WHEN MEIOSIS
GOES WRONG
IMPORTANT TERMS
•
•
•
•
•
•
•
•
•
Deletion
Translocation
Duplication
Inversion
Non-disjunction
Monosomy
Trisomy
Prenatal
Karyotype
LEARNING GOALS
1. To understand the ways that meiosis can go
wrong
2. To be familiar with some of the more common
genetic disorders that occur due to chromosomal
malfunction
REVIEW
• Meiosis is used to make gametes
• When it works (and it usually does) each gamete
ends up with a haploid number of cells (23 for
humans
What do
you notice
about the 4
gametes?
MEIOSIS ERRORS
• But sometimes it doesn’t go according to plan
• Mistakes happen
• There are two main types of errors
1. Errors caused by changes in chromosome
structure
2. Errors caused by changes in chromosome number
ERRORS IN CHROMOSOME STRUCTURE
• Usually occurs during the process of crossing over
• Sometimes when parts of chromosomes detach and
switch places, small pieces get removed or inverted (put
in upside down)or switched to a completely different
chromosome. Sometimes only one chromosome loses a
piece and another chromosome gets one
ERRORS IN STRUCTURE
• When any of these things happen there is a chance
that a baby that forms from one of the gametes will
be deformed
ERRORS IN STRUCTURE
• Structural disorders can lead to many abnormalities
and increase the likelihood of some diseases,
including cancer
• Table 4.1 on page 177 in your textbook describes
some of these READ IT!!!!
Cri du chat
Syndrome is
a deletion
disorder
ERRORS IN NUMBER
• Occurs when homologous chromosomes or sister
chromatids don’t separate properly in Anaphase (I
or II)
This is called
nondisjunction
ERRORS IN CHROMOSOME NUMBER
• When non-disjunction occurs, one gamete will lose
a chromosome and one will get an extra one
• Both cases can cause defects
• As you can see from the karyotype chart, the later
chromosomes are shorter
Since they’re shorter, they’re
more likely to get moved to
the wrong place
NON-DISJUNCTION DISORDERS
Trisomy 21
– Down Syndrome• Cause – 3 chromosome #21
• Chances – 1 in 800 births (more
common with older mothers)
• Signs- flattened face,
intellectual disabilities, short
stature
Trisomy 18
-Edward Syndrome• Cause – 3 chromosome #18
• Chances – 1 in 18000
• Signs- facial abnormalities,
intellectual and physical
disabilities
NON-DISJUNCTION OF SEX
CHROMOSOMES
Klinefelter Syndrome
XXY
• 1 in 1000 males
• Unable to produce
sperm
• Female features (e.g.
breast development)
Turner Syndrome
XO
• 1 in 5000 females
• Short stature, webbed
neck, sexual immaturity