GENES AND CHROMOSOMES

It seems like every day there is a news announcement about the discovery of a new gene . But what exactly are genesA segment of DNA that contains the instructions to make a specific protein (or part of a protein). Genes are contained on chromosomes. Chromosomes, and the genes on those chromosomes, are passed on from parent to child. Errors in the DNA that make up a gene are called variants and can lead to diseases.?

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 cellsThe smallest living unit. Cells make up all organs and tissues in multi-cellular organisms, like humans. They can also live independently, as in bacteria and other microorganisms. At a minimum, a cell is surrounded by a membrane, contains DNA at some stage in its life, and is able to replicate itself into two equal parts. individually, our genes are located or housed on chromosomesA strand of DNA contained within a cell. Each chromosome contains many thousands of genes. In humans, there are a total of 46 chromosomes, half of which come from each parent. The combined total of all chromosomes in a cell is the genome.. 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 DNADeoxyribonucleic acid (DNA) is a molecule found in the chromosomes that carries genetic information. DNA is composed of four units (called bases) that are designated A, T, G, and C. The sequence of the bases spell out instructions for making all of the proteins needed by an organism. A gene is a section of DNA that holds the instructions for a specific protein. A change in one or more of the DNA bases making up a gene is called a mutation. Some mutations change the protein instructions and can lead to particular health problems or disorders. Each parent passes half of their chromosomes, and thus half of their DNA instructions, onto their children. It is these instructions that cause certain traits, such as eye or hair color, to be inherited. 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 codeThe genetic code determines how the sequence of bases in a gene code for the sequence of amino acids in a protein. A gene is made of bases that are designated A, T, G, and C. Each series of three bases is essentially a word that codes for one of the 20 amino acids that make up all proteins. For example, the sequence AAG within a gene tells a cell to insert the amino acid lysine into a growing protein.,”  a sort of hereditarySomething that is passed on from a parent to the child, usually through the genes. 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 “genomeAll of the genetic material (DNA) contained in a full set of chromosomes in an organism. In humans, about three billion base pairs make up our 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 inheritedAcquiring a trait from one’s parents. Most traits, such as eye color or hair color, are inherited from a parent through genes. metabolic disorders are caused by a pair of recessive genes and are called “autosomal recessiveMost of the metabolic disorders that can be detected by newborn screenings are inherited in an “autosomal recessive” pattern. Autosomal recessive conditions affect both boys and girls equally. How autosomal recessive inheritance works: 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..”

 

EnzymesA molecule that helps chemical reactions take place. For example, enzymes in the stomach speed up the process of breaking down food. Each enzyme can participate in many chemical reactions without changing or being used up. 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 disorderPeople with metabolic disorders are not able to break down certain substances in food. Too much or too little of a certain substance (like fat, carbohydrate or protein) can cause serious health problems. Fatty acid oxidation disorders (FAODs), amino acid disorders (AAs) and organic acid disorders (OAs) are three types of metabolic disorders..

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.

INHERITANCE PATTERNS

Autosomal Recessive Inheritance

Most of the metabolic disorders that can be detected by newborn screeningA screening test that looks for different disorders using a small sample of blood taken from a newborn’s heel. A positive or abnormal newborn screening result means that there are slight differences that were found in the baby’s blood, and further testing is needed to figure out if the baby has a metabolic disorder. 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 carriersA person who has one copy of a gene mutation for a particular autosomal recessive disorder (remember genes come in pairs). Carriers are not affected by the disorder. However, they can pass on the gene variant to their children. Children who inherit two such gene variants will be affected by the disorder. The term variants is now used in place of the term mutation. 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 DominantGenes come in pairs. If one gene of a pair causes, by itself, a particular trait or disorder to be present, it is called ‘dominant.’ If the gene is found on one of the first 22 pairs of chromosomes, it is called autosomal dominant (chromosomes 1 through 22 are called ‘autosomes’). Although genes are always in pairs, in autosomal dominant inheritance, you only need to inherit one copy of a dominant gene in order to show a particular trait.Autosomal dominant conditions can be inherited from one parent, who also has that condition or the gene for that condition, or can occur in one person in the family for the first time (called a ‘new mutation’). If neither parent has the autosomal dominant gene change that the child has, it is a ‘new mutation’ just present in that child, and there is a low risk for it to affect other siblings. However, the person with the autosomal dominant gene change has a 50% chance to pass it on to each of his or her children. Inheritance

Currently none of the metabolic disorders that can be detected by newborn screeningThe process of testing for disease in a person who does not show signs of having the disease (nonsymptomatic or asymptomatic person). The goal of screening is to catch the disease in its early stages. are inherited in an “autosomal dominant” pattern. However, some of the causes of Critical CongenitalThis means ‘present at or before birth.’ It usually refers to health conditions or birth defects that are present in a baby at or prior to birth. Heart Defects can be geneticRelating to (or due to) genes and heredity or the field of studying genes and heredity. 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 proteinA molecule that makes up many parts of every cell in the body. Examples of proteins include hormones, enzymes, hair, and antibodies. Proteins are made up of 20 different types of individual units called amino acids. It is the order of these amino acids in a protein that determines what form and function a protein has. Each gene holds the instructions for making a single 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.

X-linkedA mode of inheritance. X-linked genes are found on the X chromosome. They have a different inheritance pattern than other genes because women have two X chromosomes while men only have one. Any mutation on the X chromosome may not cause a disease in women if the gene on the other chromosome is normal. However, that same mutation on one X chromosome in men will cause the disease because they have no second copy of the gene to compensate. Inheritance

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 chromosomeOne of the two chromosomes that are responsible for determining the sex of an organism. The other sex chromosome is called the Y chromosome. Both the X and the Y chromosome contain several genes, only some of which are involved in determining sex. In humans, the X chromosome is the larger of the two sex chromosomes. Females usually have two X chromosomes and males usually have one X chromosome and one Y 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 chromosomeOne of the two sex chromosomes that is responsible for determining the sex of an organism. The other sex chromosome is called the X chromosome. Both the X and the Y chromosome contain several genes, only some of which are involved in determining sex. In humans, the Y chromosome is the smaller of the two sex chromosomes. Most females have two X chromosomes and most males have one X chromosome 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 recessiveThis is a pattern of inheritance in which a gene for a particular trait or disorder is located on the X chromosome. Genes that are ‘recessive’ cause traits or conditions only when they are paired with a dominant gene. Genes usually come in pairs, except on the sex chromosomes in males. Males have one X chromosome that they inherit from their mothers and one Y chromosome that they inherit from their fathers. If a gene on the X chromosome causes a particular trait or disorder, a male will always show that trait or condition as they do not have another gene to cover up its effects. Females usually have a normal copy of that gene on their other X chromosome which covers up the effects of the recessive gene. Some common X-linked recessive disorders include hemophilia, Duchenne muscular dystrophy and color blindness. 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.