Mendelian genetics describes phenotypes that are due to a single gene. A cross between individuals is described using:
- P = parental generation
- F = filial generations (F1, F2, etc; note capital F and subscripted arabic numbers).
For example, a flower produces red or white flowers depending on the allele at a single locus. Red is dominant, and white is recessive. Possible genotypes are:
RR rr Rr [The dominant allele (red) is given as a capital (R) and the recessive allele (white) as lower case (r).]
Mendelian crosses can be determined using a Punnett square.
The P generation’s phenotypes are red and white. Their genotypes are RR and rr.
The F1 progeny are all Rr (genotype) and red (phenotype), since red is dominant.
