Assignment: Deep Vein Thrombosis (DVT)

Genetic Principles What determines how a genotype is expressed to create a particular phenotype? This question has not yet been fully answered (Moore, 2015). However, a number of genetic principles have been discovered, among them those of dominant and recessive genes, sex-linked genes, and polygenically determined characteristics.

Dominant and Recessive Genes In some cases, one gene of a pair always exerts its effects; in other words, it is dominant, overriding the potential influence of the other gene, which is called the recessive gene. This is the dominant-and-recessive genes principle. A recessive gene exerts its influence only if the two genes of a pair are both recessive. If you inherit a recessive gene for a trait from each of your parents, you will show the trait. If you inherit a recessive gene from only one parent, you may never know that you carry the gene. Brown hair, farsightedness, and dimples override blond hair, nearsightedness, and freckles in the world of dominant and recessive genes. Can two brown-haired parents have a blond- haired child? Yes, they can. Suppose that each parent has a dominant gene for brown hair and a recessive gene for blond hair. Since dominant genes override recessive genes, the parents have brown hair, but both are carriers of blondness and pass on their recessive genes for blond hair. With no dominant gene to override them, the recessive genes can make the child’s hair blond.

Sex-Linked Genes Most mutated genes are recessive. When a mutated gene is carried on the X chromosome, the result is called X-linked inheritance. It may have implications for males that differ greatly from those for females (Simon & others, 2016). Remember that males have only one X chromosome. Thus, if there is an absent or altered, disease-relevant gene on the X chromosome, males have no “backup” copy to counter the harmful gene and therefore may develop an X-linked disease. However, females have a second X chromosome, which is likely to be unchanged. As a result, they are not likely to have the X-linked disease. Thus, most individuals who have X-linked diseases are males. Females who have one abnormal copy of the gene on the X chromosome are known as carriers, and they usually do not show any signs of the X-linked disease. Fragile X syndrome, which we will discuss later in the chapter, is an example of X-linked inheritance (Karmiloff-Smith & others, 2016).

Polygenic Inheritance Genetic transmission is usually more complex than the simple examples we have examined thus far (Moore, 2015). Few characteristics reflect the influence of only a single gene or pair of genes. Most are determined by the interaction of many different genes; they are said to be polygenically determined. Even a simple characteristic such as height reflects the interaction of many genes as well as the influence of the environment. Most diseases, such as cancer and diabetes, develop as a consequence of complex gene interactions and environmental factors. The term gene-gene interaction is increasingly used to describe studies that focus on the interdependent process by which two or more genes influence characteristics, behavior, diseases, and development (Cho & Suh, 2016; Hodge, Hager, & Greenberg, 2016). For example, recent studies have documented gene-gene interaction in immune system functioning (Heinonen & others, 2015), asthma (Hua & others, 2016), alcoholism (Yokoyama & others, 2013), cancer (Wu & others, 2016), cardiovascular disease (Musameh & others, 2015), arthritis (Hohman & others, 2016), and Alzheimer disease (Ebbert & others, 2016).