doodle_333
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I'm not a student but a learning support assistant and something was taught in a lesson I was in today that makes no sense to me, I want to either be able to understand why it is true so I can help the kids understand or to know it is false so I can make sure the kids are being fed wrong information

basically we were looking at genes and the question was: cystic fybrosis is a recessive gene , a man has cystic fybrosis (so has two faulty genes) and a woman is a carrier (so one faulty one normal gene) and they have two children who are both carriers... they have a third child, what is the probability the child has cystic fybrosis

now my understanding was that the probability of each child having the disorder would remain (at year 9 biology level at least) fairly constant so each child would have a 50/50 chance regardless of how many children were born (in the same way as you pretty much have a 50/50 chance of flipping a coin and getting heads even if you just got heads 10x in a row) but the teacher insisted it was a 75% chance?
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Eloades11
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(Original post by doodle_333)
I'm not a student but a learning support assistant and something was taught in a lesson I was in today that makes no sense to me, I want to either be able to understand why it is true so I can help the kids understand or to know it is false so I can make sure the kids are being fed wrong information

basically we were looking at genes and the question was: cystic fybrosis is a recessive gene , a man has cystic fybrosis (so has two faulty genes) and a woman is a carrier (so one faulty one normal gene) and they have two children who are both carriers... they have a third child, what is the probability the child has cystic fybrosis

now my understanding was that the probability of each child having the disorder would remain (at year 9 biology level at least) fairly constant so each child would have a 50/50 chance regardless of how many children were born (in the same way as you pretty much have a 50/50 chance of flipping a coin and getting heads even if you just got heads 10x in a row) but the teacher insisted it was a 75% chance?
What was the reasoning behind the 75% chance?

Regardless of whether the first two children had CF or not, the probability of the third having CF remains 50%, independent of previous results.
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Chlorophile
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(Original post by doodle_333)
I'm not a student but a learning support assistant and something was taught in a lesson I was in today that makes no sense to me, I want to either be able to understand why it is true so I can help the kids understand or to know it is false so I can make sure the kids are being fed wrong information

basically we were looking at genes and the question was: cystic fybrosis is a recessive gene , a man has cystic fybrosis (so has two faulty genes) and a woman is a carrier (so one faulty one normal gene) and they have two children who are both carriers... they have a third child, what is the probability the child has cystic fybrosis

now my understanding was that the probability of each child having the disorder would remain (at year 9 biology level at least) fairly constant so each child would have a 50/50 chance regardless of how many children were born (in the same way as you pretty much have a 50/50 chance of flipping a coin and getting heads even if you just got heads 10x in a row) but the teacher insisted it was a 75% chance?
If we call the recessive (CF causing) gene f and the dominant (not CF causing) gene F, then the father has ff and the mother has Ff. Therefore, the 4 possibilities for children - each having a 25% chance - are Ff, Ff, ff, ff. ff causes CF, so the probability of a child having CF is 25% x 2 = 50%. So in this situation, the chance is indeed 50%.
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Mule
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Yes it's an independent event, having or not having a child with CF does not affect whether the next child will have one, so I agree I think it should be 50%.
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geneticist
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I'm not sure if it's mendelian.There might be a different inheritance pattern.I attached an example of eye color inheritance in drosophila (fruit fly).In male drosophila y chromosome doesn't carry genes related to somatic traits/characters.But if it's mendelian your teacher is wrong.

(Btw the cross is red×red and W_ phenotype is red w_ phenotype is white, red eye probability is % 75)
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doodle_333
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(Original post by Eloades11)
What was the reasoning behind the 75% chance?

Regardless of whether the first two children had CF or not, the probability of the third having CF remains 50%, independent of previous results.
in all honesty I don't think anyone in the room including me understood what on earth the teacher was talking about, I didn't want to challenge her directly and undermine her without being sure since she has a degree and I only have a GCSE in bio

it was something along the lines of there being 4 possibilities like chlorophile stated but because two healthy children have been born they have 'used up' the carrier options? it made literally no sense to me

(Original post by Chlorophile)
If we call the recessive (CF causing) gene f and the dominant (not CF causing) gene F, then the father has ff and the mother has Ff. Therefore, the 4 possibilities for children - each having a 25% chance - are Ff, Ff, ff, ff. ff causes CF, so the probability of a child having CF is 25% x 2 = 50%. So in this situation, the chance is indeed 50%.

(Original post by Mule)
Yes it's an independent event, having or not having a child with CF does not affect whether the next child will have one, so I agree I think it should be 50%.
okay good so I'm not being thick! I thought that was how it worked, thanks, I will try and bring it up, these are struggling kids they don't need their grasp on basics like probability challenged and confused
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Eloades11
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(Original post by geneticist)
I'm not sure if it's mendelian.There might be a different inheritance pattern.I attached an example of eye color inheritance in drosophila (fruit fly).In male drosophila y chromosome doesn't carry genes related to somatic traits/characters.But if it's mendelian your teacher is wrong.

(Btw the cross is red×red and W_ phenotype is red w_ phenotype is white, red eye probability is % 75)
There's no difference in inheritance pattern, Cystic Fibrosis in recessive and the gene isn't located on the chromosomes, otherwise the outcomes would be different depending on which parent had CF. Also, I don't think they would explain that in secondary school level at risk of it being too confusing
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