WEEK 8

• Mendelian patterns of inheritance


  Principles regarding the inheritance of traits formulated by Gregor Mendel
• Gregor Mendel performed experiments on pea plants


  1856-1863
• Mendel's conclusions from his studies

  • The hereditary determinants are finite factors called genes
  • Each parent has a gene pair in each cell for each trait studied
  • One member of a given gene pair segregates into a gamete, and each gamete carries only one allele for each gene
  • During fertilization, gametes unite randomly, independent other genes
• Alleles


  Alternatives to Dominance and Recessiveness
• Alternatives to Dominance and Recessiveness

  • Incomplete dominance
  • Codominance
• Mendel implied that only two alleles, one dominant and one recessive, could exist for a given gene. We now know that this is an oversimplification.
• Multiple alleles


  Although individual humans (and all diploid organisms) can only have two alleles for a given gene, multiple alleles may exist at the population level such that many combinations of two alleles are observed
• Multiple alleles for rabbit coat color

  • Wild-type (C+C+)
  • Chinchilla (cchc ch)
  • Himalayan (chc h)
  • Albino (cc)
• Dominance hierarchy


  Wild-type allele is dominant over all the others, chinchilla is incompletely dominant over Himalayan and albino, and Himalayan is dominant over albino
• The complete dominance of a wild-type phenotype over all other mutants often occurs as an effect of "dosage" of a specific gene product
• Mutant allele interfering with the function of a wild-type gene

  • Antennapedia mutation in Drosophila - mutant allele expands the distribution of the gene product, resulting in legs on the head instead of antennae
• Blood types


  Certain chemical substances within the red blood cells may serve as antigens
• Blood-type gene systems


  • ABO
  • MN
  • Rh
• Rh-negative individuals


  Lack the Rh antigen
• A woman who is Rh-negative carries a fetus that is Rh-positive


  The first such child may have no difficulty, but later similar pregnancies may produce severely anemic newborn infants
• Blood groups


  Alleles are A, B and O. The A allele is dominant over the O allele.
• Blood group inheritance


  • If a mother has the alleles A and O (AO), her blood group will be A because the A allele is dominant. If the father has two O alleles (OO), he has the blood group O.
• Genotype


  The combination of alleles that you have
• Phenotype


  The observable trait that you have
• Carrier


  A person has one changed (q) and one unchanged (Q) copy of a gene, and they do not have the condition associated with that gene change
• Recessive inheritance pattern


  The condition is not expressed if there is a functioning copy of the gene present
• Two people are carriers (Qq) of the same recessive genetic condition

  There is a 25% chance that they may both pass the changed copy of the gene on to their child (qq), who will then develop the condition
• Autosomal recessive genetic conditions


  • Cystic fibrosis
  • Phenylketonuria (PKU)
• Co-dominant genes


  Each allele in the gene pair carries equal weight and will show up as a combined physical characteristic
• Blood group inheritance from parents with blood groups B (BO) and A (AO)


  • Each child has a 25% chance of having blood group AB, A, B or O
• For some conditions, family members with the same mutation may not have the same symptoms. For other conditions, individuals with different mutations can have similar characteristics.
• Mendelian inheritance


  Diseases caused by mutations in a single gene are usually inherited in a simple pattern, as observed by Gregor Mendel in garden pea plants
• Basic modes of inheritance for single-gene disorders

  • Autosomal dominant
  • Autosomal recessive
  • X-linked dominant
  • X-linked recessive
• Dominant mutations


  Expressed when only one copy of that mutation is present
• Recessive mutations


  Require two mutated copies for disease to develop
• 1. linked inheritance


  Diseases caused by mutated genes located on the X chromosome can be inherited in either a dominant or recessive manner
• Fathers cannot pass X-linked traits to their sons; fathers only pass X chromosomes to their daughters and Y chromosomes to their sons. Mothers pass X-linked genes to both sons and daughters.