In Drosophila melanogaster, black body (b) is recessive to...

In Drosophila melanogaster, black body $(b)$ is recessive to gray body $\left(b^{+}\right),$ purple eyes $(p r)$ are recessive to red eyes $\left(p r^{+}\right),$ and vestigial wings $(v g)$ are recessive to normal wings $\left(v g^{+}\right) .$ The loci encoding these traits are linked, with the following map distances: (FIGURE CANNOT COPY) The interference among these genes is $0.5 .$ A fly with a black body, purple eyes, and vestigial wings is crossed with a fly homozygous for a gray body, red eyes, and normal wings. The female progeny are then crossed with males that have a black body, purple eyes, and vestigial wings. If 1000 progeny are produced from this testcross, what will be the phenotypes and proportions of the progeny?

Key Concepts

Genetic Linkage

Genetic linkage refers to the tendency of genes that are located close together on the same chromosome to be inherited together. This being the case, alleles at linked loci do not assort independently, and recombination frequencies between these genes (measured in map units) provide critical information about their relative positions on the chromosome.

Recombination Frequency and Map Units

Recombination frequency is a measure of the likelihood that a crossover event occurs between two loci during meiosis, which is expressed in map units or centiMorgans. These distances help predict the proportions of recombinant and parental types expected in offspring, playing a central role in genetic mapping and the analysis of linked traits.

Dominance and Recessiveness

The concepts of dominance and recessiveness explain how different alleles influence an organism's phenotype. A dominant allele will mask the effect of a recessive allele in heterozygotes, meaning that only recessive homozygous individuals display the recessive phenotype, which is vital in predicting outcomes in genetic crosses.

Testcross Analysis

A testcross involves breeding an individual with an unknown genotype to an individual that is homozygous recessive for the traits in question. This approach is used to reveal the unknown genotype based on the phenotype ratios observed in the offspring, particularly when analyzing the inheritance of linked or multiple traits.

Interference in Crossover Events

Interference is a phenomenon wherein the occurrence of a crossover event in one region of a chromosome reduces the probability of another crossover happening nearby. This adjustment impacts the expected ratios of recombinant offspring, making it a crucial factor to consider in genetic mapping and the calculation of crossing-over frequencies.

Transcript

00:01 Today we're talking about drosophilia again, which is a fruit fly.

00:07 They're commonly used in biology and genetics problems for a variety of reasons, mainly dealing with.

00:13 There's not really any laws about torturing them, and they're really easy to have reproduced quickly.

00:23 Okay, so we're giving some information.

00:26 Genetics problems are always a little scary, but you just need to slowly go through them.

00:30 I have typed out a bunch right here, the information that we need combined with the stuff i figured out.

00:39 Okay.

00:41 So from your textbook, i figured out that this is right here.

00:46 Sorry, i'm grabbing my.

00:58 Okay.

01:03 So this textbook, okay.

01:08 Moving on, it's asking us basically, we have recessive gray body, purple, eyes pr, red eyes pr plus, residual wings vg, and that those are recessive to normal wings, which are vg plus.

01:25 The loci encoding these straights is linked, and then your map distance is these units right here.

01:33 It's usually done as mu.

01:38 That's map units.

01:40 So our given interference is 0 .05, and we're trying to figure out if, a fly that has a black body purple eyes and vestigial wings, crossed with a homozygous for gray body, red eyes, and normal wings.

01:57 And then that female progeny is crossed with males that have a black body purpleized and vestigial wings.

02:02 Then we're trying to figure out if a thousand progeny are produced by this test cross, what will the phenotypes be and proportions of the progeny? okay.

02:15 So pause at any point, if i'm going to quickly.

02:17 Basically, we have to write down the information we need.

02:20 We know that the coefficient of coincidence is 0 .5 because it's just one minus interference, which is 0 .05, so it's 0 .05, i mean 0 .5, sorry, 0 .5.

02:37 As a recombinant frequency comes from both single and double crossover progenies.

02:42 First, we have to calculate the number of double crossover progeny.

02:47 The answer is 6 or 0 .06 % and for the bwpr adds 13 or 13 % and we know that the coefficient of coincidence is 0 .5, right? we did that earlier.

03:08 And so observed double crossovers over expected double crossovers.

03:14 We know that there's a thousand total.

03:16 0 .05 equals observed double crossovers.

03:18 Divided by 0 .06 divided, i mean, times 0 .13 times 1 ,000.

03:30 Okay? those are all underneath.

03:42 Extra parentheses, so you're not confused.

03:44 Parentheses are your friend.

03:46 We do that math.

03:48 Figure out 7 .8.

03:49 Observe double crossovers equals that.

03:51 3 .9, rounded to 4.

03:54 Best around genetic problems because they're theoretical anyway...