A.  Punnett Square
           1.  Purpose - To use a diagramatic method for predicting genotypes and phenotypes

           2.  Example:

                    Key to the Punnett Square:
                        Gray Boxes - Parental Alleles
                        White Boxes - Possible Offspring Genotypes

     B.  Monohybrid Cross
           1.  Definition - a genetic cross in which only one trait of the parents is of interest

           2.  Pea Plant Example:
                a.  Purple Flower (PP) * White Flower (pp) - P₁Cross-Pollination

                                F₁ Genotypic Outcome = All Pp
                           F₁ Phenotypic Outcome = All Purple Flowers 

                     b.  Purple Flower (Pp) * Purple Flower (Pp) - F₁Self-Pollination          

                          F₂ Genotypic Outcome = 1PP : 2Pp : 1pp
                          F₂ Phenotypic Outcome = 3 Purple : 1 White

           3.  Alternative Method - Employs a mathematical calculation for allelic possibilities in the male and female gametes, rather than a Punnett Square
                a.  Purple Flower (Pp) * Purple Flower (Pp) - F₁Self-Pollination
                b.  (1/2P + 1/2p)(1/2P + 1/2p) = 1/4PP + 1/2Pp + 1/4pp
                c.  F₂ Genotypic Outcome = 1PP : 2Pp : 1pp
                     F₂ Phenotypic Outcome = 3 Purple : 1 White

           4.  Genotypic Outcome:
                ** Monohybrid Cross = 1 : 2 : 1 

           5.  Phenotypic Outcome:
                ** Monohybrid Cross = 3 : 1

     C.  Test Cross
           1.  Purpose - To determine an organism's genotype when the  dominant phenotype is expressed.  The organism is crossed with an individual that is homozygous recessive for the trait in question.

           2.  Pea Plant Example - What is the genotype of a Purple Flowered Second Filial Plant?  It could be Pp or PP.

                                 Option #1 - Heterozygous        

                       Option #2 - Homozygous Dominant

           3.  Results:
                a.  If there are any offspring from the test cross that show the recessive phenotype (white flowers here), then the tested individual must be heterozygous.
                b.  If all of the offspring from the test cross show the dominant phenotype (purple flowers here), then the tested individual must be homozygous dominant.

      D.  Dihydrid Cross
          1.  Definition - A genetic cross in which two traits of the parents are of interest.

          2.  Product Rule - The probability of independent events occurring at the same time is the product of their individual probabilities.  Therefore, a dihybrid cross is basically the product of two monohybrid crosses.
               Example - The probability of tossing two coins and getting two tails is.....(1/2)*(1/2) = 1/4

          3. Law of Independent Assortment - The alleles for one trait may be distributed to the gametes independently of the alleles for other traits.  

                   

          4.  Pea Plant Example
                 a.  Smooth, Yellow (SSYY) * Wrinkled, Green (ssyy)- P₁True-breeding Cross-Pollination

                                F₁ Genotypic Outcome = All SsYy
                           F₁ Phenotypic Outcome = All Smooth, Yellow 

                     b.  SsYy * SsYy - F₁Self-Pollination          

                          F₂ Genotypic and Phenotypic Outcome:
                                    9  S_Y_ Smooth Yellow
                                    3  S_yy  Smooth Green
                                    3  ssY_  Wrinkled Yellow
                                    1  ssyy   Wrinkled Green
                       
          5.  Phenotypic Outcome:
                ** Dihybrid Cross = 9 : 3 : 3 : 1

     E.  Incomplete Dominance
          1.  Definition - A genetic situation where neither allele masks the presence of the other; therefore, the heterozygote has a different genotype and phenotype from both the dominant homozygote and the recessive homozygote.

          2.  Example - Flower Color in Snapdragons
               a.  Alleles Involved:
                    R = Red Flower
                    r = White Flower
               b.  P₁ - RR (red) * rr (white)
               c.  F₁ - Rr (pink)
               d.  F₂ - 1 RR (red) : 2 Rr (pink) : 1 rr (white)

      

          3.  Clue - Heterozygote has a phenotype that is intermediate between a homozygous dominant and homozygous recessive phenotype.

     F.  Codominance
          1.  Definition - A genetic situation in which both dominant and recessive phenotypes are expressed independently in the heterozygote, rather than producing an intermediate phenotype as incomplete dominance does.

          2.  Example - Sickle Cell Anemia in Humans
                a.  Alleles Involved:
                    N = Normal Red Blood Cell (circular in shape)
                           

                    n = Sickled Red Blood Cell (crescent moon shape)
                       

                b.  Phenotypes Involved:
                    NN = Normal Red Blood Cells
                    nn = Sickle Cell Anemia (Sickled Red Blood Cells cannot carry enough oxygen resulting in this condition)
                    Nn = Normal AND Sickled Red Blood Cells

          3.  Clue - Heterozygote has a phenotype that shows both the recessive and dominant phenotypes

     G.  Linkage Groups
          1.  Definition - Genes that are inherited as a pair or group because they are located close together on the same chromosome.

          2.  Clue - Genotypic and phenotypic ratios from a cross involving linked genes will differ from those normally expected from monohybrid and dihybrid crosses.

     H.  Crossing-over
          1.  Definition - The exchanging of DNA segments between homologous chromosomes during Prophase I of Meiosis I.

               

          2.  Recombinant Chromosomes - chromsomes with new combinations of alleles which result from crossing over
                a.  The farther apart two genes are on a chromosome, the greater their chances of crossing over.
                b.  The closer together two genes are on a chromosome, the more often they are linked.
                c.  Physical locations for genes can be determined simply by knowing the crossing over percentages.
                    Example:
                        Crossing Over Percentage:
                             1)  AB = 90%
                             2)  BC = 7%
                             3)  AC = 12%

                        Physical Location on Chromosome:
                                            A
                                            C
                                            B

To test your knowledge about Genetic Crosses and Squares, click on the Cross Questions Link at the top of this page.  After you answer the questions, be sure to check your responses by clicking on the Cross Answers Link.

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