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#1
08-22-2012
 USMLE Forums Addict Steps History: Not yet Posts: 114 Threads: 23 Thanked 20 Times in 18 Posts Reputation: 39
Why do Cousins Share 1/8 of genes?

Hey please can anyone explain me (Kaplan notes) why siblings shares half and first cousin shares 1/8 and 2nd cousin shares 1/32 of their genes in this relations why every generation we multiply by 1/2 by 1/2?

Thank you
 The above post was thanked by: harry999 (08-25-2012), Lairinis (10-31-2013)

#2
08-24-2012
 USMLE Forums Scout Steps History: Not yet Posts: 89 Threads: 3 Thanked 49 Times in 35 Posts Reputation: 59

its hard to explain , hmm let me try,

generation 1 A and B (couple)

Generation 2 M and N are siblings ok .. they both share 1/2 genes ,, they marry outsiders , and produce the next gen

Generation 3 P and Q ( first cousins) P is child of M so it gets 1/2 of the genes from M and other half from its other parent ok..
similarly Q gets 1/2 of genes from N ( its parent) ...

so no of shared genes between P and Q would be 1/2 x 1/2 x 1/2 = 1/8..

similarly if u take 1 more generation , shared genes in 3rd gen were 1/8, 4th gen child 1 gets 1/2 from his parent and child 2 gets half from his parent.. so its 1/8 x 1/2x 1/2 = 1/32...

if u go to next gen it would be 1/32 x 1/2 x1/2 and so on..................

I tried my level best sorry if that was confusing......:sorry:
 The above post was thanked by: harry999 (08-25-2012), Saiyeda (10-22-2012)
#3
08-24-2012
 USMLE Forums Addict Steps History: Not yet Posts: 114 Threads: 23 Thanked 20 Times in 18 Posts Reputation: 39

realllly thannks but still confusiiing:d lool

#4
08-24-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 1,186 Threads: 119 Thanked 821 Times in 434 Posts Reputation: 831

Quote:
 Originally Posted by dr.nitowl its hard to explain , hmm let me try, generation 1 A and B (couple) Generation 2 M and N are siblings ok .. they both share 1/2 genes ,, they marry outsiders , and produce the next gen Generation 3 P and Q ( first cousins) P is child of M so it gets 1/2 of the genes from M and other half from its other parent ok.. similarly Q gets 1/2 of genes from N ( its parent) ... so no of shared genes between P and Q would be 1/2 x 1/2 x 1/2 = 1/8.. similarly if u take 1 more generation , shared genes in 3rd gen were 1/8, 4th gen child 1 gets 1/2 from his parent and child 2 gets half from his parent.. so its 1/8 x 1/2x 1/2 = 1/32... if u go to next gen it would be 1/32 x 1/2 x1/2 and so on.................. I tried my level best sorry if that was confusing......:sorry:
though its little bit complicated you explained well....actually the question asked the sharing of cousins not generations....i felt very complicated to dealt about cousins....if u have any idea plz explain it
#5
08-25-2012
 USMLE Forums Scout Steps History: Not yet Posts: 89 Threads: 3 Thanked 49 Times in 35 Posts Reputation: 59

Quote:
 Originally Posted by venky2600 though its little bit complicated you explained well....actually the question asked the sharing of cousins not generations....i felt very complicated to dealt about cousins....if u have any idea plz explain it
I was talking about cousins only.. .. i mentioned generations because i couldnt draw a pedigree chart.. without the chart i cant label cousins and their parents so i represented in generations....
when you are talking about cousins sharing genes we need to figure out the common ancestors thats the first generation .. when u have common ancestors in generation 1 , 3rd generation kids become first cousins, 4th generation kids are 2nd cousins .. so shared genes in generation is nothing but the cousin sharing...! ! ..
Its hard to explain like this in text hmm.. but ill give a final try ,

lets calculate how many genes you and your first cousin are sharing alright,
(Ps.these are all statistically calculated , we are talking about chance )

Similarly if you want to calculate how many genes your son and ur first cousins son will share , then,,
u preserved 1/8 genes of the pool , ur son randomly gets half of ur genes . and his second cousin gets 1/2 genes from ur first cousin so
1/8 x 1/2 x 1/2 = 1/32....

Thats it ! ! that was my last try , I promise I wont confuse any of my forum mates in future....
 The above post was thanked by: harry999 (08-25-2012), Lairinis (10-31-2013), Saiyeda (10-22-2012), sharmi_das (10-22-2012), venky2600 (08-25-2012)
#6
10-21-2012
 USMLE Forums Scout Steps History: 1 + CS Posts: 82 Threads: 10 Thanked 76 Times in 31 Posts Reputation: 86
Picture

Quote:
 Originally Posted by cage92 Hey please can anyone explain me (Kaplan notes) why siblings shares half and first cousin shares 1/8 and 2nd cousin shares 1/32 of their genes in this relations why every generation we multiply by 1/2 by 1/2? please help Thank you

Every generation you always split your genes, half from mom and half from dad that is why every generation is 1/2.
The easiest way to get how many genes someone in the family tree is related is (1/2)^n
to get n count the lines that connect them.

for Mom or dad 1 line connects you = 1/2 your genes are the same
For a sibling you have 1 lines to your sibling = 1/2 again
Grandparents would give you 1 line to mom/dad then a 2nd line to grandparents meaning n=2 (1/2)^2 = 1/2 x 1/2 = 1/4
Goes on like that-See picture attached for example with 2nd degree cousins.
You can use this method to count number of genes shared with any one in the family tree no matter what generation they are in. Hope it helps!
Attached Thumbnails

#7
10-21-2012
 USMLE Forums Scout Steps History: 1 + CS Posts: 82 Threads: 10 Thanked 76 Times in 31 Posts Reputation: 86
Won't let me edit previous post

Quote:
 Originally Posted by freebielady Every generation you always split your genes, half from mom and half from dad that is why every generation is 1/2. The easiest way to get how many genes someone in the family tree is related is (1/2)^n to get n count the lines that connect them. for Mom or dad 1 line connects you = 1/2 your genes are the same For a sibling you have 1 lines to your sibling = 1/2 again Grandparents would give you 1 line to mom/dad then a 2nd line to grandparents meaning n=2 (1/2)^2 = 1/2 x 1/2 = 1/4 Goes on like that-See picture attached for example with 2nd degree cousins. You can use this method to count number of genes shared with any one in the family tree no matter what generation they are in. Hope it helps!
Made a mistake with a picture that was for a different post. The previous picture = 1st degree half cousins.
I am now uploading pictures for 1st degree cousins and second degree
Attached Thumbnails

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