Does a punnet square HAVE to have four genotypes?

Question

Can it have one allele from each parent or must it have two from each?

Answer 1

You will always have two alleles from each parent for each gene you are crossing for. Because each allele goes into determining whether or not the individual expresses the dominant or recessive genotype. However, if either or both of the parents are homozygous dominant or homozygous recessive, you can do a cross using just a single allele from either or both parents. Every offspring would get the same allele from one parent if only one is homozygous or the same allele from both if both parents are homozygous.

Answer 2

If you are only investigating one particular gene (and not doing a dihybrid cross), you can actually make a punnet "square" with only one allele from a parent IF that parent is homozygous (eg. AA or aa) - the answers you get from this are identical to those you get if you made a square but simply wrote the same allele twice for a homozygous parent (but you'll probably piss your teacher off!)
If a parent is heterozygous (eg. Aa) then any of their eggs or sperm could have either allele, so you need to use two alleles in your punnet "square".

Answer 3

It must have two. Or else the spilt chromosones won't work out evenly.

Answer 4

hmm, i just learned this is school and i beilieve u can have just one from a parent but it depends on if it were a dominant or indominant trait