# Di Hybrid

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Brown eyes(B) are dominate over blue eyes(b)

Dark hair (D) dominate over blonde hair(b)

Determine the genotype of women with blue eyes and hybrid dark hair and a man with hybrid dark hair and hybrid brown eyes.

I believe the genotype is bbDd x DdBb

^is that correct

I also need help determining the possible gametes

I believe its Db db Db db < female DB Db dB db < Male

^is that correct?

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Yes, you have got the genotypes and gametes correct. No explanation needed, seems like you already understand it

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For phenotypic ratio I got

Dark hair blue eyes 3:8

Dark hair brown eyes 3:8

blonde hair blues eyes1:8

blonde hair brown eyes1:8

I have reduced the ratio from 16 to 8 , please tell me if i'm correct

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Yes that's correct, although you don't write each ratio as x:8. If you want to write the fractions you could write

dark hair blue eyes 3/8

dark hair brown eyes 3/8

blonde hair blues eyes 1/8

blonde hair brown eyes 1/8

The ratio is 3:3:1:1 (or 6:6:2:2 before you reduced it)

When you write the ratio make sure you also write what it's the ratio of, e.g.

dark,blue : dark,brown : blonde,blue : blonde,brown

3:3:1:1

Edited by Blahah
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Thanks a lot. I just need some clarification. I have one more question seen we're on the topic.

Does the f1 generation have 8 possible genotypes?

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No there are only 6 possible genotypes:

DDBb, DdBb, DDbb, Ddbb, ddBb, ddbb

If you draw the punnet square you can count the different genotypes:

Remember that DdBb and DdbB are the same genotype, and it's easier to always write the dominant first so you don't accidentally count them as separate. Does that make sense?

Edited by Blahah
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Makes a lot of sense. One more explanation? Its about Mendel. I need to find the phenotypic ratio for f1 generation from a cross of a pure yellow round x green wrinkled

My findings show me the following YR YR YR YR and yr yr yr yr

so the phenotypic ratio would be yellow and round 4:8

green wrinkled 4:8

Thats for the f1 generation.

Hope you don't mind me asking so many questions. I only had one lesson on this, so I need to make sure I understand it clearly.

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So if we denote:

pure yellow as Y

green as y

round as R

wrinkled as r

And you have a pure yellow round: YYRR

and a pure green wrinkled: yyrr

So the gametes of the pure yellow round will be: YR YR YR YR

And the green wrinkled gametes: yr yr yr yr

Then we do the punnet square (only need to do a small one since they are all the same!):

You can see all F1 offspring will have YyRr.

But if you then do a dihybrid cross of two F1 offspring, they will each have gametes: YR Yr yR yr

Then the punnet square looks like:

So you can see the F2 generation has a ratio of 9:3:3:1 (I coloured the phenotypes to make it easier to count)

Does that make it clear?

So from those results (and lots more) Mendel was able to work out the laws of inheritance. Actually, it has been shown that Mendel probably guessed the laws quite early on in his experiments, and then made up the data to support his guesses! It would have been extremely unlikely that he would get the ratios so perfectly in his results with a few thousand plants. Not very scientific (but still, science is grateful to him).

If you want to read about it in more detail, this page describes it clearly.

Edited by Blahah
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Thank you so much for your help.

Makes it a lot more clear.

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