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BACKCROSS HELP

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BACKCROSS HELP

Postby star21 » Wed May 04, 2005 10:11 pm

hi there everyone,
i'm new so if ths goes wrong sorry, at the moment i am studying backcrossing and genes ect. i know what backcrossing is and the results but please please please can some one tell me how to set a backcross up for animals/fruite flies.
thank you for reading hope u can help. :D
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Postby Cross Country Runner » Wed May 04, 2005 11:24 pm

Backcrossing involves finding the offspring from mating an organism from the parent generation and the F1 generation. The resulting offspring is part of the F2 generation.
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Postby Cross Country Runner » Wed May 04, 2005 11:33 pm

Here's a example:
Say that you cross a Homozyogous Dominant parent for black fur(noted by BB) and a Homozygous Recessive parent for white fur(noted by bb)

The cross between them would produce a genotype of Bb, or Heterozygous. The Phenotype (what it looks like) would be black fur because black fur is dominant to white fur.

The Punnet Square looks like this

B B

b Bb Bb

b Bb Bb
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Postby Cross Country Runner » Wed May 04, 2005 11:49 pm

Finally for backcrossing just take the offspring-Bb which which ever parent you want:

The cross with the father would produce 50% Bb and 50% BB which both would be black.

The cross with the mother would produce 50% Bb and 50% bb which would be 50% black and 50% white.
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thanks

Postby star21 » Wed May 04, 2005 11:59 pm

thank you so much, following a little more now
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Postby star21 » Thu May 05, 2005 12:03 am

but if you have the f1 generation and the perent with the recessive gene you breed these to higlight the recessive gene how do you do it? if you where backcrossing fruit flies then how would you go about actualy carrying it out?
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Postby Cross Country Runner » Thu May 05, 2005 11:58 am

For highlighting a recessive trait you must cross the Homozygous recessive parent and the F1 generation offspring.

It would still look like this

_B _ b
b Bb bb
b Bb bb

50 % of the time the recessive trait would show up
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Postby Cross Country Runner » Thu May 05, 2005 12:09 pm

To backcross with fruit flies you would use the same method>>>>>>>>>

Say that red eye color is dominant
RR=Homozygous Dominant=Red eye color
Rr=Heterozygous=Red eye color
rr=Homozygous recessive=Green eye color

That is only if it is complete dominance.

If you need help with incomplete dominance or co-dominance or two-trait cross just ask!!!
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Postby star21 » Tue May 10, 2005 10:14 pm

hi again, thanks again. the question i'm lookin at solving as a female fruit fly with red eyes and a grey body, the male as brown eyes and a yellow body. and all the f1 offspring have taken after the mother all say 47 are red eyed and have grey bodys. and this is what i need to set up a back cross for. i know the ressesive is the male but thats about it.
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Postby Cross Country Runner » Wed May 11, 2005 1:12 pm

The only way somthing like that where all offspring show dominant traits is if the mother was completely dominant meaning her genotypes for RR for red eyes and GG for grey bodies. Then the male would have to be homozygous recessive for both of the traits.
rr for brown eyes and gg for yellow body. Finnally the offspring would all have heterozygous. Rr for red eyes and Gg for grey bodies.

Anything else? :)
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Postby star21 » Thu May 12, 2005 1:00 am

yay i'm getting it, i cannt thank you enough this was one thing i just couldnt get my head around, theres just one thing i STILL dont get, and thats if your setting up a back cross to breed all f1 offspring females with the farther(i'm i right) then how can you determin which are male which are femal and make sure they dont breed with there bros and sisters?
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Postby Cross Country Runner » Fri May 13, 2005 1:17 pm

That question is a hard one to answer. Back Crosses take an offspring and cross with a parent. So theres no way that the offspring could breed with it's brother or sister in a backcross problem. A female offspring can only cross wtih the male parent. And a male offspring can only cross with a female parent. You must take all the possible genotypes of the offspring and cross them with the parent. Since you cannot determine what will be female in the F1 generation you must except that all of them would be female. Simply cross the male parent with every type of offspring.
Note that recessive traits are implied with backcrosses
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