Question

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

Answer: Approximately 89% of the offspring will have Down syndrome.

Step-by-step solution

Identify the Chromosome Combinations of Parents

The female has Down syndrome, which means she has an extra chromosome #21. So her chromosome #21 combination is 21, 21, 21. The normal male has 46 chromosomes, so his chromosome #21 combination is 21, 21.

Represent the Possible Mating Combinations During Meiosis

During meiosis, each chromosome pair separates and forms gametes. Let's represent the possible gametes for each parent: Female: [21, 21], [21, 21], [21] Male: [21], [21]

Calculate Punnett Square

Using these gametes, we can form a Punnett square to find the possible combinations of offspring. Female | 21 | 21 ------------------- 21,21 | 21,21,21 | 21,21,21 21,21 | 21,21,21 | 21,21,21 21 | 21,21,21 | 21,21,21 The Punnett square for offspring of this mating is as follows: 1. 21,21,21 (Down syndrome) 2. 21,21,21 (Down syndrome) 3. 21,21,21 (Down syndrome) 4. 21,21,21 (Down syndrome) 5. 21,21,21 (Down syndrome) 6. 21,21,21 (Down syndrome) 7. 21,21,21 (Down syndrome) 8. 21,21,21 (Down syndrome) 9. 21,21 (Normal)

Calculate the Fraction of Offspring with Down Syndrome

Now, we calculate the proportion of offspring with Down syndrome: 8 offspring have Down syndrome (21,21,21) out of the total 9 offspring. Proportion = \frac{8}{9} So, we can expect 8 out of the 9 offspring or approximately 89% of the offspring to have Down syndrome.