Classical genetics

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Mendel Lecture for CHS IB BIO

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Classical genetics

  1. 1. Classical Genetics
  2. 2. Mendel And The Laws Of Inheritance• Gregor Johann Mendel (1822-1884) - father of genetics• Austrian monk• Conducted landmark studies from 1856-1864, - thousands of crosses• Kept meticulously accurate records that included quantitative analysis
  3. 3. Mendel Chose Pea Plants as His Experimental Organism• Hybridization – The mating or crossing between two individuals that have different characteristics • Purple-flowered plant X white-flowered plant• Hybrids – The offspring that result from such a mating – Presumed to be a blending of the parent traits – Often observed to be different than either parent (hybrid vigor)• Mendel observed them to be like one of the parents with respect to some traits – no blending
  4. 4. Mendel Studied Seven Traits That Bred True• The morphological characteristics of an organism are termed characters or traits• A variety that produces the same trait over and over again is termed a true- breeder
  5. 5. Mendel’s Experiments• Crossed two variants differing in only one trait – a monohybrid cross
  6. 6. DATA FROM MENDEL’S MONOHYBRID CROSSESP Cross F1 generation F2 generation RatioTall X All tall 787 tall, 2.84:1dwarf stem 277 dwarfRound X All round 5,474 round, 2.96:1wrinkled seeds 1,850 wrinkledYellow X All yellow 6,022 yellow, 3.01:1Green seeds 2,001 greenPurple X All purple 705 purple, 3.15:1white flowers 224 whiteAxial X All axial 651 axial, 3.14:1terminal 207 terminalflowersSmooth X All smooth 882 smooth, 2.95:1constricted 229 constrictedpodsGreen X All green 428 green, 2.82:1yellow pods 152 yellow
  7. 7. Interpreting the Data• For all seven traits studied 1. The F1 generation showed only one of the two parental traits 2. The F2 generation showed an ~ 3:1 ratio of the two parental traits• These results refuted a blending mechanism of heredity
  8. 8. Interpreting the Data• Data suggested a particulate theory of inheritance• Mendel postulated the following:
  9. 9. Law of Segregation• A pea plant contains two discrete hereditary factors, one from each parent• The two factors may be identical or different• When the two factors of a single trait are different – One is dominant and its effect can be seen – The other is recessive and is masked• During gametogenesis (meiosis), the paired factors segregate randomly so that half of the gametes received one factor and half of the gametes received the other
  10. 10. Review a few modern terms– Mendelian factors are now called genes– Alleles are different versions of the same gene– An individual with two identical alleles is termed homozygous– An individual with two different alleles, is termed heterozygous– Genotype refers to the specific allelic composition of an individual– Phenotype refers to the outward appearance of an individual
  11. 11. • Prevalent alleles in a population are termed wild- type alleles – These typically encode proteins that • Function normally • Are made in the right amounts• Alleles that have been altered by mutation are termed mutant alleles – These tend to be less common in natural populations – They are likely to cause a reduction in the amount or function of the encoded protein – Such mutant alleles are often inherited in a recessive fashion• A particular gene variant is not usually considered an allele of a given gene unless it is present in at least 1% of the population. – Rare gene variants (<1%) are termed polymorphisms rather than allelic variants
  12. 12. Modern Genetic Terminology• Recessive – Null – no functional protein is produced – loss-of-function alleles (LOF) • a protein that doesn’t function as much as or in the same way as the wildtype protein – Wildtype alleles are dominant to recessive alleles• Dominant – Gain-of-function (GOF) • protein functions is a new way • more protein is made than in wildtype • protein can not be regulated as in wildtype – Dominant-negative (DN) • a mutated protein disrupts the function of wildtype proteins – dominant alleles are dominant to wildtype

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