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.

Key concepts

trisomy 21

Trisomy 21, commonly known as Down syndrome, is a genetic disorder caused by the presence of an extra chromosome 21. Normally, humans have 46 chromosomes, arranged in 23 pairs. A person with trisomy 21, however, has a total of 47 chromosomes due to an additional chromosome 21. This condition happens during cell division when chromosomes fail to separate properly, a mistake known as nondisjunction. The extra genetic material from chromosome 21 alters the course of development and results in the characteristic features associated with Down syndrome.

Individuals with Down syndrome often experience developmental delays, but they can lead fulfilling lives with the right support and educational strategies in place. Every person with Down syndrome is unique and can possess different strengths and challenges. Understanding the genetic basis of trisomy 21 can help in planning for proper care and maximizing the potential of individuals with this genetic disorder.

chromosomal abnormalities

Chromosomal abnormalities are deviations from the normal number or structure of chromosomes. They are responsible for a range of genetic disorders and conditions.

There are two main types of chromosomal abnormalities:

• Numerical abnormalities: These occur when there is a change in the number of chromosomes. An example is trisomy, where there is one extra chromosome (as seen in trisomy 21).
• Structural abnormalities: These involve changes in the structure of chromosomes, such as deletions, duplications, translocations, or inversions. Such changes can disrupt the function of genes on the affected chromosome.

Chromosomal abnormalities can happen as random events during the formation of reproductive cells or in early fetal development. The effects of these abnormalities vary widely. Some may result in developmental delays, cognitive disabilities, or even miscarriage during pregnancy. Genetic testing and counseling can provide valuable information for families affected by chromosomal abnormalities, helping them understand potential outcomes and plan accordingly.

Punnett square analysis

The Punnett square is a tool used in genetics to predict possible genotypes of offspring from the mating of two parents. It provides a visual representation of all potential genetic combinations, helping to understand hereditary patterns.

In scenarios involving trisomy 21, such as a mating between a female with Down syndrome and a normal male, the Punnett square shows the genetic outcomes for their children. By putting the gametes (sperm and egg) of each parent on different axes of the square, we can fill in the potential genetic makeups for the offspring.

For example, in this particular exercise, the female provides eggs with the chromosome combination for Down syndrome, and the male provides normal sperm. When you complete the Punnett square:

• Eight out of nine gametes combinations lead to offspring with trisomy 21.
• Only one combination results in a genetically typical child.

The Punnett square highlights the likelihood that their children will inherit Down syndrome, showcasing genetic inheritance’s complex yet predictable nature. This predictive ability is invaluable for genetic counseling and understanding genetic risks.