lec 5-Gene Interaction I: Types of Mutation and Departure from Mendelian Ratios

The study of mutation is important in genetics, particularly distinguishing between dominant and recessive alleles. The understanding of why certain mutations are dominant or recessive has greatly developed by analyzing how these mutations affect the function of the proteins they encode. Mutations can be classified into the following main groups based on their functions:

Loss of Function Mutations

• Amorphic (Null) Mutations: These mutations result in no functional protein at all or produce a protein that completely lacks any activity. They are often recessive because a single normal copy of the gene can often provide enough function for normal health.   - Case Study: Cystic Fibrosis Transmembrane Conductance Regulator (CFTR): The CFTRΔ508 allele is an example of an amorphic mutation. The CFTR protein typically acts as a pump to remove chloride (Cl⁻) ions from cells, which helps maintain a thin mucus layer. In CFTRΔ508 mutations, the protein doesn't leave the endoplasmic reticulum, leading to thick mucus that causes severe lung and digestive issues, drastically shortening affected individuals' lifespans. Normal CFTR is dominant, while CFTRΔ508 is recessive.

• Hypomorphic (Leaky) Mutations: These mutations reduce protein function significantly but do not eliminate it entirely. This may happen either due to reduced protein production or decreased biochemical activity of the protein. They also tend to be recessive because one normal copy usually suffices.
    - Example: Tyrosinase Alleles: In the pathway to produce melanin from tyrosine, the wild-type tyrosinase allele (C) provides full activity and dark pigmentation, whereas the mutant allele (cᶜh) leads to pigment reduction.

Gain of Function Mutations

• Hypermorphic Mutations: These mutations boost the biological activity of a protein, either by increasing protein production or enhancing its function. They are usually dominant because the normal protein is insufficient if a hyperactive version exists.   - Example: Hereditary Pancreatitis (HP): Trypsin-1 is an enzyme secreted by the pancreas. If it activates too early, it can damage the pancreas itself. Normally, a mutation at Arg117 prevents self-inactivation, resulting in uncontrolled digestion and severe inflammation.

• Antimorphic (Dominant Negative) Mutations: These mutations disrupt normal protein functions, interfering with the products of normal alleles, often in a multimer form.   - Example: Marfan Syndrome (FBN1): Mutations in FBN1, which encodes fibrillin-1 for connective tissue, can cause a truncated protein that hinders the assembly of fibrous structures, leading to traits like tall stature and long fingers.

• Dominant Lethal Mutations: These mutations are rare because they only persist if individuals can reproduce before the lethality appears, which often happens later in life as in Huntington's Disease (HD). In HD, a CAG triplet repeat beyond 36 repeats leads to neurodegeneration, and this condition is dominant as the affected individuals develop symptoms regardless of the presence of a normal allele.

• Neomorphic Mutations: These mutations create entirely new functions or forms for the protein, and are typically dominant.   - Example: Antennapedia in Drosophila: A mutation causes the antenna gene to be expressed in the head, resulting in legs developing instead of eyes.

Departures from Mendelian Ratios

• Often, the clear dominant and recessive patterns observed by Mendel aren't consistent, with dominance sometimes being incomplete, yielding ratios that differ from traditional Mendelian genetics.

• Incomplete (Partial) Dominance: The heterozygous phenotype blends characteristics of the two homozygous types.
    - Example: Four O’clock Plants: A cross of red ($CC$) and white ($cc$) flowered plants results in pink ($Cc$) offspring, leading to a $1 : 2 : 1$ ratio of red to pink to white in the next generation, differing from the expected Mendelian $3 : 1$ ratio.

• Co-dominance: Here, both alleles manifest equally in the phenotype.   - Example: ABO Blood Group: The I locus has three alleles (Iᴀ, Iᴮ, and i) resulting in various blood types, with type AB expressing both A and B antigens equally. Antibody responses in blood types lead to transfusion significance; for example, type O produces both anti-A and anti-B antibodies.

Lethal Alleles and Segregation Ratios

• Some lethal alleles may distort expected Mendelian ratios as certain genotypes do not survive long enough to be counted.

• Lethal Alleles in Mice: A yellow coat mutation (Aᶴ) demonstrates a lethality in the homozygous (AᶴAᶴ) form, with only a 1:1 ratio of yellow to wild-type produced from a cross, indicating that the yellow allele is dominant, but lethal when homozygous.

• Manx Cats: All Manx cats are heterozygous for a dominant allele that affects tail formation, leading to the non-viability of homozygous individuals due to severe spinal deformities.

• Human Genetic Load: The cumulative impact of lethal alleles in a human population is termed the genetic load. Of 1,000,000 zygotes, around 85% lead to live births, while 15% are miscarried, with 7.5% due to chromosomal abnormalities and another 7.5% homozygous for lethal genes.

Sex-Related Inheritance Patterns

• X-Linkage (Sex-Linkage): Genes located on the X chromosome follow distinct inheritance patterns since males receive their single X from their mothers, making them hemizygous.   - Example: Hemophilia A: Affects mainly males due to a mutation in the clotting Factor VIII gene. Female carriers can pass it to half of their sons.

• Sex-Limited Traits: These traits' expression is confined to one sex even though the genes are on autosomes.
    - Examples include milk production in cows and the number of eggs laid by chickens.

• Sex-Influenced Traits: Genes are autosomal, but the expression varies with the individual's sex, often due to hormones.   - Example: Male Pattern Baldness: The B allele is dominant in males due to high testosterone but recessive in females, leading to different baldness patterns.

Cytoplasmic and Maternal Inheritance

• Mitochondria and chloroplasts carry their circular genomes (mtDNA) and are inherited exclusively from the mother.

• Mitochondrial Cytopathies: These conditions stem from mutations in mtDNA and primarily affect organs with high ATP demand, like muscles and nerves.   - Examples:     - MELAS: Causes muscle weakness, neurological symptoms, and episodes resembling strokes.     - LHON: Affects vision loss due to optic nerve disorders.   - Inheritance Rules: They impact both sexes and are passed on solely from mothers. Affected mothers transmit the disorder to all children, while affected fathers do not pass it on.

Penetrance and Expressivity

• There can be differences between genotype and phenotype due to incomplete penetrance and variable expressivity.

• Incomplete Penetrance: This occurs when some individuals with the mutant genotype do not show the expected phenotype.   - Example: Polydactyly: Dominant mutations appear in only 25-30% of carriers, leading to extra digits.

• Variable Expressivity: The extent to which the phenotype is expressed can vary significantly among individuals.
    - Example: Waardenburg Syndrome: This autosomal dominant condition can manifest in various features like hearing loss, mixed eye color, and white hair, depending on the individual.

• Control Factors: Both penetrance and expressivity are influenced by other genetic loci and environmental conditions.