Why are predicted ratios from a punnet square not always the actual ratiosin a populations?

Answer 1

Depending on other factors that influence genetics, crossing over, genetic drift and linkage as examples, certain characteristics may be more or less common in a population than demonstrated in a punnet square where the genetics of only 2 individuals are being studied and often the genetic make up is known.

Further, in an entire population, the gene pool can be much more or less varied than the results seen in experiments because the gene frequencies are not known. There may be more than 2 alleles at a single locus which will also influence ratios.

Also, if certain characteristics are selected for or against in a population the number of individuals with specific characteristics may be increased or diminished.

The Hardy-Weinberg Principle states that an allele that is neither selected for or against will remain at the same frequency in a population. The equations are q squared+2pq+p squared=1 and p+q=1

For example. If p=.8 and q=.2 A population of 100 individuals will have 64 individuals who are homozygous for the p characteristic 4 individuals that are homozygous for the q characteristic and 32 heterozygous individuals. Note that only 4 homozygous individuals are produced in this population even though a full 20% of the alleles are required to produce this result.

If p is the recessive characteristic 64% of the population will be consistently homozygous recessive because 80% of the alleles in the population are recessive.

If p is the dominant characteristic 96% of the population will display the dominant trait.

If q is the dominant trait 37% of the population will display the dominant trait.

The larger the population the more stable the gene frequency.