Population Genetics...
     the Hardy-Weinberg Equilibrium 
              1908 G.H. Hardy, English mathematician  &  G. Weinberg, German physician
 

 
Law of Genetic Equilibrium...
         mathematically describes the gene pool (i.e., all the alleles present)
         defines... the ideal case for a NON-evolving population.

             for an ideal case...  a number of criteria must be met... 
                  - must be an infinitely large populations (large sample sizes)
                  - should exhibit random mating
                  - absence of forces of evolution, which can change allele frequencies
                         such as...               no migration (in/out)
                                                      no mutation
                                                      no selection
                                                      each allele is equally viable (no lethals)

 

 

 

 

 

 

 

   

HW law states --> 
           original percentage of a genotypes alleles remains CONSTANT

   HW Equilibrium...  is defined algebraically, by the binomial expansion
           any gene with 2 allelic forms...       A  and  a
                   let frequency of one allele                      (A)    =     p
                     & frequency of other allele                   (a)    =     q

           then by definition,      p + q   =   1  

           HW equation...        (p + q)    p2   +   2 pq   +   q2   =  1
                                                         GG           Gg       gg

 

 

 

 

 

 

 

 

  

In population of canes...   Orange is dominant (GG) to Green (gg) 

        

   of 1000 canettes, we observe that...                  
            40 are Green (gg)     & thus      960 are Orange    (GG or Gg)
               q2 =  freq homozygous recessive (green  gg) = 0.04 
                                
40/1000 = [0.04] x 1000 = 40
               q   =  freq of recessive allele sq.root of 0.04        =   0.20  
               p   =  freq dominant allele [G] = 1 - q = 1 - 0.2        =   0.80
           2pq   =  freq of heterozygote [Gg]
                              =  2  (0.2) (0.8)  = [0.32 x 1000]             =    320
           p2 = freq of homozygous dominant 
                              =  (0.8)2 = [0.64 x 1000]                         =     640
 
(an example)     cf

 

 

 

 

 

 

 you may skip this point on. - 

agents of evolution... 
            But..... Allele frequencies do change over time via....

   Mutation... 1/10,000, random, non-directional
   Gene Flow... migration of breeders... in/out
   Genetic Drift... random loss of alleles  - due to failed matings  
(23.4)
            in very small populations, it's a statistical anomaly  
                                can lead to fixation or deletion of alleles
   Bottleneck Effect... natural disasters leave  survivors    
                    which are not representative of whole population     
(23.5)
   Non-Random Mating... in-breeding lessens heterozygosity...
           Founder's Principle...
little dispersal - new allele predominates - small human tribes
   Selection... better fit individuals are better reproducers
           Artificial - animal husbandry selects best -  mustards-      
(22.6)
           Natural - acts on individuals of populations

 

 

 

 

 

 

 

 

 

  factors that may affect selection... 
   
     ABIOTIC - non-living factors
                           temp, humidity, presence heavy metals, Chernobyl
        BIOTIC - living factors
                           predators, parasites, population density, growth rates

Types of Selection      (23.11
       STABILIZING - limits extremes of population
                    one optimum phenotype - ex: human birth weight
       DIRECTIONAL- one best phenotype, not the mean
                              gradual replacement one by another
       DIVERSIFYING (disruptive)- increases the fitness of extremes
                    no optimum phenotype (2 or more) - patchy environments
                              ex: sexual dimorphism 

 

 

 

 

 

 

 

 

 

 


How do you win the game of evolution:


           the mother of all bumper stickers......

          
the one who leaves behind the best fit genes, wins...

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
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 Cystic Fibrosis  (cc)
   
is a recessive GENETIC DISEASE of childhood that is characterized by
    respiratory & digestive problems and is usually fatal. 
   
The average life span of its victims is only about 24 years

    CF symptoms include heavy production of thick mucus in respiratory tracts, which increases
    susceptibility to respiratory infections;  90 percent of all patients die of chronic lung disease.
    
Secretions that block pancreatic ducts cause important digestive enzymes to fail to reach the
    small intestine.  Treatment is directed toward relief of symptoms, and no cure is yet known.

    CF apparently is caused by the inability of chloride ions to cross the specialized epithelial cells
  
of salivary, mucus, and sweat glands and the pancreas.  The ductal systems become clogged with
    thick secretions. 

    CF is a recessive condition from a mutation to a single gene, which produces a
    dysfunctional chloride channel protein. 
 
 

 

 

 

 

 


 Gene lies on Chromosome 7
     
CFTR gene: cystic fibrosis transmembrane conductance regulator protein
          mutation:   ΔF508 ; deletion of 3 nucelotides at positions 507-508 in the CFTR protein

   A
 T       ISOLEUCINE 506
 C
   A
 T      ISOLEUCINE 507
 C                                   --->    ISOLEUCINE  507      
   T                                              
 T      PHENYLALANINE 508    --> NO 508
 T
   G
 G      GLYCINE 509   
 T
 G
 T      VALINE 510
 T

           
            

 

 

 

 

 

 

 

 

The disease is a commonly inherited disease among Caucasians; 

in U.S. population 1/2,500 whites have the disease   cc   (0.0004 %)   

How many carriers   Cc   are there is U.S. population?

 

 

 

 

 

 

 

 

 

 

 

 

   frequency of cc  is 1/2500 =  0.0004%  

            thus freq of
  "c" is  sq.root  =  0.02%

       if         freq  of c  =  0.02 then freq of C1 - 0.02  =  0.98%
 
           
    then     freq of  2(Cc) =  2 (.98) (.02) = 0.0392

  or 4% of American whites are asymptomatic carriers of the allele
                    
        4 out of 100 whites are carriers (Cc) 
                    
        1 out of 25 in U.S. Caucasian population is a carrier.
     

back to human genetics                                              to evolution -->

 

 

 

 

 

 

 

 

 

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