Lecture 4

• Who is quoted as saying, "Nothing in biology makes sense except in the light of evolution"?

  Theodosius Dobzhansky
• What are the learning objectives of the BI324 Genetics and Evolution course?

  • Understand the origins of genetics and heredity
  • Understand genotype and phenotype and their linkage
  • Know the three Laws of Mendelian Genetics
  • Develop understanding of when and why genetics follows these laws
• What are Mendel's three laws of genetics?

  1. First Law: Segregation
  2. Second Law: Independent Assortment
  3. Third Law: Dominance
• What does the First Law of Segregation state?

  Two copies of each particle are inherited, one from each parent, and one is passed on at random in each sperm or egg.
• What does the Second Law of Independent Assortment state?

  Traits are inherited independently of one another.
• What does the Third Law of Dominance state?

  The inherited particles may be dominant or recessive.
• What is the phenotypic ratio in the first generation cross of Mendel's pea plants?


  1. 1
• What is the genotypic ratio in the second generation cross of Mendel's pea plants?

  1. 2:1
• What does a double mutation (homozygous recessive) indicate in terms of its effect on a biochemical pathway?

  A double mutation has a severe effect on the pathway.
• Who identified Alkaptonuria as a Mendelian recessive disorder?

  Archibald Garrod
• What is required for the disease associated with Alkaptonuria to manifest?

  Two mutations are required for the disease, indicating it is recessive.
• How does the presence of one functioning allele of the HGD gene affect the biochemical reaction?

  One functioning allele is enough to carry out the reaction.
• What happens when both copies of a gene are nonfunctional?

  The pathway is blocked, leading to disease.
• What is the phenotype of a pea plant with both copies of the R gene nonfunctional?

  The seeds are wrinkled.
• What is the phenotype of a pea plant with both copies of the V gene nonfunctional?

  The flowers are white.
• What does dosage dependence of gene products imply?

  If the phenotype is dosage sensitive, having one working allele is different from having none.
• What is the effect of losing both copies of a gene?

  You lose that function, requiring two copies of a loss-of-function allele to exhibit the phenotype.
• What is the significance of BRCA1 mutations in relation to breast cancer?

  A single BRCA1 mutation increases the chance of developing breast cancer by ~50%-65% by age 70.
• What happens in cases of homozygous loss of BRCA1?

  It seems to prevent the embryo from developing, causing early miscarriage.
• How do Mendel's laws relate to the mechanics of sexual reproduction in diploid species?

  • Mendel's laws are based on the segregation and independent assortment of alleles during gamete formation.
  • They depend on how phenotypes are defined and measured.
  • They serve more as guidelines than strict laws.
• What is the First Law of Segregation again?

  Two copies of each particle are inherited, one from each parent, and one is passed on at random in each sperm or egg.
• What is the Second Law of Independent Assortment again?

  Traits are inherited independently of one another.
• What is the Third Law of Dominance again?

  The inherited particles may be dominant or recessive.
• What is the significance of the quote by Theodosius Dobzhansky in the context of genetics?

  • Highlights the importance of evolution in understanding biology.
  • Suggests that genetic principles are rooted in evolutionary processes.
• What is the relationship between genotype and phenotype?

  • Genotype refers to the genetic makeup of an organism.
  • Phenotype refers to the observable traits or characteristics.
  • The genotype influences the phenotype.