Non-Mendelian Genetics

• Mendelian inheritance


  Not always the case
• Alleles are transmitted from parent to offspring according to Mendelian principles, but they often do not display the clear-cut dominant/recessive relationship observed by Mendel
• Reasons why Mendelian inheritance is not always the case

  • Two or more genes can influence the phenotype of a single characteristic
  • Genes located on the X chromosome can behave differently
• Phenotypes


  Not simply determined by genetics; the environment also plays a role
• Even if two individuals have the same genes, they may not have the same phenotype if they are raised in different environments
• Exceptions to Mendelian inheritance can lead to different phenotypic ratios from those produced by standard monohybrid, dihybrid, and trihybrid crosses
• Incomplete, or Partial, Dominance


  Neither allele is dominant
• Incomplete dominance


  • Four-o'clock or snapdragon plant with red flowers crossed with white-flowered plant yields pink flowers

  
• Phenotypic Blending


  The heterozygous phenotype exhibits a blending of traits from both alleles, resulting in an intermediate form
• Incomplete dominance


  Best understood through quantitative gene expression; the heterozygote produces an intermediate level of functional gene product, leading to a modified phenotype
• In incomplete dominance, the phenotypic ratio directly mirrors the underlying genotypic ratio (1:2:1), unlike the 3:1 ratio seen in complete dominance
• Tay-Sachs disease


  Rare but devastating inherited disease that progressively destroys nerve cells (neurons) in the brain and spinal cord; caused by a mutation in the gene that codes for an enzyme called hexosaminidase A (HEX-A)

  
• Without enough HEX-A, gangliosides build up to toxic levels within the nervous system, leading to irreversible damage and ultimately death
• Tay-Sachs disease carriers


  Have an intermediate enzyme level, but no external disease symptoms
• Codominance


  A type of inheritance where both alleles of a gene are fully expressed in a heterozygote
• MN Blood Group


  Example of codominance; two possible forms of a cell surface glycoprotein (M and N), and individuals can have one or both
• Coat Color in Cows

  • Coat color can be inherited in a codominant pattern; cows with the RW genotype have intermingled red and white hairs, giving the characteristic roan appearance
• Multiple Alleles


  A single gene may have three or more alleles in a population
• ABO Blood Group System

  • Determined by a single gene with three alleles: IA, IB, and i; IA and IB are codominant, i is recessive
• The A and B antigens are carbohydrate molecules attached to the surface of red blood cells
• People with type O blood are universal donors because their red blood cells lack A and B antigens; people with type AB blood are universal recipients because their plasma contains antibodies against neither A nor B antigens
• Lethal Alleles


  Specific gene mutations that result in the death of an organism when expressed in particular genotypes
• Recessive Lethal Alleles


  Fatal only in the homozygous state; heterozygous individuals may survive due to the presence of a functional wild-type allele; the timing of death is determined by the developmental stage at which the gene product becomes indispensable
• Recessive Lethal Alleles with Dominant Phenotypic Expression


  • Exemplified by the yellow coat color in mice; the yellow allele (AY) exhibits dominance over the wild-type agouti allele (A) in determining coat color, but homozygosity for the AY allele results in embryonic lethality
• Dominant Lethal Alleles


  Fatal when present in a single copy; the persistence of dominant lethal alleles in a population is often dependent on their expression after reproductive age, allowing for transmission to subsequent generations
• Dominant Lethal Alleles


  • Huntington disease in humans, characterized by delayed-onset neurodegeneration
• Polygenic inheritance


  Determination of a phenotypic trait by multiple genes, each with a small, additive effect; contrasts with Mendelian inheritance where single genes have discrete effects; traits exhibit continuous variation, often resembling a bell curve; susceptible to modulation by environmental influences
• Epistasis


  A form of gene interaction where the expression of one gene is modified by one or several other genes; the gene that does the masking is termed the "epistatic gene," while the gene whose expression is altered is the "hypostatic gene"; can result in suppression, enhancement, or the emergence of entirely novel phenotypes
• Recessive Epistasis


  A recessive allele at one locus masks the expression of both dominant and recessive alleles at another locus
• Recessive Epistasis


  • Coat color in Labrador Retrievers, Bombay phenotype
• Epistasis


  A form of gene interaction where the expression of one gene is modified by one or several other genes
• Epistatic gene


  The gene that does the masking
• Hypostatic gene


  The gene whose expression is altered
• Epistatic interactions


  • Can result in suppression, enhancement, or the emergence of entirely novel phenotypes
• Recessive epistasis


  • Coat color in Labrador Retrievers
  • Bombay phenotype
• Dominant epistasis


  • Fruit color in squash (white is dominant and masks yellow or green color)
• Novel phenotypes in squash

  Emergence of fruit shapes distinct from the parental phenotypes due to epistasis
• Genes involved in novel phenotypes in squash

  • Gene A/a controls one aspect of fruit shape, dominant allele (A) promotes disc-shaped fruit
  • Gene B/b controls another aspect of fruit shape, dominant allele (B) also promotes disc-shaped fruit
• Emergence of novel phenotypes in squash

  1. Parental genotypes: AABB (disc-shaped) and aabb (long-shaped)

  2. F1 generation: AaBb (disc-shaped)

  3. F2 generation: AABB, AABb, AaBB, AaBb (disc-shaped), AAbb, Aabb (sphere-shaped), aabb (long-shaped)
• Pleiotropy


  A phenomenon where the expression of a single gene has multiple phenotypic effects