Introduction to Mendelian Genetics

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Introduction to Mendelian Genetics
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I'm one of those people you hate because of
genetics. It is the truth. - Brad Pitt
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Early Observations

• Aristotle-pangenesis
• Contributions from all parts of the body come
  together in gonads

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As far back as biblical times.

• Jacobs Flock

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Two opposing theories

• Von Leeuwenhoek, et al
• Homunculus-father gives all traits, mother an
  incubator
• DeGraaf-Mother gives all traits, father gives a
  starting substance

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Blending

• Early belief that both parents contribute to
  offspring
• Blending, ie. Blue paint and yellow paint make
  green paint
• Traits disappear

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Gregor Mendel

• An Austrian monk, 1860s
• At the time, clergy were the educated
• He was a mathematician and amateur gardener

http//ridge.icu.ac.jp/gen-ed/mendel-gifs/01-mende
l-himself.JPG
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Father of Genetics

• Wasnt a geneticist!
• Analyzed traits in garden peas mathematically
• Over many generations

http//www.shsu.edu/bio_aad/bio345.htm
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Pea Plants

• Have traits that two forms
• Ie. Tall vs. short
• Round vs. wrinkled peas
• Purple vs. white flowers

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Controlled pollination

• Peas have covered flowers
• Male partstamens
• Female partpistil
• Cut the stamens from plants, obtained pollen
• Dusted pollen onto the pistil of a selected plant

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Figure 14.x1 Sweet pea flowers
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Controlled pollination
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Monohybrid cross

• Mating of two organisms to follow one trait, ie.
  Round vs. wrinkled seeds, or purple vs. white
  flowers
• Ppurple
• pwhite
• Phenotypewhat look like
• Genotypethe genes
• you have
• Alleleversion of gene

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Figure 14.x2 Round and wrinkled peas
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First generation

• Cross purple flower with white flower
• P generation first or parental generation
• F1 offspring of P generation (kids)
• F2 offspring of F1 generation

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Quantitative Data

• When Mendel crossed
• purple vs. white, got all
• purple in the F1
• let the F1s self cross
• got a 31 ratio of
• purple white in the F2

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Mendels hypotheses

• Unseen entity, called them factors
• We now call them genes
• factors retained their individual identity
• Did not blend!

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Two copies of each

• Either parent could affect the phenotype
• One factor came from each parent

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The factors separate from each other

• During gamete formation the 2 factors from the
  parents are separated from each other
• Recall that meiosis wasnt discovered until the
  1870s!

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Principle of Dominance

• Of the 2 factors present, one is often seen and
  the other is not seen

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Mendels Principle of Segregation

• During gamete formation, factors separate from
  each other

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http//ridge.icu.ac.jp/gen-ed/mendel-gifs/18-mende
l-cartoon.JPG
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Test Cross

• Cant tell by looking whether PP or Pp
• How did he know P generation was pure?
• Crossed with recessive, pp

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Why wasnt Mendel accepted in his time?

• spoke abstractly and mathematically
• Chromosomes and genes not discovered yet
• couldnt physically show his audience what
  factors were
• Readers were lost
• Paper in a respected journal, but his paper not
  widely read