Nothing is more popular around the dinner table than crediting or blaming your family for who you are. You are so good at math — just like your dad. You sing like a dream — just like your mom. Always late like your Aunt Ellen! Grandma’s eyes, Grandpa’s hair, Aunt Susie’s wit, Uncle Bert’s moods … and your sister’s funny little toe can all be found in you, and it must be because of your genes. But how exactly do genes work, and why are they important if you are trying to have a baby?
Grasping Genetics Basics
Genetics has become a very popular word. With that popularity has come myth and misunderstanding. But the concept of the field of genetics is really quite simple. Genetics deals with the instruction manual on how to build a human. To be exact, a Google search gave the definition of genetics as “the study of heredity and the variation of inherited characteristics.”
What are genes and chromosomes?
Genes are a long string of four chemicals called nucleotides and lettered as A (adenine), C (cytosine), G (guanine), and T (thymine). Words in the code are made from just these four chemicals, and the alphabet has only four letters. That’s really a small alphabet to create a person. The English alphabet has 26 letters, and if you limit the number of syllables a word could have to 14, over 2.75963 x 107 words are possible. Fortunately for those spelling whizzes, the English language has only a little over 200,000 words.
The genetic code has only four letters, and the words (codons) can be only three letters long, so at most 64 possible combinations are used for the genetic code. The directions for building a human, the genetic code, is a string of three-letter words. However, the string is not one continuous string but rather 23 strings of code. These 23 separated strings are called chromosomes (see Figure 3-1). Each time a cell wants to divide, it must accurately create two copies of the genetic code, and it does this one word (codon) at a time.
To complicate this even further, a person inherits one set of chromosomes from each parent so that each cell has two copies of each chromosome. Thus, inheritance is a demanding task of accurately repeating the copying of the code over and over to create the 37 trillion or so cells that make a human.
What do genes do?
Genes are used to direct a cell to make proteins. Proteins are strings of molecules called amino acids, and there are 20 that are used to make proteins. Proteins are the workhorse of constructing a human. The DNA uses a different type of genetic material called RNA to assemble proteins from the amino acids. Each gene determines which amino acids are to be used and in what sequence. The way in which the amino acids are strung together determines the three-dimensional structure of the protein.
FIGURE 3-1: Diagram of a chromosome.
Inheriting Infertility — Really?
The construction of a human is immensely complicated. For proper functioning, all of the various parts need to work together. Any part that does not do its job properly can throw the person out of balance and thus create disease. So normal human functioning means that the systems are in equilibrium and working properly together. Any part not functioning in equilibrium causes the disease. The genetic code determines the basis for the equilibrium; any error in the code can cause the person’s equilibrium to be disturbed, and disease follows. For people having problems conceiving, a question that needs to be answered is whether errors in the genetic code are causing the problem of getting pregnant.
There are a number of different types of genetic errors. Sometimes entire chromosomes may be missing or duplicated. There may be deletions of parts of the chromosome or parts that are misplaced or turned around. There can be errors in the letters of the code, thus causing the wrong amino acid to be used. These errors are called single-nucleotide polymorphisms (SNPs). SNPs are the most common form of variation among people. The error is the use of the wrong letter such that an A (adenine) is switched to a T (thymine). Most of these will not alter the functioning of the person, but some can cause severe disease such a sickle cell anemia. There are many types of sickle cell disease depending upon the gene mutation, but one form is caused when the sequence GAG is changed to GTG — one single letter can cause the destructive disease.
The two most common chromosomal problems causing sterility in females are 47 XXX and 45 XO (Turner’s syndrome). The 47 XXX syndrome occurs in 1 in 1,000 female births and causes premature ovarian deficiency. Turner’s syndrome occurs in 1 in 2,000 female births. Turner’s syndrome is a disease caused by an entire chromosome being absent. Turner’s syndrome results when a person has only one sex chromosome — an X chromosome. This person develops as a female with characteristics such as short stature, a web neck, and a low hairline, and about one-third will have heart defects. These people do not make eggs, so they are menopausal from birth and thus are sterile. However, some people with this problem have a mixture of cells with some having only one but some having two X chromosomes.
