In the snail Limnaea peregra, shell coiling is determined by...

In the snail Limnaea peregra, shell coiling is determined by the nuclear alleles S (dextral) and s (sinistral). This gene is a maternal effect gene that influences egg formation, which in turn influences whether the snail that hatches from the egg has a shell that coils to the right (dextral) or left (sinistral). A female sinistral snail is mated with a sinistral sperm-donor snail. The genotypes of both are unknown. All the F 1 progeny are dextral. When the members of the F 1 generation are crossed, they yield an F 2 generation that is 3/4 dextral and 1/4 sinistral. What was the genotype of the mother of the female snail? Ss or ss SS or Ss Ss ss SS

Transcript

00:01 Alleles can interact in several different ways.

00:03 We're going to look at some of the more basic ways that alleles can interact on a singular gene.

00:09 So we're not talking about epistasis.

00:11 We're not talking about multiple genes, just on one gene.

00:17 So the first thing we're going to look at is complete dominance.

00:22 This is classic mandalian genetics kind of stuff.

00:30 So in complete dominance, you have a dominant allele.

00:36 And a recessive allele.

00:39 And if you were to cross heterozygous individuals, you would find the classic punnet square where, you know, a one out of four chance of being homozygous dominant, you have a two out of four chance of being heterozygous like the parents, and finally a one out of four chance of being homozygous recessive.

01:11 What you end up with is here three, three, three out of four chances for the offspring to have the dominant trait and only one out of four to have the recessive trait.

01:28 And just to show you something else as well, if we were to cross, say, one parent that's homozygous dominant, one that's homozygous recessive, you would end up with 100 % of the offspring being heterozygous.

01:43 And so that's some of the stuff that you would look for with complete dominance.

01:48 Now, another option is co -dominants.

01:51 This is where you have two dominant alleles at the same g, at the same locus.

01:57 And they interact in a way where i'm going to do this little cross again.

02:11 So you see the same kind of stuff here where one out of four have two r alleles.

02:20 And so they would have a certain color.

02:25 Another quarter would have these two white, two w alleles and be white in color.

02:32 But if you're heterozygous with co -dominance, you're going to have something in the middle.

02:39 So if we mix red and white, you're going to get pink.

02:45 So with co -dominates, you would expect to see some kind of in -between option.

02:50 So if cats can have a short or a long tail, you know, and there's co -dominance with these alleles, you might have some cats with a medium -length tail.

03:02 Just to show you another option here, if we do the same thing where we have parents, one is homozygous for the r -ales, one is homozygous for the white alleles, the w, you would end up with 100 % heterozygous individuals, just like you did with a complete dominant.

03:24 But with complete dominance, all of those heterozygotes had the dominant phenotype.

03:36 Whereas in this case, with co -dominants, heterozygotes will have the co -dominant phenotype, meaning the in -between, the new phenotype.

03:51 Now there is something interesting, there is something sort of in -between where you have complete dominance and co -dominants.

04:02 It's not seen at a lot of genes that we look at, but an example is, say, blood types.

04:16 If you don't know, four blood types, you can have three different alleles possible at that locus...

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