Lecture 5

• Who is the author of the quote “Nothing in biology makes sense except in the light of evolution”?

  Theodosius Dobzhansky
• What are the learning objectives of BIOS3240 – Genetics and Evolution?

  • Draw mono-hybrid and di-hybrid Punnett squares
  • Understand epistatic interactions between genes
  • Learn complex types of dominance: incomplete dominance, overdominance, underdominance
  • Understand pleiotropy and polygenic inheritance
• What is the key reading for the course BIOS3240?

  Campbell Biology chapter on Mendelian genetics (12th edition, chapter 14, pages 319-328)
• What does the first law of inheritance state?

  Each parent passes on only one of its two alleles with an equal chance of passing on either.
• What trait is considered recessive in the context of the first law?

  The "wrinkled" trait is recessive relative to the "round" trait.
• What are the results of a second generation cross (F1 → F2) when both parents are Rr?

  • Phenotypic ratio: 3:1
  • Genotypic ratio: 1:2:1
• What is the genotype of a plant that gives wrinkled peas?

  Must be rr genotype.
• How can you determine the genotype of a plant that gives round peas?

  You cannot determine it; it could be either RR or Rr.
• What is the purpose of a test cross?

  To determine the genotype of an unknown parent by crossing it with a homozygous recessive.
• What happens if the unknown parent in a test cross is homozygous RR?

  All peas in the offspring will be round.
• What happens if the unknown parent in a test cross is heterozygous Rr?

  The offspring will have 50% wrinkled peas.
• What is the prediction for dihybrid crosses regarding gamete combinations?

  • If genes segregate independently, F1 dihybrid plants should produce equal numbers of gametes for all four combinations: RV, Rv, rV, rv.
• What is the phenotypic ratio for a dihybrid cross with dual heterozygotes (Rr, Vv)?

  1. 3:3:1
• What is the chance of a child having ginger hair if both parents have dark hair but are carriers?

  There is a 1 in 4 chance of having a child with ginger hair.
• What is the chance that Arya, who has dark hair, is a carrier for the recessive ginger hair trait?

  There is a 2/3 risk that she is a carrier.
• What is the risk of Alfred and Barbara having a child with cystic fibrosis (CF)?

  The overall risk to their future children is 1 in 8.
• What are the steps to determine the risk of cystic fibrosis in children of Alfred and Barbara?

  1. Parents of a patient must be carriers.
  2. One grandparent must be a carrier.
  3. Calculate probabilities based on carrier status.
  4. Overall risk is 1 in 8 for having a child with CF.
• What are the complex types of dominance that extend Mendel’s Third Law?

  • Incomplete dominance
  • Overdominance
  • Underdominance
• What is pleiotropy and polygenic inheritance?

  • Pleiotropy: A single gene influences multiple traits.
  • Polygenic inheritance: Multiple genes influence a single trait.