Chapter 24: Medical Genetics: Human Inheritance Patterns and Genetic Disorders

What underlies every aspect of human health?

Our genes

What is important for understanding diseases?

Knowledge of how genes work together and interact with the environment

What impact will genetics have on medicine?

It will revolutionize the diagnosis, treatment, and prevention of diseases

How many genetic tests are currently in clinical use?

Several hundred

What are examples of genetic diseases that can be tested?

Sickle-cell anemia, Huntington disease, cystic fibrosis

How many genetic diseases are estimated to afflict people?

Approximately 12,000

What is often the cause of many genetic diseases?

A mutation in one gene

What types of diseases may involve multiple genes?

Diabetes, asthma, mental illness

What is the focus of the study of human genetic diseases?

Diseases that result from defects in single genes

What inheritance patterns do mutant genes often obey?

Simple Mendelian inheritance patterns

What is concordance in genetics?

The percentage of twin pairs in which both twins exhibit the disorder or trait

What is a pedigree?

A diagram that shows the relationship among family members and their hereditary condition

What are the five main types of traits determined by a single gene?

Autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, Y-linked

What is a characteristic of autosomal dominant traits?

Trait usually present in every generation

What is a major symptom of Huntington Disease?

Degeneration of certain types of neurons in the brain

What is the result of the mutation in Huntington Disease?

It adds a polyglutamine tract to the huntingtin protein

What is albinism?

Absence or partial deficiency of melanin in the skin, eyes, and hair

What is cystic fibrosis?

The most common lethal genetic disease among Caucasians

What does the CFTR protein regulate?

Ion transport across the cell membrane

What is the average life expectancy of a person with cystic fibrosis?

~50 years

What is the inheritance pattern of autosomal recessive traits?

Traits tend to skip generations

What is a common feature of disorders involving defective enzymes?

They typically have an autosomal recessive mode of inheritance

What is haploinsufficiency?

The heterozygote has 50% of the normal protein, which is insufficient for a normal phenotype

What are gain-of-function mutations?

Mutations that change a protein so it gains a new function

What are dominant negative mutations?

Altered gene products that act antagonistically to the normal product

What is the significance of studying twin concordance?

It helps evaluate the genetic basis of diseases

What does it mean if a disease does not spread to individuals sharing similar environmental situations?

It suggests a genetic basis for the disease

What is the role of pedigree analysis in genetics?

To determine the inheritance pattern of human traits

What is the relationship between identical twins and disease?

Identical twins share the disease more often than fraternal twins

What is Tay-Sachs disease?

A neurodegenerative disorder caused by a mutation in the hexosaminidase A gene.

When do symptoms of Tay-Sachs disease typically appear?

At 4 to 6 months of age.

What are common symptoms of Tay-Sachs disease?

Cerebral degeneration, blindness, and loss of motor function.

What is the typical life expectancy for individuals with Tay-Sachs disease?

3 to 4 years of age.

What enzyme is deficient in Tay-Sachs disease?

Hexosaminidase A (hexA).

What lipid accumulates in Tay-Sachs disease?

GM2-gangliosides.

What is a characteristic of X-linked recessive traits?

They may skip generations.

Who are most affected individuals in X-linked recessive traits?

Most affected individuals are male.

How do affected males inherit X-linked recessive traits?

From affected mothers or mothers who are carriers.

How do affected females inherit X-linked recessive traits?

From affected fathers and affected or carrier mothers.

What is hemophilia?

A disorder where blood cannot clot properly.

Why is hemophilia referred to as the 'royal disease'?

It has affected many members of European royal families.

What are the two types of hemophilia?

Hemophilia A and Hemophilia B.

What causes Hemophilia A?

A defect in clotting Factor VIII.

What causes Hemophilia B?

A defect in clotting Factor IX.

What is a common symptom of hemophilia?

Severe internal or external bleeding from common injuries.

What is a characteristic of X-linked dominant traits?

They do not skip generations.

How do affected males inherit X-linked dominant traits?

From affected mothers.

What is Vitamin D-resistant rickets?

A disorder involving softening and weakening of bones due to lack of phosphate.

What is the only type of trait that affects males?

Y-linked traits.

What is an example of a Y-linked trait?

Hypertrichosis of the ear.

What does locus heterogeneity refer to?

A disease caused by mutations in two or more different genes.

What is penetrance?

The proportion of individuals with a specific genotype who manifest the corresponding phenotype.

What is complete penetrance?

When all individuals with a genotype express the phenotype.

What is incomplete penetrance?

When not all individuals with a genotype express the phenotype.

What is expressivity?

The degree to which a penetrant gene is phenotypically expressed.

What is phenylketonuria (PKU)?

An autosomal recessive disorder caused by a defect in the phenylalanine hydroxylase gene.

