Can anyone explain the Hardy Weinberg Theory/ Equilibrium to me? And maybe do a sample problem?

The Hardy-Weinberg Principe states that, under certain conditions, the relative number of alleles in a population will remain the same.  These 5 conditions can be found on any website or any biology text.  If a population's alleles are found to have the same frequencies in the population from one generation to the next, then the population is said tobe in Hardy-Weinberg Equilibrium;  it is not changing.  However, one of the conditions for HW equilibrium toi hold true is "No Natural Selection."  Since Natural Selection is always at work, populations are never in HW equilibrium.

An example you might see:  In a population of 1000 plants, 160 are short;  calculate the allelic frequencies.  Let T=tall and t=short.

The main HW equation is p+q=1, where p=frequency of dominant allele and q=frequency of recessive allele.

Since 160 are short, the frequency of tt is 160/1000 or 0.16.  The frequency of t is sqrt(0.16) or 0.4=q.  Therefore, the frequency of T=1-0.4 or 0.6.

The second HW equation shows how to calculate the genotypic frequencies, (p+q)^2=1 or (p^2+2pq+q^2)=1.

p^2= TT or (0.6)(0.6)=0.36
2pq= 2(0.6)(0.4)=0;.48
q^2= tt or (0.l4)(0.4)=0.16 (which was already calculated)

The number of homozygous dominants in the pop is 0.36*1000=360
The number of herterozygotes in the pop is 0.48*1000=480
The number ofr homozygous recessive  in the pop is 0.16*1000=160 (which was given), and everything checks.