Non-Mendelian Genetics

Non-Mendelian Genetics

Overview and Mendelian Requirements

  • Mendel's Laws Review:

    • Law of Dominance: In a heterozygote, one trait will conceal the presence of another trait for the same characteristic. Rather than both alleles contributing to a phenotype, the dominant allele will be expressed exclusively.

    • Law of Segregation: Two alleles for a heritable character segregate during gamete formation. This occurs during Anaphase I of meiosis when homologous chromosomes separate.

    • Law of Independent Assortment: Each pair of alleles segregates independently of any other pair of alleles during gamete formation. This applies to genes on non-homologous chromosomes or genes very far apart on the same chromosome.

  • Non-Mendelian Inheritance: These are patterns where the relationship between genotype and phenotype is more complex than simple dominance. This includes cases where one gene has multiple alleles, one gene affects multiple phenotypes (Pleiotropy), or multiple genes affect a single phenotype.

Probability Rules in Genetics

  • Multiplication Rule: Used to determine the probability that two or more independent events will occur together in some specific combination (P(A \text{ and } B) = P(A) \times P(B)).

    • Example: For a cross AaBb \times AaBb, the probability of getting an aabb offspring is (1/4 \times 1/4 = 1/16).

  • Addition Rule: Used to determine the probability that any one of two or more mutually exclusive events will occur (P(A \text{ or } B) = P(A) + P(B)).

    • Example: The probability of a heterozygote Aa in a monohybrid cross is the probability of A from mother and a from father (1/4) plus a from mother and A from father (1/4), totaling 1/2.

Degrees of Dominance

Incomplete Dominance

  • Definition: Neither allele is completely dominant, and the F_1 hybrids have a phenotype somewhere between those of the two parental varieties.

  • Molecular Basis: Usually, the dominant allele codes for a functional enzyme, and the recessive allele is non-functional. In heterozygotes, only half the amount of enzyme is produced, which is insufficient to produce the full red color, resulting in pink.

  • Ratios: The typical Mendelian 3:1 phenotypic ratio becomes a 1:2:1 ratio.

Codominance

  • Definition: Two dominant alleles affect the phenotype in separate, distinguishable ways. Both traits appear side-by-side.

  • Example: In human MN blood groups, individuals can have M molecules, N molecules, or both (MN) on their red blood cells.

Multiple Alleles and Blood Types

  • ABO Blood Groups:

    • Alleles: I^A, I^B, and i.

    • Carbohydrates: The alleles code for an enzyme that adds specific carbohydrates to the surface of red blood cells (A-type, B-type, or none for O).

    • Phenotypes: Type AB is a classic example of codominance because both A and B carbohydrates are present.

Complex Genetic Patterns

Pleiotropy

  • Definition: A single gene having multiple phenotypic effects.

  • Example: In diseases like Cystic Fibrosis or Sickle-cell disease, a single genetic mutation leads to a cascade of symptoms affecting multiple organ systems (lungs, pancreas, digestive tract, etc.).

Epistasis

  • Definition: A gene at one locus alters the phenotypic expression of a gene at a second locus.

  • Classic Ratio: In a dihybrid cross involving recessive epistasis (like Labradors), the ratio is often 9:3:4 instead of the expected 9:3:3:1.

  • Mechanism: One gene acts as a 'master switch.' If the switch gene is recessive (ee), the pigment-producing gene cannot express its color.

Polygenic Inheritance and Multifactorial Traits

  • Definition: Two or more genes have an additive effect on a single character.

  • Quantitative Characters: These characters do not fall into discrete categories but vary along a continuum.

  • Multifactorial Traits: Phenotypes that are influenced by both genetic and environmental factors (e.g., heart disease, diabetes, and alcoholism).

Sex-Linked Inheritance

X-Linked Genes

  • Structure: The X chromosome is much larger than the Y and carries many genes unrelated to sex.

  • X-Linked Disorders: Hemophilia, Duchenne muscular dystrophy, and Red-green color blindness.

