In Drosophila, an X -linked recessive mutation, scalloped (sd), causes irregular wing margins. D

In Drosophila, an X -linked recessive mutation, scalloped...

Key Concepts

Recessive Mutations

A recessive mutation is one that only exhibits its phenotype when an individual possesses two copies of the mutant allele (in autosomal cases) or a single copy in the hemizygous state (as in the X-linked context in males). This concept is critical for analyzing genetic crosses because it determines how and when the mutant phenotype appears in the offspring, influencing the expected ratios within the F1 and F2 generations.

Punnett Squares and Cross Analysis

Punnett squares are used to predict the outcomes of genetic crosses by enumerating all possible allele combinations from the parental genotypes. They are fundamental tools in genetics for visualizing how alleles segregate and combine during mating, including the analysis of F1 and F2 generations. In the context of this question, Punnett squares help illustrate the differences in inheritance patterns between X-linked and autosomal traits, especially when considering crosses involving recessive mutations.

X-linked Inheritance

X-linked inheritance refers to the transmission of genes that are located on the X chromosome. Because males have one X chromosome and one Y chromosome, a single copy of a recessive mutation on the X chromosome is enough for the trait to be expressed in males. In contrast, females have two X chromosomes and must inherit two copies of the recessive allele to show the phenotype. This concept is key to understanding why the outcomes of crosses differ between the sexes and between various types of crosses involving X-linked traits.

Autosomal Inheritance

Autosomal inheritance pertains to genes that are located on non-sex chromosomes, meaning they are inherited equally by both male and female offspring. In autosomal inheritance, the appearance of a recessive phenotype requires that an individual inherit two copies of the recessive allele, one from each parent. The inheritance patterns follow typical Mendelian ratios, without the differences that arise due to sex-linked transmission.

Transcript

00:01 So this is a question about fruit flies, and we're told that there are two alleles related to wing size.

00:08 The little s allele leads to small wings, and then an s1 allele leads to normal wings.

00:17 So those are the wings, alleles we're talking about.

00:19 And keep in mind, these are sex lengths, so they're going to show up on the x chromosome.

00:24 So in the first question, which i'm just going to call a, even though the book doesn't, we are told to do a cross between normal females, which means they are with s1 on both chromosomes, homozygous, and we're crossing those to males who have small wings, which means that they are little s.

00:51 All right, and the results are pretty easy to see because the females only have s1 to give, doesn't matter which chromosome you get.

00:59 And of course, to be female, you have to get two x chromosomes.

01:03 And so this male only has that one to get.

01:05 So all the females, right, will have be heterozygous for this allele and should have normal wings.

01:17 For the males, of course, they're again going to get an s1 from the female, from the mother, then they get a y from their father.

01:25 So these are males.

01:27 And you'll notice with the s1, they also would be normal.

01:31 So they're hemisigus.

01:32 They only have one copy of the allele because it's on the x chromosome.

01:37 But fortunately for them, they end up with normal wings.

01:41 So for part b, what we're going to do is cross these two.

01:44 Remember, this is our f1 generation here.

01:49 So we're going to cross those.

01:51 So we're going to do an s1s female and crossing that to the s1y males.

02:09 And there are different ways to do this.

02:12 I'm going to use a punnet square, but you can also just sort of work it out in other ways as well.

02:18 It's up to you.

02:25 So above this column, i'm going to put s1, and above this column i'm going to have the chromosome.

02:32 Remember to always keep track of the chromosomes when you're doing sex -link problems.

02:36 Otherwise, you get into monstrous amounts of trouble.

02:40 It just doesn't work.

02:42 And so keep track of my y chromosome.

02:44 That's a y.

02:45 Look like it.

02:46 All right.

02:47 So what do we have? well, we know these two or have y's in them.

02:51 And then this will have the x little s and this will have the x with s1 on it.

02:58 Number s1 produces normal...

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