Question

How many loci are responsible ... (picture attached)

Answer 1

We are talking about just one trait, so how many loci do you think are involved?

Answer 2

I'm not really sure what a loci is..
i am thinking one locus represents 1 trait?

Answer 3

oh, and i need t oeat dinner now
srry! i will come back as soon as possible if u are still here

Answer 4

Enjoy your dinner, we will resume this afterward. ;D

Answer 5

I'm back :) waiting for u

Answer 6

Have you given any thought to how many loci are likely responsible for this trait?

Answer 7

no idea ... never learn this thing... and is my first time to see this word "loci“...

Answer 8

Cool, a locus is literally a location on a chromosome. It is sometimes used synonymous with the term 'gene' or 'piece of DNA of interest.' So the question is actually asking how many genes control eye color in flies. Clear?

Answer 9

yea i get what u said

Answer 10

but a color can have more than 1 gene control?

Answer 11

Yes, it happens. Sometimes whether a gene is expressed is itself controlled by the expression of a different gene, in which case two genes are responsible...

Answer 12

oh...then how do we find out ... using genotype?

Answer 13

Yes, exactly.

Most of the wild type flies have red eyes, so whatever gene or genes are responsible for white eyes we can probably associate red with the dominant phenotype and white with the recessive phenotype. Emphasis on phenotype, not at this point genotype. Clear?

Answer 14

but the phenotype is just white..u cant really tell whether it is 1 , 2 , or 3 loci.. sorry ..still kinda confused

Answer 15

since red has 9, white has 7, so red has more loci than white..?

Answer 16

Trust me, we are working that way. : D

The assumption we make about the dominance / recessiveness of the phenotypes is a hypothesis which we test out using Punnett squares. Can you see why, given the initial info about many red eyed flies and a few white eyed flies, we make that assumption?

Answer 17

yes, red has be hypothesize as the dominant trait because more red eye is produced than white eye

Answer 18

Cool.

The hunch about red = dominant and white = recessive is supported by the F1 cross. White eyed flies crossed with red eyed flies produce all red eyed offspring.

Then we look at the F2 crosses and note that all result in the same ratio, and that all those ratios are roughly equal to 1/2. Clear thus far?

Answer 19

yes, clear so far

Answer 20

Now the fun begins.

The simplest approach is to test the simplest case (that there is one gene driving eye color) and see if it works. Because the ratio is so nice and even, we suspect that this is the case: ratios of 1/2 or 1/4 tend to come up in one gene crosses...

Answer 21

but the ratio is actually 7/9 ..is it still a nice number..

Answer 22

One of the key concepts in genetics is "close enough." Biology never gives you the exact ratios you expect; there are statistical tests called "Chi square procedures" which let you measure when close is close enough and when it isn't, but I'm not going to get in to that.

Answer 23

oh.. so roughly say it is 1/2
but do u know what is the total loci responsible for the eye color trait
or what does 1/2 exactly mean..?

Answer 24

like 2 loci for red and 1 locus for white...?

Answer 25

Nope. The 1/2 is the proportion of F2 flies which have each eye color, not the number of loci driving eye color. Then we start thinking about cases where we see 1/2 phenotypic ratios and the answer is, when heterozygous individuals are crossed with homozygous recessive individuals.

That is Ww x ww = 1/2 Ww and 1/2 ww. You can draw out a Punnett square to convince yourself of that if you like...

Answer 26

i know that :)
i understand what u've said

Answer 27

Cool, so the next question to ask is, is it possible under the one locus assumption for there to be 1/2 Ww and 1/2 ww flies in the F2 generation?

Answer 28

no... i guess heterozygous and homozygous has a different locus?

Answer 29

Heterozygous refers to organisms which have different alleles (i.e., one dominant and one recessive) of the same gene at the same locus but on different chromosomes. Homozygous refers to organisms which have the same allele (either both dominant or both recessive) of the same gene at the same locus on different chromosomes.

Answer 30

So if the parent generations were WW and ww, all the F1s are Ww. When you cross F1s with each other the cross is Ww x Ww = (3/4) red and 1/4 white. Which is not what you want to see, is it?

Answer 31

Are you there?

Answer 32

yea, thinking it in my brain lol

Answer 33

Cool, take your time.

Answer 34

and yes i get it
cuz u want Ww x ww , but not Ww x Ww ?

Answer 35

Yes, indeed. So we throw out the one locus hypothesis. But it is usually worth it to try the simplest case first.

The next thing I'd do is think about the biochemical conditions under which white eyed flies are born. Eye pigments are probably formed in a series of reactions catalyzed by a series of different enzymes. Mutations in any one or more of the enzymes would probably change red color to white...

So that suggests a second approach: perhaps the three different strains of true breeding white eyed flies have that phenotype because they have different mutations in different loci. Within each strain, one locus is responsible. But different loci are responsible the phenotype in different strains.

Clearish?

Answer 36

yess, i get it

Answer 37

So when two different strains are crossed, all the resulting offspring are heterozygous for the mutant loci of both their parents. That is, the flies from the strain with the white allele at mutant A get a wild type allele of that gene from strain B, while strain B gets a wild type allele of the mutant gene B from strain A.

Take your time with that one if you need to.

Answer 38

originally the loci are in the same position on the chromosome (because of the same phenotype)
and u say the loci change because of mutation, and it results in a different phenotype?

Answer 39

Yes, mutant genes like the white allele come about through mutations in one chromosome which are passed on to offspring.

Answer 40

"wild type allele" means a different allele or loci from the parent?

Answer 41

The two alleles occupy the same position on the chromosome - it's just that they differ in their DNA sequences. One is wild type, the other is mutant. Same gene. Same location, different allele.

Answer 42

I think i get what u said

Answer 43

The F1 flies are heterozygous for both alleles. At the first locus they have a wild type allele from the parent which was normal for that and one mutant allele from the parent which was not normal for that. The same at the second locus. Clear?

Answer 44

yes

Answer 45

So when they cross the F1 flies with each other, they get the expected ratio 9:3:3:1 dihybrid ratio. The 9 flies have the dominant phenotype. The heterozygous flies in the two 3 groups have two different genotypes (homozygous recessive for different mutant alleles) but the different genotypes produce the same phenotype, white eyes. And of course the fly recessive for both traits has white eyes.

So 9 flies have red eyes. 3 + 3 + 1 flies = 7 flies have white eyes.

Bingo. I think that is what you're looking for.

And because there were three different strains of flies and the crosses of all different strains produced the same ratio, there were mutations at three different loci.

I am sorry that it took me a while to get that.

Answer 46

I think it will take me quite a while to fully understand this .... dont need to wait for me :D
and thank you so much
I seem like i need to read ur answer all over again lol

Answer 47

Read it over. I have to go but I'll check back later (i.e., a few hours). If you are confused, it ALWAYS helps to draw Punnett squares. If only because you get to have a break from thinking while drawing straight lines on a grid. :D

Answer 48

omg, dont know how many times i should say thank you, but thanks blues!!
I'm reading it over now and it is so much more clear :)