1.  Define the following terms and utilize these terms in an appropriate context:
Allele                              autosome                      codominance                              continuous variation
dominance                    epistasis                        gene interaction                         discontinuous variation
hemizygous                   lethal allele                     multiple allele                              incomplete dominance
sex-influenced              sex-limited                      X-linked inheritance
additive allele               polygenic inheritance

2.  Describe what is meant by Neo-Mendelian Genetics.

3.  Demonstrate the ability to utilize various labeling methods used to designate allelic variants from the wild type allele.

4.  Contrast incomplete dominance and codominance.

5.  With regard to the ABO blood types in humans, determine the genotypes of male and female parents if given their blood type, at least one of their parents blood types or a childs blood type.  In addition be able to predict the blood types of the offspring that a given couple may have and the expected proportion of each.

6.  Describe lethal genes and explain, in general, how they produce harmful effects.

7.  Explain why lethals are ordinarily recessive, semidominant (incomplete dominant) or co-dominant, but rarely dominant.

8.  Define epistasis, explain what F2 results indicate that epistasis may be involved and outline a cross in which epistasis is operating.

9.  Discuss the reasoning behind the statement that there are multiple alleles of any gene.

10.  Differentiate between inheritance controlled by multiple alleles and polygenic inheritance.

11.  Explain why the inheritance of eye color in humans must be affected by several sets of alleles.

12.  From the fraction of the F2 like either parent, and from the number of F2 phenotype classes, state the number of pairs of multiple genes affecting a trait.

13.  Using the three-contributing-allele hypothesis, explain the inheritance of human skin color.

14.  Distinguish between discontinuous and continuous variation.  To which category does epistasis belong?

15.  Define and discuss the following terms:  polygenes, additive alleles, multiple factor hypothesis.

Resources:  Text Chapter 4,  6
 
 

    Neo-Mendelian Genetics
            -investigations didn't precisely conform to expected ratios
            -Assumptions:     phenotype under control of one or more genes

                                          principles of segregation and independent assortment apply

1. Alleles- alternate forms of the same gene, contain modified genetic info to specify an alteration of the original gene product.

            Ex. 350+ different alleles of genes that specify changes that create cystic fibrosis

            Over 100 different alleles of genes that specify the protein portion of human hemoglobin

            Once manufactured, product of an allele may or may not be functional

2. Wild type allele, occurs most frequently or is the form labeled "normal"

3. Mutation- source of new alleles, recognized by change in phenotype which results from change in functional activity of cell product controlled by gene

                -change may reduce, enhance or eliminate functional capacity of a gene product

                Therefore, phenotype may or may not be altered in a discernable way

4. Allele symbols-             + = wild type/normal,   letters usually indicate the mutation

                                            e = ebony body in D. melanogaster
                                            e+e+ = normal (or +/ +)
                                            e+e = heterozygote (or +/e)
                                            e e = ebony (or e/e)

                works well when alleles are dominant or recessive, however,

                Pg. 61, multiple alleles (codominant) blood
                Pg. 62, multiple alleles at white locus of D. melanogaster

                Even more complicated is no dominance clearly exists

Incomplete Dominance (Partial Dominance)
    - observation of intermediate phenotypes generated by cross w/ contrasting traits

     "Hey Greg, what the heck is going on? We told you this stuff was blending"

        Four O'Clocks, snapdragons Petal Color

                P generation         R1R1 (Red)  X R2R2  (White)

                F1                                 R1R2 (Pink)

                F2                         1/4 R1R1 (red); 1/2 R1R2 (pink); 1/4 R2R2 (white)

    - Phenotype under control of single pair of alleles w/ no dominance

    - incomplete dominance relatively rare

    - Even when complete dominance is evident, careful examination of the level of gene product rather than phenotype shows some intermediate.

            Ex. Tay-Sachs disease
                    -homozygous recessive severly effected, heterozygote phenotypically normal
                    -affected individuals almost no activity of exzyme Hexosaminidase- A

heterozygotes express ~50% of enzyme activity of homo dominant

*Illustrates somewhat arbitrary nature of dominance/ recessive

*Incomplete dominance tends to produce phenotypes that appear to be blended or intermediate, however . . . . . .

