11.4

Mendelian Patterns of Inheritance in Humans

Overview of Mendelian Inheritance

Mendelian inheritance describes how traits are passed from parents to offspring through genes, following the principles first described by Gregor Mendel. While many human traits follow these patterns, studying inheritance in humans is more complex due to long generation times and ethical constraints on breeding experiments.

• Mendelian traits: Traits controlled by a single gene with clear dominant and recessive alleles.

• Pedigree analysis: A tool used to study inheritance patterns in families by tracking traits across generations.

• Applications: Understanding genetic disorders, predicting inheritance, and genetic counseling.

Pedigree Analysis

Constructing and Interpreting Pedigrees

Pedigrees are diagrams that show the occurrence of phenotypes in several generations of a family. They help geneticists determine the mode of inheritance for specific traits.

• Symbols: Squares represent males, circles represent females. Shaded symbols indicate individuals expressing the trait; unshaded symbols indicate those who do not.

• Generations: Each row represents a generation, with Roman numerals (I, II, III, etc.).

• Lines: Horizontal lines connect mates; vertical lines connect parents to offspring.

Pedigree analysis can reveal whether a trait is dominant, recessive, autosomal, or sex-linked.

Example: Pedigree for a Recessive Trait

• Recessive traits often skip generations and appear only when an individual inherits two copies of the recessive allele.

• Carriers: Individuals with one dominant and one recessive allele (heterozygotes) do not show the trait but can pass it to offspring.

Human Genetic Disorders

Recessively Inherited Disorders

Many genetic disorders are inherited in a recessive manner, meaning an individual must inherit two copies of the mutant allele to express the disorder.

• Examples: Cystic fibrosis, sickle-cell disease, Tay-Sachs disease.

• Carrier parents: Two carriers have a 25% chance of producing an affected child.

Dominantly Inherited Disorders

Some disorders are caused by dominant alleles. Only one copy of the mutant allele is needed for the disorder to be expressed.

• Examples: Achondroplasia (a form of dwarfism), Huntington's disease.

• Lethal alleles: Dominant lethal alleles are less common because they often result in death before reproductive age.

Multifactorial Disorders

These disorders are influenced by multiple genes and environmental factors. They do not follow simple Mendelian inheritance patterns.

• Examples: Heart disease, diabetes, cancer, alcoholism, schizophrenia.

Case Study: Sickle-Cell Disease

Genetic Basis and Inheritance

Sickle-cell disease is caused by a mutation in the hemoglobin gene. It is inherited in a recessive manner.

• Homozygous recessive: Individuals have sickle-cell disease.

• Heterozygous (carriers): Usually healthy but may have some symptoms (sickle-cell trait).

• Heterozygote advantage: Carriers are resistant to malaria, explaining the persistence of the allele in certain populations.

Symptoms and Treatment

• Symptoms include anemia, pain, organ damage, and increased risk of infection.

• Treatments: Blood transfusions, medications, and bone marrow transplants.

Genetic Counseling and Testing

Assessing Genetic Risk

Genetic counselors help families assess the risk of inherited disorders using family history, pedigree analysis, and genetic testing.

• Applications: Predicting the likelihood of a child inheriting a disorder, making informed reproductive choices.

• Testing: Can be performed before or during pregnancy to assess risk.

Key Terms and Concepts

• Genotype: The genetic makeup of an individual.

• Phenotype: The observable traits of an individual.

• Homozygous: Having two identical alleles for a gene.

• Heterozygous: Having two different alleles for a gene.

• Carrier: An individual who is heterozygous for a recessive disorder.

Sample Punnett Square for a Recessive Disorder

When both parents are carriers (heterozygous, Aa):

	A	a
A	AA (normal)	Aa (carrier)
a	Aa (carrier)	aa (affected)

There is a 25% chance of an affected child (aa), 50% chance of a carrier (Aa), and 25% chance of a normal child (AA).

Additional info:

• Pedigree analysis is a critical tool in human genetics due to ethical and practical limitations on experimental breeding.

• Understanding inheritance patterns helps in diagnosing, managing, and preventing genetic disorders.