M2P4

• Pleiotropism


  The phenomenon of multiple effect (multiple phenotypic expressions) of a single gene

  
• Phenylketonuria (PKU)


  • Affected individuals secrete excessive quantity of amino acid phenylalanine
  • Short stature, mentally deficient, widely spaced incisors, pigmented patches on skin, excessive sweating, and non-pigmented hairs and eyes
• Marfan syndrome


  • Caused by autosomal dominant mutation in the gene encoding the connective tissue protein fibrillin
  • Fibrillin is important to the structural integrity of the lens of the eye, to the lining of vessels such as the aorta, and to bones, among other tissues
  • Lens dislocation, increased risk of aortic aneurysm, and lengthened long bones in limbs

  
• Sickle cell anemia


  • Mutation of a beta-globin gene that can cause various organ problems

  
• Environmental influence on gene expression

  • Coat color distribution in Himalayan rabbits and Siamese cats in response to temperature
  • Temperature-dependent sex determination in freshwater turtle Emys orbicularis embryos
  • Anthocyanin production in maize in response to temperature
• Environmental influence on gene expression

  • Birth deformities in babies caused by chemicals ingested or drugs taken by pregnant women (e.g. thalidomide)
  • Birth abnormalities like spina bifida caused by deficiency in folic acid in pregnant women

  
• Environmental influence on gene expression

  • Male pattern baldness influenced by hormones testosterone and dihydrotestosterone
  • Development of mammary glands in women
• Penetrance


  The extent to which a particular gene or set of genes is expressed in the phenotypes of individuals carrying it, measured by the proportion of carriers showing the characteristic phenotype

  
• Complete penetrance


  • Most homozygous dominant and recessive genes and many completely dominant genes even in heterozygous conditions give their complete phenotypic expressions

  
• Complete penetrance


  • In pea: alleles RR for red flowers and rr for white flowers
  • In guinea pigs: dominant allele B for black coat in homozygous and heterozygous condition
• Incomplete penetrance


  • Some genes in homozygous as well as in heterozygous conditions fail to provide complete phenotypic expression
• Incomplete penetrance


  • Polydactyly has about 70% penetrance
  • Retinoblastoma has 75% penetrance
  • Tendency to develop diabetes mellitus

  
• Expressivity


  The degree of effect produced by a penetrant genotype, determined by the proportion of individuals with a given genotype who also possess the associated phenotype

  
• Expressivity


  • Polydactylous condition may be penetrant in the left hand (6 fingers) and not in the right (5 fingers)
  • Retinoblastoma can affect both eyes while in some, only 1 eye is affected
• Both penetrance and expressivity are affected by the environment
• The expression of a gene can be influenced by the diet and lifestyle
• Expressivity of a completely penetrant gene can be influenced by temperature, with lower temperature producing most effect
• Severity of symptoms of an inheritable allergy or the difference in height of identical twins raised in different homes (vary in diet)
• Identical twins


  Derived from 1 zygote that splits during development

  
• Fraternal twins


  Come from 2 different eggs fertilized separately at the same time
• Only a small percentage of monozygotic twins have identical genomes
• Dizygotic (DZ) twins share 50% genes (just like regular siblings), and if they are raised together, also have the same environment during their early years

  
• Since dizygotic twins are expected to have different sets of friends and careers, environment can then differ greatly

  
• Concordance


  The probability that a pair of individuals will both have a certain characteristic, given that one of the pair has the characteristic
• Discordance


  The degree of dissimilarity in a pair of twins with respect to the presence or absence of a disease or trait

  
• Monozygotic (MZ) twins have more similar genes; dizygotic (DZ) twins share 50% of their genes

  
• Twins


  • Raised together, have the same environment during their early years
  • Expected to have different sets of friends and careers, so environment can then differ greatly
• Twinning


  Comparison of monozygotic and dizygotic twins in terms of genes and environment

  
• Monozygotic (MZ) twins


  • Have more similar genes
  • Dizygotic (DZ) twins share 50% of their genes

  
• Concordance


  Higher in monozygotic twins compared to dizygotic twins

  
• Beckwith-Wiedemann syndrome


  • Overgrowth syndrome, larger than normal, taller than peers during childhood, becomes less apparent over time, increased risk of cancerous and noncancerous tumors
• Russell-Silver syndrome


  • Slow growth before and after birth, low birth weight, asymmetric growth of some body parts, thin, digestive abnormalities, episodes of hypoglycemia, short adults, increased risk of delayed development, speech and language problems, learning ability affected
• Discordance in metabolism in monozygotic twins

  • One normal, one with metabolic problems and obesity
• Twin studies teach us about genetics
• Rule of Multiplication (product rule)


  The probability that independent events will occur simultaneously is the product of their individual probabilities
• Rule of Addition (sum rule)

  The probability of an event that can occur in two or more independent ways is the sum of the separate probabilities of the different ways
• Mendelian cross between pea plants heterozygous for stem length (Tt)

  • Probability of producing tt offspring is 1/4
• Mendelian cross between pea plants heterozygous for stem length (Gg)

  • Probability of dominant phenotype offspring
• Probability (p)


  The chance that an event will occur = the case (a) divided by the total number of cases (n)
• Probability of heads or tails on a coin

  • Heads = 1/2, Tails = 1/2, Sum = 1