chapter 13a

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Chapter 13a


  • Chapter 13: Extensions of Mendelian Principles:

    Multiple alleles

    Modifications of dominance relationships

    Gene interactions

    Essential genes and lethal alleles

    Gene expression and the environment


  • Multiple alleles:

    Not all genes have two forms (alleles), many have multiple alleles.

    Diploid individuals have only two alleles, one on each chromosome.


    ABO blood groups

    Drosophila eye color

    Fig. 13.3

  • Human ABO blood groups:

    4 blood phenotypes: O, A, B, & AB

    3 alleles: IA, IB, I

    IA and IB are dominant to i.

    IA and IB are codominant to each other.

    PhenotypeGenotypeRBC-antigenBlood-antibodyOi/inone (H)anti-A & BAIA/ IA or IA/iAanti-BBIB /IB or IB /iBanti-AABIA/IBA and Bnone

  • ABO inheritance is Mendelian:

    Possible parental genotypes for type O offspring:

    i/i x i/i

    IA/i x i/i

    IA/i x IA/i

    IB/i x i/i

    IB/i x IB/i

    IA/i x IB/i

  • Drosophila eye color:

    > 100 mutant alleles for the eye color locus on the X chromosome.

    w+wild type, redw mutant, white-eyewemutant, eosin (reddish-orange)

    1912, Thomas H. Morgan

    Crossed eosin-eyed female with a white-eyed male:

    All F1 had eosin eyes.

    P Crossw (X)Ywe (X)we/wXXwe/YXYwe (X)we/wXXwe/YXY

  • Drosophila eye color:

    Alfred H. Sturtevant (1913) observed:

    Red (w+) is dominant to white (w) and eosin (we).Eosin (we) is recessive to red (w+), but dominant to white (w).Concluded eosin (we) and white (w) are multiple alleles of the same gene.

    Confirmed by crossing F1 female with wild type red-eyed male:

    w+(X)Ywe (X)we/w+XXwe/YXYw (X)w/w+XXw/YXY

  • Molecular basis of multiple alleles and dominance relationships:

    Different alleles of the same gene reflect different activity and expression of the gene product.

    Drosophila homozygotePhenotypeRelative eye pigmentw+wild type1.0000wwhite0.0044wttinged0.0062waapricot0.0197wblblood0.0310weeosin0.0324wchcherry0.0410wa3apricot-30.0632wwwine0.0650wcocoral0.0798wsatsatsuma0.1404wcolcolored0.1636

  • Number of alleles (n) and number of genotypes (Table 12.3):

    # genotypes = n(n + 1)/2

    Homozygotes = nHeterozygotes = n(n - 1)/2

    # alleles# genotypesHomozygotesHeterozygotes11102321363341046515510

  • Different types (modifications) of dominance relationships result fromDifferent molecular patterns of gene expression.

    Complete dominance

    Incomplete dominance


  • 1. Complete dominance (complete recessiveness)

    One allele is completely dominant to another.

    Phenotype of the heterozygote is the same as homozygous dominant.

    Recessive phenotype is expressed only when the organism is homozygous recessive.

    e.g., Mendels pea traits (Fig. 11.5)

  • 2. Incomplete (partial) dominance

    One allele is not completely dominant to another.

    Phenotype of the heterozygote is intermediate between the phenotypes of homozygotes for each allele.

    e.g., plumage color in chickens and palomino horses

  • Fig. 13.7, Incomplete dominance in chickens

  • 3. Codominance

    Alleles are codominant to one another.

    Phenotype of the heterozygote includes the phenotype of both homozygotes.

    e.g., ABO blood groups & sickle-cell anemia

    Fig. 4.9

  • Molecular explanations for dominance relationships:

    Complete dominance

    Dominant allele creates full phenotype by one of two methods:

    Half the amount of gene product produced by homozygote is sufficient (haplosufficient), OR

    Expression of dominant allele in heterozygote is up-regulated to match the homozygote.

    Incomplete dominance

    Recessive allele is not expressed in heterozygote:

    Homozygote:2 doses of a gene product

    Heterozygote:1 dose of a gene product


    Both alleles are expressed equally resulting in a combined phenotype.

  • Gene interactions and modified Mendelian ratios:

    Phenotypes result from complex interactions of genes (molecules).

    e.g., dihybrid cross of two independently sorting gene pairs, each with two alleles (A, a & B, b).

    9 genotypes (w/9:3:3:1 phenotypes):


    Deviation from this ratio indicates the interaction of two or more genes producing the phenotype.

  • Two types of gene interactions:

    Multiple genes control the same trait and by their interactions produce a new phenotype.

    Epistasis - one or more genes mask the expression of other genes and alter the phenotype.

