## Plants - genetics

Genetics as it applies to evolution, molecular biology, and medical aspects.

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ashanair70
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Joined: Tue Dec 18, 2012 12:13 am

### Plants - genetics

I am not sure this is the right category to post. I can't think of anything else.

Can someone help me to find a good explanation/answer for the following question:

An inbred strain of plants has a mean height of 24 cm. A second strain of the same species also has a
mean height of 24 cm. When these plants are crossed, the F1 are also 24 cm. However, when the F1 plants
are crossed, the F2 plants show a wide range of heights; the majority of F2 are like P1 and F1, but
approximately 4 of 1000 are only 12 cm tall and 4 of 1000 are 36 cm tall. What fraction of the F2 plants will
be 27 cm in height? [Assume that for the genes involved in determining plant height, each allele contributes
the same amount.]
A. 3/4
B. 9/16
C. 56/256
D. none
E. cannot be determined from the information given

schinker96
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Joined: Tue Dec 18, 2012 1:18 am
E is what i think because i see not really enough info given or it is C

ashanair70
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Posts: 4
Joined: Tue Dec 18, 2012 12:13 am

### Re: Plants - genetics

Answer is C. But I don't know how to do this calculation. Can someone explain?

JackBean
Inland Taipan
Posts: 5694
Joined: Mon Sep 14, 2009 7:12 pm
4 in 1000 is the same as 1 in 250, which corresponds to 1 : 8 : 28 : 56 : 70 : 56 : 28 : 8 : 1 (from the Pascal's triangle http://en.wikipedia.org/wiki/Pascal%27s_triangle )
that is 9 options for 24 cm difference (36 - 12), thus the consequtive steps are of 3 cm. Thus 27 cm will be 56 in 256
http://www.biolib.cz/en/main/

Cis or trans? That's what matters.

ashanair70
Garter
Posts: 4
Joined: Tue Dec 18, 2012 12:13 am

### Re: Plants - genetics

I tried my best to understand Jack Bean's answer but could not.

Can some one explain it in a simpler way?

JackBean
Inland Taipan
Posts: 5694
Joined: Mon Sep 14, 2009 7:12 pm
You know that the number of offspring is shown by Pascal's triangle, right?
For two alleles you have 1 : 2 : 1
For three alleles 1 : 3 : 3 : 1
For four 1 : 4 : 6 : 4 : 1
etc.
So, if 4 in 1000 are 12 and 36 cm tall, that means that every in 250 plants is tall like that. And on which level of Pascal's triangle you have sum of 250?
For two alleles: 1 + 2 + 1 = 4
For threee: 1 + 3 + 3 + 1 = 8
For four: 1 + 4 + 6 + 4 + 1 = 16
etc. until 1 : 8 : 28 : 56 : 70 : 56 : 28 : 8 : 1
Here you see that you have 9 options of height. The difference is 36 cm - 12 cm = 24 cm. 24 cm /9 = 3 cm, so 1 plant will 12 cm, 8 plants will be 15 cm, 28 plants will be 18 cm, 56 plants will be 21 cm, 70 plants will be 24 cm (i.e. "the majority of F2 are like P1 and F1") and 56 plants will be 27 cm tall out of 250 plants. Thus C is the closest.
http://www.biolib.cz/en/main/

Cis or trans? That's what matters.

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