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Pedigree chart question...

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

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Pedigree chart question...

Postby said212 » Sat Oct 13, 2007 9:26 pm

Image
Image

Based on these two pages can someone explain to me how generation 2 (4) is completely dominant and not heterozygous and how is generation 3(3) completely recessive when both parents are fully homo.
Im confuzzled and yeah. Any help is greatly appriciated...ive been scratching my head over this for a couple of hours :?
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Postby Jammerz » Sun Oct 14, 2007 5:31 am

Generation II, individual 4 is heterozygous. A shaded symbol means that the individual is affected and expresses the trait. The half-shaded symbol is generally used for recessive traits (see the picture you first posted, that's what it says).

Usually, when an individual expresses a trait, the symbol is fully shaded, especially if his/her genotype is unknown.

The reason for half shading is that, with a recessive trait, you can't tell solely by looking at phenotype whether someone is a carrier or not (they don't express the trait). With a dominant trait, someone with the dominant allele will always express the trait; you know that they must carry the allele of interest.

As a result, you can infer that individuals 4 and 5 of generation II are both heterozygotes, which is how they are able to have an unaffected (homozygous recessive) offspring, individual 3 of generation III.
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Postby said212 » Mon Oct 15, 2007 4:58 am

Thanks a bunch. Now it actually makes sense...
say i was to make a pedigree chart where diabetes is the recessive trait...
if the mother is normal by phenotype would the circle be shaded in or left empty cause in the normal/short sighted vision chart...being "affected" with normal vision meant a blank circle ?
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Re: Pedigree chart question...

Postby Revenged » Wed Oct 17, 2007 2:48 pm

said212 wrote:Based on these two pages can someone explain to me how generation 2 (4) is completely dominant and not heterozygous and how is generation 3(3) completely recessive when both parents are fully homo.
Im confuzzled and yeah. Any help is greatly appriciated...ive been scratching my head over this for a couple of hours :?


One more point to add...

In the first generation...

Ia - homozygous recessive (not short sighted)
Ib - heterozgous (short sighted)

It is not 'SS' as you wrote down in your diagram... if Ib was homozygous dominant (as you suggest) then all the second generation would be heterozygous (short sighted)... As 'SS' with 'ss' can only give 'Ss' children...
Last edited by Revenged on Wed Oct 17, 2007 2:58 pm, edited 2 times in total.
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Postby Revenged » Wed Oct 17, 2007 2:53 pm

said212 wrote:say i was to make a pedigree chart where diabetes is the recessive trait...


Firstly diabetes stands for many disorders... Diabetes mellitus (type I, type II), diabetes insipidus (cranial, nephrogeneic), MODY...etc.

And none of these are genetic recessive diseases... So please don't model diabetes on a pedegree chart!
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Postby whizzbee » Mon Oct 22, 2007 4:05 am

ok..but let's treat as if diabetes is a genetic disease. since said212 make it as a recessive trait then i think the mother's genotype can be homozygous (XX) or heterozygous (Xx)

in the case of the shortsightedness, it is a dominant trait, so one who is "affected" with a normal sight is symbolized as an empty blank.

(i'm just a beginner, correct me if i'm wrong), thanx..
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