  • Different genes control the same trait and collectively produce a new phenotype, e.g., comb shape in chickens.

    4 phenotypes resulting from dominant and recessive alleles at 2 loci:


    Cross true-breeding rose-combed (RR/pp) and pea-combed (rr/PP) chickens.

    Interaction of two dominant alleles (R & P), produces a third phenotype (walnut), all F1 are walnut-combed (Rr/Pp).

    Fourth phenotype (single-comb, rr/pp) appears in the F2.

    F2 is 9:3:3:1 (walnut:rose:pea:single) and fits Mendelian ratios.

    Multiple genes involved, and interaction of two dominant alleles (R & P) produce factors that modify comb shape from a simple (rose/pea) to more complex form (walnut).


  • Fig. 13.9

  • Epistasis

    No new phenotype is produced, but one gene (epistatic) masks the phenotypic expression of another gene (hypostatic).

    Dominant epistasis, A masks the effect of B.

    Recessive epistasis, caused by recessive alleles, aa masks the effect of B at another locus.

    Can occur with two genes, requiring A and B to produce a phenotype (duplicate dominant or recessive epistasis).

  • Recessive epistasis, coat color determination in rodents:

    Three loci involved (agouti = color banded hairs, ~grey):

    C allele determines pigment (C- = pigment, cc = albino)A allele determines agouti factor (A- = banded, aa = solid)B allele determines color(B- = black, bb = brown)

    A allele is epistatic over B locus, inserts bands of color between black and brown (appears grey).

    C allele is epistatic over A and B loci, as cc is albino regardless of its genotype at the A and B loci.


  • Recessive epistasis, coat color determination in rodents (cont.):

    Assume for this cross that all mice have one B allele (B- = black) and there are no brown mice (bb).

    Cross true-breeding black-agouti (AA/CC) with albino (aa/cc).

    All F1 are agouti Aa/Cc.

    In the F2, A-/cc and aa/cc individuals show the same albino phenotype.

    F2 phenotypic ratio is 9:3:4 instead of 9:3:3:1.

  • Fig. 13.11,Recessive epistasisF2: 9:3:4

  • Essential genes and lethal alleles:

    Essential gene=may result in a lethal phenotype when mutated.

    Lethal allele=mutation that results in death.(can be dominant or recessive)

    Dominant lethal alleleAa and AA dieRecessive lethal alleleaa dies

  • Yellow body color, an example of a lethal allele in mice:

    Yellow mice never breed true.

    Cross yellow x non-yellow, F1 is 1:1 yellow and non-yellow (all yellow mice are heterozygotes, AY/A).

    Cross yellow x yellow (AY/A x AY/A), F2 is 2:1 yellow:non-yellow instead of the predicted 3:1 ratio.

    Homozygotes (AY/ AY) are aborted in utero.

    Yellow is dominant with respect to coat color, but acts as a recessive lethal allele.

    The AY allele has a large deletion and is fused to the promoter of a nearby (Raly) gene (Raly is inactivated).

  • Fig. 13.17, Lethal alleles in mice,Yellow body color

  • Why do lethal alleles persist in the population?

    Recessive lethal alleles are not eliminated; rare alleles occur in the heterozygote (protected polymorphism).

    Allele frequency q = 0.01Expected frequency of double recessive homozygotes, q2 = 0.0001Expected frequency of heterozygotes, 2pq = 0.0198

    For complete recessive allele at equilibrium ( = mutation rate and s = selection coefficient):q = (/s)

    If homozygote is lethal (s = 1) then q =

    If s < 1 the frequency of the allele will be higher.

  • Two other important terms:

    Penetrance describes how completely an allele corresponds with a trait in the population (0-100%) ~ Frequency (+/-)

    Expressivity describes variation in expression of a gene or genotype (can be constant or variable) ~ Variability

    Important because of the influence of the environment (internal & external) and development---lots of factors influence gene expression.

  • Fig. 13.18, Penetrance and expressivity

  • Some effects of the environment:

    Age of onset (male pattern baldness)

    Sex(male pattern baldness)

    Temperature (influences enzymes, coloration in Siamese cats, sex determination in reptiles)

    Chemicals (phenocopy, chemicals mimic phenotype produced by rare recessive alleles)

    Measles during the first 12 weeks of pregnancy produces fetal cataracts, deafness, and heart defects.

    Thalidomide (1959-1961), prescribed as a sedative for expectant mothers suppressed limb-bone development.

  • Male Pattern Baldness(Fig. 13.20)

    OMIM 109200


    Dominant in males

    Recessive in females

    Influenced by testosterone

  • Male Pattern Baldness(Fig. 13.20)

    OMIM 109200


    Dominant in males

    Recessive in females

    Influenced by testosterone

  • Epig