We are accustomed to thinking about *organisms* in a population. To study what is called population genetics, we need to expand this concept and think about *alleles* in a population. For example, for organisms, we could ask: "*Of all the people in the world, what percent have cystic fibrosis?*" The corresponding question for alleles is: "*Of all the F and f alleles of CFTR in the world (two alleles per person), what percentage are f ?*" These two questions have different answers, of course, but they are related. Let's see how.

**1. Under what conditions can we expect "allele
frequencies" (or "allele percentages") in a large population
to remain constant from generation to generation? **

In general, allele frequencies will remain constant from generation to generation
if . . .

(1) if the members of the large population mate randomly,

*and*

(2) if all genotypes survive and reproduce equally well,

*and*

(3) if new alleles are not entering into the population (either via new mutations
or migration of individuals).

Obviously, this set of conditions is never met completely. However, in large
populations it may be "close enough" such that allele frequencies
do not change noticeably over some observable number of generations. When this
is the case, we refer to the situation as "Hardy-Weinberg equilibrium".
{Named after the two people who first realized this.}

2. In such a randomly mating population, what is the graph of "genotype
frequencies" as a function of "allele frequencies" (for the case
of two alleles of gene *alpha*)?

This is the "p&q" graph of Problem S-12. This relationship between allele frequencies and genotype frequencies is
an expression of the "Hardy-Weinberg principle". p is the *A*
allele frequency, q is the *a* allele frequency, p squared is the *AA*
genotype frequency, q squared is the *aa* genotype frequency, and 2pq is
the *Aa *genotype frequency.

**3. What valuable calculation about cystic fibrosis (or any single gene inherited
disease) can we make from the "genotype frequency vs. allele frequency"
graph? **

Page 730 gives the calculation. If we know what fraction of the population has
the disease (i.e., how many people are genotype *ff* ), we can calculate
how many people are heterozygous carriers of a defective allele.

4. How can we extend this to a three allele case, such as that for human blood
type?

Page 731

Figure 17.12.

5. How can we extend this to X-linked genes?

Page 734.

Figure 17.16.