Most commonly, traits are influenced by multiple genes and the environment. So for most traits, patterns of inheritance are complex and unpredictable. For instance, when a sexually reproducing organism inherits a defective or "broken" allele that codes for a non-functional protein, the second copy can often make up for the loss. This type of interplay between alleles, especially when traits are influenced by multiple genes, makes predicting inheritance difficult, if not impossible.
A small number of traits are influenced primarily by a single gene. Traits influenced mainly by one gene usually have predictable patterns of inheritance. However, most of the time even supposed "single-gene" traits are influenced by multiple genes, as in the case of eye color. And often times, a single gene can influence multiple traits. For example, some genes involved in eye color also influence hair and skin color.
Why do so many textbooks pretend that eyes come in just two colors, brown and blue? Eye color is influenced by two major genes and several minor genes. Variations in these genes determine whether your eyes are brown, blue, green, gold, hazel, greyish-blue, brown with green flecks, blue with a gold ring...you get the idea.
In rare cases, changes in DNA sequences can cause disease. The pattern of disease inheritance can be predictable or unpredictable, depending on whether the disease is influenced mainly by one gene or by several genes. See Genetic Disorders
Sex chromosomes
It's not quite accurate to say that everyone inherits two copies of every gene. In most mammals, the X and Y sex chromosomes determine whether an individual is male or female. Females have two X chromosomes, and therefore two copies of every gene. Males, however, have one X and one Y chromosome. For genes that appear only on the X chromosome or only on the Y [some do appear on both], males inherit just one copy.
Other living things have sex chromosomes too. Birds and reptiles have Z and W sex chromosomes. Unlike with X and Y, males have two Z chromosomes, and females have one Z and one W. Some insects and a few mammals have only an X chromosome. Females have two copies and males have one. However, sex is not always determined by chromosomes. With alligators, crocodiles, and most turtles, it's egg incubation temperature. And some fish can change gender in response to cues from the environment. Sex determination happens in many different ways.
Traits that involve genes on the sex chromosomes have different inheritance patterns than non-sex-linked traits. Color-blindness and Duchenne muscular dystrophy, for example, are linked to genes on the X chromosome. Both of these disorders are more common in males because they have just one X chromosome. They have no second allele to mask the effects of a defective gene.
There is a similar rate of recurrence among all first-degree relatives [parents, siblings, offspring of affected child]. It is unusual to find a substantial increase in risk for relatives related more distantly than second degree to the index case.
◆The risk of recurrence is related to the incidence of the disease.
◆Some disorders have a sex predilection, as indicated by an unequal male:female incidence. Pyloric stenosis, for example, is more common in males, whereas congenital dislocation of the hips is more common in females. With an altered sex ratio, the risk is higher for the relatives of an index case whose gender is less often affected than relatives of an index case of the more frequently affected gender. For example, the risk to the son of an affected female with infantile pyloric stenosis is 18%, compared with the 5% risk for the son of an affected male. An affected female presumably has a greater genetic susceptibility, which she can then pass on to her offspring.
◆The likelihood that both identical twins will be affected with the same malformation is