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Classical Genetics.



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Using a Punnett square:

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Punnett square

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Step 1


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Step 2

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Dihybrid crosses

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f1 generation

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f2 generation

Probability & Genetics

The probability for each toss individually resulting in heads is 1/2.

Thus: to obtain the probability of two successively one multiplies their individual probabilities. (1/2) X (1/2) = 1/4

If both parents are Bb what is the probability of the offspring being bb?

The offspring must receive a b gamete from both parents.

The probability of a b sperm is 1/2

The probability of a b ovum is also 1/2

Thus the probability of a bb offspring is the product of the two individual probabilities :(1/2)X(1/2)=1/4


There are 2 mutually exclusive ways this can occur:

  1. Heads the first time, tails the second.
  2. Tails the first time heads the second.

The probability for each event (H/T) is 1/2

Thus the probability of situation 1 can be derived using the product law:

(1/2) X (1/2) = 1/4

The probability of situation 2 can thus be the same:

(1/2) X (1/2) = 1/4

Since the individual events are mutually exclusive, the total probability of either occurring can be derived using the sum law:

(1/4) + (1/4) = 1/2


Chromosomes and Classical Genetics

Sex Linkage:

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Crossing Over / Recombination

As seen before, genetic recombination can occur during synapsis of meiosis. This can explain much of the variability which challenged the principles of independent assortment through chromosome migration.

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Gene Mapping


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Example Gene Map

Other Factors Effecting Phenotypic Expression:

Multiple alleles:

ABO Blood typing


Gene interaction:

Continuous variation:

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Continuous Variation of Human Height

Environmental effects:



Incomplete Dominance.

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Mr. Stanley

Last updated: March 11, 2004