Heredity: It Runs in the Family
Were you ever told when you were younger that you looked just like your mother or father? This is because we inherit genes from our parents that determine what we look like. Just like diseases or health risks, physical traits like your hairline and the shape of your nose can be determined by genes that have been passed on from generation to generation in your family. The passing of traits through generations is called heredity.
Genes are passed from parents to children. Every person has two sets of chromosomes in their cells. When a man and woman conceive a baby, one set of chromosomes from each parent combine to make up the DNA of their child. Remember: everyone has two copies of every gene. Because we receive only half of our mother's genes (one copy of each gene) and half of our father's genes, we are not identical to either of our parents.
With each pregnancy, the chromosomes from our mother and father combine in different ways. This is why we are similar to, but not identical to our siblings; we receive a different combination of our parent's genes.
As you may have guessed, identical twins are the exception. Each twin gets the same set of genes from their mother and fatherâ€”making them physically and genetically identical.
Genetic Risk for Sex-Specific Diseases Comes from Both Sides of the Family
Although an individual may look similar to a relative, it does not mean they will develop the same diseases and health problems as that relative. Each gene is passed separately, so if you've always been told you look just like your mother, that doesn't mean you will have all of the same health problems. For example, even the look, shape and color of your eyes is controlled by different genes than those that affect how your eyes work, so having the same shape and color eyes as a parent doesn't mean you will also inherit age-related macular degeneration from them.
In addition, genetic risk factors for diseases that affect only males or only females can be transmitted by both genders. For example, a male can have and pass on to his children a genetic variant that increases the risk of disease that only occurs in females, such as ovarian cancer.
If a man has an extensive family history of ovarian cancer, it is possible he inherited a genetic variant associated with ovarian cancer. Even though he cannot develop ovarian cancer, he could still pass on this variant to his children. His daughters may have an increased risk for developing ovarian cancer and his sons may in turn pass the variant on to their daughters, increasing their risk for ovarian cancer.
Similarly, females can have and pass on to their children genetic variants that increase the risk of diseases that only occur in males. For example, a woman can pass on a genetic variant associated with an increased risk of prostate cancer to her sons, even though she cannot ever get prostate cancer herself.