Mendelian Genetics

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Mendelian Genetics

• Monohybrid Cross - one gene and one traite
• Dihybrid Cross - two genes and two traits
• Basic Probability
• Pedigree Analysis
• Variations

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Mendels Innovations

• True-breeding lines
• Discrete characters
• Controlled Crosses
• Counting

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Mendels Basic Monohybrid Cross
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Genetic Explanation of monohybrid cross
RR
rr
Rr
1 RR2 Rr
rr
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Explanation by Punnett Square
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F3 Test of Random Segregation
Round (565) Wrinkled 193 round only
(1/4) all wrinkled 372 31 round winkled
(2/4) (1/4)
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Mendels First Law (Random Segregation
In the formation of gametes, the paired
hereditary determinants (alleles of genes)
separate (segregate) in such a way that each
gamete is equally likely to contain either member
of the pair
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Genetic terminology

• Gene - the unit of heredity
• Allele - An alternative form of a gene
• gamete - a reproductive cell - sperm or egg
  pollen or ovum
• genotype - the genetic constitution of an
  organism or cell
• phenotype - the physical manifestation of a
  genotype

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Genetic terminology continued

• Homozygote - an individual possessing two
  identical alleles
• Heterozygote - an individual possessing two
  different alleles
• Dominant allele - one which will be expressed
  phenotypically in homozygotes and heterozygotes
• Recessive allele - one which will be expressed
  only in homozygotes

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Hypotheses regarding the behavior of two genes

• The genes segregate together, such that all
  gametes received paired parental alleles (e. g.
  RY or ry)
• Predicted Ratio 3 round yellow 1 wrinkled
  green
• The genes assort independently, such that the
  combinations RY, Ry, rY, and ry occur with equal
  frequency
• Predicted ratio 9 round yellow 3 round green3
  wrinkled yellow1 wrinkled green

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Actual results of Mendels Dihybrid Cross

• Round, yellow 315
• Round, green 108
• wrinkled, yellow 101
• wrinkled, green 32

Very Close to 9331 prediction of
independent assortment
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The Law of Independent Assortment
Segregation of the members of any pair of
alleles is independent of the segregation of
other pairs in the formation of reproductive cells
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Methods for Ratio Prediction

• Punnett squares
• Forked line diagrams

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The Testcross
A/a B/b C/c X a/ab/bc/c
Genotype Phenotype Ratio AaBbCc ABC 1/8 aaBbCc a
BC 1/8 AabbCc AbC 1/8 AaBbcc Abc 1/8 aabbCc ab
C 1/8 aaBbcc aBc 1/8 aabbCc abC 1/8 aabbcc a
bc 1/8
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Significance of the test cross
Unlike an F2 Cross, it results in a 11
correspondence between genotype and
phenotype. Thus, it is the preferred
experimental design for genetic analysis
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Preliminary Probability Considerations in Genetics

• Two events are mutually exclusive if the
  occurrence of one excludes the occurrence of the
  other the probability of obtaining one or the
  other outcomes is determined by adding their
  separate probabilities.
• Two events are independent if the occurrence of
  one has no affect on the probability of the other
  occurring the probability of two such events
  occurring simultaneously is determined by
  multiplying their probabilities.

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