[morgan_kay]

If in horses, black coats (B) and walking gaits (W) are dominant to white coats (b) and pacing gaits (w).

How do I cross a male horse that is homozygous for both dominant traits and a female horse that is homozygous for both recessive traits into a dihybrid cross?

[spmorgan]

The father's genotype is BbWw, the mother's is bbww. Make a punnett square where the male's genotype is down the side, the mother's across the top.

[spmorgan]

The father's genotype is BBWW, the mother's bbww. Draw a punnett square where the father's genotype is down the side, the mother's across the top.

[spmorgan]

the father's genotype is BbWw, the mother's is bbww. Make a punnett square where the male's genotype is down the side, the mother's across the top.

Sorry, forgot the father was homozygous.

[Unknown008]

I don't know what a punnett square is, but the only possible outcome is a heterozygous allele if you take a homozygous dominant and a homozygous recessive allele.

[asking]

A punnett square is a simple table with all the possible gamete (sperm or egg) genotypes of one parent on the left side and those of the other parent running across the top. Each square shows the genotype of the offspring that would result from that particular egg and sperm. You add them up to get the proportions of DD, Dd, and dd, for example. You can do this with one gene or several.

In this example, the sex of each parent is unimportant. It only matters if the genes are sex linked, which appears not to be the case here.

The Punnett square is basically an alternative to doing simple algebra.

[asking]

The question is odd, since it asks how do I cross these two horses?
(Not what is the genetic outcome of the cross.)

I would say, in the usual way!

[Unknown008]

Lol! I noticed it.. not too sure what that meant either :p

I may have been using that 'punnett square' without knowing it's called as such...

I usually do it as such, except that I have voluntarily expanded fully even though the same allele is repeated:



Then I would do a similar 'drawing' for the other allele in which all the off springs are found to have the combination Ww.

Then combining the two to have the answer...

[asking]

Nice. I wish I knew how to make quick diagrams like that!

The punnett square gives you the proportions of each. With several genes, that can be helpful.

Here's a calculator, for example:
Punnett Square Calculator

[Unknown008]

I did it using Open Office Drawing :o

Hey, it's simpler :p Thanks for the link! I think I may have seen that once or twice... but I rarely used that.

That makes a whole year I didn't do biology now... I sort of miss it a little...

[asking]

Okay. That does it, I was just wishing yesterday that I had a simple drawing program like the old MacDraw (you probably are too young to have any idea what that is). Anyway, I will get open office. Ironically, just last week I told my son to download it for his homework. I didn't know it had a drawing app.

Are you in college or high school? Biology is fun and you seem to be good at it. :)

[Unknown008]

I'm in High School, the Cambridge system of education.

Last year, I was doing 3 science subjects, 2 language and Maths.
This year, We have to take 3 main, 2 subsidiary subjects. Those came out as chemistry, physics, maths for the main, and general paper (srot of english language) and french language as subsidiary.

Some took Biology instead of French in Sub, but for many people, having 3 science subjects here is way too bulky to handle! So, I come here quite often, to see if there are something interesting to see :)

[asking]

Sounds like you are getting a great education.

You might enjoy Science Now once in a while.
Science/AAAS | ScienceNOW: The Latest News Headlines from the Scientific World

I just noticed this question was asked in March. So I don't have to feel guilty about coopting the thread.

[Unknown008]

Lol! And I was thinking that spmorgan was another one stuck on the problem at first. Thanks for the link :)

[katieokell]

The cross would be:

BW
Bw BbWw

Since both are homozygotes, you know there can only be one possible genetic zygote offered by each parent. All offspring will exhibit the dominant trait. However, the F2 offspring cross would be:

BW Bw bW bw
BW BBWW BBWw BbWW BbWw
Bw BBWw BBww BbWw Bbww
BW BbWW BBWb bbWW bbWw
Bw BbWw Bbww bbWw bbww

Where the phenotypic ratios are:
12/16 B
4/16 b
12/16 W
12/16 w

Which I think is fairly cool, and in an extremely loose sense exhibits the 'alteration of generations' phenomenon (which I know won't follow in the next generation, and isn't a sex-linked trait... )