Appendix: Patterns of Inheritance
Most cells of the human body contain 23 pairs of chromosomes, half of which are inherited from each parent. Only the human reproductive cells, the sperm in males and the egg in females, have 23 individual chromosomes, not pairs. Scientists number them 1 through 22. The 23rd chromosome is a sex-determining chromosome. It is named X in an egg. The sex chromosome in a sperm may be either the same as in an egg, X, or a different chromosome called Y.
When the reproductive cells, the sperm and egg, combine at fertilization, the fertilized egg that results contains 23 chromosome pairs. A fertilized egg that will develop into a female contains chromosome pairs 1 through 22, and the XX pair. A fertilized egg that will develop into a male contains chromosome pairs 1 through 22, and the XY pair.
There are hundreds of diseases caused by mutations in a single gene. These diseases are usually inherited in one of several straightforward patterns, depending on the gene involved. If the gene is located on the X chromosome, the inheritance is called X-linked. If the gene is located on a non-sex chromosome (autosome), the inheritance is called autosomal.
The inheritance of a gene can also be dominant or recessive. Recessive inheritance means that two copies of the gene (one on each of the chromosome pair) must be altered in order for the disease to occur. By contrast, dominant inheritance means that mutations in one copy of the gene are sufficient to cause the disease.
The mutated gene is located on an autosome (non-sex chromosome). One mutated copy of the gene in each cell is sufficient to cause a disease. Each affected person usually has one affected parent.
The mutated gene is located on an autosome. Two mutated copies of the gene are present in each cell when a person has an autosomal recessive disorder. An affected person usually has unaffected parents who each carry a single copy of the mutated gene (and are referred to as carriers).
The mutated gene is located on the X chromosome. Females are more frequently affected than males, because they have two X chromosomes.
The chance of passing on an X-linked dominant disorder differs between men and women. A striking characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons (no male-to-male transmission).
The mutated gene is located on the X chromosome. Sons of a carrier mother have a 50 percent chance of inheriting the disorder. Daughters also have a 50 percent chance of inheriting the defective gene but usually are not affected, since the healthy X chromosome they receive from their father can offset the faulty one received from their mother. Therefore, males are more frequently affected than females.
The chance of passing on the disorder differs between men and women. A father cannot pass X-linked traits to his sons.
In codominant inheritance, two different versions (alleles) of a gene can be expressed, and each version makes a slightly different protein.
This type of inheritance, also known as maternal inheritance, applies to genes in mitochondrial DNA. Because only egg cells contribute mitochondria to the developing embryo, only females can pass on mitochondrial conditions to their children.