Genetics Test Cross

[fob12]

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Hi guys, I am having trouble w/ this problem:

In watermelons, the unlinked genes for green color (G) and for short length (S) are dominant over alleles for striped color (g) and long length (s). Predict the phenotypes and their ratios for the cross Ggss X ggSs.

The answer is:

1:1:1:1
green short: striped short: green long: striped long

Why is this so?

 

[syn_apse]

punnett square worked for me...

 

[Kremlar]

Hi guys, I am having trouble w/ this problem:

In watermelons, the unlinked genes for green color (G) and for short length (S) are dominant over alleles for striped color (g) and long length (s). Predict the phenotypes and their ratios for the cross Ggss X ggSs.

The answer is:

1:1:1:1
green short: striped short: green long: striped long

Why is this so?

These are the gametes they can produce.
Gs gs x gS gs
when you cross you get :
GgsS 1) Green Short
Ggss 2) Green Long
ggSs 3) striped short
ggss 4) striped long

hope that helps

 

[cdpiano27]

Simply use the "branch" method on this question. I just learned this from the beginning of my genetics class.


Note that this is a dihybrid cross of Ggss X ggSs.

So we will set up two branches. One for Gg X gg and another one for ss X Ss

like this

First cross Gg*gg. We get 1/2 Gg and 1/2 gg.
Next cross Ss*ss. We get 1/2 Ss and 1/2 ss

So the branch looks like:

1/2 Ss
.5 Gg
1/2 ss

1/2 Ss
.5 gg
1/2 ss

now multiply the probabilities in each branch to get
1/4 GgSs, 1/4 Ggss, 1/4 ggSs, 1/4 ggss
this is clearly a 1:1:1:1 ratio.

Maybe this is not taught in gen bio though in this easy way.

But use it. It works every time, and is very easy for a dihybrid cross rather than writing out 16 squares.

 

[teefRcool]

So the branch looks like:

1/2 Ss
.5 Gg
1/2 ss

1/2 Ss
.5 gg
1/2 ss

now multiply the probabilities in each branch to get
1/4 GgSs, 1/4 Ggss, 1/4 ggSs, 1/4 ggss
this is clearly a 1:1:1:1 ratio.

I didn't understand how u made the branch could u explain a little further thanks

 

[booboo]

i dunno if i did it right..if i am wrong, please let me know..thanks

 

[cdpiano27]

OK, the branch did not come out correctly when I posted it due to the spacing of the word processor on this setup.

But here is the procedure.

First cross the G's.

So the first branch will contain 1/2 Gg and 1/2 gg.
2 elements.

The S's compose the second branch. which will have four elements.

2 (Ss and ss with 1/2 probabilities) for each of Gg and gg which are the two possible genotypes for the g's

Hence: the First branch will have 1/2 Gg and 1/2 gg.

The second branch will have (1/2 Ss 1/2 ss) which pairs with 1/2 Gg and (1/2 Ss 1/2 ss) again which pairs with 1/2 gg.
The spacing will not come out when I send it so I have tried ot explain what the branch loooks like.
Hence,to get the ratio multiply each the probabilities in each branch.
We get 1/4 GgSs, 1/4 Ggss, 1/4 ggSs, 1/4 ggss.
Which is a 1:1:1:1 ratio. You can easily match the phenotypes to the genotypes in this problem.

 

[cdpiano27]

Not quite. The final probabilities should have 1/4 for each not 1.

But 1/4:1/4:1/4:1/4 reduces down to a 1:1:1:1 ratio.

remember all of the probabilities in the last step must add to 1.

 

[booboo]

isnt 1/2 x 1/2 = 1/4

 

[cdpiano27]

Yes, but each different genotype in the final result has 1/4 probability.

You wrote 1 GgSs, 1 Ggss, 1 ggSs, and 1 ggss. It should be 1/4 GgSs, 1/4 Ggss, 1/4 ggSs, and 1/4 ggss.

That is the only mistake in that picture. Everything else is correct.

We notice that if we had up to final (equal in this case) probabilities of the phenotypes resulting in the dihybrid cross we will get 1.

This method is much easier than using a 4X4 cross.

Imagine if we have a trihybrid cross we would have 8X8 cross. or 2^n*2^n where n is the number of loci.

This is the reason for the "branching" method.

A good question for the DAT would be a trihybrid cross. This would test if someone knew this method or did not.

 

[booboo]

but didint u do the same exact thing? i was doing this on computer and thats why i didnt put 1/4 value.

 

[syn_apse]

that branching technique doesn't look like a shortcut to me

 

[teefRcool]

that branching technique doesn't look like a shortcut to me

it is if u have tri hybrids and more

 

[syn_apse]

but this isn't a trihybrid, hence my comment

 

[teefRcool]

Well the punnet square can get a little messy this technique singles out everything.