Inheritance Patterns

Inheritance Patterns

  • Understanding how traits are passed from parents to offspring.

Overview of Inheritance

  • Humans possess two copies of almost every gene, one inherited from each parent.

  • Mutations: Genetic changes that affect the functioning of a gene. These mutations can lead to diseases that follow specific inheritance patterns.
      - The pattern of inheritance is influenced by the location of the gene on the chromosomes.
      - The requirement of one or two copies of the gene also impacts inheritance patterns.

  • This concept is often referred to as Mendelian inheritance.

Key Vocabulary

  • Heterozygous: Refers to an individual possessing two different alleles of a particular gene, represented as (Ll).

  • Homozygous: Refers to an individual having two identical alleles of a particular gene, represented as (LL or ll).

  • Alleles: Variants of a DNA sequence on a gene; individuals inherit two alleles, one from each parent.

Patterns of Inheritance

Dominant Inheritance

  • A trait appears when at least one copy of the mutated gene is present.
      - Affected individuals typically have an affected parent.
      - Examples of dominant diseases include:
        - Huntington’s Disease: A progressive neurodegenerative disorder.
        - Achondroplasia: A genetic disorder resulting in dwarfism.
      - Dominant traits are represented by uppercase letters (e.g., A).

Recessive Inheritance

  • A trait is observed only when an individual has two copies of the mutated gene.
      - Affected individuals often have parents who are carriers (unaffected but possess one copy of the mutated gene).
      - Traits can skip generations since carriers are often not affected.
      - Examples include:
        - Sickle Cell Anemia: A blood disorder caused by a mutation in the HBB gene.
        - Cystic Fibrosis: A genetic disorder affecting the respiratory and digestive systems.
      - Recessive traits are symbolized by lowercase letters (e.g., a).

Types of Inheritance Based on Chromosomes

Autosomal Inheritance

  • Genes located on non-sex chromosomes (chromosomes 1-22).

Sex-linked Inheritance

  • Involves genes located on sex chromosomes.
      - X-linked Inheritance:
        - X-linked Dominant: A mutation in one X chromosome leads to disease in both males and females.
          - Example: Fragile X Syndrome.
        - X-linked Recessive: A mutation typically affects males more than females. Males have only one X chromosome (XY), while females have two (XX). Thus, a single mutated copy on X would result in the disease in males.
          - Example:
            - Hemophilia A: A bleeding disorder due to a deficiency in clotting factors.
            - Duchenne Muscular Dystrophy: A severe form of muscular dystrophy.
      - Important point: Fathers cannot pass X-linked traits to their sons (since sons inherit the Y chromosome from their father).

Characteristics of Different Inheritance Patterns

  • Autosomal Dominant
      - Affected individuals usually have affected parents.
      - Seen in every generation.
      - Examples: Huntington's disease, neurofibromatosis, achondroplasia.

  • Autosomal Recessive
      - Typically involves two carriers as parents and may skip generations.
      - Examples: Tay-Sachs disease, sickle cell anemia.

  • X-linked Dominant
      - Affected males will transmit the trait to all daughters (affected) but no sons.

  • X-linked Recessive
      - Males are more frequently affected than females; affected males usually appear in each generation.

  • Mitochondrial Inheritance
      - Passed only by females since all mitochondria are inherited from the mother.
      - Can affect both males and females; condition appears in every generation.
      - Examples include: Leber's hereditary optic neuropathy, Kearns-Sayre syndrome.

Pedigree Analysis

  • Pedigrees: Diagrams used to show the presence or absence of traits in a family over generations.
      - Helps analyze inheritance patterns of a trait within a family using standardized symbols (e.g., squares for males, circles for females).

Common Pedigree Symbols

  • Male: ☐

  • Female: ○

  • Identical twins: ≈

  • Fraternal twins: 𝄒

  • Marriage: -

  • Consanguineous marriage: ⊕

  • Affected individuals: shaded symbols

  • Unaffected individuals: non-shaded symbols

  • Carrier: half-shaded symbols

  • Deceased: symbol with a line through it

  • Individuals within a generation are numbered (Arabic numerals), and generations are indicated using Roman numerals.

Examples of Genetic Inheritance

Autosomal Dominant Inheritance

  • Example: Freckles
      - Genotypes: FF (homozygous dominant), Ff (heterozygous), ff (homozygous recessive).
      - Affects all genders; observed in every generation.

Autosomal Recessive Inheritance

  • Affects individuals that come from unaffected parents who are carriers.

  • Traits can skip generations.

X-linked Recessive Inheritance

  • More prevalent in males than females, with unaffected parents having affected children.

Pedigree Example and Analysis

  • Pedigree 1: Analyzed for individual II-4, inferring gender, offspring, and whether affected or unaffected.

  • Pedigree 2: Analyzed for determining the inheritance of an autosomal dominant condition, evaluating the frequency of affected individuals across generations.
      - Key characteristics of autosomal dominant: trait seen in every generation, affected individuals often have affected parents.