Fundamentals of Genetics

Chapter 10

I.         Gregor Mendel (mid 19^th century)

            A.        Austrian monk, managed farms

            B.        Trained in science/math

            C.        Applied statistics to study of plants

II.        Experiments

            A.        Used self pollination to develop pure strains

            B.        Studied 14 characteristics having 2 traits

            C.        Crossed 2 traits and studied F₁ and F₂ generations

III.       Results

            A.        F₁ generation showed only 1 trait

            B.        F₂ generation showed both traits

            C.        F₂ ratio 3:1

IV.      Conclusions

            A.        Something controlled expression (factor)

            B.        Each characteristic had two factors

            C.        Expression of characteristic result of interaction of factors

            D.         Dominance Principle

            E.        Law of segregation

            F.        Law of independent assortment

V.        Dominance Principle

            A.        F₁ results show 1 factor can mask another

            B.        F₂ results show 2 factors involved

            C.        Masking factor is dominant

            D.        Masked factor is recessive

            E.        Recessive expressed when both factors recessive

VI.      Law of Segregation

            A.        F₂ results show 2 factors involved

            B.        Each individual has 2 factors

            C.        \Factors must separate during meiosis

VII.     Law of Independent Assortment

            A.        Dihybrid cross shows 2 characteristics not linked

            B.        Factors distributed independently

VIII.    Mendel’s Work not Accepted

            A.        No mechanism for it to work

            B.        Poor understanding of statistics

            C.        Lack of prestige/publication

IX.      Walter Sutton - 1900

            A.        Observed meiosis and moving chromosomes

            B.        Discovered Mendel’s writings

            C.        Proposed Chromosome Theory to link the two

X.        Chromosome Theory

            A.        Chromosomes carry inheritable units (genes) which occur as pairs of alleles

            B.        Allele is Mendel’s factor

                        1.         Capital letter is dominant

                        2.         Lower case is recessive form

XI.      Genotype

            A.        Genetic makeup of organism

            B.        Each characteristic has only 2 alleles

            C.        Homozygous: both the same

            D.        Heterozygous: alleles different

            E.        Multiple: characteristic affected by >3 alleles

XII.     Phenotype

            A.        Expression of characteristic

            B.        Homozygous (Dominant): dominant trait

            C.        Heterozygous: dominant trait

            D.        Homozygous (recessive): recessive trait

XIII.    Probability

            A.        Likelihood of event happening

            B.        Divide occurrences of 1 event by total events

            C.        Example

                        1.         Draw an ace from deck of cards

                        2.         4 aces and 52 cards

                        3.         Probability is 4/52 or 1/13

            D.        Small number of trials may skew events

XIV.   Punnet Square

            A.        Probability diagram

                        1.         Label column w/paternal alleles

                        2.         Label row w/maternal alleles

                        3.         Write pairs in blocks

            B.        Pairs predict genotype/phenotype of cross

XV.     Monohybrid Cross

            A.        Only one characteristic

            B.        Homozygous/Homozygous

                        1.         Heterozygous offspring

                        2.         Show dominant trait

            C.        Homozygous (D)/Heterozygous

                        1.         2 Homozygous (D) and 2 heterozygous

                        2.         Show dominant trait

            D.        Homozygous (R)/Heterozygous

                        1.         2 Heterozygous and 2 Homozygous (R)

                        2.         Heterozygous show dominant trait

                        3.         Homozygous show recessive trait

            E.        Heterozygous/Heterozygous

                        1.         1 homozygous (D), 1 homozygous (R), 2 heterozygous

                        2.         Homozygous (D), heterozygous show dominant

                        3.         Homozygous (R) shows recessive

XVI.   Test Cross

            A.        Used to determine unknown genotype

            B.        Cross individual shown dominant trait with homozygous recessive

            C.        Distribution of phenotype determines genotype

            D.        Homozygous: all offspring heterozygous, dominant trait

            E.        Heterozygous: ½ offspring heterozygous, dominant and ½ homozygous, recessive

XVII.  Incomplete Dominance

            A.        Neither allele dominant

            B.        Homozygous shows pure traits

            C.        Heterozygous shows intermediate trait

XVIII. Codominance

            A.        2 nonidentical alleles specify characteristics

            B.        Both are expressed when heterozygous

            C.        Blood

                        1.         3 alleles: I^A, I^B, I

                        2.         Type A: I^AI^A or I^Ai

                        3.         Type B: I^BI^B or I^Bi

                        4.         Type AB: I^AI^B

                        5.         Type O: ii

XIX.   Dihybrid Cross

            A.        Cross involving 2 independent characteristics

            B.        Each individual has 4 possible combinations

            C.        Use 4x4 punnet square

            D.        Characteristics pass independently

XX.     Homozygous to Homozygous

            A.        Produce heterozygous offspring

            B.        All show dominant trait

XXI.   Heterozygous to Heterozygous

            A.        9/16 offspring show dominant trait

            B.        3/16 show one dominant and one recessive trait

            C.        3/16 show reverse of B.

            D.        1/16 show double recessive trait