adriannawinco
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Can someone help me with a biology related question? I'm offering good points!!!!! Help me out please.
In some bees, thin stripes (S) are dominant over thick stripes (s) and black eyes (B) are dominant over gray eyes (b). Complete a dihybrid cross for parents with the genotypes: SsBB × ssBb and answer the following in complete sentences.

Describe how you would set up a Punnett square for this cross.
List the likelihood of each possible offspring genotype.
List the likelihood of each possible offspring phenotype.

Respuesta :

Apocrophon
First, write out all combinations for the parents:
SsBB:
SB
sB
SB
sB

ssBb:
sB
sb
sB
sb

So now that you know what to put on as the x and y for the Punnett Square, set it up:

     |    SB   |    sB    |   SB    |  sB     
sB | SsBB |  ssBB |  SsBB |  ssBB 
sb | SsBb |  ssBb  |  SsBb |  ssBb 
sB | SsBB |  ssBB |  SsBB |  ssBB 
sb | SsBb |  ssBb  |  SsBb |  ssBb 

Now, listing each possible genotype and the number of times they show up in the square:
SSBB
0 times
SSBb/SSbB
0 times
SSbb
0 times
SsBB/sSBB
4 times
SsBb/SsbB/sSBb/sSbB/
4 times
Ssbb/sSbb
0 times
ssBB
4 times
ssBb/ssbB
4 times
ssbb
0 times

Since there are only 4 genotypes are possible, set up ratios of those four:

SsBB:
4 times/16 total = .25 = 25%

SsBb:
4 times/16 total = .25 = 25%

ssBB:
4 times/16 total = .25 = 25%

ssBb:
4 times/16 total = .25 = 25%

This means that the genotypes have a 25% : 25% : 25% : 25% likelihood.


Now, let's figure out the phenotypes of the 4 genotypes:
SsBB:
Ss = dominant
BB = dominant

SsBb:
Ss = dominant
BB = dominant

ssBB:
ss = recessive
BB = dominant

ssBb:
ss = recessive
Bb = dominant

So we have 2/4 that are S-dominant B-dominant and 2/4 that are S-recessive B-dominant.
This means that the phenotypes are:
2/4 = 50% Thin stripes with black eyes, and
2/4 = 50% Thin stripes with gray eyes.

I hope this helped you out :)

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