Human Genetics: Mendelian Inheritance and Disorders

What are the two general classes of genetic disorders in humans?

Autosomal disorders and X-linked disorders.

What are the two modes of inheritance for genetic traits?

Dominant and recessive.

What is a pedigree analysis?

A family tree that describes the phenotypes and relationships of parents and children across generations.

What can pedigree analysis reveal about inheritance?

It can reveal whether traits are dominant or recessive and whether they are X-linked or autosomal.

What is the purpose of developing a hypothesis when interpreting pedigrees?

To determine the mode of inheritance and write down possible genotypes based on that hypothesis.

What is the total number of base pairs in the human genome per diploid cell?

6.6 billion base pairs.

How many genes are present in the nuclear genome?

Approximately 45,000 genes

What are the two types of chromosomes in the human genome?

Autosomes (chromosomes 1 to 22) and sex chromosomes (X and Y).

What is the size of the mitochondrial genome and how many genes does it contain?

The mitochondrial genome is small (~17 kilobases) and contains 38 genes.

What percentage of spontaneous abortions are caused by chromosome disorders?

Approximately 50%.

What is the most common chromosome disorder?

Down syndrome.

What causes Down syndrome?

Trisomy for chromosome 21.

What is the incidence of Down syndrome at birth?

Affects 1 in 850 births.

What are single-gene disorders?

Disorders due to mutation in one or both copies of a single gene, affecting about 2% of the population.

What is an example of a single-gene disorder?

Cystic fibrosis.

What are multifactorial disorders?

Disorders that involve the combined effect of multiple genetic defects, often influenced by the environment.

What is an example of a multifactorial disorder?

Bipolar disorder or coronary artery disease.

What is a spontaneous somatic mutation?

A mutation that affects an individual but is not passed on to offspring.

What is a spontaneous germ line mutation?

A mutation that can be passed on to offspring and future generations.

What is the typical cause of trisomy for chromosomes?

Usually caused by meiotic nondisjunction.

What is the role of genetic testing and counseling in human genetics?

To assess the risk of genetic disorders and provide information to families.

What is the significance of pedigree symbols in genetic analysis?

They help identify relationships and inheritance patterns in family trees.

What is the cause of Down Syndrome?

It is caused by meiotic nondisjunction, resulting in a zygote with 3 copies of chromosome 21.

What are the types of single-gene disorders?

They can be autosomal or X-linked, and can be dominant or recessive.

What inheritance patterns do single-gene disorders often display?

They often display standard Mendelian patterns of inheritance.

Name an example of an autosomal recessive disorder.

Sickle cell anemia, cystic fibrosis, or Tay-Sachs disease.

What are examples of X-linked recessive disorders?

Hemophilia A and B, red-green color blindness, and Duchenne muscular dystrophy.

What is an example of an autosomal dominant disorder?

Achondroplasia or Huntington disease.

What is the significance of being a carrier for autosomal recessive disorders?

Phenotypically normal individuals may be homozygous dominant (AA) or heterozygous carriers (Aa).

What is sickle cell disease?

It is caused by autosomal recessive mutations in the b-globin gene on chromosome 11, leading to abnormally shaped red blood cells.

How common is sickle cell disease among African-Americans?

Approximately 10% are carriers.

What recent advancements have been made in treating sickle cell disease?

There has been progress toward curing the disease using gene therapy.

What causes cystic fibrosis?

It is due to recessive autosomal mutations in the CFTR gene on chromosome 7.

What is the carrier frequency of cystic fibrosis among European Americans?

1 in 25 are carriers.

What is the effect of cystic fibrosis on the body?

It leads to increased mucus in the lungs due to high levels of extracellular chloride ions.

How can carriers and affected individuals of cystic fibrosis be identified?

Through DNA tests.

What is the probability that Joe and Jill's child will have cystic fibrosis if Joe is a carrier and Jill's father had CF?

The child has a 25% chance of having CF and a 50% chance of being a carrier.

What are X-linked recessive genetic disorders caused by?

Mutations on the human X chromosome.

Why can't males be heterozygous for X-linked recessive disorders?

Males have only one X chromosome.

What is hemophilia A?

It is due to a mutation in an X-linked gene that encodes a blood clotting factor (factor VIII).

Who was a notable carrier of hemophilia?

Queen Victoria.

What is the probability that Billie and Bob's child will have hemophilia if Billie is healthy but her father had hemophilia?

The probability is 0% for a son and 50% for a daughter to be carriers.

What are dominant inherited disorders characterized by?

Serious genetic disorders caused by dominant alleles, which are rare due to strong selective pressure against them.

Name two examples of dominant inherited conditions.

Achondroplasia and Huntington's disease.

What is Huntington's disease caused by?

An autosomal dominant mutation in the Huntingtin gene.

At what age does Huntington's disease typically strike individuals?

Between 35 and 45 years old.

What are the main effects of Huntington's disease?

Loss of motor control, cognitive problems, dementia, and death.

Can genetic testing be done for Huntington's disease?

Yes, genetic testing can be done prior to the appearance of symptoms.

What is the probability that Joe's child will develop Huntington's disease if Joe's father had it?

There is a 50% probability that Joe's child will develop the disease.

What are some examples of autosomal recessive disorders?

Sickle cell anemia, cystic fibrosis, and Tay-Sachs disease.

What are some examples of X-linked recessive disorders?

Hemophilia A and B, red-green color blindness, and Duchenne muscular dystrophy.

What is the role of inherited mutations in diseases like heart disease and cancers?

They predispose an individual to a disease.

