The Inheritance of Retinal Degenerations

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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
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).

46 chromosomes of a normal female
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.

46 chromosomes of a normal 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 January 23, 2000