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Mendel's single-gene inheritance or not?

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

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Mendel's single-gene inheritance or not?

Postby hth610 » Fri Jan 15, 2010 5:18 am

"Mendel believed that a single gene was responsible for one single trait all by itself and now we know that many genes have control over the production of traits"

I am confused by the part "many genes have control over the production of traits"
If gene H determines one's height, so its alleles would be HH, Hh, and hh. these alleles are from one gene,
then how can many genes have control over height?

Please Help!
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Re: Mendel's single-gene inheritance or not?

Postby mith » Fri Jan 15, 2010 6:48 am

Because Hh and Bb are just simplifications for learning inheritance patterns in biology. Think about it, your DNA is coding for specific proteins not for specific traits.

Is there such a thing as a tall protein?
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Postby JackBean » Fri Jan 15, 2010 7:07 am

Basically, qualitative traits are usually coded by only one gene, whereas quantitative traits (as is height) are usually coded by several genes
http://www.biolib.cz/en/main/

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Postby hth610 » Sat Jan 16, 2010 8:48 pm

i am still confused..so if quantitative traits such as height is usually coded by several genes then what do the H, and h etc in the punnett square represent? alleles from one gene or several genes?
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Postby JackBean » Sat Jan 16, 2010 8:59 pm

of course of one gene. And another one can be e.g. D and d or T and t, but usually the real genes have a little longer names ;)
http://www.biolib.cz/en/main/

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Postby hth610 » Sat Jan 16, 2010 9:23 pm

so the gene coding for height could essentially have alleles t,i,d,a...etc.? and what does this have any connection with polymorphism?
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Postby JackBean » Sun Jan 17, 2010 12:17 pm

IMHO, polymorphism is other word for several alleles.
http://www.biolib.cz/en/main/

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Postby NeilC413 » Tue Feb 02, 2010 8:15 am

It's not that the "gene for height" has many different alleles, it is that there are many different genes that contribute to the height of the organism. Each of these genes has its own alleles and the combination of the different alleles (along with things like nutrition) determine the mature height of the organism.

So while there are traits which are determined by a single gene (like pea plant pod color), others are determined by a number of different genes. You could do a punnett square to determine offspring genotype probabilities but it would have to be bigger than the 2x2 square we use for single gene crosses. If you've done dihybrid crosses, the concept is the same, just expanded.

For instance, let's say there's 5 genes that contribute to height. We'll give them the letters A B C D and E. One possible combination in a diploid organism is AaBBccDdEE. Another is aabbccddEe, a third is AaBbCCDDEE. Let's say we had a plant that was heterozygous for all these genes (AaBbCcDdEe) and crossed it with itself. If you wanted to do a punnett square you would have to write all the possible gametes around the borders and you would end up with a total of 1024 squares!

I hope this helps (and that my math above is right... it's late). The main thing to understand is that while some traits are determined by a single gene, many are determined by several genes.

edit: Also you are correct that genes can have more than the two alleles commonly explored when learning Mendel's principles of inheritance.
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