Mendel And The Gene Idea

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  • 1. Mendel and the Gene Idea
  • 2. Gregor Mendel: The Man
    • Austrian monk
    • Began breeding peas in 1857 to study inheritance
    • Kept very accurate records of his laboratory work and used very large sample sizes
  • 3. Why Peas?
    • Available in many varieties
      • Flower color, seed color, flower position, pod color, seed shape, pod shape, stem length
    • Mendel could control which plants mated with which
    • Peas grow quickly!
  • 4. Mendel’s Procedure
    • Crossed male pea parts (stamens) and female pea parts (carpels) with opposite traits
    • Carpel matures to first-generation offspring (F 1 )
  • 5. Mendel’s Predictions
    • Crossing purple and white flowers would result in an intermediate phenotype
      • Mendel thought light purple flowers would be the result!
  • 6. Mendel’s Findings
    • Alternative versions of genes (different alleles) account for variations in inherited characters
      • The gene for flower color (example) exists in two versions – purple and white
      • Each version is called an allele
  • 7. Mendel’s Findings
    • For each characteristic, an organism inherits two alleles, one from each parent
      • An allele is a part of a chromosome
      • Each parent contributes one chromosome of each homologous pair
  • 8. Mendel’s Findings
    • If the two alleles differ, then one, the dominant allele , is fully expressed in the organism’s appearance; the other, the recessive allele , has no noticeable effect on the organism’s appearance.
  • 9. Mendel’s Findings
    • The two alleles for each trait segregate during gamete production.
      • An egg cell or sperm cell receives only one allele
      • Each parent passes on only one of his/her 2 alleles
      • This is Mendel’s Law of Segregation
  • 10. Some Terminology
    • Punnett Square:
      • A diagram used to predict the results of a genetic cross
    • Homozygous vs. Heterozygous:
      • Homozygous/pure: identical alleles (HH or hh)
      • Heterozygous/hybrid: different alleles (Hh)
    • Genotype vs. Phenotype:
      • Genotype: genetic makeup (Tt)
      • Phenotype: physical appearance (tall)
  • 11. TestCross
    • A testcross is used to determine the genotype of a parental organism
    • Cross the organism with the unknown genotype with an organism with the recessive phenotype
  • 12. Law of Independent Assortment
    • Each pair of alleles segregates into gametes independently
      • Just because an organism gets one allele doesn’t mean it will get a certain other one
      • Example:
        • Seed color (yellow or green) vs. Seed shape (round or wrinkled)
        • Yellow is NOT always with Round, etc…
        • Blonde hair does not HAVE to go with blue eyes
  • 13. Probability
    • Probability scale ranges from 0 to 1
      • If an event is certain to happen, it has a probability of 1
      • If an event is certain NOT to happen, it has a probability of 0
      • Getting heads on a coin toss is ½ (one out of two)
  • 14. The Rule of Multiplication
    • To determine the chance that two or more independent events will occur together in a specific combination, compute the probability for each independent event and then multiply the individual probabilities to get the overall probability
    • Example: Rolling two dice and rolling a 3 on each
      • 1/6 X 1/6
      • 1/36
  • 15. The Rule of Addition
    • The probability of an event that can occur in two or more different ways is the sum of the separate possibilities of those ways
    • Rolling an odd number using a dice:
      • 1/6 + 1/6 + 1/6 = 3/6 (or ½)
  • 16. Incomplete Dominance
    • F 1 hybrid is intermediate between the two parents
    • 1:2:1 ratio
    • red: pink: white
  • 17. Codominance
    • Both alleles are separately manifested in the phenotype
      • Example: Horses
        • Brown hairs
        • Black hairs
        • Brown and Black hairs
  • 18. Multiple Alleles
    • Genes that exist in more than two allelic forms
    • Example: ABO Blood Typing
      • I A I A , I A i
      • I B I B , I B i
      • I A I B
      • ii
  • 19. Pleiotropy
    • Def’n:
      • The ability of a gene to affect an organism in many ways
    • Example:
      • Alleles that cause sickle-cell anemia also cause other symptoms
  • 20. Epistasis
    • Def’n:
      • A gene at one locus (location) alters the phenotypic expression of a gene at another locus (location)
      • BB/Bb/bb determines coat color…BUT…
      • CC/Cc/cc determines pigment or not
  • 21. Polygenic Inheritance
    • Many characteristics, including human skin color and height, vary along a continuum among the population
    • Polygenic inheritance is the effect of two or more genes put together on a single phenotypic characteristics
    • Example: Height determined by 3 genes
      • AABBCC: very tall person (6’2”)
      • aabbcc: very short person (4’11”)
      • AaBbCc: intermediate height person (5’5”)
  • 22. Pedigrees
    • A pedigree is a family tree that shows the interrelationships of parents and children across the generations
    • Used to predict patterns in the future (risk assessment)
  • 23. Recessive Genetic Disorders
    • Cystic Fibrosis (cc)
      • Recessive disorder; most common in Caucasians
      • Cc (carrier)
    • Tay-Sachs Disease (tt)
      • Recessive disorder; most common in Ashkenazi Jews
    • Sickle-Cell Anemia (aa)
      • Recessive disorder; most common among African-Americans
  • 24. Societal Factors…
    • The prevalence of recessive genetic disorders greatly increases when closely-related relatives interbreed
    • This is why many countries and cultures have laws against intermarriage among close relatives (cousins, etc.)
  • 25. Dominant Genetic Disorders
    • Dwarfism:
      • DD or Dd = dwarf phenotype
    • Huntington’s Disease:
      • Aa or AA
      • Current research can now tell us whether or not a person has Huntington’s before symptoms set in
      • Ethical dilemma??