KARYOTYPING AND NON-DISJUNCTION
What is karyotyping?
A method of identification of chromosomes Pictures of chromosomes are taken as the
cell undergoes mitosis The image is enlarged Individual chromosomes are cut up Chromosomes are matched up based on:
Size (largest to smallest) Centromere position G-banding
A mess of chromosomes…
After karyotyping… a normal male
After karyotyping…normal
Non-disjunctionnondisjunction Non-disjunction is a failure of chromosomes
to properly separate during either Stage 1 or Stage 2 of meiosis
Upon fertilization the zygote may have one too many chromosomes ( trisomy) or one too few chromosomes (monosomy)
All but one monosomy case results in death
Non-disjunction
Non-disjunction occurs quite often among humans
Impact is so severe to the zygote that miscarriage occurs very early in the pregnancy
If the baby survives, it develops a set of traits that we call a syndrome
Down’s Syndrome
Most commonly known trisonomy 1:700 births; 1 in 6 die within 1st year Average age is 16.2 years Common facial feature Short stature Stubby fingers and toes Large tongue – makes speech difficult
Down’s syndrome
Down’s Syndrome
One of the most common causes of mental disability (IQ is in the 25-75 range)
Prone to heart defects, respiratory problems and leukemia
Down’s Syndrome
Odds of having a Down’s child increases with the age of the mother
1 in 1500 if mom is in early 20’s 1 in 70 if mom is over 35 1 in 25 if mom is over 45
Patau Syndrome
1:15,000 births as most fetuses die before term
Of those that survive, 5% live to age 3; 45% die within the first month
Serious eye, brain, and circulatory defects
Patau’s Syndrome
Edward’s Syndrome
Only 10% survive past one year
All die early in infancy
Many complications
These are the only known trisonomy genetic disorders that result in offspring surviving for
a short period of time
Non-disjunction of the sex chromosomes These can be fatal Most do survive just fine
Klinefelter’s Syndrome
Affects 1:500 males XXY Tall, sterile males Normal intelligence
Has female characteristics
Klinefelter’s Syndrome
Jacob’s Syndrome (super male) XYY Somewhat taller
than average Slightly below
normal intelligence
1:1000 males Extra
testosterone
XXX (super female?)
1:1000 live births Normal intelligence Fertile No physical problems
There are some women who are XXXX and XXXXX – each increasing X results in lesser intelligence and fertility
Monosomy – Turner’s Syndrome (XO) 1:2700 births Live normal lives
but do not mature sexually at puberty
Sterile Short stature Short broad neck Broad chest
Other chromosomal issues
Deletion – a segment of the chromosome is missing
Example: Cri-du-chat (1:1,000,000) Improperly developed larynx Severely mentally handicapped
Other chromosomal issues
Duplication
Ex. Fragile X1:1500 males, 2500 femalesMost common form of mentally handicapped offspring