What is the significance of BRCA1 and BRCA2 mutations?

Heterozygosity for these mutations increases the risk of breast and ovarian cancers.

What is the breast cancer risk associated with BRCA1 and BRCA2 mutations?

BRCA1 mutation: 60% risk; BRCA2 mutation: 45% risk.

What is the ovarian cancer risk associated with BRCA1 and BRCA2 mutations?

BRCA1 mutation: 40% risk; BRCA2 mutation: 15% risk.

What are BRCA1 and BRCA2 classified as?

Tumor suppressor genes involved in DNA repair.

When is genetic testing for BRCA1 and BRCA2 mutations recommended?

For individuals with a family history of breast cancer before age 50, multiple breast cancers, or male family members with breast cancer.

What is Hereditary Nonpolyposis Colorectal Cancer (HNPCC)?

A type of hereditary colon cancer due to autosomal dominant mutations in DNA repair genes.

What percentage of colorectal cancers is attributed to HNPCC?

2-4% of all colorectal cancers.

What is the likelihood of heterozygotes developing colon cancer in HNPCC?

Up to 80% of heterozygotes develop colon cancer.

What are the possible outcomes of genetic tests?

1. No mutation or benign mutation detected, 2. Identification of a mutation known to cause disease, 3. Identification of a variant of uncertain significance (VUS).

What is the Genetic Information Nondiscrimination Act (GINA)?

A law that prohibits the use of genetic information in hiring, firing, and promotion decisions and prevents denial of health insurance coverage based on genetic information.

What does GINA not cover?

Life, disability, or long-term care insurance.

What are some practical and ethical issues related to genetic testing?

Impact on patients and their families, paternity issues, and potential impact on insurance coverage.

What is a variant of uncertain significance (VUS)?

A genetic variant whose association with disease is not clearly established.

What is genetics?

The study of heredity and variation.

What specifies the traits of an organism?

Genes carried on chromosomes.

What were the dominant models of heredity in the early 19th century?

Blended inheritance and inheritance of acquired characteristics.

What is blended inheritance?

Hereditary material mixed in offspring, which cannot be separated once mixed.

What is the inheritance of acquired characteristics?

Parents modify their traits based on use, and these modified traits are passed on to offspring.

Who is Gregor Mendel?

An Austrian monk who studied garden peas and developed the particulate theory of heredity.

What is the particulate theory of heredity?

Parents transmit heritable factors ('genes') that remain distinct from one generation to the next.

What is Mendelian genetics?

Transmission genetics often referred to as Mendelian genetics, based on Mendel's work.

What types of traits did Mendel study?

Qualitatively different traits, such as flower color, rather than quantitatively different traits.

What is a monohybrid cross?

An experiment where plants that differ by only one character trait are crossed.

What did Mendel's monohybrid cross results suggest?

Results were inconsistent with the blending theory of heredity.

What are alleles?

Alternative versions of heritable factors that specify traits.

What is the difference between homozygous and heterozygous organisms?

Homozygous organisms have two identical alleles, while heterozygous organisms have two different alleles.

What determines the appearance of an organism when two different alleles are present?

One allele is dominant and determines the appearance, while the other allele is recessive and has no effect.

What is the dominant allele for flower color in Mendel's study?

Purple is dominant, represented by the allele P.

What is the recessive allele for flower color in Mendel's study?

White is recessive, represented by the allele p.

What is the difference between genotype and phenotype?

Genotype is the genetic makeup, while phenotype is the physical appearance.

What does Mendel's Law of Segregation state?

The two alleles for each character trait are separated during gamete production and reunited by fertilization.

What is the significance of Mendel's conclusions?

They fit his data and provided a foundation for modern genetics.

What is the role of true-breeding plants in Mendel's experiments?

True-breeding plants ensure that offspring are always identical to the parent when self-pollinated.

What is the importance of accurate quantitative records in Mendel's work?

They allowed for reliable analysis and conclusions about inheritance patterns.

What is the relationship between chromosome behavior during meiosis and Mendel's Laws?

Chromosome behavior during meiosis explains how alleles segregate and assort independently.

What is the phenotypic ratio observed in Mendel's monohybrid crosses?

3:1 (3 dominant : 1 recessive)

What is the genotypic ratio observed in Mendel's monohybrid crosses?

1:2:1 (1 homozygous dominant : 2 heterozygous : 1 homozygous recessive)

What tool did Mendel use to predict the outcomes of genetic crosses?

Punnett Square

What were the dominant and recessive traits Mendel studied in seed color?

Dominant: yellow; Recessive: green

What were the dominant and recessive traits Mendel studied in seed shape?

Dominant: round; Recessive: wrinkled

What phenotype did F1 progeny always exhibit in Mendel's monohybrid crosses?

Dominant phenotype

What phenotypic ratio did Mendel observe in the F2 generation of his monohybrid crosses?

3:1 (dominant : recessive)

What genotypic ratio did Mendel observe in the F2 generation of his monohybrid crosses?

1:2:1 (homozygous dominant : heterozygous : homozygous recessive)

What is a dihybrid cross?

A genetic cross examining the inheritance of two traits simultaneously.

What traits did Mendel examine in his dihybrid cross?

Seed shape (Round vs. wrinkled) and seed color (Yellow vs. green)

What is the genotype of the F1 generation in Mendel's dihybrid cross?

Yy Rr (heterozygous for both traits)

What principle did Mendel propose based on the results of his dihybrid crosses?

The principle of independent assortment.

What does Mendel's Law of Independent Assortment state?

The segregation of alleles for one trait is independent of the segregation of alleles for another trait.