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Chapter 25: Problem 2

Explain, at the molecular level, why human genetic diseases often follow a simple Mendelian pattern of inheritance, whereas most normal traits, such as the shape of your nose or the size of your head, are governed by multiple gene interactions.

Short Answer

Expert verified
Human genetic diseases often follow a simple Mendelian pattern of inheritance because they are caused by a mutation in a single gene. The presence of that mutation determines if a person develops the disease. Also, those traits are often hidden or recessive, which is a key part of Mendelian principles. In contrast, normal traits like nose shape or head size are determined by multiple genes and their interactions (polygenic), and often the environment also plays a role in these traits. As such, polygenic traits do not follow the simple one gene-one trait Mendelian pattern.

Step by step solution

01

Understanding Mendelian Inheritance

Mendelian inheritance refers to the pattern of inheritance that was first described by Gregor Mendel based on his observations of pea plants. Mendel found that certain traits follow an inheritance pattern, where an offspring receives one allele from each parent. Consequently, certain diseases are caused by a single mutation in a single gene and thus follow Mendelian inheritance. Examples include cystic fibrosis and sickle cell anemia.
02

Understanding Polygenic Inheritance

In contrast, polygenic inheritance refers to the inheritance of traits that are determined by more than one gene. These traits exhibit a continuous distribution in the population, such as height or skin color, and do not follow Mendelian inheritance. This is because these traits are influenced by multiple genes and their interactions, and often also by environmental factors.
03

Applying Mendelian and Polygenic Inheritance to the Question

Human genetic diseases often follow a simple Mendelian pattern of inheritance because they are caused by a mutation in a single gene. This means that the presence of one or two copies of the mutant allele determines whether an individual will develop the disease. On the other hand, normal traits like the shape of a person's nose or the size of their head are determined by multiple genes and their interactions. These traits are polygenic, so they do not follow a simple Mendelian pattern of inheritance. Instead, the combined effect of many genes, as well as environmental factors, determines these traits.

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Most popular questions from this chapter

In general, why do changes in chromosome structure or number tend to affect an individual's phenotype? Explain why some changes in chromosome structure, such as reciprocal translocations, do not.

Achondroplasia is a rare form of dwarfism caused by an autosomal dominant mutation that affects the gene that encodes a fibroblast growth factor receptor. Among 1,422,000 live births, the number of babies born with achondroplasia was 31. Among those 31 babies, 18 of them had one parent with achondroplasia. The remaining babies had two unaffected parents. How do you explain those 13 babies, assuming that the mutant allele has \(100 \%\) penetrance? What are the odds that these 13 individuals will pass this mutant gene to their offspring?

We often speak of diseases such as phenylketonuria (PKU) and achondroplasia as having a genetic basis. Explain whether the following statements are accurate with regard to the genetic basis of any human disease (not just PKU and achondroplasia). A. An individual must inherit two copies of a mutant allele to have disease symptoms. B. A genetic predisposition means that an individual has inherited one or more alleles that make it more likely that she or he will develop disease symptoms than other individuals in a population will. C. A genetic predisposition to develop a disease may be passed from parents to offspring. D. The genetic basis for a disease is always more important than the environment.

Like Hurler syndrome, Fabry disease involves an abnormal accumulation of substances within lysosomes. However, the lysosomes of individuals with Fabry disease show an abnormal accumulation of lipids. The defective enzyme is \(\alpha\)-galactosidase \(\mathrm{A}\), which is a lysosomal enzyme that functions in lipid metabolism. The defect causes cell damage, especially to the kidneys, heart, and eyes. The gene that encodes \(\alpha\)-galactosidase \(A\) is found on the \(X\) chromosome. Let's suppose a phenotypically unaffected couple produces two sons with Fabry disease and one phenotypically unaffected daughter. What is the probability that the daughter will have an affected son?

With regard to pedigree analysis, make a list of observations that distinguish recessive, dominant, and \(\mathrm{X}\)-linked patterns of inheritance.
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