Question

For several years, Hans Nachtsheim investigated an inherited anomaly of the white blood cells of rabbits. This anomaly, termed the Pelger anomaly, is the arrest of the segmentation of the nuclei of certain white cells. This anomaly does not appear to seriously burden the rabbits. a. When rabbits showing the Pelger anomaly were mated with rabbits from a true-breeding normal stock, Nachtsheim counted 217 offspring showing the Pelger anomaly and 237 normal progeny. What is the genetic basis of the Pelger anomaly? b. When rabbits with the Pelger anomaly were mated with each other, Nachtsheim found 223 normal progeny, 439 with the Pelger anomaly, and 39 extremely abnormal progeny. These very abnormal progeny not only had defective white blood cells, but also showed severe deformities of the skeletal system; almost all of them died soon after birth. In genetic terms, what do you suppose these extremely defective rabbits represented? Why were there only 39 of them? c. What additional experimental evidence might you collect to test your hypothesis in part $b$ ? d. In Berlin, about 1 human in 1000 shows a Pelger anomaly of white blood cells very similar to that described for rabbits. The anomaly is inherited as a simple dominant, but the homozygous type has not been observed in humans. Based on the condition in rabbits, why do you suppose the human homozygous has not been observed? e. Again by analogy with rabbits, what phenotypes and genotypes would you expect among the children of a man and woman who both show the Pelger anomaly? (Data from A. M. Srb, R. D. Owen, and R. S. Edgar, General Genetics, 2 nd ed. W. H. Freeman and Company, $1965 .)$

Name: Problem 26, Chapter 6: Introduction to Genetic Analysis
Uploaded: 2020-07-24T14:48:30-08:00
Duration: 8 min 34 s
Channel: Jessica Wooten

For several years, Hans Nachtsheim investigated an inherited anomaly of the white blood cells of rabbits. This anomaly, termed the Pelger anomaly, is the arrest of the segmentation of the nuclei of certain white cells. This anomaly does not appear to seriously burden the rabbits.
a. When rabbits showing the Pelger anomaly were mated with rabbits from a true-breeding normal stock, Nachtsheim counted 217 offspring showing the Pelger anomaly and 237 normal progeny. What is the genetic basis of the Pelger anomaly?
b. When rabbits with the Pelger anomaly were mated with each other, Nachtsheim found 223 normal progeny, 439 with the Pelger anomaly, and 39 extremely abnormal progeny. These very abnormal progeny not only had defective white blood cells, but also showed severe deformities of the skeletal system; almost all of them died soon after birth. In genetic terms, what do you suppose these extremely defective rabbits represented? Why were there only 39 of them?
c. What additional experimental evidence might you collect to test your hypothesis in part $b$ ?
d. In Berlin, about 1 human in 1000 shows a Pelger anomaly of white blood cells very similar to that described for rabbits. The anomaly is inherited as a simple dominant, but the homozygous type has not been observed in humans. Based on the condition in rabbits, why do you suppose the human homozygous has not been observed?
e. Again by analogy with rabbits, what phenotypes and genotypes would you expect among the children of a man and woman who both show the Pelger anomaly?
(Data from A. M. Srb, R.
D. Owen, and
R. S. Edgar, General Genetics, 2 nd ed. W. H. Freeman and Company, $1965 .)$

Introduction to Genetic Analysis

Anthony J. F.… 11th Edition

Chapter 6, Problem 26

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Solved by Expert Jessica Wooten

07/24/2020

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For several years, Hans Nachtsheim investigated an inherited anomaly of the white blood cells of rabbits. This anomaly, termed the Pelger anomaly, is the arrest of the segmentation of the nuclei of certain white cells. This anomaly does not appear to seriously burden the rabbits. a. When rabbits showing the Pelger anomaly were mated with rabbits from a true-breeding normal stock, Nachtsheim counted 217 offspring showing the Pelger anomaly and 237 normal progeny. What is the genetic basis of the Pelger anomaly? b. When rabbits with the Pelger anomaly were mated with each other, Nachtsheim found 223 normal progeny, 439 with the Pelger anomaly, and 39 extremely abnormal progeny. These very abnormal progeny not only had defective white blood cells, but also showed severe deformities of the skeletal system; almost all of them died soon after birth. In genetic terms, what do you suppose these extremely defective rabbits represented? Why were there only 39 of them? c. What additional experimental evidence might you collect to test your hypothesis in part $b$ ? d. In Berlin, about 1 human in 1000 shows a Pelger anomaly of white blood cells very similar to that described for rabbits. The anomaly is inherited as a simple dominant, but the homozygous type has not been observed in humans. Based on the condition in rabbits, why do you suppose the human homozygous has not been observed? e. Again by analogy with rabbits, what phenotypes and genotypes would you expect among the children of a man and woman who both show the Pelger anomaly? (Data from A. M. Srb, R. D. Owen, and R. S. Edgar, General Genetics, 2 nd ed. W. H. Freeman and Company, $1965 .)$

