BIOL 2500 - Topic 7 (part 4)

Nondisjunction in meiosis I and meiosis II

It only occurs once throughout meiosis, such that if it occurs in meiosis I, then meiosis II will proceed normally and vice versa

Nondisjunction in meiosis I

It results in the failure of homologous chromosomes to separate; therefore, after meiosis I, one of the daughter cells has both chromosomes while the other has none

Resulting gametes when nondisjunction occurs in meiosis I

½ of gametes will be n + 1 (trisomic)

½ of gametes will be n -1 (monosomic)

Nondisjunction in meiosis II

It results in the failure of sister chromatids to separate, therefore nondisjunction only occurs in one of the two daughter cells after meiosis I

Resulting gametes when nondisjunction occurs in meiosis II

½ will be n (euploid)

¼ will be n-1 (null?)

¼ will be n+1

Monosomics

Missing one copy of a specific chromosome, while all the other chromosomes are considered normal (2n-1)

Monosomics in animals

When it occurs in autosomes, it often leads to abortions/miscarriages, but it can be somewhat tolerated in sex chromosomes

Monosomics in a sex chromosome example

Turner syndrome (one X instead of two X’s)

Why is X0 significant if one of the X’s is inactivated anyway?

2 chromosomes are still needed for normal development in females, as the genes on a single X chromosome are not enough for development

Can turner syndrome occur in males

Yes, as a result of missing parts of the Y chromosome, but it is rare

How does turner syndrome occur

It occurs when one of the gametes has an n-1

Trisomics

Having an extra copy of a specific chromosome (2n + 1)

Trisomy at different chromosomes

There are potentially 24 different kinds of trisomy, but mostly in 13, 18, 21, and the sex chromosomes

Trisomy vs. monosomy

Trisomy can occur in autosomes and still be passed on, such that some individuals are able to grow and reach adulthood, which does not occur in monosomy

Trisomics examples

1.) Triple X syndrome (XXX)

2.) Klinefelter syndrome (XXY)

Can XXY individuals produce gametes?

They they can, but they may not be viable

Trisomy 21 (autosomal or sex-chromosome?)

Autosomal but… *ASK J*