Errors During Meiosis
Abnormal Meiosis
Errors in the normal process of meiosis can occur
during:
1. Crossing Over
2. Independent Assortment
2 Types of Errors:
-changes in chromosome structure
-changes to chromosome number
1. Changes in Chromosome Structure
Pieces of chromatids do not attach properly or reform during crossing
over
Four types of incorrect re-attachment:
1. Translocation - segment of one chromosome becomes attached to
a different chromosome Ex. Chronic Myelogenous Leukemia
2. Deletion - a piece of chromosome is deleted Ex. Cri du Chat
3. Duplication - a section of a chromosome appears two or
more times in a row Ex. Charcot-Marie- Tooth Disease
4. Inversion - a section of chromosome is inverted Ex FG
syndrome
. Chronic Myelogenous
Leukemia – Translocation
Chromosome 9
and 22 are
translocated
Cri du Chat - Deletion
“Cat-like crying”
Mental/Physical
Delays
Microencephaly
(smaller head)
Caused by a small
deletion on the 5th
chromosome
(hence 5psyndrome)
Charcot-Marie-Tooth
Disease - Duplication
Neurological
condition
Affects motor
control and can
cause
deformations
Duplication in
chromosome 17
FG Syndrome - Inversion
A section of the
X chromosome
becomes
inverted
Mental/Physical
delays
Macroencephaly
(larger head)
2. Changes in Chromosome
Number
Monosomy: missing
one
chromosome
(2n=45)
Non-disjunction can occur
when homologous
chromosomes or sister
chromatids do not separate
during anaphase I or II of
meiosis
Trisomy: One extra
chromosome
(2n=47)
Gametes will have either
extra or missing
chromosomes
Homologous chromosomes fail to separate
Sister chromatids fail to separate
Autosomal Conditions
Trisomy 21 – Down Syndrome
Trisomy 18 – Edward Syndrome
Trisomy 13 – Patau Syndrome
Sex Chromosome
Conditions
XXY – Klinefelter
XYY – Jacobs
XXX – Triple X
XO - Turner
Down Syndrome
Trisomy 21
Affects approximately 1 in 800 live births
Characteristic facial appearance
Mild to moderate intellectual disability
Short height
Risk increases with age of mother
Edwards Syndrome
Trisomy 18
Severe intellectual disability
Low birth weight, heart defects
Small head, small jaw and mouth
Clenched fists
Early Death
Patau Syndrome
Trisomy 13
Severe intellectual disabilities and
physical abnormalities
Variety of defects in organs
Large triangular nose
Extra fingers or toes, cleft lip
Early Death
Klinefelter
Child inherits 2 X chromosomes (female) and a single Y
chromosome (male)
Child appears male at birth but at puberty produces high
levels of female sex hormones
Small testes, breast enlargement, infertility
Testosterone injections can help treat the condition
Turner Syndrome
Female with single X chromosome
Short stature, webbed neck
Appear female but do not develop sexually
Homework
Pg. 181 # 12 - 15