# Punnett Square Help Pls (DH Snow Boas)



## piehunter (Jul 10, 2007)

Hey Guys,

Been trying to teach myself the punnett square method, but i'm a bit puzzled by one thing......

You put letters corresponding to each pair of genes along each side. I can't work out how the letters have been allocated on the Ralph Davis site.

This is the example of a DH Snow pairing. Ralph Davis Link

---*---MR-----Mr-------mR-------mr*
*MR*--MMRR----MMRr----MmRR------MmRr
*Mr*--MMRr-----MMrr----MmRr-----Mmrr
*mR*--MmRR----MmRr----mmRR----mmRr
*mr*--MmRr-----Mmrr-----mmRr---mmrr


I'm really confused with the allocation of letters (around the outside) to gene mutations/types, could someone explain a little further for me please? 

Thanks


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## alan1 (Nov 11, 2008)

everyone is different , but this is why i do it 'my way'...

your way...

---*---MR-----Mr-------mR-------mr*
*MR*--MMRR----MMRr----MmRR------MmRr
*Mr*--MMRr-----MMrr----MmRr-----Mmrr
*mR*--MmRR----MmRr----mmRR----mmRr
*mr*--MmRr-----Mmrr-----mmRr---mmrr

my way...

---*---MR-----M-------R-----N*
*MR*--MMRR---MMR----MRR---MR
*M*----MMR----MM-----MR----M
*R*----MRR-----MR-----RR-----R
*N*----MR------M-------R-----N

same thing, but soo much easier to read
all i do is remove the 'small case'


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## DavidStaffs (Jul 13, 2009)

With the punnett square technique you can really pick any letters you want to correspond to the given trait but people will often pick specific ones like M and R as the capital and small case letters look very different i.e. M m and R r where as things like s' look similar even as capital and small case i.e S s (not so bad when typed but when hand written can become very confusing) 

The second important thing to note is that a capital letter always indicates a dominant allele and a small case a recessive allele of the gene and thats why its important to know whether its a dominant or recessive trait/gene

Anyway as i was saying you can really pick any letter you choose but this is why its best to include a key as to what each letter means ...... (which i cant see on the link at all though dont wish to malign anyones work lol) 

Now i dont know anything really about boas or their genetics but from looking at the link to ralph davis site i'd say the two traits you're looking at are albino and anerythristic (tell me if i;m wrong lol) anyway it would appear that both traits are recessive so will only be phenotypic (i.e. will be a visible trait) when the animal has both recessive alleles and is whats called heterzygous recessive ........ in this case it appears the letter M denotes the albino gene so mm is a visual albino and the letter R denotes the anerythristic so rr is visual anerythristic ...... if would appear your snow boa is an albino anerythristic (again tell me if i'm wrong lol) and there fore must be both heteryzygous recessive for albino (mm) and heterzygous recessive for anerythristic (rr) so the offspring with the gene alleles mmrr will be the visual for both and a visible baby snow boa

you can see from the link that its a 1 in 16 ratio so for every 16 babies produced only one is statistically likely to be snow ..... although we all know it never works like that in real life 

hope that has helped a bit and not confused you even more lol and shows why its always useful to include a key for your punnett squares ........... wouldnt my biology teachers be proud haha


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## paulh (Sep 19, 2007)

Ralph Davis does not use the standard gene symbol rules that were taught me when I worked in university genetics lab. Oh well, I'll stick with his for this explanation.

A DH snow X DH snow mating. This is a two gene pair problem. Snow is a combination of albino and anerythristic. DH for snow is heterozygous albino and heterozygous anerythristic. Heterozygous albino means a normal gene (M) paired with an albino gene (m). Heterozygous anerythristic means a normal gene (R) paired with an albino gene (r). So both parents have Mm Rr gene pairs. 

By the way, the M normal gene and the R normal gene are different genes. The normal gene is the most common gene in a given gene pair in the wild population of snakes. Different gene pairs have different normal genes.

Each sperm cell gets one gene from each of the father's gene pairs. Half of the sperm cells get an M gene, and the other half of the sperm cells get an m gene. And half of the sperm get an R gene, and the other half of the sperm get an r gene. So each sperm cell gets either an M gene or an m gene, and each sperm cell gets either an R gene or an r gene. Whether a given sperm cell gets an M gene or an m gene has no effect on whether the same sperm cell gets an R or an r gene. 

If an M gene is in a given sperm, then either an R gene or an r gene is in the same sperm. (Disregard the dots in the following diagram. They are just to keep the R and r genes to the right of the M and m genes.)

..........R = sperm with an M gene and an R gene
M <
..........r = sperm with an M gene and an r gene

If an m gene is in a given sperm, then either an R gene or an r gene is in the same sperm.

..........R = sperm with an m gene and an R gene
m <
..........r = sperm with an m gene and an r gene

So the father has four different combinations of genes (genotypes) in his sperm -- M R, M r, m R, or m r. Each genotype is present in equal numbers of sperm cells.

And each baby gets one gene from each of the mother's gene pairs. The mother has four different genotypes in her eggs -- M R, M r, m R, and m r. Each genotype is present in equal numbers of egg cells. The reasoning is the same as the one used to assign genotypes to the father's sperm cells.

Each type of sperm cell has an equal chance of fertilizing each type of egg cell. An M R sperm could fertilize either an M R egg or an M r egg or an m R egg or an m r egg. An M r sperm could fertilize either an M R egg or an M r egg or an m R egg or an m r egg. And so on with an m R sperm and an m r sperm.

A baby snake comes from the fertilization of an egg by a sperm. This fertilization reestablishes the gene pairs in the baby snake.

A Punnett square is a tool to diagram all the possible outcomes in the babies. The 4 types of sperm are at the left side, and the four types of eggs are on the top. Each box in a row gets genes in the sperm at the left side of the row. And each box in a column gets the genes in the egg cell at the top of the column. 

A decent genetics text does a better job of explaining all this than any herper genetics web site that I have seen. Hope this helps.


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## DavidStaffs (Jul 13, 2009)

Just realised in the above i said mm and rr are heterozygous recessive ......... sorry should say homozygous recessive (**** meaning the same)


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