question on gene mutations

[gib65]

If a gene mutates in an offspring, will that mutation be permanent? Suppose you had a sheep that was born with two heads. If that sheep was to have offspring of his own, would those offspring have at least a 1 in 4 chance of inhereting an extra head?

 

Of course, evolution wouldn't work if mutations weren't permanent from one generation to the next, but I'm wondering if this is only true for some mutations, while others get "restored" back to the original DNA configuration after one generation.

[Bluenoise]

No there is no restoration in the sence you are talking about.

[Dak]

any mutations that are outside of the germline will not get passed on, so i suppose, in theory, if you got a mutation that occoured early during development you could see it's phenotypic effects, say, in the arm, but the mutation would not be in the gamete-producing cells, so it couldn't be passed on.

 

Other than that, proabably very rare, example, i don't think theres any mechanisms for once-only mutations.

[gib65]

So, then, would fetal alcohol syndrome be an example of a genetic mutation that occurs sometime during development - or is it a genetic mutation at all?

[Dak]

A quick google-scholar search indicates that ethanol interferes with developmental pathways and regulation, possibly by competetively inhibiting retinol, which is (usually) converted into retanoic acid, which acts as a developmental regulator: hence, ethanol = less retinoic acid = wonky regulation of development. there are a few other possible mechanisms, none of which involve genetic mutation as far as i can tell.

 

So no

[lucaspa]

If a gene mutates in an offspring' date=' will that mutation be permanent? Suppose you had a sheep that was born with two heads. If that sheep was to have offspring of his own, would those offspring have at least a 1 in 4 chance of inhereting an extra head?

 

Of course, evolution wouldn't work if mutations weren't permanent from one generation to the next, but I'm wondering if this is only true for some mutations, while others get "restored" back to the original DNA configuration after one generation.[/quote']

 

There is a difference here between "genetic mutation" and "sheep with 2 heads". The first is the genotype and the second is the phenotype -- how the animal looks.

 

You can get a phenotype without a genetic mutation. As Dak pointed out, retinoic acid is a potent regulator of development. There are several papers involving chick embryos where retinoic acid was injected into a limb bud during development. The result was two limbs. Two right legs or two left legs, depending on which side got the extra retinoic acid.

 

So, if there was some developmental anomaly, the sheep with two heads could be a phenotype and not a genotype.

 

Evolution works on genotypic changes -- changes in the sex cells -- that show up as phenotypes. However, it's much more complicated than usually presented. Most traits are NOT due to a SINGLE gene. They are the sum of the effects of several genes (also most genes have more than one effect). We tend to give the simple examples of one gene = one trait, but those are the exceptions, not the rule.

 

So, let's say there was a mutation in the sex cells that produced a sheep with 2 heads. Let's even assume that the mutation is in one gene to produce a different allele (form) of the gene. That allele, in combination with all the other particular alleles, produced a sheep with 2 heads.

 

When you are saying "1 in 4 chance" you are assuming that the allele is recessive, but also that the mate of the 2 headed sheep will also have that recessive allele. But the mate does NOT have that allele, does it? We have a new mutation and only 1 sheep has it.

 

So, it's possible that none of the offspring of that sheep and a 1-headed sheep would have two heads. If the mutation was dominant and worked only by straight Mendelian genetics, then the odds are that 1/2 the offsring would be 2 headed. But if recessive then none of the offspring would have 2 heads because there is no second recessive allele.

 

Instead, half the offspring would have the recessive allele to form 2 heads. The next generation might have another sheep with two heads -- when 2 one-headed sheep mated and combined the alleles necessary to give a 2 headed sheep.

 

But to answer your last comment: No, mutations do not get "restored". However, in the absence of strong positive selection pressure, they remain at very low frequency in the population. They may be eliminated by either negative selection or genetic drift (chance).