Mendelian Inheritance

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Mendelian Inheritance

• Chapter 4, part 1

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Objectives

• Describe the four postulates drawn from Mendels
  pea experiments.
• Perform monohybrid and dihybrid crosses.
• Define and perform a test cross.

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History of Genetics

• 1. Hippocrates (500BC)
• -"humors act as bearers of hereditary traits
• -drawn from various parts of the body to become
  semen
• -can be healthy or diseased, accounts for sick or
  deformed babies
• -humors can be altered, so it is possible to pass
  on acquired traits

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• 2. Aristotle (300 BC)
• -generative power of sperm resided in the VITAL
  HEAT it contained
• -Vital heat had capacity to produce offspring in
  same form as parent
• -every part of new organism found in menstrual
  blood of mother and activated by semen of
  father
• -VH cooked and shaped menstrual blood which was
  physical substance that gave rise to offspring

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Two Schools of Thought 1600-1850

• No sperm or egg visible prior to 1800
• 1. William Harvey Theory of Epigenesis
• -an organism is derived from substances present
  in the egg, which differentiate into adult
  structures during embryonic developments.

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• 2. Preformationists
• -inside of gametes was an intact organism with
  all parts (just really small) called HOMUNCULUS.
• -next generation existed inside gametes
• gtspermists- organism is contained in sperm
• gtootists - homunculus is contained in egg

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Other Prevailing Theories

• Blending Theory of Inheritance
• Spontaneous generation
• Fixity of species
• Theory of Natural Selection Charles Darwin
• published 1859
• gap in theory believed in inheritance of
  acquired characteristics
• lacked understanding of genetic basis of
  inheritance and variation

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Mendelian Genetics Transmission Genetics

• 1866 Mendel determined that distinct units of
  inheritance existed
• was able to predict the behavior of these units
  during gamete formation
• 1856 started study
• 1866 published
• 1868 stopped study, elected abbott
• 1879 first chromosome viewed
• 1884 Mendel dies

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What Mendel Did

• Studied seven characters (visible features) each
  represented by two contrasting forms
• true (pure) breeding strains
• mature in single season

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Pea Anatomy
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Monohybrid Cross

• Generations P1, F1, F2
• Example
• Tall X dwarf P1
• All Tall F1
• 787 tall, 277 short F2
• Ratios accepted way to express genetic data
• Didnt matter which sex had trait, but Mendel
  checked. Called reciprocal crosses

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Conclusions from Monohybrid Crosses

• 1. Postulate of Unit Factors in Pairs
• Genetic factors are controlled by unit factors
  that exist in pairs in individual organisms
• gteach diploid organism receives one factor
  from each parent

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• 2. Postulate of Dominance and Recessiveness
• When two unlike unit factors responsible for a
  single character are present in a single
  individual, one unit factor is dominant to the
  other, which is recessive.
• -trait expressed in F1 results from presence of
  dominant unit factor
• -trait not expressed in F1 but reappears in F2
  recessive unit factor

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• 3. Postulate of Segregation
• During the formation of gametes, the paired unit
  factors separate, or segregate, randomly so that
  each gamete receives one or the other with equal
  likelihood.
• gtif TT, offspring will receive T
• gtif Tt, offspring has 50 likelihood of
  getting T or t
• -wasnt linked to meiosis till 1902

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Mendel in Modern Day Vocabulary

• 1. Phenotype
• 2. Gene
• 3. Allele
• 4. Genotype
• 5. Homozygous/homozygote
• 6. Heterozygous/heterozygote

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Punnett Square in Monohybrid Crosses
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Test Crosses
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Mendels Fourth Postulate

• Used dihybrid crosses (two traits)
• crossed pea plants with contrasting traits to
  see if one trait influenced the expression of the
  other. They didnt and he came up with The
  Postulate of Independent Assortment
• During gamete formation, segregating (separating)
  pairs of unit factors assort independently of
  each other
• all possible combinations will form with freq

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