CHAPTER 2

• Mendelian inheritance


  Inheritance patterns that obey two laws: Law of segregation and Law of independent assortment

  
• Simple Mendelian inheritance


  • Involves a single gene with two different alleles
  • Alleles display a simple dominant/recessive relationship

  
• In this chapter, we will examine traits that deviate from the simple dominant/recessive relationship
• The inheritance patterns of these traits still obey Mendelian laws, but they are more complex and interesting than Mendel had realized

  
• Wild-type alleles


  Prevalent alleles in a population that typically encode proteins that function normally and are made in the proper amounts
• Mutant alleles


  Alleles that have been altered by mutation, are rare in natural populations, and are likely to cause a reduction in the amount or function of the encoded protein
• Genetic diseases are caused by mutant alleles, and in many human genetic diseases, the recessive allele contains a mutation that prevents the allele from producing a fully functional protein
• Incomplete dominance


  The heterozygote exhibits a phenotype that is intermediate between the corresponding homozygotes
• Incomplete dominance example


  • Flower color in the four o'clock plant, where the two alleles CR (red) and CW (white) produce a pink flower in the heterozygote

  
• In incomplete dominance, the 3:1 phenotypic ratio observed in simple Mendelian inheritance is not observed, but rather a 1:2:1 ratio

  
• Whether a trait is dominant or incompletely dominant may depend on how closely the trait is examined
• Pea shape


  • Mendel visually concluded that RR and Rr genotypes produced round peas and rr genotypes produced wrinkled peas, but microscopic examination reveals that not all round peas are "created equal"
• Incomplete penetrance


  A dominant allele does not always influence the outcome of a trait in a heterozygote individual
• Incomplete penetrance example


  • Polydactyly, an autosomal dominant trait where affected individuals have additional fingers and/or toes, but in some cases, individuals carry the dominant allele but do not exhibit the trait
• Penetrance


  The measure of how often a dominant allele "penetrates" into the phenotype of the individual, described at the population level
• Expressivity


  The degree to which a trait is expressed, such as the number of extra digits in polydactyly
• Environment


  Environmental conditions may have a great impact on the phenotype of the individual, such as coat color changes in the arctic fox or symptoms of phenylketonuria (PKU)
• Overdominance


  The phenomenon in which a heterozygote is more vigorous than both of the corresponding homozygotes, also called heterozygote advantage
• Overdominance example


  • Sickle-cell anemia, where heterozygotes (HbAHbS) have an advantage over both homozygotes (HbSHbS and HbAHbA) due to resistance to malaria
• The sickle cell allele has been found at a fairly high frequency in parts of Africa where malaria is found, because heterozygotes are more resistant to malaria
• Multiple alleles


  Many genes have three or more different alleles within natural populations, but a maximum of two alleles are found in any particular diploid individual
• Multiple alleles example


  • Coat color in rabbits, with four different alleles (C, cch, ch, c) that determine the pattern and presence of pigmentation
• The dominance hierarchy for rabbit coat color alleles is C > cch > ch > c
• Temperature-sensitive conditional allele


  An allele where the encoded enzyme is only functional at low temperatures, as seen in the Himalayan pattern of coat color in rabbits and the Siamese pattern of coat color in cats
• 1. tyrosine


  Precursor to synthesis of
• Eumelanin


  Pigment that produces black/brown color
• Phaeomelanine


  Pigment that produces red/yellow color
• Coat color genotypes


  • C (full coat color)
  • cch (chinchilla pattern)
  • ch (himalayan pattern)
  • c (albino)
• Dominance hierarchy of coat color genotypes


  C > cch > ch > c
• Coat color phenotypes


  • (a) Full coat color
  • (b) Chinchilla coat color
  • (c) Himalayan coat color
  • (d) Albino coat color
• Himalayan coat color


  • Example of a temperature-sensitive conditional allele
  • Enzyme encoded by this gene is functional only at low temperatures
  • Dark fur will only occur in cooler areas of the body
• Other examples of temperature-sensitive conditional alleles

  • Siamese pattern of coat color in cats
• ABO blood group


  • Determined by the type of antigen present on the surface of red blood cells
  • Antigens are substances recognized by antibodies produced by the immune system
• ABO blood group alleles

  • IA (produces antigen A)
  • IB (produces antigen B)
  • i (no surface antigen produced)
• Codominance


  • Allele i is recessive to both IA and IB
  • Alleles IA and IB are both expressed in a heterozygous individual
• ABO blood types


  • O (no antigens, antibodies against A and B)
  • A (antigen A, antibodies against B)
  • B (antigen B, antibodies against A)
  • AB (antigens A and B, no antibodies)
• For safe blood transfusions, the donor's blood must be an appropriate match with the recipient's blood
• If a type O individual received blood from a type A, type B or type AB blood, antibodies in the recipient blood will react with antigens in the donated blood cells, causing the donated blood to agglutinate and potentially resulting in a life-threatening situation
• 1. linked genes


  • Traits governed by genes on the sex chromosomes
  • Pedigree for an X-linked disease shows mostly affected males with their mothers as carriers
• Pedigree for Duchenne muscular dystrophy (DMD)


  • Affected individuals shown with filled symbols, female carriers shown with half-filled symbols