Ex. M N Blood Groups (pg 61) - glycoproteins found on surface of RBC's

-provide immunological identity and will elicit an antibody response if present on transfused or transplanted tissues

Genotypes/ Phenotypes LMLM ----> M
                                          LMLN ----> MN
                                          LNLN ----> NN

*Incomplete dominance on gross phenotypic level and codominance on subcellular level
 
 

        **Multiple alleles can only be studied in populations

       A B O blood groups- IA, IB, Io (sometimes i) I= isoagglutinogen (antigen)

            A and B codominant to each other and both dominant to o

            IAIB- universal recipient (no antibodies)
            i i - universal donor - (no antigens)

          Bombay - hh, h group cannot attach sugar (responsible for A or B antigen) allows oo phenotype despite A, B inheritance

            Drosophila- white (w) one of 100 alleles at this locus

            Cystic Fibrosis
 

Lethal Alleles- Mutations alter gene product to nonfunctional state

        when genetic defects cause 100% mortality

        Generally recessive which means homozygous recessive dies

        However, dominant lethals exist, override expression of wildtype

Ex. coat color in mice

        Y+ X Y+
        resulting ratios 2/3 yellow, 1/3 agouti

        Should have been 3:1

        YY genotype lethal

Huntingtons disease (dominant)

Pleiotropy- one gene effects many different traits; biochemical pathways usually blocked secondary or other effects

        Coat color/ survival

        seedcoat/ starch grains

        pg 262 PKU

                lack enzyme that metabolizes phenylalanine
                PKU individuals ----> higher levels of phenyl. effects IQ, headsize, hair color plus others

              Marfan Syndrome (Flo Hyman)

tall, thin, long legs and arms, nearsighted and wall of aorta thin causing it to enlarge and split; all traced to defective gene responsible for production or normal connective tissue

Expressivity- the degree or range in which a phenotype for a given trait is expressed.

***both penetrance and expressivity have significant influences from the environment and other genes

Combinations of two gene pairs

        -up to now, modifications of monohybrid (3:1)
        -combining any two inheritance patterns will modify the 9:3:3:1
        -Mendels independent assortment still apply provided genes not on same chromosome

        Ex. mate 2 humans heterozygous for albanism and both of blood type AB

                Aa IAIB X Aa IAIB

All offspring

        3/4 pigmented ----> 1/4 A-----> 3/16 Normal, A
        1/2 AB -----> 3/8 = (6/16) Normal, AB
        1/4 B -----> 3/16 Normal B

        1/4 albino ---------> 1/4 A -----> 1/16 albino, A
        1/2 AB -----> 2/16 = (1/8) albino, AB
        1/4 B -----> 1/16 albino, B

        3: 6: 3: 1: 2: 1 ratio, not classical 9:3:3:1
            albanism classical
            blood type multiple allele

Neo-Mendelian genetics show more sophistacated inheritance
        *phenotypic characters can be influenced by more than one gene (and more than one gene product)

Discontinuous Variation

Epistasis- interaction between non-allelic genes such that one interferes (influences) with or prevents expression of the other.

**Bateson/Punnett crossed two varieties of white-flowered peas

        F1-----> all purple!

        F2-----> 9/16 purple, 7/16 white

    2 genes working one dominant of each would produce purple color

        9/16 C_P_                 7/16 C_pp; ccpp
            purple                             white

       Complimentary Gene Action (pg 64-)

BIOCHEMICAL EXPLANATION (PG 67-68)
        -inheritance of dominant produces functional enzyme in a pathway

        -inheritance of recessive produces nonfunctional enzyme along pathway

X (colorless) ------> C dom -----> Y (colorless) -----> P dom ----->purple pigment

X (colorless) ------> C dom -----> Y (colorless) -----> p rec -------> no pigment

*in humans, if you inherit two recessives for albanism it matters little what your genes for eye color are!!

Novel Phenotypes

        flies: Aabb X aaBB
                 brown    scarlet

            F1 AaBb wildtype

            F2 9:3:3:1

                A_B_         wildtype         both enzyme products produced

                A_bb          brown             only brown produced

                aaB_         scarlet             only scarlet produced

                aabb          white                no pigments produced

Continuous Variation
        Quantitative Inheritance- genes make additive contributions to gene expression

Polygenic inheritance- "Mailman/ Milkman/ Postman/ UPS man/ Schwann man Syndrome"

1. Characters can be quantified by measuring, weighing, counting, etc.

2. Two or more pairs of genes influence phenotype in an additive way (more genes, more variation)

3. Each gene locus occupied by either additive or non-additive allele (dominant/ recessive)

4. Effect of each allele equivalent to all others (no epistasis)

5. Together genes produce substantial variation

6. Variation enhanced by environmental factors Ex. height ---> diet; plants ---> soil, waterfall

7. analysis of polygenic traits requires study of large populations. (large sample base)

Ex. wheat kernel color
4 additive alleles                 3 additive                 2 additive                 1 additive allele
AABB 1/16 Red                 AaBB or AABb         AaBb 1/16 pink         aaBb or Aabb 4/16 light pink
                                                4/16 light red

0 additive alleles
aabb 1/16 white

Variation will fit a bell shaped curve
 
 
 
 
 
 
 

height, weight, stature in all plants and animals, grain yield in crops, beef and milk production, egg production, skin color etc.
 

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