Why do Cousins Share 1/8 of genes? - USMLE Forums
USMLE Forums Logo
USMLE Forums         Your Reliable USMLE Online Community     Members     Posts
Home
USMLE Articles
USMLE News
USMLE Polls
USMLE Books
USMLE Apps
Go Back   USMLE Forums > USMLE Step 1 Forum

USMLE Step 1 Forum USMLE Step 1 Discussion Forum: Let's talk about anything related to USMLE Step 1 exam


Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 08-23-2012
USMLE Forums Addict
 
Steps History: Not yet
Posts: 114
Threads: 23
Thanked 19 Times in 18 Posts
Reputation: 38
Genetics 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?

please help

Thank you
Reply With Quote Quick reply to this message
The above post was thanked by:
harry999 (08-25-2012), Lairinis (10-31-2013)



  #2  
Old 08-24-2012
dr.nitowl's Avatar
USMLE Forums Scout
 
Steps History: Not yet
Posts: 89
Threads: 3
Thanked 48 Times in 34 Posts
Reputation: 58
Default

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:
Reply With Quote Quick reply to this message
The above post was thanked by:
harry999 (08-25-2012), Saiyeda (10-22-2012)
  #3  
Old 08-24-2012
USMLE Forums Addict
 
Steps History: Not yet
Posts: 114
Threads: 23
Thanked 19 Times in 18 Posts
Reputation: 38
Default

realllly thannks but still confusiiing:d lool
Reply With Quote Quick reply to this message
  #4  
Old 08-24-2012
venky2600's Avatar
USMLE Forums Master
 
Steps History: 1+CK+CS+3
Posts: 1,186
Threads: 119
Thanked 815 Times in 434 Posts
Reputation: 825
Send a message via Yahoo to venky2600
Post

Quote:
Originally Posted by dr.nitowl View Post
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
Reply With Quote Quick reply to this message
  #5  
Old 08-25-2012
dr.nitowl's Avatar
USMLE Forums Scout
 
Steps History: Not yet
Posts: 89
Threads: 3
Thanked 48 Times in 34 Posts
Reputation: 58
Default

Quote:
Originally Posted by venky2600 View Post
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 )

Your grandparents had a pool of genes, your dad and his bro have 50% of shared genes.. You inherit 50% of genes your dad had, similarly your first cousin will inherit some random 50% genes his dad had.... so by calculation of the chance of sharing the common pool between you and your first cousin will become 1/2 x 1/2 x1/2 =1/8

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....
Reply With Quote Quick reply to this message
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  
Old 10-21-2012
freebielady's Avatar
USMLE Forums Scout
 
Steps History: 1 + CS
Posts: 82
Threads: 10
Thanked 76 Times in 31 Posts
Reputation: 86
Default Picture

Quote:
Originally Posted by cage92 View Post
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
Why do Cousins Share 1/8 of genes?-consanguinity.png  
Reply With Quote Quick reply to this message
  #7  
Old 10-21-2012
freebielady's Avatar
USMLE Forums Scout
 
Steps History: 1 + CS
Posts: 82
Threads: 10
Thanked 76 Times in 31 Posts
Reputation: 86
Default Won't let me edit previous post

Quote:
Originally Posted by freebielady View Post
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
Why do Cousins Share 1/8 of genes?-1st-degree-cousins.png   Why do Cousins Share 1/8 of genes?-2nd-degree-cousins.png  
Reply With Quote Quick reply to this message



Reply

Tags
Genetics-

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the USMLE Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:
Medical School
Choose "---" if you don't want to tell. AMG for US & Canadian medical schools. IMG for all other medical schools.
USMLE Steps History
What steps finished! Example: 1+CK+CS+3 = Passed Step 1, Step 2 CK, Step 2 CS, and Step 3.

Choose "---" if you don't want to tell.

Favorite USMLE Books
What USMLE books you really think are useful. Leave blank if you don't want to tell.
Location
Where you live. Leave blank if you don't want to tell.

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes


Similar Threads
Thread Thread Starter Forum Replies Last Post
More protiens than there are genes? Yallah USMLE Step 1 Forum 3 11-29-2015 07:00 AM
Tumor Suppressor Genes cordam-12 USMLE Step 1 Mnemonics 5 06-30-2013 10:41 PM
Homeobox containing genes! dr akram USMLE Step 1 Forum 4 08-20-2011 02:56 AM
immunoglobulin genes doctorF USMLE Step 1 Forum 4 04-07-2010 09:57 PM
Tumor Suppressor Genes YoramFishke USMLE Step 1 Forum 1 10-13-2009 11:02 PM

RSS Feed
Find Us on Facebook
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.

USMLE® & other trade marks belong to their respective owners, read full disclaimer
USMLE Forums created under Creative Commons 3.0 License. (2009-2014)