Biology Forum Molecular Biology Punnett Square

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• #7647
arensdt
Participant

I could really use some help on a question from my biology class concerning the Punnett Square (which I am having a hard time understanding). The problem is…

On the planet “Starlight” live the colorful Paces. Their hair may be either blue (dominant) or yellow (recessive), and their eyes may be green (dominant) or red (recessive). A blue-haired woman with green eyes and a blue-haired man with red eyes marry. Make a Punnett Square to determine all possible genotypes of their future children and then determine the possible phenotypes. (When the dominant expression of a trait is expressed, you should always assume heterozygosity.) Submit the possible genotypes and phenotypes and their ratios of this mating. Do not submit the Punnett Square.

Can anyone help me with this? I sure hope so.

• #72725
James
Participant

Its a dihybrid cross

So BB=blue hair, Bb= blue hair, bb=yellow hair
GG= green eyes, Gg= green eyes and gg= red eyes

parental= B_G_ x B_gg

It says assume heterozygousity so its
Bb/Gg x Bb/gg

the possible gametes are

BG or Bg or bG or bg x Bg or bg

…….Bg……bg
BG
Bg
bG
bg

So for example the very bottom right square would be bb/gg. The very top left combination is BB/GG. Fill it out for each combination then you know the geneotypes. Then work out the phenotypes and the ratios.

• #72734
arensdt
Participant

James,

Thank you very much for the help! But I still don’t understand how this all comes together. What about the hair color? And how do you figure the ratio’s?

Anybody…

• #72736
fox914
Participant

The ratios you figure out like this:

Say (for example) the results for the Punnett Square were BB, Bb, Bb, and bb. The ratio is 1:2:1 because, out of 4 results in a Punnett square, ONE is BB (homozygous dominant), TWO are Bb (heterozygous), and ONE is bb (homozygous recessive). SO the ration is ONE:TWO:ONE.

Did that help? 🙂

• #72739
James
Participant

Ah sorry to confuse you, I’ve edited it. Check out wikipedia to understand the punnett sqaure http://en.wikipedia.org/wiki/Punnett_square. When you have the square of genotypes, translate the genotypes to their phenotypes. Then the ratio between the eg blue hair and green eyes: blue hair red eyes: yellow hair blue eyes etc.. will be evident.

• #72741
kotoreru
Participant

I think it’s important for you to know that the question you have been set is purely for understanding theory – eye and hair heredity are not as simple as this.

There was a discussion on this a little while ago in one of the other sub-forums…use the search function 🙂

• #72787
arensdt
Participant

James and everyone else,

Thanks for the help on my square. Based on the problem and the information that you gave me this is what I can up with. Right…Wrong??

Dave

Capital=Dominant
Lowercase=recessive
BB=Blue hair
Bb=Blue Hair
bb=yellow hair
GG=green eyes
Gg=green eyes
gg=red eyes

Woman=BbGg
Man=BBgg
Child=BbGg x BBgg

……Bg……………….bg
BG BB/Gg Bb/Gg
Bg BB/gg Bb/gg
bG bB/Gg bb/Gg
bg bB/gg bb/gg

8 possible genotypes.

Blue Hair/Green eyes=2 (25% possibility) 1:4 ratio
Blue Hair/Red eyes=2 (25% possibility) 1:4 ratio
Yellow hair/green eyes=2 (25% possibility) 1:4 ratio
Yellow hair/red eyes=2 (25% possibility) 1:4 ratio

• #72788
James
Participant

A dominant characteristic requires only one allele to be expressed. bB is the same as Bb.

BB/Gg= blue, green
Bb/Gg= blue, green
BB/gg= blue, red
Bb/gg = blue, red
bB/Gg= blue, green
bb/Gg = yellow, green
bB/gg= blue, red
bb/gg = yellow, red

3:3:1:1

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