mutations can be passed down from fathers to sons.

Like autosomal mutations, X-linked diseases can be recessive or dominant.  This is important, because in the case of X-linked recessive conditions, such as hemophilia, the chance of a child having the disease is different for boys and girls.

Males are XY, if a man’s X chromosome has a mutated gene, even if it is a recessive trait, he will have the disease, since that is his only X chromosome.
A female has two X chromosomes and would need to have a mutation on both of her X chromosomes to have an X-linked recessive disease. If she has only one X chromosome with the gene mutation and the other X chromosome is normal, she a carrier. This means she will not have the disease or she will have a milder form of the disease, but will be able to pass it to her children.

The chance of having an affected child with an X-linked recessive disease breaks down like this:

  • Carrier mother and healthy father: There is an equal chance of having a healthy girl, a healthy boy, an affected boy, or carrier girl. Statistically, this means that half of the children, boys and girls together, will be unaffected. For the other half of the children, the girls will be carriers and the boys will be affected.
  • Non-carrier healthy mother and affected father: All the girls will be carriers since the father can only pass his mutated X chromosome to the girls, while none of the boys will have the disease since the father passes his Y chromosome.
  • An affected mother with 2 copies, and affected father with one copy: All children will be affected. This is rare.
  • Carrier mother with one copy and an affected father: Half of the girls will be carriers and half of the girls will be affected. Half of the boys will be affected and half of the boys will be unaffected.
We hope you found this information helpful. For the next chapter in this series please click here.

For more information about genetic testing in the setting of infertility treatment please visit reproductivefacts.org
ReproductiveFacts.org
is a patient education website of ASRM.