If you have ever gone through a high school biology course, you were sooner or later treated to the story of Gregor Mendel and his pea plants. Once upon a time, circa mid 1800s, an Augustinian monk named Father Mendel lived in what is now the Czech Republic.
In his spare time he enjoyed a bit of gardening. He liked to grow pea plants and good-naturedly wait for them to crossbreed in specific combinations.
Working with seven characteristics of pea plants; plant height, pod shape and color, seed shape and color, and flower position and color he made some unusual observations. For example with seed color, he found that when a yellow pea and a green pea were bred together their offspring plant was always yellow. On the other hand, in the next generation of plants, the green peas would reappear in a ratio of 1:3. Working on his theories of the inheritance of traits, he eventually coined the terms recessive and dominant, in reference to certain traits.
As (bad) luck would have it, he was promoted to abbot and as such no longer had the time to devote to his experiments. In addition to the foregoing, scientists of the time, including theorists like Charles Darwin, considered his scientific findings worthless. It was not until several decades later that his findings were considered seriously and reproduced, showing that he was correct all along in his findings that certain dominant genes will elicit certain traits to evidenced in a plant or animal.
Genetic Alphabet Soup
From there the next step was to recognize DNA as the carrier of the genetic information, and ultimately apply the theory of dominant genes to the human body. Modern science has connected genetic dominance and birth defects via an out-and-out alphabet soup of markers and abbreviations that sometimes makes it hard to understand just what is being said. In order to make some sense out of all the abbreviations, here is the relationship between dominance and birth defects in a nutshell:
Dominance of a genetic trait simply means that you will see it brought to expression in the appearance of a body when it is pitted against another similar but recessive gene.
An easy to understand example is the color of the iris which determines your eye color. Brown is a dominant trait, an as such when genes with the brown coding are present, the child will have brown eyes. Conversely, if the dominant brown trait is absent, the next most dominant gene, in this case green, will determine the color of the eyes. Green is a recessive gene, but when compared to other recessive genes determining eye color, it is almost always dominant.
Therefore, if a child receives a green recessive iris determining gene from his father, but a brow dominant iris determining gene from his mother, the odds are good that the child will have brown eyes.
Of course, this is highly simplified and is rarely, if ever, as simple. Dominance may sometimes refer to a condition where a conceived child will inherit identical dominant genes, while other times the dominant gene will be pitted against a different dominant gene. Then again, at times a dominant gene may be offset by a certain combination of recessive genes. For example, there are two classes of inheritance, Mendelian, and non-Mendelian. Non-Mendelian inheritance is an all-purpose term referring to any pattern of inheritance where traits do not separate out according to Mendels laws. His laws describe the inheritance of traits linked to single genes on chromosomes in the nucleus. Mendelian inheritance birth defects are disorders that are caused by mutations in a single gene.
Birth defects that may be linked back to a dominant combination of genes are Huntingtons disease which has been located as a gene present on chromosome four, Pfeiffer syndrome, and also hereditary multiple exostoses which refers to bony masses that form and attach themselves to the various bones in the body of a child. There are a host of other diseases associated with the dominant gene, and many of these will be diagnosed successfully during genetic counseling prior to conception. Non-Mendelian birth defects are more common than Mendelian.