Basic Principles of Heredity - DOC

							                             Basic Principles of Heredity

The father of genetics
    Mendel’s unique credentials allowed him to discern the basic principles of genetics
    He worked with the common garden pea - Pisum sativum

    Mendel’s botanical background provided him with an understanding of flower
      anatomy and reproductive physiology

    Mendel also had access to pure-breeding varieties
    In fact, he spent over 2 years determining which particular varieties (traits) to
       investigate

    Peas are normally self-fertilizing

    Mendel’s Experiment
         o      So Mendel had to manipulate the flowers in order to achieve cross-
             fertilization
         o      Initially he conducted a series of monohybrid crosses investigating the
             inheritance of a single trait

          o     The phenotype was the same as one of the parents
                  It was not a blend
                  It was not like both of the parents
          o     Not only that, but the reciprocal crosses gave the same result

          o     What happened to the wrinkled seed trait?
          o     Mendel decided to analyze the F generation as well - pure genius!
                                                 2


    Mendel’s conclusions
        The F1 plants must have inherited genetic factors from both parents
         o     Therefore, each plant must possess two genetic factors (alleles) for each
             characteristic

         The two alleles in each plant separate when gametes are formed
          o    This occurs with equal probability
          o    One allele is the dominant form; the other is the recessive form

    Further generations confirmed Mendel’s original conclusions

    We can now relate Mendel’s concepts on genetic factors to chromosome segregation
      in meiosis

    A cross between a F genotype and either of the parental genotypes is a backcross
                         1
 Probability in genetics
      o      The Punnett square can determine the probability of obtaining offspring of
          various phenotypes and genotypes

       o   We can use the multiplication rule when we are looking at the
         probability of obtaining two independent events
       o   We can use the addition rule when we are looking at the probability of
         obtaining any one of two or more mutually exclusive events

       o    Binomial expansion
       o    For more complex problems, we can use other tools

 Consider albinism
 Albinism is inherited as an autosomal recessive trait
       o     So very easily, we can determine that two heterozygous parents will have
          a 1/4 chance of having an albino child

      It starts to get more difficult when the questions become more complex

       o    All three children with albinism?
              1/4 x 1/4 x 1/4 = 1/64

       o    Three children, one with albinism?
              Here, we need to consider three alternate situations

                        1/4 x 3/4 x 3/4 = 9/64
                        3/4 x 1/4 x 3/4 = 9/64
                        3/4 x 3/4 x 1/4 = 9/64
                        9/64 + 9/64 + 9/64 = 27/64

       o    Five children, two with albinism?

                 Our best bet is to expand the binomial (a + b)n
                 (a + b)5 = a5 + 5a4b + 10a3b2 + 10a2b3 + 5ab4 + b5

                 If we set
                 a = probability of a child with albinism = 1/4
                 b = probability of a child without albinism = 3/4
                 Then …
                 a5 = probability of having 5 children with albinism = 1st term

                      10a2b3 = probability of having 5 children, 2 with albinism = 4th
                        term
                      = 10(1/4)2(3/4)3 = 270/1024 = .26
           o     P = probability of event X with probability p occurring s times and event Y
               with probability q occurring t times

                      Event X = probability of a child with albinism = 1/4
                      Event Y = probability of a child without albinism = 3/4
                      s = number with albinism
                      t = number without

Testcrosses
    Testcrosses allow us to determine the genotype of individuals with ambiguous
       phenotypes

Genetic symbols
    Different genetic systems have adopted different symbols to identify alleles
    One, two, or three letter combinations
    Dominant allele is usually upper-case
    + is often used to designate wild-type

Ratios
    If a cross involves a single locus, recognition of distinctive ratios can quickly reveal
       the underlying genetics

Multi-loci crosses
           o     Mendel also conducted and analyzed such dihybrid crosses

           o     Mendel found 315 round, yellow; 101 wrinkled, yellow; 108 round, green;
               32 wrinkled, green seeds

Principle of independent assortment
    Mendel obtained the same 9:3:3:1 ratio from several different dihybrid crosses
    He concluded that alleles at different loci assort themselves independently of each
       other

    The caveat is that the different genes must be located on different chromosomes

    For these dihybrid crosses, we can consider the two monohybrid crosses
      independently

Observed vs. expected ratios
    Our predictions for genetic crosses yield probabilities, not certainties
    Often, the observed results are different from the expected results
    To determine if the observed results are reasonable with respect to the expected
      results, we can apply the goodness-to-fit chi-square test

    The chi-square test can tell us the probability that any difference between the
      observed value and the expected value is due to chance