How do you complete a Mendelian cross?

Is there a difference between Mendelian monohybrid cross and test cross?

• Why is the inheritance ratio for linked genes the same as the Mendelian 3:1 ratio for a monohybrid cross but it's 1:1 for a test cross? Super confused.

• Answer:

  In a typical Mendelian monohybrid cross we start with two true breeding homozygotes and produce a heterozygous F1, we then cross the heterozygotes to get the 3:1 ratio in the F2. AA x aa ---> Aa, then Aa x Aa--> AA, Aa, aA, aa A test cross is using a recessive individual for one or more loci (you can use it in a dihybrid, trihybrid or test for linkage cross), to find out what the genotype of the other 'test' parent is. Using a recessive means that we can tell the difference between a homozygote and heterozygote. If you test a heterozygote Aa x aa then you will get Aa and aa, 1:1. If your test parent is a homozygote you will get all Aa and there will be no recessive phenotypes. If your were to cross AABbccDdEEffGgHh x aabbccddeeffgghh you would in one go be able to work out the genotype for every single locus in the test parent.