BIO 7:  Preview for Lecture 23

Extensions of Mendel's Ideas based on Gene (DNA) à RNA à Protein à Phenotype

Phenotypic ratios differ from what Mendel observed, but genotypic ratios are the same

 ¨       Lack of Dominance:  in a heterozygote neither allele is expressed over the other.  2 reasons:  

Incomplete Dominance:  one allele codes for active protein, other allele codes for inactive protein and the amount of active protein shows up in the phenotype.

e.g. In snapdragons:      R = seq in DNA coding for an active protein for making red pigment                                                       

                                           R' = seq in DNA coding for an inactive protein for making red pigment

                                                      Color phenotype depends on the amount of red pigment present

                 Possible Genotypes:

                 Possible Phenotypes:

 Co-dominance:  one allele codes for active protein, other allele codes for active protein slightly different in activity, and both activities show up in the phenotype.

 e.g.  In humans:              I^A = seq in DNA coding for an active protein for attaching A antigen                                                      

                                           I^B = seq in DNA coding for an active protein for attaching B antigen

                                                 Blood type phenotype depends on which antigens are attached to cells

                 Possible Genotypes:

                 Possible Phenotypes:

Contrast with Complete Dominance (What Mendel observed):  dominant allele codes for active protein, recessive allele codes for inactive protein, and one dose of active protein works as well as two doses.

 In humans:           T = seq in DNA coding for an active enzyme to remove wastes from neurons                                            

                            t = seq in DNA coding for an inactive enzyme to remove wastes from neurons

               Having just a little active enzyme is sufficient for waste removal and a normal phenotype; having no active enzyme results

               in waste accumulation and Tay-Sachs Disease

                 Possible Genotypes:

                 Possible Phenotypes:

Multiple Allelism:  there can be more than 2 forms of a gene, because there are many ways to change the DNA and protein, and different protein activities may cause different phenotypes.

 e.g.  In humans:          I^O = seq in DNA coding for an inactive protein for attaching A and B antigens

 Possible Genotypes:

         Possible Phenotypes:

Gene Interaction is something else Mendel did not know about.  Several genes can determine one trait because many proteins encoded by genes work together in metabolic pathways to produce one final outcome.

e.g. Human hair color is determined mainly by 2 pairs of genes, which are on different pairs of chromosomes

D codes for active protein for making dark pigment and  d codes for inactive protein for making dark pigment                       

R codes for active protein for making red pigment  and   r codes for inactive protein for making red pigment

        When dark pigment is present, you cannot see the red pigment.

        Genotypes:          D_ R_                    D_ rr                    dd R_                    dd rr

        Phenotypes:

        What is the expected phenotypic ratio from a mating of        Dd Rr  x  Dd Rr?

Complex Genetic Traits are due to many pair of interacting genes + the environment

• Different combinations of genes + environmental factors can produce the same phenotype
• Phenotypes of offspring are difficult to predict from phenotypes of parents

        e.g. height, weight, IQ test performance

• Identified by twin studies using

monozygotic twins

dizygotic twins

concordance;  the percentage of the time a pair of individuals shows the same form of the trait

 

                                                                    CONCORDANCE FOR

TRAIT                                    MONOZYGOTIC TWINS        DIZYGOTIC TWINS

Schizophrenia                                        55%                                            10%

Alcoholism                                            55%                                            28%

Blood Type                                         100%                                            66%

Epilepsy                                                72%                                            15%

Diabetes                                                65%                                            18%

Cleft Lip                                                42%                                             5%

Death by infection                                    1%                                              1%

 

[Which of the traits above is a complex genetic trait, a simple genetic trait, not determined by genes?]

 

 

 

 

 

 

 

Genes and Environment Interact:  one genotype may produce different phenotypes in different environments.

[Are cuttings genetically identical or different?]