What happens to individuals with PKU if untreated?

Phenylalanine accumulates, causing mental and physical abnormalities.

What is a phenocopy?

An organism whose phenotype has been modified to resemble that of a different mutant organism.

What caused the Thalidomide syndrome?

Use of the drug Thalidomide during pregnancy, leading to birth defects.

What is the teratogenic effect of (S)-Thalidomide?

It causes birth defects.

What are the desirable properties of (R)-Thalidomide?

It acts as a sedative and antinausea drug.

Approximately how many genetic diseases are currently known to afflict humans?

12,000

What is the primary reason geneticists rely on pedigree analysis rather than controlled crosses in humans?

Controlled matings are not possible for ethical and practical reasons.

What term describes identical twins formed from the same sperm and egg?

Monozygotic twins.

Fraternal twins are also known by what term?

Dizygotic twins.

What does the term 'concordance' refer to in the study of twin pairs?

The percentage of twin pairs in which both individuals exhibit the same disorder or trait.

Why is the actual concordance of a genetic disorder often lower than its theoretical value?

Environmental factors also contribute to the development of the disease.

Which observation regarding environmental spread suggests a disease has a genetic basis?

The disease does not spread to individuals sharing similar environmental situations.

In a pedigree, what term is used for the specific person from whom the family tree is traced?

Propositus or proposita.

What is a characteristic feature of autosomal dominant traits regarding generational presence?

The trait is usually present in every generation.

In an autosomal dominant pedigree, what is the significance of two affected parents producing an unaffected child?

It proves that both parents are heterozygotes carrying a recessive normal allele.

On average, what proportion of children born to one affected parent in an autosomal dominant inheritance pattern will be affected?

Half of the children.

Which specific protein is encoded by the gene mutated in Huntington Disease?

Huntingtin.

What specific type of mutation in the huntingtin gene causes Huntington Disease?

The addition of a polyglutamine tract to the protein.

What is the physiological cause of the symptoms in Huntington Disease?

The aggregation of mutant huntingtin protein causes the degeneration of certain neurons in the brain.

Define 'haploinsufficiency' as an explanation for autosomal dominant disorders.

A condition where the heterozygote has 50% of the normal protein, which is insufficient for a normal phenotype.

How does a 'gain-of-function' mutation result in an autosomal dominant disorder?

The mutation changes the protein so that it acquires a new, abnormal function.

What is the mechanism of a 'dominant negative' mutation?

The altered gene product acts antagonistically to the normal protein product.

What is a common generational characteristic of autosomal recessive traits?

The trait tends to skip generations.

In autosomal recessive inheritance, what is the expected phenotype of children born to two affected parents?

All children will be affected.

Why do disorders involving defective enzymes typically follow an autosomal recessive inheritance pattern?

Heterozygotes possessing 50% of the normal enzyme level typically maintain a normal phenotype.

Which protein is defective in individuals with Cystic Fibrosis?

Cystic fibrosis transmembrane conductance regulator (CFTR).

What is the primary function of the CFTR protein in cell membranes?

It regulates ion transport across the membrane.

What is the approximate carrier frequency of Cystic Fibrosis among Caucasians?

3%.

Tay-Sachs Disease is caused by a mutation in the gene encoding which enzyme?

Hexosaminidase A (HexA).

The accumulation of which specific category of lipids causes neurodegeneration in Tay-Sachs Disease?

GM2-gangliosides.

Where in the cell do unbroken lipids accumulate in Tay-Sachs patients?

In the lysosomes.

What is the typical lifespan for a patient with Tay-Sachs Disease?

3 to 4 years of age.

What term describes males regarding X-linked genes since they only have one X chromosome?

Hemizygous.

In X-linked recessive inheritance, who does an affected male result from?

An affected mother or a mother who is a carrier.

Why can an X-linked recessive trait never be transmitted directly from father to son?

Fathers only pass their Y chromosome to their sons.

What protein is defective in Duchenne muscular dystrophy?

Dystrophin.

Hemophilia A is caused by a defect in which specific clotting factor?

Clotting factor VIII.

Which clotting factor is defective in Hemophilia B?

Clotting factor IX.

In Androgen Insensitivity Syndrome (AIS), what is the chromosomal sex of the affected individual?

XY.

What is the phenotype of an individual with Androgen Insensitivity Syndrome?

External features are feminine, but internally they have undescended testes and no uterus.

Which inheritance pattern is characterized by affected males passing the trait to all daughters but no sons?

X-linked dominant.

In X-linked dominant inheritance, what is the expected outcome for the children of an affected homozygous female?

All children will be affected.

Which bone disorder is an example of an X-linked dominant trait?

Vitamin D-resistant rickets.

What is the defining characteristic of Y-linked inheritance?

Traits are passed from father to all sons.