  • Patterns:

    • A female needs two copies of the allele to express the trait (Homozygous).

    • A male needs only one copy of the allele to express the trait (Hemizygous).

X-Inactivation in Females

  • Barr Body: One X chromosome in each female cell becomes highly condensed and inactive.

  • Mosaicism: If a female is heterozygous for a gene on the X chromosome, she will be a mosaic for that character (e.g., tortoiseshell cats have patches of orange and black fur because different X chromosomes are active in different patches of skin cells).

  • Xist Gene: The "X-inactive specific transcript" gene is active only on the chromosome that will become the Barr body.

Linkage and Mapping

Linked Genes

  • Discovery: Work by Thomas Hunt Morgan on fruit flies (Drosophila) showed that some traits are inherited together because they are located on the same chromosome.

  • Parental vs. Recombinant Types: Offspring that match the parental phenotypes are parental types; those that do not are recombinant types.

Recombination and Mapping

  • Crossing Over: Happens during Prophase I. It breaks the physical connection between genes on the same chromosome.

  • Map Units: The distance between genes. The farther apart two genes are, the higher the probability that a crossover will occur between them.

  • Sturtevant's Rule: Recombination frequencies can be used to map gene sequence and distance. A recombination frequency over 50\% indicates the genes are on different chromosomes or so far apart they assort independently.

Abnormalities and Organelle Inheritance

Genomic Imprinting

  • Definition: A phenomenon in which expression of an allele in offspring depends on whether the allele is inherited from the male or female parent.

  • Methylation: Most imprints are critical for embryonic development and involve the silencing of one allele via DNA methylation.

Non-Nuclear Inheritance

  • Cytoplasmic Genes: Mitochondria and chloroplasts are inherited solely from the maternal parent (the egg).

  • Variegation: In plants, white and green patches on leaves can be caused by mutations in chloroplast DNA inherited through the cytoplasm.

Chromosomal Deviations

  • Nondisjunction: Failure of chromosomes to separate.

    • Monosomy: Missing one chromosome (2n-1).

    • Trisomy: Having an extra chromosome (2n+1) (e.g., Trisomy 21).

  • Structural Alterations:

    • Deletion: Removes a chromosomal segment.

    • Duplication: Repeats a segment.

    • Inversion: Reverses the orientation of a segment within a chromosome.

    • Translocation: Moves a segment from one chromosome to a non-homologous chromosome.

Statistical Analysis: Chi-Square (X^2)

  • Application: Genetics problems use Chi-Square to see if the results of a cross match the predicted Mendelian ratios.

  • Interpretation:

    • If your calculated X^2 is lower than the critical value, you fail to reject the null hypothesis (the traits follow the expected pattern).

    • If the X^2 is higher than the critical value, the difference is statistically significant, and you reject the null hypothesis (potentially indicating linkage or epistasis).

Epigenetics and Variable Expression

Incomplete Penetrance

  • Definition: A situation in which a person carries a specific genetic variant (genotype) but does not display the expected trait (phenotype).

  • Penetrance Calculation: Quantified as the percentage of people with a genotype who express the phenotype. For example, if 80\% of individuals with a disease-causing mutation show symptoms, the gene has 80\% penetrance.

  • Variable Expressivity: This refers to the degree or intensity to which a genotype is expressed in different individuals. While penetrance is "all or nothing," expressivity is a scale of severity.

Epigenetics

  • Definition: The study of heritable changes in gene expression that do not involve changes to the underlying DNA sequence.

  • Mechanism 1: DNA Methylation: The covalent addition of methyl groups to DNA (often at CpG sites), which generally acts to suppress gene transcription.

  • Mechanism 2: Histone Modification: Chemical alterations (like acetylation or phosphorylation) to the histone proteins around which DNA is wrapped. This changes the chromatin structure, making genes more or less accessible for transcription.

  • Environmental Factors: Epigenetic marks can be influenced by external factors such as diet, toxins, and stress, potentially providing a link between the environment and gene expression across generations.