Genetics BIOL 2120 - Lecture Notes

I. Gregor Mendel's experimental design
    A. Choice of organism
        1. Short life cycle
        2. Inexpensive
        3. Many varieties
        4. Many offspring
    B. Focused on specific traits
        1. Many varieties in peas
        2. Distinctly contrasting phenotypes
        3. The expression of one trait did not influence the expression of a second trait
    C. Used pure lines
    D. Method of reproduction
        1. Selfing versus cross pollination
    E. Collected large amounts of data
    F. Analyzed data using mathematics
II. Types of crosses
    A. Parental generation
        1. Start with pure breeding (homozygous) lines
        2. Several generations of selfing prior to start of experiment
        3. Clearly defined traits
    B. First filial generation
        1. Will be heterozygous at all gene loci being studied
    C. Second filial generation
        1. If one trait, monohybrid cross
        2. If two traits, dihybrid cross
    D. Reciprocal cross
        1. Same traits, but switch the parents expressing each trait
            a. Eg. colorblind male x normal female
            b. Reciprocal cross: normal male x colorblind female
        2. Important in identifying sex-linked traits
III. Mendel's conclusions
    A. Inheritance is particulate, not a blending
        1. We call the particles 'alleles'
    B. Adult peas have two alleles at one gene locus - a gene pair
        1. Homozygous: two alleles the same
        2. Heterozygous: two alleles that are different
    C. Mendel's law of dominance
        1. Dominant alleles: expressed whether homozygous or heterozygous
        2. Recessive alleles: expressed only in when homozygous
    D. Mendel's law of independent assortment
        1. Members of gene pair separate (segregate) equally - randomly - into
            gametes
        2. Random assortment of alleles identical to behavior of chromosomes in
            meiosis
        3. Each gamete carries only one member of gene pair
        4. Random joining of gametes to form zygote
            a. No selection of which sperm will combine with which egg
            b. Equal chance of either of a gene pair from either parent
        5. Exceptions if genes located near each other on the same chromosome
        6. Note: chromosomes were not shown to carry genes until 1931- 55 years
            after Mendel's experiments!
IV. Important terminology in Mendelian genetics
    A. Gene vs. allele
    B. Homozygous vs. heterozygous
    C. Genotype vs. phenotype
    D. Dominant vs. recessive
    E. Monohybrid vs. single trait crosses
        1. Monohybrid: both parents are heterozygous for a trait
            a. Genotypic ratio is 1AA:2Aa:1aa
        2. Single trait cross - each parent can be either homozygous or heterozygous
            a. Genotypic ratio depends on the cross
        3. Test cross: A_ x aa
            a. Purpose is to determine the genotype of an individual expressing
                dominant trait
            b. If results are all A_ (and large number of progeny), cross was AA x aa
            c. If results are 1A_:1aa, cross was AA x aa
    F. Dihybrid vs. double trait crosses
        1. Dihybrid: AaBb x AaBb
        2. Double trait crosses: any combination involving 2 genes
V. Methods of predicting the outcome of a cross
    A. Punnett square
        1. Good for one gene
        2. More difficult for two or more genes
    B. Branch diagrams
        1. Also more complex when three or more genes
    C. Probability calculations
        1. Useful for numerous genes
        2. Calculate some probabilities without determining all possibilities
        3. And - multiply
        4. Or - add
        5. Spend time in lab mastering the rules
VI. Mendelian Genetics in Humans
    A. Problems with making planned crosses
        1. Can't tell who to mate with whom
        2. Offspring not in large numbers
    B. Alternative approach
        1. Use of pedigrees
        2. Symbols used in pedigrees - figure 2-15, p. 44
    C. Autosomal recessive traits
        1. Skip generations
        2. Unaffected parents produce affected offspring
        3. Trait shows up in consanguineous marriage
    D. Autosomal dominant traits
        1. No affected offspring without an affected parent
        2. Does not skip generations
        3. Seen in about half the offspring of affected individuals
    E. NOTE:
        1. Recessive traits can occur frequently
        2. Dominant traits can be rare
        3. Occurrence has to do with frequency of allele in population
        4. NOT with whether allele is dominant or recessive
    F. Continuous vs. discontinuous traits
        1. Continuous traits show no discrete classes - such as hair color or height in
            Humans
        2. Continuous traits have multiple causes
            a. numerous genes
            b. environmental influence
    3. Discontinuous traits more readily dissected genetically

For questions, comments and additional information, contact mfhicks@pstcc.edu
Last Updated: June 22, 2001
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