Mendelian Genetics
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
 Peas are normally self-fertilizing
Mendel’s Experiment
 Initially he conducted a series of monohybrid crosses investigating the inheritance of
a single trait
 The phenotype was the same as one of the parents
What happened to the wrinkled seed trait?
Mendel decided to analyze the F generation as well - pure genius!
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Mendel’s conclusions
 The F1 plants must have inherited genetic factors from both parents
 Therefore, each plant must possess two genetic factors (alleles) for each
characteristic
 The two alleles in each plant separate when gametes are formed
 This occurs with equal probability
 One allele is the dominant form; the other is the recessive form
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
Further generations confirmed Mendel’s original conclusions
We can now relate Mendel’s concepts on genetic factors to chromosome segregation in
meiosis
Remember, the chromosome theory of heredity wasn’t developed until the early 1900s
Probability in genetics
 The Punnett square can determine the probability of obtaining offspring of various
phenotypes and genotypes
 We can use the multiplication rule when we are looking at the probability of
obtaining two independent events
 We can use the addition rule when we are looking at the probability of obtaining
any one of two or more mutually exclusive events
Binomial expansion
 For more complex problems, we can use other tools
 Our best bet is to expand the binomial (a + b)n
Testcrosses
 Testcrosses allow us to determine the genotype of individuals with ambiguous
phenotypes
Multi-loci crosses
 Mendel also conducted and analyzed such dihybrid crosses
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
 We start by determining the expected values based on the hypothesis that the
expected results should match a particular ratio (e.g., 1:1, 3:1, 9:3:3:1, etc)
 This is called the null hypothesis (H0)
 The X2 value allows us to determine the probability value (p)
 p allows us to evaluate the likelihood that the observed differences are due to chance
Pedigree
 A pedigree is a schematic diagram of a family history, often with phenotypic and
genotypic data
Autosomal recessive
Consanguinity
 In the case of rare autosomal recessive traits, the trait is more likely to occur when
two people within the same family mate
 They will have a greater chance of both possessing the rare recessive allele
Autosomal dominant