ABNORMAL MEIOSIS
Global competencies: Reflection and communication
INTRODUCTION
■ Can lead to extra chromosomes = trisomy
■ Can lead to missing chromosomes = aneuploidy or monosomy
■ Chromosomes failing to separate = chromosome non-disjunction
■ If it occurs in the sex chromosomes (X and Y)
■ Fertilization with one of these abnormal gametes results in zygote with extra
or missing chromosome = aneuploidy
■ Diagnosis = identifying the illness
■ Prognosis = how the illness will develop
KLINEFELTER’S SYNDROME
■ Boy is born with an extra copy of the X
chromosome → XXY
■ Sex chromosome within the egg cell splits
unevenly
■ Very rare that this will occur in the sperm
cell
■ Symptoms in males with this syndrome
are not very obvious
■ Symptoms may include:
– Sparse body hair
– Enlarged breasts
– Small testicles and penis
– Not very deep voices
– Infertile (cannot father children)
TURNER SYNDROME
■ Only affects females
■ One of the X-chromosomes is
missing (XO instead of XX)
■ Not hereditary – occurs
randomly due to incomplete
separation during meiosis
■ Infertile – cannot pass along to
next generation/offspring
■ Symptoms:
– Webbed neck
– No growth spurts
– Delayed development
– Infertility
DOWN SYNDROME
■ Extra copy of chromosome 21
■ Also called trisomy 21
■ Rare for males to be the cause of Down’s
syndrome
■ Occurs during oogenesis
■ The two number 21 chromosomes don’t
separate properly during anaphase – both
go into the same daughter cell instead of
one into each of the two daughter cells.
■ Egg now has two number 21
chromosomes → fertilized egg has three
number 21 chromosomes (2 from mother
and one from father)
■ New embryo develops by mitosis – has 47
chromosomes
Characteristics of Down syndrome
■ Varying degrees of intellectual and
developmental delay
■ Distinctive flattened facial features
■ Slanted eyes (due to folds of skin at
corners of eyes)
■ Other physical features
– short, stubby fingers
– Wide space between big and
second toe
– Large head with different
developing ears
■ Heart defects and other health
problems
■ Have a happy, loving nature
How common is Down syndrome?
■ 1 in 900 births
■ Chances increase with increasing age of the mother:
– At age 35 = 1 in 350 chance
– At age 45 = 1 in 30 chance
■ Known as maternal age effect = chromosomes have difficulty in separating with
aging
Children with genetic defects
■ Grow and develop slower
■ May need special needs classrooms – as still need support
■ Offer encounter patronising attitudes and discrimination
Aborting the foetus
■ Combination of blood test and
ultrasound examination
■ Able to detect 11 weeks after
conception
■ More time to allow for decision
■ May also have amniocentesis –
small sample of fluid from amniotic
cavity in uterus is removed
KARYOTYPE
■ A set of chromosomes in a cell
■ Shows the number, size and shape of chromosomes during metaphase of mitosis
■ Show whether a cell originates from male or female
■ Clearly see chromosomal abnormalities
Creating a karyotype
■ Nuclei of cultures white blood cells
■ Stimulated to divide by growing in a culture medium for several days
■ Treated with a drug that arrests mitosis in metaphase (easy to identify
chromosomes)
■ Chromosomes are dyed, then photographed
■ Software then arranges them into homologous pairs