Directions: Use the information provided and your knowledge of Life Science to answer the following questions. Show all work where necessary.
Directions: Use the information provided and your knowledge of Life Science to answer the following questions. Show all work where necessary.
During human reproduction, cells undergo a specialized type of division called meiosis, which reduces the chromosome number by half so gametes (egg and sperm) each contain one copy of every chromosome. When fertilization occurs, the resulting embryo receives two complete sets—one from each parent. This carefully controlled process ensures stable inheritance patterns across generations. However, when meiosis does not proceed correctly, chromosomes may fail to separate evenly. Such events are known as nondisjunction, and they can lead to inheritable genetic variations in offspring.
One of the best-known examples of nondisjunction is Down syndrome, a condition caused by the presence of an extra copy of chromosome 21, resulting in trisomy 21. Normally, individuals inherit two copies of chromosome 21, but nondisjunction can occur during meiosis I or meiosis II in a parent’s gamete formation. If an egg or sperm cell ends up with two copies of chromosome 21 instead of one, and this gamete is involved in fertilization, the resulting zygote will contain three copies. This change is present in every cell of the developing embryo and remains throughout life.
Diagram 1.
Source: https://fity.club/lists/suggestions/nondisjunction-in-mitosis/
Down syndrome is characterized by recognizable physical traits, intellectual disability, increased risk of certain medical conditions, and characteristic developmental patterns. Since the extra chromosome is part of the individual’s DNA, the variation is inheritable in the sense that it originates in gamete formation and is passed to every cell of the offspring. Although individuals with Down syndrome typically do not pass trisomy 21 to their children, the trisomic condition itself originates as a meiotic genetic variation.
Diagram 2.
Source: https://www.genome.gov/genetics-glossary/Down-Syndrome-Trisomy-21
Population-level data show clear patterns: the frequency of nondisjunction leading to trisomy 21 increases with maternal age, especially after age 35. This relationship provides an opportunity to analyze how meiotic errors contribute to genetic variation. Because nondisjunction occurs randomly and cannot be reversed once it has happened, it is considered a viable error in meiosis that leads to inherited chromosomal variation.
Table 1.
Maternal Age (years) | Trisomy 21 Incidence per 1000 Births |
|---|---|
20 | 1 |
25 | 1.2 |
30 | 1.5 |
35 | 2.5 |
38 | 5 |
40 | 8 |
42 | 15 |
45 | 28 |
Graph of Information - Figure 1.
Table 2.
Error Type | Percentage of Trisomy Cases (%) |
|---|---|
Meiosis I | 75 |
Meiosis II | 25 |
Graph of Information - Figure 2.
According to Table 2, during which meiotic division do most nondisjunction events that lead to trisomy 21 occur?
Using Table 1 and Figure 1, describe the mathematical relationship between maternal age and trisomy 21 incidence.
Using the reading and Diagram 1, explain how errors in chromosome separation affect cell function after fertilization.
Based on Diagram 2, what chromosomal condition characterizes Down syndrome?
Using Table 2 and Figure 2, compare the percentages of nondisjunction events that occur in meiosis I and meiosis II. How does this pattern contribute to inherited chromosomal variation?
Explain how nondisjunction during meiosis leads to inheritable chromosomal variation such as trisomy 21.
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