Question

Hello, I am just wanting some help with clarifying genetic crosses to use to test different aims of genetics and inheritance using 2 genes. I have included part a, b, c, d for a question (without values as Im confident with this myself) and Included my answers / confusion for each. I would love you to clarify if this is the correct cross.  

a. To test for the dominance of the mutant phenotype at each locus: Dihybrid cross using 2 pure breeding parents (one homozygous recessive and one homozygous dominant phenotype)

b. Test whether each locus is autosomal or X-linked: Reciprocal cross to a

c. Test whether these 2 loci are assorting independently: I am confused by 2 options. 1. Should a test cross be used as I know Test crosses can also be used to determine if two genes are linked or unlinked by mating with a known heterozygous. If there is an equal ratio of the four potential phenotypes, the two genes are likely unlinked (independent assortment). OR I am very confused if I should use the F1 Heterozygotes from the dihybrid cross in cross a, and then if a 9:3:3:1 there is independent assortment? Or does this only show complete dominance?

d. Conduct a Chi-Squared test- I have values but I know how to calculate it, I just want to check that if the p value is in the not significant range then we do NOT reject the null hypothesis? And can chi squared be used for both a test cross and di hybrid cross?

Thank you so very much for your time. You are a lifesaver!

Answer 1

a. To test dominance you should cross a homozygous recessive wit heterozygous dominant

b. Yes you are right a reciprocal cross is required. Cross pure breeding male showing trait with purebreeding normal female and another cross vice versa.If resulting ratio of both cross are same then it is autosomal otherwise it is sex linked trait

c. Yes you should use test cross .No need to confuse cross purebreed homozygous recessive with heterozygous dominant of f1 generation .You will get both 1:1:1:1 genotypic and phenotypic ratio if there is no linkage

d. Yes chi square test can be used for dihybrid and test cross

Chi square = summation of ( observed value)²_ (expected value)² /expected value

If result is not in significant range of p(degree of freedom) then null hypothesis is rejected.

I hope l have cleared your doubts