Mendelian genetics

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.

  R R
r Rr Rr
r Rr Rr

The F1 progeny are all Rr (genotype) and red (phenotype), since red is dominant.

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Wild-type alleles

The wild-type allele is the most common allele seen for an organism. This is often denoted by ‘+’. Any allele that is not wild-type is shown as ‘­–’. The 2 symbols are separated by a forward slash (/). Therefore, a diploid organism that is homozygous for a wild-type allele at locus R can be shown as +/+. Sometimes the locus abbreviation is given, followed by superscripted wild-type denominators: R+/+.

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