Genetics Ch. 5

• Complete Dominance


  Phenotype of heterozygote is the same phenotype of one of the homozygotes
• Incomplete dominance


  Phenotype of the heterozygote fall between the phenotypes of the two homozygotes
• Co-dominance


  Phenotype of the heterozygote includes the phenotypes of both of the homozygotes
• Variations of dominance occur at a single locus and do not change how the alleles are inherited, only changes how the phenotype relates to the genotype
• Sickle Cell Disease

  • An example of Codominance, can look "recessive", Heterozygotes are "haplosufficient"
• Heterozygotes for HgbS

  Do not have a large increase in these risk factors
• Many populations that have high concentrations of Sickle Cell are in malaria endemic regions
• Penetrance

  if you have the gene for that trait, to what degree does it show up
• Expressivity

  to what degree/ how much is being expressed
• Penetrance and Expressivity are caused by other genes and environmental factors
• Polydactyly


  • Exhibits incomplete penetrance and variable expressivity
• Lethal Alleles

  an allele that causes the death of an organism during development normally before birth; reason why some genotypes don't appear in offspring
• Recessive Lethal Allele


  where only the homozygous (DD,TT,LL) die
• Dominant Lethal Allele

  where both the homozygous and heterozygous die
• Multiple Alleles

  • More than 2 possible alleles for a given gene are present in a population
  • Any individual diploid organism will only have two alleles, no matter how many there are in a population
• To calculate the # of genotypes possible:

  n(n+1)/2, n is the number of different alleles
• Compound Heterozygotes

  • different alleles can yield the same phenotypes
  • a person who has two different alleles at the same locus that results in the recessive phenotype
• Codominance w/ Multiple Alleles - ABO Blood Type

  • A Blood Type: Antigen- A, Antibodies: Anti-B
  • B Blood Type: Antigen- B, Antibodies: Anti-A
  • AB Blood Type: Antigen- A & B, Antibodies: None
  • O Blood Type: Antigen- None, Antibodies: Anti-A & Anti-B
• Red Blood cells that don't react with the recipient antibody remian evenly dispersed - Donor and Recipient are compatible
• Red Blood Cells that react with the recipient antibody clump together -Donor and recipient are not compatible
• Gene Interaction

  Genes assort independently but have their expression influenced by genes at other loci
• Recessive epistasis

  Complete dominance at both gene pairs; however, when one gene is homozygous recessive, it hides the phenotype of the other gene
• Dominant epistasis

  Complete dominance at both gene pairs; however, when one gene is dominant, it hides the phenotype of the other gene
• Epistasis

  The effect when one gene masks the effect of another gene at a different locus
• Epistatic gene

  The gene that does the masking
• Hypostatic gene

  The gene that is masked
• Recessive epistasis

  For the epistasis to occur you must be homozygous for the epistatic gene
• Without a functional deposition enzyme, it doesn't matter if the pigment is produced
• Sex-influenced characteristics


  Determined by autosomal genes, inherited by Mendel's principles but expressed differently in males and females
• Sex-limited characteristics

  Encoded by autosomal genes that are expressed in only one sex
• Sex-influenced and sex-limited characteristics

  • Bearded vs. beardless goats (sex-influenced)

  Feather plumage (cock feathering) in chickens (sex-limited)

  Male-limited precocious puberty in humans (sex-limited)
• Recessive Epistasis

  for epistasis to occur you must be homozygous for the epistatic gene
• Dominant Epistasis

  only one copy of the epistatic allele is required for epistasis to occur
• Duplicate Epistasis

  occurs when two recessive alleles at either locus can cause epistasis of the phenotype
• Cytoplasmic Inheritance

  • Mitochondrial diseases
  • DNA is found on the chloroplast mitchrondira
  • Mitochondrial genes are passed by the egg only
• Genetic Maternal Effect

  • Genes inherited from both parents, but phenotype is determined by the genotype of the mother
  • offspring inherit genes for characteristic from both parents, but offspring's phenotype is determined not by its own genotype but by its mother
• Genomic Imprinting

  • Occurs when the expression of the genetic material is dependent upon whether it is inherited from the mother or father
  • Epigenetics plays a key role
  • The expression of some genes are effected by the parental origin
• Epigenetics


  the study of changes in gene function that are heritable (mitotically or meiotically) that doesn't involve a change in DNA nucleotide sequence
• Mutation

  Change in DNA nucleotide sequence
• Mitotically


  passed to other cells (self)