4.2.4 – 4.3.1 Non-disjunction, karyotyping, chorionic
villus sampling, amniocentesis, theoretical genetics
Tyler and Tahjai
4.2.4
Explain that non-disjunction can lead to changes in chromosome
number, illustrated by reference to Down syndrome (trisomy 21).
4.2.4
Non-disjunction- failure of chromosomes to properly separate during
Anaphase I
 Leads to aneuploidy or polyploidy within gametes
 Aneuploidy – having one extra chromosome or missing one
chromosome
 Extreme cases can result in total non-disjunction, where all
homologous pairs fail to separate
Total non-disjunction
causes polyploidy, where
an organism has a
complete extra set of
chromosomes. (3n)
4.2.4
Synapsis- pairing up of homologous chromosomes during
Prophase I
 Resulting
pairs of homologous chromosomes are called
tetrads or bivalents
4.2.4
Trisomy 21 (Down syndrome)
 Down syndrome is present when there are 3
chromosomes of chromosome 21
 Occurs
from non-disjunction and aneuploidy in either
parent gamete
 One
21
parent gamete will carry two copies of chromosome
Causes of non-disjunction
 Non-disjunction
can occur randomly, but age of female
gametes/eggs influences chances
 Female
gametes are produced before birth and are not used
until ovulation, leaving egg cells to endure damage from
environment over time
 Because
sperm cells are constantly produced, the age of the
male parent has less of an effect on non-disjunction
A 20 year old woman’s egg
cells will have collected 20
years of damage and with
more years, comes more
damage
4.2.5
State that, in karyotyping, chromosomes are arranged in pairs
according to their size and structure
4.2.5
Karyotyping- the process of finding the chromosomal
characteristics of a cell
Process:
 Chromosomes within cell are stained with a dye to reveal
structural banding
 Chromosomes are then photographed. Chromosomes are cut
from picture, then paired and arranged based on size and
structure
4.2.6
State that karyotyping is performed using cells collected by
chorionic villus sampling or amniocentesis, for pre-natal diagnosis
of chromosome abnormalities
Chorionic Villus Sampling
Amniocentesis
4.2.6
What is Chorionic Villus?
Chorionic villi cells are stored within the placenta of the zygote and are used
to analyze an organism’s chromosomes
4.2.6
Chorionic Villus Sampling
 Can
be done 11-12 weeks into pregnancy
 Process
of taking chorionic villi cells in order to obtain cells
from original zygote tissue (chromosomes)
 1%
risk of miscarriage
4.2.6
Amniocentesis
 Can
be done during 16th week of pregnancy
 Amniotic
fluid sample is taken to obtain chromosomes
 The
sample cells begin to divide and are photographed in
order to produce karyotype
 0.5%
risk of miscarriage
4.2.7
4.3.1
Theoretical Genetics Definitions
Genotype- the alleles of an organism
Phenotype- includes all the characteristics of an organism
Dominant Allele- one which has the same effect on the phenotype whether it
is present in the homozygous or heterozygous state
Recessive Allele- one which only has an effect on the phenotype when present
in the homozygous state
Codominant Alleles- a pair of alleles that both affect the phenotype when
present in a heterozygote
Locus- the particular position on homologous chromosomes of a gene
Homozygous- having two identical alleles of a gene
Heterozygous- having two different alleles of a gene
Carrier- a heterozygous individual that has one copy of a recessive allele that
causes a genetic disease in individuals that are homozygous for this allele
Test Cross- testing a suspected heterozygote by crossing with a known
homozygous recessive
4.3.1
Genotype- the alleles of an organism
Phenotype- includes all the characteristics of an
organism
4.3.1
Dominant Allele- one which has the same effect on the
phenotype whether it is present in the homozygous or
heterozygous state
Recessive Allele- one which only has an effect on the
phenotype when present in the homozygous state
4.3.1
Codominant Alleles- a pair of alleles that both affect the
phenotype when present in a heterozygote
Locus- the particular position on homologous
chromosomes of a gene
4.3.1
Homozygous- having two identical alleles of a gene
Heterozygous- having two different alleles of a gene
4.3.1
Carrier- a heterozygous individual that has one copy of
a recessive allele that causes a genetic disease in
individuals that are homozygous for this allele
Test Cross- testing a suspected heterozygote by
crossing with a known homozygous recessive
Works Cited



"Amniocentesis - What Is an Amniocentesis Video." How-to Videos: How-to
and DIY Videos - About.com Videos. N.p., n.d.Web. 22 Feb. 2012.
<http://video.about.com/pregnancy/Amniocentesis.htm>.
"Chorionic Villus Sampling - What Is Chorionic Villus Sampling - CVS
Video." How-to Videos: How-to and DIY Videos - About.com Videos. N.p., n.d.
Web. 22 Feb. 2012. <http://video.about.com/pregnancy/Chorionic-VillusSampling.htm>.
"HowStuffWorks Videos "Conspiracy Test: Gulf War Illness Test Results"
."HowStuffWorks Videos "Video Channel" . N.p., n.d.Web. 22 Feb. 2012.
<http://videos.howstuffworks.com/discovery/32199-conspiracy-test-gulfwar-illness-test-results-video.htm>.
Karyotype game

http://learn.genetics.utah.edu/content/begin/traits/karyoty
pe/