The allele for a hitchhiker's thumb is recessive compared to straight thumbs, which are dominant. in a population of 1000 individuals, 510 show the dominant phenotype. how many individuals would you expect for each of the three possible genotypes for this trait?

TT=90
Tt=420
tt= 490

When there is 2 type of allele, there will be 3 possible genotypes: homozygous dominant, heterozygous, homozygous recessive. Dominant phenotype will be expressed for homozygous dominant and heterozygous. That mean, the number of homozygous would be:

TT+Tt= 510
total population= TT + Tt + tt
1000= 510 + tt
tt=490

The frequency of recessive allele would be:
t= √(490/1000)= √0.49= 0.7

After that, you can find the frequency of the dominant allele
T+t= 1
T+0.7=1
T=0.3

Using both frequency, you can find the number of homozygous dominant (TT) and heterozygous (Tt) population
TT= 0.3 *0.3 * 1000 individuals = 90 individuals
Tt= 2 * 0.3 * 0.7 * 1000 individuals= 420 individuals

The answer is b. hope this !