Talk of the many mutations present in the DNA of any one individual should not strike fear in the hearts of those who are planning families. The vast majority of babies are born healthy. In populations in which the parents are not related to each other, more than 97% of the children can be expected to be born without birth defects, and more than 99% will be free of major birth defects. This reflects, at least in part, the rarity of dominant alleles that can cause birth defects, but it also reflects just how often the hidden genetic defects (the recessive alleles) in one person are different from the hidden genetic defects in another. So what happens when people who are related have children together? They have an increased chance of sharing defective alleles, and the rate of birth defects may double. These numbers tell us that, even in the case of first-cousin marriages, most babies are born healthy. According to the March of Dimes, some of the most common birth defects include cerebral palsy, spina bifida, cleft lip/palate, lack of one or both kidneys, obstruction of the small intestine or urine passage, diaphragmatic hernia or abdominal wall malformations, chromosomal anomalies of which Down's syndrome is the most common, and limb malformations. Although there are genetic components to some birth defects, many people born with birth defects go on to have normal children, and good prenatal care reduces the chance of some types of birth defects. One of the most noticeable advances came about when it was discovered that addition of folic acid to the diet of pregnant women reduces the frequency of spina bifida.
the process that originally generated each of the alleles that we follow from one generation to the next when we look at inheritance of two different versions of a trait. Perhaps that mutation happened thousands of years ago and has been handed down to many descendants who all hold that mutation as something that makes them different from the rest of the human race. Or perhaps the mutation responsible for a particular trait happened in the sperm or egg that created you.
A mutation is a stable and heritable change in the genome of an organism, which in the case of a human being means a change in the sequence of the DNA that spells out the genetic blueprint. Not only is mutation a genetic process that actually alters the base sequence of the DNA, it is also the term used for the result of the process—a new sequence that is different from what was present in the DNA of that individual's parents. In the case of a human, a mutation can be either a change in the sequence of the DNA of the chromosomes in the nucleus or a change in the sequence of the mitochondrial chromosome. Most of the mutations are on the chromosomes in the nucleus, since the vast majority of the DNA is there. Mutations include changes from one base pair to another (for example, A-T to G-C), deletion of one or more base pairs, or insertion of one or more bases. In Chapter 14, we talked about the basic concept of how mutations affect us—through the production or lack of production of a gene product or altered gene product. However, we have not yet talked about the many different kinds of mutations or how they come about.
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