Non-Mendelian Genetics Notes
Mendel's Postulates
- 1. Principles of Paired Factors
- 2. Principle of Dominance
- 3. Law of Segregation or Law of Purity of Gametes (Mendel's First Law of Inheritance)
- 4. Law of Independent Assortment (Mendel's Second Law of Inheritance)
Extensions of Mendelian Genetics
- Incomplete Dominance
- Codominance
- Multiple Alleles
- Multiple Genes
- Sex-limited traits
- Sex-influenced traits
- Sex-linked traits
Assumptions of Mendelian Inheritance
- Traits are determined by genes with two alternative alleles.
- Complete Dominance
- Alleles are found on autosomes.
- No Gene Interaction: A single phenotype is affected by more than one set of genes.
- Complete Penetrance: Percentage that individuals express the trait given their genotype.
Incomplete Dominance
- Heterozygote produces an intermediate phenotype.
- Example: Snapdragon flower color, where R¹R¹ is red, R²R² is white, and R¹R² is pink.
- The wild-type allele (R1) produces an enzyme for red pigment synthesis, while the mutant allele (R2) produces a non-functional enzyme. The heterozygote produces about half the pigment, resulting in a pink phenotype.
Codominance
- Both alleles are clearly evident in a heterozygote.
- Example: MN blood group, where M and N alleles control a glycoprotein on the surface of RBCs.
- ABO blood group
Multiple Alleles
- Three or more alleles of the same gene are present in a population.
- Number of possible genotypes: {\frac{1}{2} n (n+1)}, where n is the number of alleles.
- ABO blood groups, controlled by chromosome 9, with A and B antigens on the surface of RBCs.
- The I designation stands for isoagglutinogen, another term for antigen.
Bombay Phenotype
- A rare recessive mutation in the FUT1 gene (fucosyl transferase).
- Prevents the synthesis of complete H substance, resulting in a functional type O blood group even if the individual is genetically type B.
Lethal Alleles
- Recessive lethal allele: Wild-type allele is sufficient for survival in heterozygotes.
- Example: Yellow coat in mice, where the yellow allele is dominant for coat color but lethal when homozygous.
- Dominant lethal allele: One copy of the allele results in death.
- Example: Huntington's disease, where the disease onset is delayed until adulthood.
Combinations of Two Gene Pairs
- Mendel’s principle of independent assortment applies if genes are not located on the same chromosome.
Gene Interaction
- Several genes influence a particular characteristic.
- Epigenesis: Each step of development increases complexity and is influenced by many genes.
- Example: Inner ear formation.
- Epistasis: One gene pair masks or modifies the effect of another gene pair.
Multiple Genes or Polygenes:
- Two or more gene pairs account for the phenotype.
- Alleles do not exhibit dominance but have a quantitative effect.
- Contributory/effective allele (C) vs. non-contributory/non-effective alleles (c).
- Effects of contributory alleles are equal but cumulative (additive).
X-Linkage in Drosophila:
- Observed by Thomas H. Morgan in 1910.
- Recessive alleles for white eye are found on the X chromosome and absent in the Y chromosome.
- Males are hemizygous for X-linked genes.
X-linked Genes
- Genes described on the X Chromosome
- Hemizygous: in males as Y chromosomes lacks homology with X chromosomes
- SRY genes (sex-determining region Y) is what makes men
Barr Bodies
- Inactivated and highly condensed X chromosome attached to the nuclear membrane.
- X chromosome inactivation varies randomly from cell to cell
Sex-Linked Traits
- Controlled by genes on sex chromosomes (X and Y).
- X-linked recessive traits: More common in males.
- X-linked dominant traits: Affected males pass the condition to all daughters but no sons.
- Y-linked (Holandric) traits: Affect every son of an affected male.
Sex-Limited Traits
- Autosomal traits expressed exclusively in one sex.
- Genes are found in both sexes but only expressed in one with suitable sex hormones.
Sex-Influenced Traits
- Autosomal traits expressed in both sexes but more frequent in one sex than the other.