genetic crossing why isnt it 1/4 :(

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biohelp22
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genetic crossing why isnt it 1/4 :(

Post by biohelp22 » Sun Apr 23, 2006 2:57 pm

this question has confuzzled me

in humans, if red hair (R) is dominant to brown hair (r) and freckles (F) are dominant to no freckles (f) what fraction of the progeny cross RrFf x RRff will have red hair and no freckles??

a. 9/16
b. 1/2
c. 3/8
d. 3/16
e. 1/16

i did the punnett square and i got 4/16 :(

d00d
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Post by d00d » Sun Apr 23, 2006 6:24 pm

Parents: Genotype- RrFf x RRff

Meiosis: Gametes- (RF Rf rF rf) x (Rf Rf Rf Rf)

Offspring: Genotypes- 4 RRFf, 4 RRff, 4 RrFf, 4 Rrff

Phenotypes- 8 Red Haired, No Freckles

therefore you have a possible 8 out of 16 with red hair and no freckles which amounts to a probability of 1/2. the answer is B.

sdekivit
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Re: genetic crossing why isnt it 1/4 :(

Post by sdekivit » Mon Apr 24, 2006 8:58 pm

biohelp22 wrote:this question has confuzzled me

in humans, if red hair (R) is dominant to brown hair (r) and freckles (F) are dominant to no freckles (f) what fraction of the progeny cross RrFf x RRff will have red hair and no freckles??

a. 9/16
b. 1/2
c. 3/8
d. 3/16
e. 1/16

i did the punnett square and i got 4/16 :(


red hair and no freckles = RRff or Rrff

chance for RR = 1/2 and ff = 1/2 --> RRff = 1/2 * 1/2 = 1/4
chance for Rr = 1/2 and for ff = 1/2 --> Rrff = 1/2 * 1/2 = 1/4

combined chance for red hair and no freckles: 1/4 + 1/4 = 1/2 :)

d00d
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Post by d00d » Tue Apr 25, 2006 12:00 am

thanks for repeating!
Since when did what we pay for coloured cloth gauge our gravity?

sdekivit
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Post by sdekivit » Tue Apr 25, 2006 7:03 pm

d00d wrote:thanks for repeating!


i didn't repeat you

--> i gave another method, which is much faster, because you don't have to draw a punnett square ;)

d00d
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Post by d00d » Tue Apr 25, 2006 9:05 pm

i take that back...sorry there man.
Since when did what we pay for coloured cloth gauge our gravity?

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