GENES AND CHROMOSOMES
It seems like every day there is a news announcement about the discovery of a new gene . But what exactly are genes?
At their most basic, genes are the units of inheritance that are passed from one generation to another. They are located in every cell of our bodies and they provide the instructions for every structure and function of our body. It is estimated that we have over 30,000 different genes. Rather than having all 30,000 bouncing around inside cells individually, our genes are located or housed on chromosomes. Chromosomes are tightly wound structures composed of DNA (deoxyribonucleic acid) which is the building block of genes.
Chromosomes, as well as the genes located on them, come in pairs. We have a total of 46 chromosomes arranged into 23 pairs. The first 22 pairs of chromosomes are called “autosomes ” and are numbered 1 to 22, from largest to smallest. The 23rd pair is called the “sex chromosomes” and consist of an X and a Y in males and two X chromosomes in females. Chromosomes can be seen under a microscope but the individual genes located on them cannot.
All 30,000 genes are located on our chromosomes. Genes are made of DNA which is a molecule made up of four different chemicals called bases. These bases are usually referred to by their initials – A for adenine, T for thymine, G for guanine and C for cytosine. These 4 bases make up the “genetic code,” a sort of hereditary alphabet. Genes are made up of small sections of DNA. If the bases are the alphabet, then genes can be thought of as sentences. Each sentence (or gene) provides a very specific instruction for our bodies. One pair of genes might tell what color our eyes will be and another might tell how long our big toe should be! When scientists want to refer to all of our genes together, they call it a “genome.” If bases are the alphabet, and genes are the sentences, then the genome is truly the book of life.
All of our genes (except those on our sex chromosomes) come in pairs. Although they all are in pairs, different genes come in different forms. Some are ” dominant” meaning that only one of a pair is necessary for that gene’s trait to be expressed. Other forms are “recessive,” and only express their traits if found in double dose – i.e., in the absence of a dominant gene. Most inherited metabolic disorders are caused by a pair of recessive genes and are called “autosomal recessive.”
Like beads on a string, every bead represents a separate gene. There are thousands of genes on each chromosome
HOW ENZYMES WORK
Enzymes are proteins that our bodies use to digest food and turn it into energy. Our bodies need over 1000 different enzymes to break down the food we eat into usable energy to fuel our bodies. If an enzyme is missing or not working properly, then an individual will have a metabolic disease. Each enzyme in our body is constructed based on instructions from a specific pair of genes. If both genes in a pair of genes are not working, then the enzyme they instruct will not form properly. If a person is lacking an enzyme, they will have an inherited metabolic disorder.
Inherited metabolic disorders are caused by alterations, or mutations, in a pair of genes leading to the absence or malfunctioning of one particular enzyme. If a particular enzyme is missing or not working properly, the substance it is supposed to metabolize will not break down and will build up in the bloodstream and/or cells. This usually interferes with normal growth and development and can cause a variety of medical problems.
Autosomal Recessive Inheritance
Most of the metabolic disorders that can be detected by newborn screening are inherited in an “autosomal recessive” pattern. Autosomal recessive conditions affect both boys and girls equally.
Everyone has a pair of genes for each enzyme in the body. A separate pair of genes is responsible for making each enzyme. A person with a metabolic disorder has one enzyme that is either missing or not working properly. The problem is caused by a pair of “recessive” genes that are not working correctly. They do not make the needed enzyme.
A person has to have two non-working “recessive” genes in order to have an autosomal recessive metabolic disorder. A person with an autosomal recessive disorder inherits one non-working gene from his or her mother and the other from his or her father. The parents are called carriers for that condition.
Parents of children with a metabolic disorder rarely have the disorder themselves. Instead, for that pair of genes, each parent has one that is working correctly and one that is not working (called the “recessive gene). People with a single non-working gene are called carriers. If one gene of the pair is working correctly, it makes up for the recessive non-working gene. Therefore, carriers usually will not have the condition.
Autosomal Dominant Inheritance
Currently none of the metabolic disorders that can be detected by newborn screening are inherited in an “autosomal dominant” pattern. However, some of the causes of Critical Congenital Heart Defects can be genetic and inherited in an autosomal dominant pattern. Autosomal dominant conditions affect both boys and girls equally.
Everyone has a pair of genes that code for each protein in the body. Each pair of genes is responsible for making different proteins that make up the body. A person with an autosomal dominant condition has one gene in a pair that is either missing or not working properly. The problem is caused by a single “dominant” gene that is not working correctly. They either cannot make the needed protein or the protein that is made does not work properly.
A person only needs to have one non-working “dominant” gene in order to have an autosomal dominant condition. A person with an autosomal dominant condition can either inherit one non-working gene from one parent or the non-working gene can be something that started new in him or her. This means that sometimes one of the parents of a child with an autosomal dominant condition can also have the condition him or herself.
Occasional disorders screened for by newborn screening can be inherited in an X-linked pattern. In this type of inheritance, the gene is located on the X chromosome, one of the sex chromosomes. Genes usually come in pairs, with each parent giving one copy to their child. The sex chromosomes, however, are different.
A male inherits one X chromosome from his mother, and one Y chromosome from his father. A female inherits two X chromosomes, one from each parent. When a female has a non-working copy of the gene on one of her X chromosomes, she will not have an X-linked disorder. This is because she has a second, working copy of the gene on her other X-chromosome.
A male with a non-working copy of the gene for an X-linked disorder on his X chromosome will have the disorder. This is because he does not have a second X chromosome with a working copy of the gene. Therefore, it is more common for males to have X-linked recessive disorders than females.
If a mother has one non-working copy of the gene, she is called a carrier. For carriers, there is a 50% chance that each male pregnancy will have the X-linked disorder. For carriers, there is a 50% chance that each female pregnancy will be a carrier, like her mother.
A father passes his Y chromosome to his sons and his X chromosome to his daughters. Therefore, if a father has an X-linked disorder, none of his sons will have the disorder and all of his daughters will be carriers.