When Meiosis goes Awry

nondisjunction

the members of a chromosome pair fail to separate at
anaphase

nondisjunction produces

gametes with an incorrect number of chromosomes

Nondisjunction can occur during

meiosis I or II

What is nondisjunction?

Failure of chromosomes or sister chromatids to separate properly during meiosis.

What fails to separate in nondisjunction during Meiosis I?

Homologous chromosomes fail to separate.

What gametes result from nondisjunction in Meiosis I?

2 gametes with n+1 chromosomes and 2 gametes with n−1 chromosomes. All gametes are abnormal

What fails to separate in nondisjunction during Meiosis II?

Sister chromatids fail to separate.

What gametes result from nondisjunction in Meiosis II?

1 gamete with n+1, 1 gamete with n−1, and 2 normal gametes (n).

Which type of nondisjunction produces all abnormal gametes?

Nondisjunction in Meiosis I.

Which type of nondisjunction can produce normal gametes?

Nondisjunction in Meiosis II.

What is another name is for down syndrome

trisomy 21

Down Syndrome

condition in which an individual has an extra chromosome 21

Aneuploidy

abnormal number of chromosomes

Down syndrome affects about

one out of every 700 children

down syndrome is revealed by

karyotype analysis

What happens if an abnormal egg cell (n+1) is fertilized by a normal sperm cell (n)?

The resulting zygote has 2n + 1 chromosomes (an extra chromosome).

What does n + 1 mean?

The gamete contains one extra chromosome.

What does 2n + 1 mean?

The zygote contains one extra chromosome compared to the normal diploid number.

What is trisomy?

A condition in which an individual has three copies of a chromosome instead of two (2n + 1).

How can nondisjunction lead to trisomy?

Nondisjunction produces an n + 1 gamete. When it fuses with a normal gamete (n), it forms a 2n + 1 zygote (trisomy).

Equation for forming a trisomic zygote?

(n + 1) + n = 2n + 1

Example of a disorder caused by trisomy?

Down syndrome is an example

How does the risk of Down syndrome change with maternal age?

The risk of having a baby with Down syndrome increases as the mother's age increases, especially after age 35.

Why are older mothers at greater risk of having a child with trisomy 21?

Their egg cells have been arrested in meiosis for many years, increasing the chance of errors in chromosome separation.

What is the relationship between maternal age and nondisjunction?

As maternal age increases, the likelihood of nondisjunction increases.