Chapter 17 Notes

Allelic Frequency


Genetic variation in organisms is studied at the level of a population not the individual. 

     A population is a group of individuals of the same species that live in the same area.


One single change in an individual is not considered evolving.  Only a population of species can evolve. 


Members of a population of species interbreed, therefore they share a common group of genes called a gene pool.


Gene Pool: all of the genes that are present in a population.  This includes all the different alleles. 


The relative (allelic) frequency of an allele is the number of times that the allele occurs in a gene pool compared to the number of times other alleles for the same gene occur.


How do new alleles arise?

Through random mutations.


In genetic terms the definition for evolution is: any change in the relative frequency of alleles in a population.

    if the frequency of the allele changes, then the population is evolving.





Ex. Estimating Allelic Frequency

Generation #1:


  Red (RR)     Red (RR)   Pink (RR)    Pink (RR)



 Red (RR)    Pink (RR)  Red (RR)      Pink (RR)


Phenotypic frequency:          

Red = 4/8 = 0.5 or 50%

Pink = 4/8 = 0.5 or 50%

White = 0/8 = 0.0 or 0%


Allelic frequency:

R = 12/16 = 0.75 or 75%

R = 4/16 = 0.25 or 25%






Generation #2:


  Red (RR)     Pink (RR)  Red (RR)     Pink (RR)



Red (RR)  White (RR)  Red (RR)      Red (RR)


Phenotypic frequency:           

Red = 5/8 = 0.625 or 62.5%

Pink = 2/8 = 0.25 or 25%

White = 1/8 = 0.125 or 12.5%


Allelic frequency:

R = 12/16 = 0.75 or 75%

R = 4/16 = 0.25 or 25%


If the number of alleles does not change from one generation to the next, then the relative allelic frequency has not changed and evolution is not occurring even if the phenotypic frequency is changing.

Generation #3:


 Red (RR)      Pink (RR) White (RR) Pink (RR)


                 White (RR) White (RR) Red (RR)  Red (RR)


Phenotypic frequency:          

Red = 3/8 = 0.375 or 37.5%

Pink = 2/8 = 0.25 or 25%

White = 3/8 = 0.375 or 37.5%


Allelic frequency:

R = 8/16 = 0.5 or 50%

R = 8/16 = 0.5 or 50%


If the number of alleles changes from one generation to the next, then the relative allelic frequency has changed and thus evolution has occurred.


NOTE: Allelic frequencies cannot change if there is not genetic variation within a species.


Sources of Genetic Variation:

1.  Mutations: Any change in a sequence of DNA. 


Mutations can occur because of:

    Random mistakes in the replication of DNA.

    Radiation or toxic chemicals in the environment.



2. Gene Shuffling: the random rearrangement and recombination of chromosomes.




Gene shuffling occurs during:

    Meiosis (formation of gametes) due to independent assortment of chromosomes.

    Crossing Over during meiosis.


Thus, sexual reproduction is a major source of variation within many populations.




Inheritable variation can be expressed by either single-gene traits or by polygenic traits.


The number of phenotypes produced for a given trait depends on how many genes control the trait.