Non-Mendelian Genetics

Non-Mendelian Genetics

  • Traits often deviate from Mendelian ratios due to:
    • Varying degrees of dominance
    • Multiple genes acting together
    • Genes on sex chromosomes
    • Genes close together on the same chromosome segregating as a unit
    • Non-nuclear inheritance (mitochondrial/chloroplast DNA)

Degrees of Dominance

  • Alleles show varying dominances:
    • Complete Dominance: Homozygous dominant and heterozygous are phenotypically the same.
    • Incomplete Dominance: Neither allele is fully dominant; results in a blended phenotype (e.g., red and white flowers produce pink).
    • Codominance: Both alleles expressed (e.g., AB blood type).
    • Multiple Alleles: More than two alleles can exist for a gene (e.g., blood types IA, IB, i).

Multiple Genes

  • Epistasis: Phenotypic expression of one gene affects another.
  • Polygenic Inheritance: Multiple genes influence one phenotype (e.g., height).

Sex Chromosomes

  • Sex-linked Genes: Genes located on sex chromosomes; X-linked genes are common.
  • X-linked traits passed from fathers to daughters but not sons; mothers pass to both.
  • Males express X-linked traits if inherited due to hemizygosity.

X-Linked Disorders

  • Examples:
    • Duchenne muscular dystrophy
    • Hemophilia
    • Color blindness

Linked Genes

  • Genetic Recombination: Offspring with new combinations of genes.
  • Linked Genes: Genes close together on the same chromosome; inherited together unless separated by crossing over.

Mapping Distance

  • Linkage Map: Based on recombination frequencies; 1 map unit = 1% recombination frequency.

Non-Nuclear DNA

  • Traits shown on mitochondrial and chloroplast DNA, maternally inherited.

Statistical Analysis: Chi-Square

  • Goodness of Fit Test: Compares observed vs. expected results to see if data fits.
    • Null hypothesis states no significant difference.
  • Formula: X^2 = \sum\frac{(O - E)^2}{E}
  • Degrees of freedom calculated as categories - 1.
  • If X^2 > critical value, reject null hypothesis.

Environmental Effects on Phenotype

  • Environmental factors can influence gene expression, leading to phenotypic variability (e.g., temperature effects on coat color).

Genetic Disorders

  • Linked to mutated alleles or chromosomal changes (e.g., Tay-Sachs, sickle cell anemia).
  • Nondisjunction: Errors in chromosome separation lead to disorders like Down Syndrome.