You have been given a virgin Drosophila female. You notice...

You have been given a virgin Drosophila female. You notice that the bristles on her thorax are much shorter than normal. You mate her with a normal male (with long bristles and obtain the following $\mathrm{F}_{1}$ progeny: $\frac{1}{3}$ short-bristled females, $\frac{1}{3}$ long-bristled females, and $\frac{1}{3}$ long-bristled males. A cross of the $\mathrm{F}_{1}$ long-bristled females with their brothers gives only long-bristled $\mathrm{F}_{2}$ A cross of short-bristled females with their brothers gives $\frac{1}{3}$ short-bristled females, $\frac{1}{3}$ long-bristled females, and $\frac{1}{3}$ long-bristled males. Provide a genetic hypothesis to account for all these results, showing genotypes in every cross.

You have been given a virgin Drosophila female. You notice that the bristles on her thorax are much shorter than normal. You mate her with a normal male (with long bristles and obtain the following $\mathrm{F}_{1}$ progeny: $\frac{1}{3}$ short-bristled females, $\frac{1}{3}$ long-bristled females, and
$\frac{1}{3}$ long-bristled males. A cross of the $\mathrm{F}_{1}$ long-bristled females with their brothers gives only long-bristled $\mathrm{F}_{2}$ A cross of short-bristled females with their brothers gives
$\frac{1}{3}$ short-bristled females, $\frac{1}{3}$ long-bristled females, and
$\frac{1}{3}$ long-bristled males. Provide a genetic hypothesis to account for all these results, showing genotypes in every cross.

Key Concepts

Heterozygote Viability versus Homozygote/Hemizygote Lethality

This concept describes how an organism that carries one copy of a deleterious allele (heterozygote) may exhibit an altered phenotype yet remain viable, while the presence of two copies (or one in a hemizygous state) can lead to lethality. This difference in viability is critical in explaining unexpected progeny ratios in crosses, as it results in the absence of certain genotypes from the surviving population.

Interpreting Segregation Ratios in Genetic Crosses

Understanding how genes segregate during mating, especially when lethality is involved, is key to formulating genetic hypotheses. The altered ratios of phenotypic classes in the offspring—as well as the survival of only particular genotypes—provide clues to the underlying genetic mechanisms such as gene location (X-linked), dominance relationships, and selective viability based on genotype.

X-linked Inheritance

This concept involves genes located on the X chromosome, which are passed from parents to offspring differently based on the sex of the individual. In many cases, a mutation on the X chromosome can have a drastic effect on males because they are hemizygous (have only one copy of the X chromosome), while females have two copies, which can lead to different survival or expression patterns in the two sexes.

Dominant Mutation with Lethal Effects

A dominant mutation is one that is expressed even when only one copy of the allele is present. In some genetic scenarios, such a mutation can be viable in heterozygous forms yet become lethal when homozygous (or hemizygous in males, in the case of an X-linked mutation). This creates a situation where the mutant phenotype is observed in carriers, but in certain crosses, individuals inheriting the mutant allele in a particular hemizygous or homozygous state die, leading to skewed phenotypic ratios among surviving progeny.

Transcript

00:01 Order to figure this out, let's take a look at what we have.

00:03 So if a short bristled female is spread to a normal, which is a long bristle, male, what the offspring look like, so this will be parental, i'm just going to do p1 because we've got to do three crosses here.

00:28 So the f1 from this combination of breeding yields one third short bristled feet.

00:38 Females, one -third long -bristled females, and one -third long -bristled males.

00:54 There's some key sort of hints in this outcome.

00:59 So first, let's look.

01:01 We have twice as many females as we do males.

01:06 Our expected sex ratio should be one -to -one if this was an autosomal non -lethal trait.

01:13 However, we have two times as many females as males.

01:19 Not only that, but where are the one -third or whatever, one -quarter short -bristled males? they're not listed here because there weren't any in the f -1 offspring.

01:33 And so this gives us a hint that this is a lethal trait or a lethal allele and it's sex -linked.

01:40 So it's found on the x chromosome.

01:42 Otherwise, we would have a one -to -one ratio of males and females, and we would have both short -bristled females and short -bristled males alongside the long -bristled females and males.

01:55 This is supported by the fact that if you take, as the problem indicated, a long -bristled female, so this p -2, and you cross her with her f -1 brothers from up here, so long -bristled males, all of your offspring in the f2 from this cross of long -bristled females to long -gristled males yields long -gristled flies, regardless of sex.

02:34 And so that tells you that this chromosome x is carrying the allele.

02:40 We can identify the location of these alleles simply by looking at females.

02:49 So if a female is long bristled, she is recessive.

02:57 So this would be the genotype of a long, bristled or normal female...

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