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Key Concepts

Dominant Inheritance

Dominant inheritance refers to the pattern in which a single copy of an allele is sufficient to express the associated phenotype. In cases like the Pelger anomaly, an individual exhibiting the anomaly carries at least one copy of the dominant allele, so heterozygotes manifest the trait even when crossed with normal individuals.

Homozygous Lethality

Homozygous lethality occurs when individuals carrying two copies of a particular allele do not survive, often because the homozygous condition disrupts essential developmental processes. In the context of the Pelger anomaly, while heterozygotes may show a mild defect, the homozygous condition leads to severe abnormalities and early death, which explains the lower than expected number of offspring exhibiting the extreme phenotype.

Mendelian Segregation and Ratios

Mendelian segregation is the principle that alleles are distributed to offspring in predictable ratios during gamete formation and fertilization. Analyzing the progeny ratios from different mating scenarios allows geneticists to infer the mode of inheritance, assess allele dominance, and determine the occurrence of lethal genotypes, as evidenced by the altered ratios in the Pelger anomaly crosses.

Experimental Genetics and Hypothesis Testing

Experimental genetics involves designing and interpreting cross-breeding experiments to test genetic hypotheses. By analyzing the outcome of matings, scientists can deduce genetic relationships and identify phenomena like homozygous lethality. Additional experiments such as larger scale crosses, embryological examinations, or molecular analyses can further validate these conclusions.

Comparative Genetics

Comparative genetics is the study of genetic similarities and differences across species, which can provide valuable insights into the underlying mechanisms of inherited traits. Observations in animal models, such as rabbits with the Pelger anomaly, can shed light on similar genetic behavior in humans, explaining why certain phenotypes (like the unobserved homozygous condition in humans) may not appear due to their lethal nature.

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Transcript

00:01 In order to answer this question, let's write down some facts.

00:04 We know from careful reading of the question that the rabbits that were normal were true breeding.

00:12 So that means that they have a homozygous state.

00:18 And so that's a key piece of information for us to solve this problem.

00:23 And so let's work on a part.

00:25 A part told us that of the offspring, there were 217 pelgar anomaly rabbits.

00:39 And there were 237 normal rabbits.

00:47 And this is a one to one ratio.

00:52 And so if we think about, okay, what were the parents of these offspring in the f1? one of those parents has to be homozygous.

01:03 So they have to be either parents were either big a, big a, or little a, little a, because that's the, that's the, that's the definition of true breeding is that normal produces always normal offspring.

01:18 That tells us that pelgar must be heterozygous because they're different than the normal, but the normal rabbits can only donate one normal allele.

01:31 And so we know that normal is true breeding.

01:34 So either big a, big a, or little a, little a must be normal and the heterozygote must be pelger.

01:42 Okay.

01:43 So then let's work through b part.

01:45 B part says what happens basically if you breed a pelgar rabbit with another pelgar rabbit.

01:58 And so we know that they're heterozygous.

02:01 So it's essentially a situation like this.

02:07 And here we can do a simple monohybrid cross and we can put our alleles in the appropriate windows.

02:16 And then we can bring them together to represent the offspring.

02:22 And so when we do that, we know that pelger is going to be the heterozygote.

02:28 So here is the pelger.

02:30 And then one of these is normal.

02:32 And i'm not sure right now we have enough information to tell.

02:35 So you're just going to have to choose one.

02:38 And so i know that pelger alleles are recessive.

02:44 So we know, at least in humans.

02:48 So here we get 223.

02:52 Normal offspring and this is in the problem 439 pelger offspring and 39 very abnormal probably won't make it to birth if they are born they die shortly after because they have extensive deformities and so here if we look this is a one to two to one ratio and that's what we expect from a breeding between two heterozygo individuals.

03:25 And so we know that this abnormal one here is lethal.

03:33 And in humans, those babies don't make it to term usually.

03:37 And so here we have the predicted ratio where big a, big a, we'll say is normal...

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