I really need help with dihybrid crosses plz!

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Jinasarangyoonhox3
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I really need help with dihybrid crosses plz!

Post by Jinasarangyoonhox3 » Thu Jan 05, 2006 4:53 am

In this problem ... it says "two pea plants heterozygous for both traits are crossed."

G-Green pea pod g-Yellow pea pod
T-Tall t-short


what are the chances of being homozygous for both traits in the F1 generation?


I'm confused on how to start it off ... like drawing the cross diagram. Thank you. :lol:

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Post by xnabizkox » Thu Jan 05, 2006 9:05 am

Dihybrid crosses involve 2 traits that will independently assort. Since the plants are heterozygous this means both plants will have the genotype of GgTt. The dihybrid cross looks a little like this.

GgTt x GgTt

______[GT]___[Gt]___[gT]___[gt]
[GT] _ GGTT _ GGTt _ GgTT _ GgTt

[Gt] _GGTt___ GGtt__ GgTt__ Ggtt

[gT] _ GgTT _GgTt _ ggTT __ ggTt

[gt] __GgTt___ Ggtt__ ggTt __ggtt

SO the chance of the F1 generation (first offspring) being homozygous (GGTT and ggtt) is 2:16.

Hope that helps.
Last edited by xnabizkox on Thu Jan 05, 2006 9:34 pm, edited 1 time in total.
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Post by Enzyme » Thu Jan 05, 2006 11:01 am

Jinasarangyoonhox3, the Punnet's square is very easy to draw. You only have to write the gametes from one parental individual in the first row and the gametes from the other parental individual in the first column. Then, only you have to combine them. xnabizkox has written it very well.

But, what were the question that problem says? If the problem ask for the probability of being homozygous for the two characters, you should answer 2/16 (because you have to take in account dominant homozygous and recessive homozygous). But if the problem ask for the probability of being dominant homozygous (GGTT) or recessive homozygous (ggtt), you should answer 1/16, as xnabizkox said.

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Post by sdekivit » Thu Jan 05, 2006 11:34 am

another one is just simply using mathematics :p --> chanches. I'll explain:

If we cross GgTt with each other, we get:

_________________Gg______ x ________Gg
_______________/___________|___________\
___________GG(1/4)______Gg(1/2)______gg(1/4)
__________/___|___\_____/___|___\_____/___|___\ ____<-- (times Tt)
____GGTT_GGTt_GGtt_GgTT_GgTt_Ggtt_ggTT_ggTt_ggtt

Because TT also has a chance of 1/4 GGTT has a chance of 1/4 * 1/4 = 1/16

Tt has a chance of 1/2, thus GGTt has a chance of 1/4 * 1/2 = 1/8 = 2/16

And so on.

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Post by Enzyme » Thu Jan 05, 2006 12:16 pm

sdekivit wrote:another one is just simply using mathematics :p --> chanches. I'll explain:

If we cross GgTt with each other, we get:

_________________Gg______ x ________Gg
_______________/___________|___________\
___________GG(1/4)______Gg(1/2)______gg(1/4)
__________/___|___\_____/___|___\_____/___|___\ ____<-- (times Tt)
____GGTT_GGTt_GGtt_GgTT_GgTt_Ggtt_ggTT_ggTt_ggtt

Because TT also has a chance of 1/4 GGTT has a chance of 1/4 * 1/4 = 1/16

Tt has a chance of 1/2, thus GGTt has a chance of 1/4 * 1/2 = 1/8 = 2/16

And so on.


He he, cool example :wink:.
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Post by Jinasarangyoonhox3 » Thu Jan 05, 2006 8:42 pm

thank you so much !!! also to get the phenotypic ratio of this problem ... im not too sure...

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Post by xnabizkox » Thu Jan 05, 2006 9:33 pm

Phenotypic ratio is 9:3:3:1

9 Green Tall
3 Green Short
3 Yellow Tall
1 Yellow Short

Do the punnett square and count how many different physical traits result from the cross.
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Post by Enzyme » Thu Jan 05, 2006 10:25 pm

Jinasarangyoonhox3 wrote:thank you so much !!! also to get the phenotypic ratio of this problem ... im not too sure...


You only have to count all the different appearance traits in the Punnet's square, as xnabizkox said.

Take in account that it is a 9:3:3:1 segregation (because you'll see that 9/16 corresponds to G_T_, 1/16 to ggtt and the others are combinations). And seeing the genotypes of each result in Punnet's square, we can deduce the phenotypes. Take a look to xnabizkox post:

xnabizkox wrote:Phenotypic ratio is 9:3:3:1

9 Green Tall
3 Green Short
3 Yellow Tall
1 Yellow Short


Did you find all the phenotypes and their probabilities in the table of genotypes (Punnet's square)?
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Post by Jinasarangyoonhox3 » Thu Jan 05, 2006 11:15 pm

okay ... i understand ... thanks ...




also my final question .... if i want to find the chances of having offspring heterozygous for both traits for the pea plants .... would the answer be 4:16 ... which reduces to 1/4. so 1/4 would be the answer? is that correct?

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Post by Enzyme » Fri Jan 06, 2006 9:25 am

xnabizkox wrote:GgTt x GgTt

______[GT]___[Gt]___[gT]___[gt]
[GT] _ GGTT _ GGTt _ GgTT _ GgTt

[Gt] _GGTt___ GGtt__ GgTt__ Ggtt

[gT] _ GgTT _GgTt _ ggTT __ ggTt

[gt] __GgTt___ Ggtt__ ggTt __ggtt


If you want to find the chances for having heterozygous for both treats, the answer would be 4/16 which we can simplify to 1/4. YES, IT IS CORRECT :wink:.
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Post by sdekivit » Fri Jan 06, 2006 1:07 pm

phenotypes can also be done in the same way:

________________Gg x Gg
_______________/________\
__________green(3/4)_yellow(1/4)
_________/_____|________|______\
_____tall(3/4)_short(1/4)_tall(3/4)_short(1/4)

thus see here the phenotypes and their probabilities:

green and tall will be: (3/4 * 3/4) = 9/16

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Post by sdekivit » Fri Jan 06, 2006 1:09 pm

Enzyme wrote:
xnabizkox wrote:GgTt x GgTt

______[GT]___[Gt]___[gT]___[gt]
[GT] _ GGTT _ GGTt _ GgTT _ GgTt

[Gt] _GGTt___ GGtt__ GgTt__ Ggtt

[gT] _ GgTT _GgTt _ ggTT __ ggTt

[gt] __GgTt___ Ggtt__ ggTt __ggtt


If you want to find the chances for having heterozygous for both treats, the answer would be 4/16 which we can simplify to 1/4. YES, IT IS CORRECT :wink:.


with my 'probability-tree': Gg = 1/2 and Tt = 1/2

--> GgTt = 1/2 * 1/2 = 1/4 = 4/16

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