Polygenic Inheritance

Hey all!!

I'm on the SNAB Bio A2 syllabus and I've got to the end of my notes and it's on Polygenic Inheritance which I'm a bit stumped by... if anyone else here is doing SNAB you'll understand how bad the SNAB book is, it's just waffle!

If anyone could clarify what it is and how it works (maybe with a worked example) that would be fantastic :smile:

Thank you very much :biggrin:

Reply 1

Mr. Pink

polygenic inheritance is when a particular characteristic is mediated by many genes (usually on different chromosomes). This means that the population will show contiunous variation for this characteristic, meaning that the characteristic cannot be easily grouped into distinct classes.

for example height: a person's height is controlled by a multitude of genes, therefore the members of a certain population will exhibit a continuous distribution on heights, not a just a few seperate heights.

An example of inheritance where only a few seperate heights is shown is in bean plants such as those studied by mendel. The plants were either tall, or dwarf, two distinct characteristics.

Reply 2

lovelee-jublee

Ok thank you, so I understand the explanation of it, but can anyone show me how it works by using the alleles, say H or h for height?

Reply 3

Mr. Pink

thats the thing for something to be due to polygenic inheritance, it means that there are a large number of genes contributing. its way beyond the scope of a level to demonstrate a polygenic genetic cross. so for height it might depend on like tons of different genes with each ones set of alleles. for mono/dihybrid inheritance, then its easy to show how height is inherited,
lets say that H is dominant and h is recessive. HH is tall, and Hh is tall, hh is short. the if two parents Hh and Hh produce offspring the offspring can be any of HH, Hh or hh, in terms of phenotype it will either be tall or short, in this case it only has a 25% chance of being short (try the genetic cross if you want).

Polygenic inheritance of height would have a whole bunch of genes involved

Reply 4

lovelee-jublee

Ah, ok. It's just in my book there's an example of a Punnet square, but to be honest I don't understand how they did it :confused:

It's hard to represent it here otherwise I'd show you.

Oh well, thanks for your help. As long as I know and understand the definition I'm sure that will be of use!

Thanks a lot :smile:

Reply 5

Mr. Pink

no problem was the punnet square perhaps using a 4x4 table, cause that would be dihybrid inheritance...

Reply 6

lovelee-jublee

Mr. Pink

no problem was the punnet square perhaps using a 4x4 table, cause that would be dihybrid inheritance...

No, it was 2 along the top, 4 down the side. The example it gives is
"A deep-blue eyed woman (Bb bb bB) has a child with light-brown eyes. The father is medium-blue eyed (Bb bb bb)."
The alleles on the top are Bbb and bbb, and down the side they are Bbb, BbB, bbb and bbB... I don't understand how they got that from the alleles of the parents :confused:

It's a really rubbish example but it's the only one they give!

Reply 7

Mr. Pink

that is weird, maybe on your syllabus they ask for that kind of stuff, but i think if you apply the rules for normal inheritance you can get it out... here's how:

first determine possible gametes that can be given from each parent, (i.e. the possible combinations of gametes using one from each pair) so for the mother

Bb bb bB
1st(pair) 2nd(pair) 3rd(pair)
B b b (possible gamete 1 - Bbb)
b b b (" " 2 - bbb)
B b B (3 - BbB
b b B (4 - bbB)

for the father

Bb bb bb
B b b (1 - Bbb)
b b b (2 - bbb)

then use the punnet square with maternal gametes on one side and paternal across the top. for each square inside the punnet square just put them together, so you get 3 pairs, the first pair, with the father first allele and mothers first, the second pair with the father's second allele and the mothers second allele etc.)
using the results from your square you can determine the probabilities of each color of eye etc.
i'm guessing it tells you what the conditions are for light brown eyes...

Reply 8

lovelee-jublee

^^^ Aaaah I get it!!! How you set that out and explained it made so much more sense than the book!!! So I just make the punnet square using the possible genotypes made from each parent, one parent on each side.

Thank you so much!!! :love:

Reply 9

Mr. Pink

no problem, i'd be happy to answer any other questions if you have them

mr. pink

Reply 10

lovelee-jublee

Thanks, I'll probably be posting some more up later when I go through my notes and don't understand something from this dreaded book! :p:

Reply 11

asianangel86

great!
now I'M confuddled.

Reply 12

lovelee-jublee

asianangel86

great!
now I'M confuddled.

What exam board are you on?

Reply 13

asianangel86

aqa; need to know it for tomorrows module 5!

Reply 14

Mr. Pink

strictly speaking the above example isn't polygenic inheritance as it requires many genes to contribute, the environment plays a role, AND continuous variation is shown, and in this case, clearly discontinuous variation is assumed. its essentially an extension on dihybrid inheritance

Reply 15

lovelee-jublee

Wanted to say than you once again Mr Pink, because a question on polygenic inheritance came up!!! Although we didn't have to do the punnet square, we had to describe what it was so your explanation helped :smile: