Problem 12 - Biology: The Unity and Diversity of Life

Question

Nondisjunction can result in ____________. a. duplications c. crossing over b. aneuploidy d. pleiotropy

Step-by-Step Solution

Answer

Nondisjunction results in aneuploidy.

Step 1: Understanding Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division, either during meiosis I or meiosis II.

Step 2: Identifying the Result of Nondisjunction

When nondisjunction occurs, it results in gametes that have an abnormal number of chromosomes. This leads to a condition called aneuploidy, where individuals have an abnormal number of chromosomes, such as trisomy 21 (Down syndrome).

Step 3: Evaluating Other Options

Let's evaluate the other options to see why they are incorrect in this context: - Duplications involve a part of a chromosome being duplicated, which is not directly a result of nondisjunction. - Crossing over is the exchange of genetic material between homologous chromosomes, which occurs normally during meiosis and is not a result of nondisjunction. - Pleiotropy refers to a single gene affecting multiple traits, unrelated to chromosome number abnormalities.

Key Concepts

Aneuploidy

Aneuploidy is a condition where the number of chromosomes in the cells is not the typical number for a species. This means there is either one extra chromosome or one missing chromosome. Humans normally have 46 chromosomes, organized into 23 pairs, so any variation can cause health issues. A common example of aneuploidy is Down syndrome, also known as trisomy 21, where there is an extra copy of chromosome 21. Other types include Turner syndrome, which involves a missing X chromosome, and Klinefelter syndrome, where there is an extra X chromosome.

• Trisomy: having an extra chromosome, leading to three copies of one chromosome instead of two.
• Monosomy: a missing chromosome, resulting in just one copy of a chromosome instead of the usual pair.

Aneuploidy usually occurs because of a mistake during cell division processes called meiosis, where there is an error known as nondisjunction. This means chromosomes don't separate the way they should, leading to these abnormalities in chromosome numbers.

Meiosis

Meiosis is a special type of cell division that produces gametes, such as sperm and egg cells. It ensures these cells have half the number of chromosomes of a regular body cell, which is essential for sexual reproduction. There are two main stages in meiosis:

• Meiosis I: Homologous chromosomes are separated, reducing the chromosome number by half. This includes crossing over, where chromosomes exchange genetic material, creating genetic diversity.
• Meiosis II: Sister chromatids (the two identical halves of a duplicated chromosome) separate, leading to four unique gametes.

Mistakes during meiosis can result in gametes with an incorrect number of chromosomes. This can cause genetic disorders or developmental issues in offspring if fertilization occurs. Nondisjunction, for instance, is a type of error in meiosis where chromosomes fail to separate properly.

Chromosomal Abnormalities

Chromosomal abnormalities refer to any event where chromosomes deviate from the normal structure or number. These abnormalities can be either numerical or structural, severely affecting an individual's development and health. Numerical abnormalities involve changes in the number of chromosomes, leading to conditions like aneuploidy. Structural abnormalities involve alterations in the physical structure of chromosomes, such as:

• Deletions: A part of the chromosome is missing.
• Duplications: A section of the chromosome is repeated.
• Inversions: A chromosome segment is reversed end-to-end.
• Translocations: A segment from one chromosome breaks away and attaches to another chromosome.

These changes can result from errors during cell division processes like meiosis or external environmental factors. Chromosomal abnormalities are a major reason for genetic conditions and can be identified using genetic testing and screening. Understanding these anomalies helps in diagnosing and managing various genetic disorders.