Daniel McDonald and Nancy Peer determined that eye spot...

Daniel McDonald and Nancy Peer determined that eye spot (clear spot in the center of the eye) in flour beetles is caused by an X-linked gene $(e s)$ that is recessive to the allele for the absence of eye spot $\left(e s^{+}\right)$ They conducted a series of crosses to determine the distance between the gene for eye spot and a dominant X-linked gene for striped $(S t),$ which causes white stripes on females and acts as a recessive lethal (is lethal when homozygous in females or hemizygous in males). The following cross was carried out (D. J. McDonald and N. J. Peer. $1961 .$ Journal of Heredity $52: 261-264$ ). (EQUATION CANNOT COPY) a. Which progeny are the recombinants and which progeny are the nonrecombinants? b. Calculate the recombination frequency between $e s$ and $S t$ c. Are some potential genotypes missing among the progeny of the cross? If so, which ones and why?

Daniel McDonald and Nancy Peer determined that eye spot (clear spot in the center of the eye) in flour beetles is caused by an X-linked gene $(e s)$ that is recessive to the allele for the absence of eye spot $\left(e s^{+}\right)$ They conducted a series of crosses to determine the distance between the gene for eye spot and a dominant X-linked gene for striped $(S t),$ which causes white stripes on females and acts as a recessive lethal (is lethal when homozygous in females or hemizygous in males). The following cross was carried out (D. J. McDonald and N. J. Peer. $1961 .$ Journal of Heredity $52: 261-264$ ).
(EQUATION CANNOT COPY)
a. Which progeny are the recombinants and which progeny are the nonrecombinants?
b. Calculate the recombination frequency between $e s$ and $S t$
c. Are some potential genotypes missing among the progeny of the cross? If so, which ones and why?

Key Concepts

Dominant Lethal Allele

A dominant lethal allele is one that causes death when present in a certain dosage, such as being homozygous in one sex or hemizygous in the other. In the context of genetic crosses, the presence of a dominant lethal allele can lead to the absence of certain genotypes or phenotypes among the progeny because individuals with these genotypes do not survive, which must be taken into account when interpreting the results of crosses and calculating recombination frequencies.

Recombination Frequency

Recombination frequency is a measure of how often crossover events occur between two loci during meiosis, calculated as the percentage of recombinant progeny out of the total progeny. This metric is used in genetic mapping to estimate the distance between genes on a chromosome; a higher recombination frequency generally indicates a greater physical distance between the loci.

X-linked Inheritance

X-linked inheritance involves genes that are located on the X chromosome, leading to unique patterns of transmission, especially between males (who have one X chromosome) and females (who have two). This concept is crucial for understanding how alleles expressed on the X chromosome behave differently in males and females, affecting the appearance of traits and the outcomes of crosses.

Recombinant and Nonrecombinant Progeny

In genetic crosses, nonrecombinant progeny are those whose phenotype or genotype matches one of the parental types, indicating that no crossover event occurred between the loci of interest. Recombinant progeny, on the other hand, display new combinations of traits that result from crossing over between the linked genes. Identifying these classes is essential in mapping the distance between genes on a chromosome.

Transcript

00:01 Right, so here we're looking at recombination and linkage, and we're looking at a particular study to figure out how recombination works.

00:11 And so we have this cross where we have a female with no eye spots and stripes for one of their alleles.

00:25 And the other one has the eye spot gene, but no stripes on it.

00:29 And that's crossed with a male that has eye spots, no stripes.

00:38 Okay, so when we do this cross, we would expect four different combinations, right? we could take this top allele with either the top and bottom of the males.

00:48 So that would give us two combinations, and then we would get two more by the second allele of the female.

00:56 So those would give us our non -recombinant pairs.

01:03 And so if we do that, we would expect to see, let's see.

01:11 So if we take the top one, no eye spots and stripes, that can get paired with either the eye spots and no stripes, or that same first allele could get paired with the y allele.

01:27 And then if we take the bottom one, we have eye spots but no stripes, and then that could get paired with the eye spots and no stripes, or we could have the eye spots and no stripes get paired with the y allele.

01:43 So these four are what would be our non -recombinant, and they're non -recombinant because they were just produced exactly from the parent genes without any changing about, right? the thing is one of these is actually missing from our list, and it's going to be this no -eye spot and stripes male.

02:24 And we'll get to why that one's missing later.

02:26 So the other three are the non -recombinant ones of the offspring, which means everything else is a recombinant one.

02:33 So the other three listed is this eye -spot stripe with eye -spot no stripe.

02:48 No eye spots, no stripe with eye spots and no stripe.

03:02 And then lastly, the other male is the no eye spots, no stripe, boy.

03:10 Okay, so that means these three are the recombinant...

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