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Trihybrid Cross

Genetics as it applies to evolution, molecular biology, and medical aspects.

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Trihybrid Cross

Postby Starry_Fruit » Sat Feb 24, 2007 11:46 pm

I'm having serious trouble with trihybrid crosses because the only examples I can find lead to a 27:9:9:3:9:3:3:1 ratio. The way I do them is using a branch diagram.

I did this on the test and got it wrong obviously because that was only an example and does'nt teach how to do it for different questions such as this one:

In fruit flies, thin antenna is caused by an allele Xt that is X-linked and recessive to an allele for normal antenna XT. White wings are caused by an allele b that is autosomal and recessive to an allele for black wings b+. A female fly that is homozygous for normal antenna and black wings mates with male fly that has thin antenna and white wings. The F1 are intercrossed to produce the F2. What are the phenotypes and proportions of F1 and F2 flies from this cross? (10 Marks)

Now I got P: XTXTBB * XtYbb for the parent generation and for the F1 generation I got F1:


Now I am totally confused as to how to do a test cross for the F2 generation!! I tried it multiple times but could not figure it out and when I asked the T.A. he just gave me the answer and said do a test cross but would'nt go through it with me step by step. I'm very upset. The answer he gave me was:

6/16 Normal antennae, black wings female
2/16 Normal antennae, white wings female
3/16 Normal antennae, black wings male
1/16 Normal antennae, white wings male
0 Thin antennae, black wings female
0 Thin antennae, white wings female
3/16 Thin antennae, black wings male
1/16 Thin antennae, white wings male

How should I have set it up so that I would get a 6:2:3:1:0:0:3:1 ratio? I'm not even sure what kind of ratio that is. Any kind of help would be appreciated.
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Postby ArtificialAngel » Sat Mar 24, 2007 3:16 am

eeeh gesh i didnt even know such a thing existed lol i only know of the dihybrid crosses. sry
Without an exit..if you intend to survive the impact, to the point where you fall apart.. Enrapture the deceits
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Re: Trihybrid Cross

Postby Revenged » Sat Mar 24, 2007 5:38 pm

Starry_Fruit wrote:Now I got P: XTXTBB * XtYbb for the parent generation and for the F1 generation I got F1:


Yes... exactly... if you mate the offspring...

XTXt Bb vs. XTY Bb

Gametes XTB, XTb, XtB, Xtb and XTB, XTb, YB, Yb

So this would still be a dihybrid cross and if you cross them you should end up with 16 possibilites... this should give the answer you were expecting...

PS. This is a dihybrid cross (as you only have two genes)... It isn't a trihybrid cross...
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