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Chapter 6: Problem 14

Certain varieties of chrysanthemums contain \(18,36,54,72,\) and 90 chromosomes; all are multiples of a basic set of nine chromosomes. How would you describe these varieties genetically? What feature do the karyotypes of each variety share? A variety with 27 chromosomes has been discovered, but it is sterile. Why?

Short Answer

Expert verified
Short answer: The chrysanthemum variety with 27 chromosomes is sterile because it is triploid, having three complete sets of chromosomes (3n). Triploid karyotypes often lead to sterility because during meiosis, the cells are unable to evenly divide the chromosomes, which is necessary for successful fertilization and offspring viability.

Step by step solution

01

Identifying the pattern of chromosome numbers

Observe that all the given chromosome numbers (\(18, 36, 54, 72,\) and \(90\)) are multiples of the basic set of nine chromosomes. The given sets can be represented as \(2\times9, 4\times9, 6\times9, 8\times9,\) and \(10\times9\).
02

Describing the genetic makeup of each variety

Since each variety is a multiple of the basic set of 9 chromosomes, we can describe these varieties genetically as diploid (\(2n\)), tetraploid (\(4n\)), hexaploid (\(6n\)), octoploid (\(8n\)), and decaploid (\(10n\)). The corresponding chromosome numbers are \(18\) for diploid, \(36\) for tetraploid, \(54\) for hexaploid, \(72\) for octoploid, and \(90\) for decaploid.
03

Identifying shared karyotype features

The karyotypes of each variety share the feature that they are all polyploid, meaning they have multiple complete sets of chromosomes. In the case of chrysanthemums, the basic set contains 9 chromosomes. The karyotypes share multiples of this basic chromosome set.
04

Sterility of the variety with 27 chromosomes

The variety with 27 chromosomes is observed to be sterile. This sterility can be explained by the fact that 27 is not a multiple of an even number of basic chromosome sets – it is represented as \(3\times9\). This variety would be triploid (\(3n\)), meaning it has three complete sets of chromosomes. Triploid karyotypes often lead to sterility because during meiosis, the cells are unable to evenly divide the chromosomes, which is required for successful fertilization and offspring viability.

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Most popular questions from this chapter

A boy with Klinefelter syndrome \((47, \mathrm{XXY})\) is born to a mother who is phenotypically normal and a father who has the X-linked skin condition called anhidrotic ectodermal dysplasia. The mother's skin is completely normal with no signs of the skin abnormality. In contrast, her son has patches of normal skin and patches of abnormal skin. (a) Which parent contributed the abnormal gamete? (b) Using the appropriate genetic terminology, describe the meiotic mistake that occurred. Be sure to indicate in which division the mistake occurred. (c) Using the appropriate genetic terminology, explain the son's skin phenotype.

What are the possible reasons behind translocations?

In a human genetic study, a family with five phenotypically normal children was investigated. Two children were "homozygous" for a Robertsonian translocation between chromosomes 19 and 20 (they contained two identical copies of the fused chromosome). They have only 44 chromosomes but a complete genetic complement. Three of the children were "heterozygous" for the translocation and contained 45 chromosomes, with one translocated chromosome plus a normal copy of both chromosomes 19 and \(20 .\) Two other pregnancies resulted in stillbirths. It was later discovered that the parents were first cousins. Based on this information, determine the chromosome compositions of the parents. What led to the stillbirths? Why was the discovery that the parents were first cousins a key piece of information in understanding the genetics of this family?

Drasophila may be monosomic for chromosome \(4,\) yet remain fertile. Contrast the \(\mathrm{F}_{1}\) and \(\mathrm{F}_{2}\) results of the following crosses involving the recessive chromosome 4 trait, bent bristles: (a) monosomic \(\mathrm{IV}\), bent bristles \(\times\) normal bristles; (b) monosomic \(\mathrm{IV},\) normal bristles \(\times\) bent bristles.

How can you explain that not all Down syndrome cases are due to nondisjunction?
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