ChloeYeo
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The colour of onion bulbs is determined by two genes, A/a and B/b.
• A is a dominant allele and codes for the production of a red pigment.
• Onion bulbs that are homozygous for the recessive allele, a, produce no pigment
and are white.
• B is a dominant allele that inhibits the expression of allele A.
• The recessive allele, b, allows the production of the red pigment.
A white onion plant was cross-pollinated with a red onion plant. All 15 offspring had the
genotype AaBb.
(i) Identify the following:
The genotype of the white onion plant.

For the genotype of the white onion plant, the mark scheme answer is aaBB.

Why is it aaBB but NOT aabb(my answer) or aaBb?

Can someone please explain?

Thank you so much!!
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theJoyfulGeek
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(Original post by ChloeYeo)
The colour of onion bulbs is determined by two genes, A/a and B/b.
• A is a dominant allele and codes for the production of a red pigment.
• Onion bulbs that are homozygous for the recessive allele, a, produce no pigment
and are white.
• B is a dominant allele that inhibits the expression of allele A.
• The recessive allele, b, allows the production of the red pigment.
A white onion plant was cross-pollinated with a red onion plant. All 15 offspring had the
genotype AaBb.
(i) Identify the following:
The genotype of the white onion plant.

For the genotype of the white onion plant, the mark scheme answer is aaBB.

Why is it aaBB but NOT aabb(my answer) or aaBb?

Can someone please explain?

Thank you so much!!
So the onion offspring are AaBb, and one parent is red and the other is white.
This means that at least one parent must have the dominant B allele - and as this inhibits red, it's the white parent.

So the white onion cannot be bb, as it has a B allele.

As the red parent is bb, and ALL offspring are Bb, the white onion parent must be BB.

So that's why it's aaBB instead of aaBb or aabb.

I hope this helps!
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ChloeYeo
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(Original post by theJoyfulGeek)
So the onion offspring are AaBb, and one parent is red and the other is white.
This means that at least one parent must have the dominant B allele - and as this inhibits red, it's the white parent.

So the white onion cannot be bb, as it has a B allele.

As the red parent is bb, and ALL offspring are Bb, the white onion parent must be BB.

So that's why it's aaBB instead of aaBb or aabb.

I hope this helps!
Is my understanding correct?
Since B is the dominant epistatic allele and A codes for red, AaBb, aaBb, AABb, AaBB, aaBB and AABB will all be white.
Because the epistatic allele masks the expression of both alleles of the other gene, so masks both Aa/AA and aa.
For the onion to be white it must be a bb which does not mask the expression of the first gene and aa codes for white so the answer would be aabb.

But as aa codes for white and Bb or BB masks the expression of the first gene won't this also come out as white? What will this mean? I still don't understand why this won't work.

Thank you!!
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theJoyfulGeek
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(Original post by ChloeYeo)
Is my understanding correct?
Since B is the dominant epistatic allele and A codes for red, AaBb, aaBb, AABb, AaBB, aaBB and AABB will all be white.
Because the epistatic allele masks the expression of both alleles of the other gene, so masks both Aa/AA and aa.
For the onion to be white it must be a bb which does not mask the expression of the first gene and aa codes for white so the answer would be aabb.

But as aa codes for white and Bb or BB masks the expression of the first gene won't this also come out as white? What will this mean? I still don't understand why this won't work.

Thank you!!
B doesn't inhibit the first gene - just the A allele (not the a). But the first part you said is correct!
However, aabb is not correct (for reasons I explained above).

I think you just misread the question part about the B inhibiting the A allele!
Last edited by theJoyfulGeek; 6 days ago
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theJoyfulGeek
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(Original post by ChloeYeo)
Is my understanding correct?
Since B is the dominant epistatic allele and A codes for red, AaBb, aaBb, AABb, AaBB, aaBB and AABB will all be white.
Because the epistatic allele masks the expression of both alleles of the other gene, so masks both Aa/AA and aa.
For the onion to be white it must be a bb which does not mask the expression of the first gene and aa codes for white so the answer would be aabb.

But as aa codes for white and Bb or BB masks the expression of the first gene won't this also come out as white? What will this mean? I still don't understand why this won't work.

Thank you!!
I'll explain why it has to be aa.

So, one parent is red (Aa or AA) and the other is white (Aa, AA, or aa (as it has the B for reasons explained above).

For ALL of the offspring to be identical, both parents must be homozygous. The white parent could hypothetically be AA or aa. However, the red parent could only be AA (if it were aa, it would be white):
If red is crossed with AA white, the offspring would be AA.
So red must be crossed with aa white.

So the only possible option is aaBB - what the markscheme says!

I hope this helped! Sorry about not being clear enough in my first post.

Last edited by theJoyfulGeek; 6 days ago
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ChloeYeo
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(Original post by theJoyfulGeek)
I'll explain why it has to be aa.

So, one parent is red (Aa or AA) and the other is white (Aa, AA, or aa (as it has the B for reasons explained above).

For ALL of the offspring to be identical, both parents must be homozygous. The white parent could hypothetically be AA or aa. However, the red parent could only be AA (if it were aa, it would be white):
If red is crossed with AA white, the offspring would be AA.
So red must be crossed with aa white.

So the only possible option is aaBB - what the markscheme says!

I hope this helped! Sorry about not being clear enough in my first post.

OHH I finally get it! Thank you!!

So the parents must be aaBB(white) and AAbb(red) to get all AaBb offsprings!
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theJoyfulGeek
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(Original post by ChloeYeo)
OHH I finally get it! Thank you!!

So the parents must be aaBB(white) and AAbb(red) to get all AaBb offsprings!
Yep! No problem!
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