Question

unit 6 - mendelian genetics

1. label and describe what is happening in each example of meiosis shown below

diagrams of meiosis and nondisjunction

1. explain each condition and how it is caused by nondisjunction:

a. turner’s syndrome
b. klinefelter’s syndrome
c. down’s syndrome

Answer:

Question 21 (Labeling and describing meiosis examples)

First Meiosis Diagram (Normal Meiosis)

• Step 1: Identify Phases

The first diagram shows a cell with homologous chromosomes (paired) in the first meiosis division (meiosis I), then separating into two cells. In the second meiosis division (meiosis II), sister chromatids separate, resulting in four haploid cells (each with \( n \) chromosomes).

• Step 2: Label and Describe
• First Meiosis Division (Meiosis I): Homologous chromosomes pair (synapsis), then separate into two daughter cells (each with duplicated chromosomes, \( 2n \) to \( n \) chromosomes, but still duplicated).
• Second Meiosis Division (Meiosis II): Sister chromatids of each duplicated chromosome separate, resulting in four haploid cells (each with \( n \) unduplicated chromosomes).

Second Meiosis Diagram (Nondisjunction in Meiosis I)

• Step 1: Identify Nondisjunction

Nondisjunction occurs in meiosis I: homologous chromosomes fail to separate.

• Step 2: Describe Outcomes
• After meiosis I: One cell has an extra chromosome (\( n+1 \)), the other is missing one (\( n-1 \)).
• After meiosis II: Two cells have \( n+1 \) chromosomes, two have \( n-1 \) chromosomes.

Third Meiosis Diagram (Nondisjunction in Meiosis II)

• Step 1: Identify Nondisjunction

Nondisjunction occurs in meiosis II: sister chromatids fail to separate.

• Step 2: Describe Outcomes
• After meiosis I: Two cells with \( n \) chromosomes (normal for meiosis I).
• After meiosis II: One cell has an extra chromosome (\( n+1 \)), one is missing (\( n-1 \)), and two are normal (\( n \)).

Question 23 (Explaining Conditions Caused by Nondisjunction)

a. Turner’s Syndrome

• Explanation: Turner’s Syndrome (XO) affects females, with a single X chromosome (missing a sex chromosome). Caused by nondisjunction during meiosis (usually in sperm or egg formation), resulting in a gamete with no sex chromosome. When fertilized by a normal gamete (X), the zygote has XO.
• Key Points: Female, sterile, short stature, webbed neck. Nondisjunction leads to missing X or Y (in sperm/egg), resulting in XO.

b. Klinefelter’s Syndrome

• Explanation: Klinefelter’s Syndrome (XXY) affects males, with an extra X chromosome. Caused by nondisjunction (meiosis I or II) in egg or sperm, producing a gamete with XX (egg) or XY (sperm). Fertilization with a normal gamete (Y or X) results in XXY.
• Key Points: Male, sterile, reduced testosterone, feminine features. Nondisjunction leads to extra X in sex chromosomes.

c. Down’s Syndrome

• Explanation: Down’s Syndrome (Trisomy 21) is caused by nondisjunction of chromosome 21 during meiosis (usually meiosis I in egg formation). A gamete with an extra chromosome 21 is produced. Fertilization with a normal gamete results in a zygote with three copies of chromosome 21 (2n+1 for chromosome 21).
• Key Points: Intellectual disability, distinct facial features, heart defects. Nondisjunction of chromosome 21 leads to trisomy.

Final Answers (Summarized)

Question 21

• First Diagram: Normal meiosis (meiosis I: homologous separation; meiosis II: sister chromatid separation) → 4 haploid cells (\( n \)).
• Second Diagram: Nondisjunction in meiosis I → 2 cells with \( n+1 \), 2 with \( n-1 \).
• Third Diagram: Nondisjunction in meiosis II → 1 cell \( n+1 \), 1 \( n-1 \), 2 \( n \).

Question 23

• a. Turner’s Syndrome: XO (missing sex chromosome) from nondisjunction (gamete with no sex chromosome).
• b. Klinefelter’s Syndrome: XXY (extra X) from nondisjunction (gamete with XX or XY).
• c. Down’s Syndrome: Trisomy 21 (extra chromosome 21) from nondisjunction of chromosome 21.