09-02-2005, 08:44 AM
It is very easy. p and q represent the frequency of two different allels. Don't confuse genotype with phenotype.
I will use some examples here:
Autosomal recessive---------AA, Aa, aa, three different genotype, but only two phenotype, (phenotypically normal, or diseased people, right?), if you use p represent A, q represent a,
then in H-W equation: P^2+2pq+qq^2=1
the gene frequency for AA is pxp=p^2, aa is qxq=q^2, Aa=2pq
In another word, prevalence of homozygous normal is p^2, prevalence of the houmozygous disease is q^2, heterozygous carrier prevalence is 2pq. Here we are talking about phenotype frequency. If the question ask you what is the prevalence of healthy people for this disease, you have to use p^2 +2pq to answer it.
In autosomal dominant: prevalence of disease is calculated through: P^2+2pq, (because AA and Aa are all phenotypically diseased) only q^2 (whose genotype is aa) is disease free.
So it is very easy. AA=p^2 Aa=pq, aa=q^2
I enjoy sovling genetics problems a lot. I am sure you will encounter at least one or two genetics Q about this. After understanding the whole H-W equation, this should be a give-away Q to you. Good luck.
I will use some examples here:
Autosomal recessive---------AA, Aa, aa, three different genotype, but only two phenotype, (phenotypically normal, or diseased people, right?), if you use p represent A, q represent a,
then in H-W equation: P^2+2pq+qq^2=1
the gene frequency for AA is pxp=p^2, aa is qxq=q^2, Aa=2pq
In another word, prevalence of homozygous normal is p^2, prevalence of the houmozygous disease is q^2, heterozygous carrier prevalence is 2pq. Here we are talking about phenotype frequency. If the question ask you what is the prevalence of healthy people for this disease, you have to use p^2 +2pq to answer it.
In autosomal dominant: prevalence of disease is calculated through: P^2+2pq, (because AA and Aa are all phenotypically diseased) only q^2 (whose genotype is aa) is disease free.
So it is very easy. AA=p^2 Aa=pq, aa=q^2
I enjoy sovling genetics problems a lot. I am sure you will encounter at least one or two genetics Q about this. After understanding the whole H-W equation, this should be a give-away Q to you. Good luck.