Modification of Mendelian Ratios

     The effect of alleles on a given phenotype.


     Gene Interaction-control of a single phenotype by more than one set of genes.






    Alternate forms of the same gene.

    Wild type(normal)-occurs most frequently in a population.                                                              

                                  -usually dominant

    Derived from mutations-deleterious                                                                                          




Symbols For Alleles

    Recessive Allele- d (dwarf)


    Dominant Allele- D (tall)


    D. melanogaster                                                                                                                       

                              -wild-type trait= superscript +                                 

-       a slash (/) between letters=same locus (location)



Factors that modify the mendelian patterns of inheritance


(1) Incomplete (partial) Dominance

     Neither allele is dominant


     F1 Heterozygotes= Intermediate (blended) phenotype (Red X White =Pink)


     Phenotypic ratio = Genotypic ratio (1:2:1)


     Tay-Sach’s disease-Enzymatic disorder (hexosaminidase)                                               

-Homozygous recessive = die                                                                                                          

-Heterozygotes- 50% of enzyme activity.




(2) Codominance

     Both alleles of a single gene are expressed.



     MN blood group                                                                                        

                              -Antigens (glycoproteins) on the surface of RBCs.                                                                               

                             -Two forms = M and N

    An Individual may exhibit on or both.




(3) Multiple Alleles

    Three or more alleles of the same gene.

          (e.g.) (a) The ABO blood group

                   (b) The Rh antigens


(a) The ABO blood group

     One of four possible phenotypes resulting from 2 antigens (A and B).


     Determined by mixing blood sample with antiserum containing A or B antibodies.


           Allele Designation- IA, IB IO


     Testing compatibility of blood transfusions


     Determining parentage


     A individual= IAIA or IAIO

           B individual= IBIB  or IBIO

     AB individual = IAIB

     O individual= IOIO

     Expression of ABO influenced by gene at the secretor locus.

     Se/Se or Se/se= secretors

     se/se= non-secretors








(b) Rh Antigens

     Erythroblastosis fetalis (form of anemia)


     Rh-positive fetus/ Rh-negative mother


Rh Antigens may result from

(1)  Multiple alleles at a single locus,  or


(2)  Three or more linked genes, each with 2 alleles



(4) Lethal Alleles

    Recessive lethal allele-Homozygous (lethal)                                                                            

                                         -Heterozygous (mutant phenotype)


        Dominant lethal allele                                                          

        -One copy of allele results in death -Huntington’s Disease

                     -late onset



(5) Combinations of Two Gene Pairs

    9:3:3:1 ratio modified


    Two modes of inheritance- 3:6:3:1:2:1


    E.g. = 2 individuals heterozygous for albinism and both have AB blood type.


(6) Gene Interaction

    Epistasis- Several genes control one phenotype



    A ratio expressed in 16 parts (9:3:4) when studying inheritance of a single character = two gene pairs are interacting.


    Examples of epistasis

    A genotype mask /suppresses expression of an allele.

    A dominant allele at one locus mask expression of alleles at a second locus.

    The homozygous condition of either recessive allele mask the expression of the dominant allele at the other locus.

    New phenotype in the F2 generation.




    Transmission of genes located on the x-chromosome.

    Crisscross pattern of inheritance.

    Males are hemizygous for all alleles on the x-chromosome.



Extranuclear Inheritance(Non-Mendelian Inheritance)


Maternal Inheritance-DNA from the mitochondria (chloroplast) determines offspring phenotype.


Maternal Effect- Phenotypic effects on the offspring produced by factors transmitted      

                                through the egg cytoplasm.

                               -Patterns (traits) established during early development.