Nondisjunction

Nondisjunction

Nondisjunction

Nondisjunction is the improper separation of chromosomes during meiosis. As learned, the tetrads separate in meiosis I and the sister chromatids separate in meiosis II.

Numerous structures ensure proper separation of chromosomes including centrioles, spindle fibers, and centromeres. If any part malfunctions, the result may be disastrous.

A = a normal meiosis has occurred; B = nondisjunction occurred during Meiosis 2; C = nondisjunction occurred during Meiosis 1

Study the drawing below. Fill in the diploid number of this fictitious creature. In the haploid gametes, how many chromosomes should be present? .

Study the drawing below. Fill in the diploid number of this fictitious creature. In the haploid gametes, how many chromosomes should be present? 

Since the first diploid cell had FOUR chromosomes, each of the resulting haploid gametes should have TWO chromosomes.

However, NONDISJUNCTION has occurred.

You will notice that the top 2 egg cells ended up with 3 chromosomes each, and the bottom 2 egg cells have ended up with 1 chromosome each.

This is the result of nondisjunction - the chromosomes have failed to separate correctly; this usually results in genetic disorders.

A normal human karyotype should show 22 autosomes, or non-sex chromosomes and 2 sex chromosomes- either an XX for female or an XY for male.

The total number of human chromosomes should include 46.

However, as shown in the diagram mistakes may occur in the process of meiosis.

For normal offspring, the diploid number must be the same as the original parent following fertilization.

In this case the diploid number is ______.

How many chromosomes would the offspring inherit if the egg cell in the top right corner was fertilized? ______.

How many if the bottom right egg was fertilized? _____.

For normal offspring, the diploid number must be the same as the original parent following fertilization.

In this case the diploid number is 4.

How many chromosomes would the offspring inherit if the egg cell in the top right corner was fertilized? 5.

How many if the bottom right egg was fertilized? 3.

Obviously, the result may be a chromosomal abnormality. The abnormalities result from nondisjunction and result in an atypical number of chromosomes, or a structural abnormality in one or more chromosomes.

• Karyotypes, or pictures of chromosomes, allow doctors and scientists to look for chromosomal abnormalities.

• Creation of a karyotype requires the condensed (visible) chromosomes, which occurs in Prophase of the cell cycle.

• The number of autosomes in this karyotype includes:

• The number of sex chromosomes in this karyotype includes:

• The total number of chromosomes in this karyotype is:

• What is the gender of the person?

• How do you know?

• The number of autosomes in this karyotype includes: 44

• The number of sex chromosomes in this karyotype includes: 2

• The total number of chromosomes in this karyotype is: 46

• What is the gender of the person? Female

• How do you know? 2 X chromosomes

Abnormal Karyotype #1

• Where is the trisomy?

• How many autosomes in this karyotype?

• How many sex chromosomes?

• What is the gender of the person?

• What is the name of this disorder, often referred to as trisomy 21?

#1

• Where is the trisomy? In pair 21.

• How many autosomes in this karyotype? 45

• How many sex chromosomes? 2

• What is the gender of the person? Male

• What is the name of this disorder? Down syndrome

Down syndrome

•  delayed development, learning disability, short stature, or speech delay in a child

Abnormal Karyotype #2 

• Gender determination may be difficult in this case, but what is the most likely gender?

• Name of condition:

• Life expectancy:

• Side effects:

Turner syndrome (X)

• Gender determination may be difficult in this case, but what is the most likely gender? Female

• Name of condition: Turner syndrome

• Life expectancy: slightly shorter than average

• Side effects: short stature, delayed puberty, infertility, heart defects, and certain learning disabilities.

Abnormal Karyotype #3 

• What is most likely the gender?

• Name of condition:

• Life expectancy:

• Side effects:

Klinefelter syndrome (XXY)

• What is most likely the gender? Male

• Life expectancy: Close to average

• Symptoms: Males born with Klinefelter syndrome may have low testosterone and reduced muscle mass, facial hair, and body hair. Most males with this condition produce little or no sperm

Abnormal Karyotype #4

• Gender:

• Name of condition:

• Life expectancy:

• Side effects:

Patau syndrome (Trisomy 13)

• Gender: Female

• Name of condition: Patau syndrome

• Life expectancy: Between 7 and 10 days

• Side effects: severe physical and mental defects

Abnormal Karyotype #5

• Gender:

• Name of condition:

• Life expectancy:

• Side effects:

Edwards' syndrome (Trisomy 18)

• Gender: Female

• Name of condition: Edward's syndrome

• Life expectancy: usually fatal before birth or within the first year of life

• Side effects: organ failure and other physical/mental defects

Multiple Choice

What is the gender of this person?

• Male

• Female

Multiple Choice

What disorder does this person have?

• Klinefelter's syndrome

• Turner's syndrome

• Down syndrome

• Edward's syndrome

• Patau syndrome

Multiple Choice

What disorder does this person have?

• Klinefelter's syndrome

• Turner's syndrome

• Down syndrome

• Edward's syndrome

• Patau syndrome

Multiple Choice

What disorder does this person have?

• Klinefelter's syndrome

• Turner's syndrome

• Down syndrome

• Edward's syndrome

• Patau syndrome