In Drosophila, males from a true-breeding stock with raspberry-colored eyes were mated to female

In Drosophila, males from a true-breeding stock with...

Key Concepts

Genetic Mapping

Genetic mapping is a technique used to determine the relative positions of genes on a chromosome by calculating recombination frequencies. The frequency of recombinant phenotypes—expressed as a percentage—can be used to estimate the map distance between genes. A lower recombination frequency indicates that the genes are closer together, while a higher frequency suggests greater physical separation along the chromosome.

Recombination and Crossing Over

Recombination is the process by which chromosomes exchange segments during meiosis, leading to new combinations of alleles. In organisms like Drosophila, crossing over only occurs in female meiosis, which significantly impacts the distribution of recombinant and parental phenotypes in offspring. Analyzing the frequency of these recombination events allows researchers to infer the physical distances between linked genes.

Sex-Linked Inheritance

Sex-linked inheritance refers to the transmission of genes located on the sex chromosomes, typically the X chromosome in Drosophila. In these cases, males, being hemizygous (having only one X chromosome), often display the phenotype associated with the allele present, whereas females (with two X chromosomes) may display the dominant phenotype if they carry one wild?type allele. This pattern of inheritance explains differing phenotypes between males and females in genetic crosses.

Drosophila Genetics

Drosophila melanogaster is a model organism widely used in genetics to study inheritance, gene function, and mapping. Its unique features—such as a relatively simple genome, distinct sex chromosomes, and the fact that recombination occurs only in females—make it ideal for dissecting genetic crosses and understanding how traits are passed from one generation to the next.

Gene Linkage

Gene linkage occurs when two or more genes are located close together on the same chromosome and tend to be inherited together during meiosis. The closer the genes are in physical proximity, the lower the chance that a recombination event will separate them. This concept is crucial for interpreting deviations from expected independent assortment ratios in genetic cross experiments.

Transcript

00:01 Okay, this question here says, as we learn in this chapter the y mutation of the zoophile started by thomas hunt, morgan is x -linked and is asymptotoy type.

00:08 So in this case, let's use this allele here, okay? so you have here the females and you have here males.

00:18 So in case of females you have xx and in case of males you have xy.

00:23 So in this case, you have these possibilities and it says that, well let me complete here for males, xy and like this.

00:37 So it says that the y mutation is recessive to wild type.

00:43 So this one is the wild allele and this is not the wild, this is the wild type allele, okay? so wild type is red.

00:49 So in this case, this one is going to call for red eyes in females and this is red eyes in males and this one is going to be white eyes in females and this one is going to be white eyes in males.

01:04 And it says, we're not going to fill this yet, okay? so it says, when true breeding white eyed males, it means white eyes is like this in males, okay? when a true breeding white eyed males can in this mutation were crossed with true breeding purple eyed females, all of the f1 progeny, so you have versus this one here, we don't know what is the genotype or the phenotype for purple.

01:35 And it says that all of the f1 progeny had wild type red eyes.

01:41 Okay, so apparently because of the information given, this is not involving only one gene.

01:47 Apparently you have two genes here, okay? so in this case, let's just delete this and well, let's continue solving here.

01:58 You're going to have a b gene too, okay? so first we have to solve here or to look at the ratios that you found in bf2.

02:08 So you have three eights that are going to be wild type females.

02:17 Then you have one quarter for wild type males.

02:26 You have three sixteenths for wild type male.

02:36 Then you have one eight for purple eyed females.

02:46 And one sixteen for purple eyed males.

02:52 So what you have to do here is to is to look at this ratios as if they are out of something, okay? it means just add the wild type females in the wild type males, also the wild males with the wild females and the purple eyed males.

03:09 And also let's give the denominator 16 because when you have two genes in a type hybrid cross you're going to get 16 possibilities.

03:17 So here you're abbreviating the ratios, okay? so instead of having one quarter you're going to have if you multiply by four you're going to get four sixteenths.

03:30 Then in this case you have to multiply by two so you have two sixteenths and that's it, okay? so for example now, oh well, and in this case you have to multiply by two also so you have six sixteenths.

03:46 So in this case wild type females with wild type males you have a total of nine sixteenths.

03:56 Then you have purple eyed females and purple eyed males.

04:03 If you add them you're going to get three sixteenths.

04:07 And finally you have four sixteenths for white males.

04:12 It means four sixteenths like this, okay? so as you can see here you have a phenotype of 934 and if you remember this normally in a type hybrid cross you get 9331.

04:26 But in this case what is happening? these three and one are getting together because normally these nine are for dominant a, dominant b.

04:34 Three are for dominant a, recessive b.

04:36 And three is for recessive a, dominant b.

04:39 And one is for recessive a, recessive b.

04:40 But in this case you are adding these three and one and producing a four.

04:45 So practically there is something called recessive epistasis, okay? whenever you have that recessive a it doesn't matter what you have at the beginning because you're going to have the recessive phenotype directly.

05:02 So this is practically what is happening here.

05:03 You have a case of recessive epistasis and it says formulate a hypothesis to explain the inheritance of these colors.

05:09 So let's change this a little bit, just the opposite.

05:15 That you have recessive a, dominant b.

05:20 You have dominant a here, recessive b and that's it.

05:24 This is practically the same, okay? so in this case as you can see here if we suppose that this is like an autosomal gene you would have that.

05:34 A homozygous dominant and a heterozygous for a are going to code for.

05:38 Well this codes for, as you can see here, that is for wild type and wild type it means for great eyes.

05:47 So this a, if you have the dominant a you're going to express.

05:52 Actually let's just leave it as it was, okay? sorry for this.

05:56 And like this and like this.

06:03 So when you have the dominant a and the dominant b what are you going to have? you're going to express a b gene.

06:09 And if you express a b gene you're going to have a dominant a, great eyes.

06:14 That is a wild type, okay? and when you have the homozygous recessive you're going to have, you will have dominant a and recessive b.

06:21 This is for this number three and this number three was for this one here.

06:24 So this is purple or purple...

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