Suppose that a geneticist discovers a new mutation in...

Suppose that a geneticist discovers a new mutation in Drosophila melanogaster that causes the flies to shake and quiver. She calls this mutation quiver, qu, and determines that it is due to an autosomal recessive gene. She wants to determine whether the gene encoding quiver is linked to the recessive gene for vestigial wings, vg. She crosses a fly homozygous for quiver and vestigial traits with a fly homozygous for the wild-type traits, and then uses the resulting F1 females in a testcross. She obtains the flies from this testcross. Phenotype Number of flies vg$^+$ qu$^+$ 230 vg qu 224 vg qu$^+$ 97 vg$^+$ qu 99 Test the hypothesis that the genes quiver and vestigial assort independently by calculating the chi-squared, $X^2$, for this hypothesis. Provide the $X^2$ to one decimal place. $X^2$ = Incorrect

Transcript

00:01 With an f1 generation who is heterozygous, meaning they have a dominant in a recessive of alleles at both genes, we're going to be doing a test cross with them, which means that the person or the individual that we're crossing them with is completely recessive.

00:18 We do this in a test cross, so that way only the genes of the f1 generation come through.

00:26 Because our test cross can only give recessive alleles.

00:29 And we know that recessive alleles don't determine phenotype.

00:32 Only the dominant ones do, whether the dominant one is present or not.

00:36 So since the f1 individual has half dominant alleles and half not, that is the one that will tell us, you know, how alleles are being distributed and what the phenotypes are for the f2 generation.

00:52 So then when we look at the f2 generation, we know that we're really looking at how the f1 alleles were distributed.

00:59 And we're going to list the offspring in the f2 generation in order from the most common to the least.

01:05 So you see i started with the 230.

01:08 I went all the way down to the least common, 97.

01:10 And there's a total of 650 offspring here.

01:14 Now, when i look at these numbers, i know that these numbers here of how many offspring we have of each phenotype are considered observed numbers.

01:24 Right? this is what we are told.

01:26 This is what we are observed.

01:27 However, we can also compare these to the expected numbers to do each high score table to see how closely our observed numbers and our expected numbers are.

01:38 Now, when we do a test cross like this where the f1 individual is heterozygous and our test cross is completely recessive, then we would expect a one to one to one ratio, meaning that there are four different phenotypes, two, three, four, and each of them happen in equal measure.

01:56 So i would expect 25 % to have both dominates, 25 % to have neither, 25 % to have one and 25 % to have just the other.

02:07 So out of 650, if i take that 25 % and i multiply it by 650, i would see that 25 % would be 162 .5...

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