The Inheritance of Retinal Degenerations
Page 2
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BASIC GENETICS
In most cells of the body, a structure, called the nucleus, contains the genetic material
responsible for inherited traits (Figure 2). The genetic material is packaged in the form
of chromosomes, rod-like structures visible only through a microscope.
Figure 2. A human cell. The nucleus contains the
genetic material
Chromosomes have various sizes and come in pairs. Every cell, except eggs and
sperm, contains 23 pairs, or 46 chromosomes. Twenty-two pairs are the same in both
males and females and are called autosomes. The sex chromosomes make up the
twenty-third pair. Females have two similar chromosomes designated the X
chromosomes (Figure 3), and males have one X chromosome and a different, smaller
chromosome (Figure 4).
Figure 3. The 46 chromosomes of a normal female
Courtesy of Marion Marcus, Genetics Laboratory, Kennedy Institute
Each egg and sperm has only 23 chromosomes, one of each pair. Every egg has 22
autosomes and one X chromosome. Each sperm has 22 autosomes and either an X or
a Y chromosome (but not both). At conception when the egg and the sperm cells
merge, the cell they form will have 46 chromosomes, half from the mother (from the
egg) and half from the father (from the sperm). If the sperm carried an X chromosome
the child will have two X's and be female. If the sperm carried a Y chromosome, the
child will have an X and a Y and be male.
Figure 4. The 46 chromosomes of a normal male
Courtesy of Marion Marcus, Genetics Laboratory, Kennedy Institute
Chromosomes are composed of genes. Genes are small pieces of hereditary material
arranged along the chromosomes much like beads on a string. Genes are made of a
chemical called deoxyribonucleic acid (DNA). Because chromosomes come in pairs,
the genes are paired as well. It may be easiest to imagine that a pair of genes carries a
set of instructions which, once read by the body, specifies an inherited trait such as
blood type. Although each pair of genes specifies one trait, each gene of the pair may
carry different instructions for that trait and each will influence how the trait is actually
produced in the body.
Occasionally there is a change in a gene, called a mutation, that alters the instructions
for that gene. A mutated gene may cause an abnormal trait or disease. Genetic
diseases are classified as dominant if only one mutated gene, when paired with a
normal gene, is needed to produce the disease and recessive if a pair of mutated
genes (two copies of the gene) is needed to produce the disease. Disease-causing
mutated genes can lie on autosomes or sex chromosomes. Retinal degenerative
diseases are due to several different mutated genes that produce dominant or recessive
diseases.
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Date last modified December 4, 1998