In a genetics problem, how to know when to do a dihybrid cross vs. monohybrid?

Remember that the gametes hold 1 of each gene. So if the genotype is Aa, the gametes can be A or a. And the genotype is AaBb, the gametes can be AB, Ab, aB or ab. In that case, your square will be twice as big (instead of 2x2 it'll be 4x4).

Ok. I'll quote by old school textbook.

--Monohybrid Cross: "A cross between two true-breeding parents that possess different forms (alleles) of one particular gene".
So, that is, you could see it as being a questions where it only one "thing", so in Sex-Linked genetics, it is colour-blindess you are doing crosses for, using the sex chromosomes.
Or, a mono-hybrid cross may come up in a question as the allele for seeds being either Round (dominant) or Wrinkles (recessive). Here, I'd make "R" be round, and "r" be wrinkled. Just do your normal monohybrid cross.

--Dihybrid Cross: "A cross between two true-breeding parents that possess different forms (alleles) of two genes".
So, you in Pea Seeds.. Round ("R") dominant wrinkled ("r"). That's all fine and dandy. But, there's also their colour to deal with now. . Yellow ("Y") dominant to green ("g") recessive allele.
So, I can't do my old monohybrid cross; how will I know if the R has a Y or a y gamete?
SO.. with RrYy parent, the gametes will be: RY or Ry or rY or ry.



MONOHYBRID CROSS:
In sex-linked conditions (drosophila in fruit flies; colour blindness in humans).
So, with Drosophila in flies, Red Eye Colour (R) is dominant to allele for White Eyes (r).
phenotype: White eye female x red eyed male
genotype: X^r X^r    x    X^R Y

[So, you do a monohybrid cross, with the appropriate R and r on the XX and XY. ]

........ X^r ...... X^r
X^R..X^R X^r.. X^R X^r
Y .... X^r Y .... X^r Y

Offspring: X^RX^r : X^rY



DIHYBRID:
Pea seeds.
Y = yellow, y = green ; R = round, r = wrinkles. These are the phenotypes, where capital letter is dominant to the recessive, lower case allele.
YyRr x YyRr will have that both parents' gametes will be YR, Yr, yR or yr  in the cross (a dihybrid cross). (you get a 9:3:3:1 ratio). Explained in 1st explanation.



I hope this helps. it's hard to type an answer for this question! I find it much easier to write and talk it through. Plus, I use hand motions a lot when I speak and explain things.

Monohybrids and dihybrids are very easy to construct. Di means you have two genes in question, mono means you have one.