Basic Genetics

Your body’s cells

Cells are the tiny, living building blocks of your body.  Every living part of you - your muscles, skin, blood, nerves, bones, and organs - is made up of living cells. New cells are made when the old cells divide in two.  Every human being starts as one cell, the cell formed when a woman's egg is fertilized by a man's sperm.

Chromosomes

Chromosomes are tiny rod shapes inside each cell of the body.  There are 23 pairs of chromosomes (46 total) in each cell.  Almost all the cells in your body have 46 chromosomes.  There are two types of cells that are different.  These are the eggs in a woman and the sperm in a man.  They only have 23 chromosomes each.

When a woman becomes pregnant, her egg joins with the father's sperm to make a new cell.  This cell will grow and divide into a baby.  This first cell has 46 chromosomes, 23 from the mother and 23 from the father.  All the instructions for how the child will grow are in the genes on these chromosomes.  Each cell of the body will contain exact copies of the 23 pairs of chromosomes that were in the first cell that was formed by the egg and sperm.

The chromosomes are kept in a special part of the cell called a nucleus.  If you had a very strong microscope, you could see the chromosomes inside the nucleus of the cell.

The chromosomes contain your genes

Genes are the blueprints for your body.  They are the instructions for building new cells.  They tell what color hair and eyes you will have.  They tell what diseases you might be likely to get.  Genes tell the body's cells how to make factor.  There are tens of thousands of genes arranged along the chromosomes. All of the genes are the same in each cell and they are all arranged the same way.

What makes a baby a boy or girl?

There are 23 pairs of chromosomes in each cell of the body.  One pair of chromosomes is different in men and women.  It is called the sex chromosome pair.  The sex chromosome pair determines if a child will be a boy or a girl.  A woman’s sex chromosome pair is labeled XX.  A man’s sex chromosome pair is labeled XY.

An egg or sperm only has half of the parent's sex chromosome pair.  An egg will always have an X chromosome, since the woman's sex chromosome pair only has Xs.  Sperm can have either an X chromosome or a Y chromosome.

If a sperm with an X chromosome fertilizes the egg, the new cell will have two X chromosomes.  One came from the father's sperm and one came from the mother's egg.  Since the sex chromosome that is made is XX, the cell will grow into a girl baby.

If a sperm with a Y chromosome fertilizes the egg, the new cell will have an X and a Y chromosome.  The X came from the mother's egg and the Y came from the father's sperm.  Since the sex chromosome that is made is XY, the cell will grow into a boy baby.

Figure 2-2. How the sex of a baby is set.  One sex chromosome comes from each parent.  Follow the arrows to see the possible match-ups. 

Genetic defects

If a gene is faulty, there will be a problem with the part of the body it directs.  This is called a genetic defect. There is a gene that tells certain cells in the body how to make clotting factor.  There are also genes that tell how to make von Willebrand factor (VWF) and how to make each part of a platelet.  If there are problems with any of those genes, the factor or platelets that are made may not work right.  There may not be enough factor made.  The person will have a bleeding disorder.  What type the person has depends on which gene has the defect.

Sex-linked disorders

A sex-linked gene is any gene that is found on the sex chromosome labeled X.  The instructions that these genes give are called sex-linked traits. If a defective gene is on the X chromosome, it is called a sex-linked disorder. It will occur almost exclusively in men.  Hemophilia and color blindness are both sex-linked disorders.  The genes that cause them are on the X chromosome.

Autosomal disorders

The 22 pairs of chromosomes that are not the sex chromosomes are called autosomes.  If a disorder is carried by a gene on one of these autosomes, it is called an autosomal disorder.  Since it is on the autosome and not the sex chromosomes, it can occur in both males and females.

Dominant and recessive genes

When the 23 chromosomes from the father's sperm and the 23 from the mother's egg meet, they pair up.  The genes on the chromosomes pair up, too.  The paired genes, one from each parent, carry the plans for the same part of the body.  For instance, the gene for hair color from the father pairs up with the gene for hair color from the mother.

Sometimes one of the genes in the pair is stronger and blocks the instructions from the other gene.  The stronger gene that takes control is said to be dominant.  The gene whose instructions are blocked is called
recessive.  A recessive gene will have a chance to send its instructions to the cells only if it is paired with another recessive gene.

Suppose a child receives a gene for brown eyes from one parent and a gene for blue eyes from the other parent.  Since the gene for brown eyes is the dominant gene, the child will have brown eyes.  The child will still carry the recessive gene for blue eyes and may pass it on to his or her children.

For each gene pair, a child can inherit two dominant genes, two recessive genes, or one dominant and one recessive gene.  Which kind of genes he or she gets from the parent is decided by chance.

If a disorder occurs in a child when only one parent had the gene, it is called a dominant disorder.  If both parents must have the gene for the child to get the disorder, it is called a recessive disorder.