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Chapter 2: Problem 15

21. Such individuals therefore ha… # Trisomy 21 or Down syndrome occurs when there is a normal diploid chromosomal complement of 46 chromosomes plus one (extra) chromosome #21. Such individuals therefore have 47 chromosomes. Assume that a mating occurs between a female with Down syndrome and a normal 46 -chromosome male. What proportion of the offspring would be expected to have Down syndrome? Justify your answer.

Short Answer

Expert verified
Answer: The expected proportion of offspring with Down syndrome is 100%.

Step by step solution

01

Represent the parental genotypes

The female has Down syndrome, which means she has an extra chromosome 21 (three in total), so her genotype can be represented as 21-21-21. The normal male's genotype, with 46 chromosomes, includes a pair of chromosome 21s: 21-21.
02

Create a Punnett square

Next, create a Punnett square to represent the possible chromosome combinations for the offspring. It should be a 3x2 matrix, representing the three chromosome 21 alleles from the mother and the two chromosome 21 alleles from the father: | | 21 (Father) | 21 (Father) | |-------|------------|------------| | 21 (Mother) | 21-21-21 | 21-21-21 | | 21 (Mother) | 21-21-21 | 21-21-21 | | 21 (Mother) | 21-21-21 | 21-21-21 |
03

Calculate the probability of offspring having Down syndrome

In the Punnett square, all six cells represent offspring with three chromosome 21s (Trisomy 21), also known as Down syndrome. The proportion of offspring with Down syndrome is 6/6 = 1, which means 100% of the potential offspring would have Down syndrome in this scenario. So, based on these calculations, the expected proportion of offspring with Down syndrome from this mating between a female with Down syndrome and a normal 46-chromosome male is 100%.

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Most popular questions from this chapter

A species of cereal rye (Secale cereale) has a chromosome number of \(14,\) while a species of Canadian wild rye (Elymus canadensis) has a chromosome number of \(28 .\) Sterile hybrids can be produced by crossing Secale with Elymus. (a) What would be the expected chromosome number in the somatic cells of the hybrids? (b) Given that none of the chromosomes pair at meiosis I in the sterile hybrid (Hang and Franckowlak, 1984 ), speculate on the anaphase I separation patterns of these chromosomes.

Describe the role of meiosis in the life cycle of a vascular plant.

How do the stages of mitosis and meiosis occur in a specific order and never alternate?

Compare chromosomal separation in plant and animal cells.

You are given a metaphase chromosome preparation (a slide) from an unknown organism that contains 12 chromosomes. Two that are clearly smaller than the rest appear identical in length and centromere placement. Describe all that you can about these chromosomes.
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