The Student Room Group

Bio help

A level 4. If a pure breeding strain of mice with brown coloured fur are allowed to breed with mice with grey coloured fur they produce offspring which all have brown coloured fur. If the F1 mice are allowed to interbreed they produce an F2 generation with fur colour in the proportion of three brown coloured to one grey coloured . Explain these results fully What would be the result of mating a brown coloured heterozygote from the F2 generation with the original grey coloured parent? ()

Scroll to see replies

what have you got so far?
Literally nothing like I don't even know where to start
can you work out what the genotype for the pure breeding strain of mice with brown coloured fur? remember that when they are crossed with grey mice, the offspring are all brown. so which, of brown and grey, is dominant, and which is recessive? can you then work out the genotype for the grey mice?
So B= brown b= grey
So Bb x Bb
B b
B Bb Bb
bBb Bb

Genotype = 100% Bb so brown
Reply 5
Assuming brown mice are Bb/bb
Assuming grey mice are bb
B is dominant whereas b is recessive

Mating a brown coloured heterozygote (Bb) from the F2 generation with the original grey coloured parent (bb):

The genetic makeup of offspring: 1/2 Bb and 1/2 bb
Therefore, half the offspring will be brown and half grey.
Reply 6
the grey mouse is bb whereas the brown mouse is Bb
Why isn't the brown mouse BB
I don't get that part if it's brown + grey -> brown offspring why isn't the brown parent BB if that's what makes brown fur
Original post by Roxibox123
So B= brown b= grey
So Bb x Bb
B b
B Bb Bb
bBb Bb

Genotype = 100% Bb so brown


"So B= brown b= grey"
good, well done, brown is dominant, grey is recessive

"So Bb x Bb
B b
B Bb Bb
bBb Bb "
not quite sure what you mean by this, which mice are you referring to? also your genetic cross doesn't quite work, remember that a B allele from parent X combined with a B allele from parent Y results in the offspring being BB it can't have a little b if there aren't any inherited from the parents!


the next part of the question talks about breeding the "pure" (i.e. homozygous) brown mice with the grey mice. this gives the results that you got in your genetic cross diagram (but you seem to have got the parent alleles a bit mixed up)

then these Bb mice are interbred... have a go at doing this genetic cross in a punnett square. your previous attempt started off with the right parent alleles, but the resulting offspring genotypes don't match up

there are 2 parts to the question to answer:

explain why the F2 generation (offspring of the Bb x Bb cross) is 75% brown and 25% grey (brown : grey ratio is 3 : 1)
you can pretty much do this by showing your workings through the whole description part show all the genetic crosses in well-labelled punnett squares, and make sure you highlight which offspring are which colours, and show the ratios/percentages. you could say that x% is BB; y% is Bb and z% is bb, meaning that overall p% is brown and q% is grey.

the result of mating a brown coloured heterozygote from the F2 generation with the original grey coloured parent
just do another genetic cross diagram. read all the components carefully, and show all the ratios for the offspring :smile:
Original post by Roxibox123
I don't get that part if it's brown + grey -> brown offspring why isn't the brown parent BB if that's what makes brown fur


remember that:

the recessive trait (grey fur, "b") is only expressed if both alleles are recessive (bb)
the dominant trait (brown fur, "B") will be expressed if at least one of the alleles is dominant (i.e. BB or Bb)
How is it 3:1 B9385D2F-8C66-4551-9041-A00110A24F88.jpg.jpeg
Also what's the f1 and f2 generation
Original post by Roxibox123
Also what's the f1 and f2 generation



F1 generation = the offspring of the first cross, between the original parents
F2 generation = the offspring of the second cross, between the F1 parents
Original post by Roxibox123
How is it 3:1 B9385D2F-8C66-4551-9041-A00110A24F88.jpg.jpeg

the cross you've drawn shows the offspring of the original parents (pure brown (BB) crossed with grey (bb))

the 3:1 ratio is referring to the offspring when F1 (offspring of BB and bb the F1 are therefore all Bb, as you have deduced in your diagram above) mice are crossed (Bb crossed with Bb)
What happens if an original parents mates with the f2
Like what's that called ... f3?
erm not sure, for clarity you'd probably just say F2 x original (or similar)
Could someone plz help me on this q
5. A species of poppy may have plain petals or petals with a large black spot near the base . A cross between unspotted and spotted plants sometimes produces offspring which all have unspotted petals Explain these results by means of genetic diagrams .
I've done the other questions but I'm just stuck on this one because I keep getting unspotted every time
4E991BF0-8D08-4953-90ED-313E6233E802.jpg.jpeg

Quick Reply

Latest