Modes of Inheritance – Dominant & Recessive
You have probably wondered about your curly hair, the color of your eyes, the tone of your skin, your height and possibly even your body type and how they relate to your parents’ more than a fair share of times. There are some traits that your mother and father have that you do not have and that your sibling has, and then there are those that you share with your parents. And if you have kids, how is it possible that perhaps they have some traits that you parents have but you do not? And perhaps it is even the other way around – you have them but your kids do not? Or your husband does not have those traits, but his parents have them. And so on. It all comes down to how the traits that your parents have are passed down to you, or how you have inherited them. This process is called the modes of inheritance and depending on the types of genes that your parents share with you, these are either dominant or recessive, and we will get into that in just a little bit.
In order to truly appreciate how characteristics are passed on or down in a family, it is important to have an understanding of the basic laws of inheritance. This is particularly useful when one tries to understand the chances of inheriting a disease, depending on who in the family has had it. It is also really important if you are at a stage of planning a family, and you would like to know the chances or perhaps even risks of your future child inheriting some undesired genetic traits.
Let us begin by saying that every one of us has two copies of almost every gene, and each copy is inherited from each of the parents. In other words, half our genetic material is our mothers and the other is our fathers. For a long time, scientists have tried to understand how genes work and how changes in the genes affect their function. Some genetic changes do not have an effect on the genes’ functions, while others, such as mutations, do have an effect and can cause diseases.
Not all diseases affect all family members the same way. Some mutations may be expressed differently among the family tree, manifesting themselves as different symptoms. On the contrary, individuals with different mutations can have similar symptoms or disease characteristics. This is because of another reason, namely that how genes are expressed is not just under the control of the genetic makeup but also as a result of environmental influences on the genotype (or the set of genetic information each one of us possesses).
Conditions that are results of mutations in one gene are inherited in a simple straightforward pattern. This, of course, is largely dependent on the location of the gene in the chromosome and whether two copies or just one copy of the gene is needed. We know this in large part because Gregor Mendel, the Augustinian monk and father of modern genetics, delineated these rules based on the study of garden pea plants. While disorders that are the result of one single gene are rare, they do affect millions of Americans. Single-gene disorders can exist in several modes, namely dominant, autosomal recessive, X-linked dominant, and X-linked recessive.
|Pattern of Inheritance||Traits||Diseases|
|Autosomal Dominant||Affected individuals also have an affected parent. Typically affects every generation.||Huntington’s disease|
|Autosomal Recessive||Affected individuals have parents who are both carriers of the mutation, and the condition does not always affect every generation.||Tay-Sachs disease, sickle cell anemia, cystic fibrosis, phenylketonuria (PKU).|
|X-linked Dominant||Predominantly affects females because all daughters and no sons of an affected man will be affected. If the mother is affected, this can lead to affected males and females in the same generation.||Hypophatemic rickets (vitamin D resistant rickets), ornithine transcarbamylase deficiency|
|X-linked Recessive||Predominantly affects males are more frequently affected, and those who are affected are often present in each generation.||Hemophilia A, Duchenne muscular dystrophy|
|Mitochondrial||Equally prevalent in male and female children. However, since mitochondria originate from the mother, it can only be passed on by females. It can appear in every generation.||Leber’s hereditary optic neuropathy, Kearns-Sayre syndrome|
In the case of dominant mutations, one copy of the mutation is enough to cause disease. This means that if you inherit one copy of a dominant disease, you will suffer from it. An example of a dominant disease is Huntington’s disease. Furthermore, diseases that are inherited dominantly typically affect every generation of the family, whereby each affected person also has one affected parent. An exception to this is something called a spontaneous mutation, whereby a dominant mutation affected an individual without prior family history of it.
In the case of recessive mutations, both copies of the gene mutation have to be present in order for the disease to develop. Each of the parents is typically a carrier, even though they do not have the disease or disease symptoms. By virtue of this and the fact that both copies of the mutation have to be present, the disease is not always present in every generation of the affected family. In other words, if each of the parents passes down a copy of the genetic mutation, the child or children will have the disease.
Patterns of inheritance are a bit more complicated when it comes to mutations sex chromosomes – X chromosomes in females and X/Y chromosomes in males. In general, females carry two X chromosomes (XX), while males carry one of each (XY). In other words, while females can carry two copies of an X-linked genetic mutations, males carry just one copy of an X-linked and a Y-linked mutations. Since females do not have a Y chromosome, they do not carry genetic mutations that are located on it and are therefore unaffected by diseases that result from them. Just like with non sex chromosomes (or autosomes), diseases that are carried on the X chromosomes can be inherited in a dominant or recessive pattern. However, since males only have on X chromosome, they will be affected by a genetic mutation on it regardless of whether it is dominant or recessive.