Genetics
Non-disjunction and Karyotypes
Created by:
Sandra, Abby, Jasmine, Sarah, and Thomas
Important Definitions
• Genotype- refers to the alleles of an organism. Written
using upper case and lower case letters.
– ie Tt
• Phenotype-includes all characteristics of an organism.
Written as a word.
– ie. Tall
• Dominant allele- has the same affect on the phenotype
whether it is present in the homozygous or the
heterozygous state.
– ie Tt and TT are tall
Definitions continued
• Recessive allele- only has an effect on the phenotype
in the homozygous state.
– ie tt is tall
• 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.
– ie tt or TT
Definitions continued
• Heterozygous-two different alleles
of a gene.
ie Tt
• 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- refers to testing a suspected heterozygote
by crossing with a known homozygous recessive.
What is Non-disjunction?
• When chromosomes do not separate properly
during meiosis, either in meiosis I (in anaphase
I) or meiosis II (in anaphase II).
• This failure to separate can lead to aneuploidy
(having one extra or missing chromosome) or
polyploidy (having one complete extra set of
chromosomes).
Non-disjunction video
http://www.biostudio.com/d_%20Meiotic%20Nond
isjunction%20Meiosis%20I.htm
Non-disjunction in Down
Syndrome
• Down Syndrome is an
example of a genetic
disease caused by
non-disjunction.
– This can be seen
through karyotyping
when there are three
chromosomes in the
21st pair.
Down Syndrome
• Down syndrome is a result of aneuploidy. If
non-disjunction occurs in either parent, one of
the 2 gametes will have 2 copies of
chromosome 21. So, when the zygote is
formed it will contain 3 copies of chromosome
21 and will be aneuploid. AKA: trisomy 21
– (tri=3 somy=chromosomes)
Factors affecting chances of
Down Syndrome
• Age
– Females= gametes develop before birth, so the age
of the woman is the age of egg cells. The older the
woman, the more damage the cells have endured.
– Males= sperm is constantly produced so the age of
father has less effect than age of mother.
• Family history
– If a relative has down syndrome, the person has an
increased chance.
Karyotyping
• Karyotyping: process of finding the
chromosomal characteristics of a cell
• Chromosomes are arranged in pairs according
to their size and structure starting with the
largest
How Karyotyping is Done
• Common pre-natal tests for diagnosis of
chromosome abnormalities:
• chorionic villus sampling (CVS)
– CVS: can be done at 11-12 weeks. Involves taking
sample of chorionic villi (finger-shaped growths
found in the placenta) to get cells from tissue that
originally came from the zygote. Cells will have
same genetic composition as cells of unborn baby so
a karyotype can be made. Can take fewer than 2
weeks.
Cont.
• Amniocentesis
– Amniocentesis: can be done around the 16th week of
pregnancy. Take a sample of amniotic fluid (which
have fetal cells) and a culture is made. Cells are
photographed and chromosomes are arranged in
homologous pairs. Can take up to 3 weeks.
How to Determine Gender from
Karyotyping
• Males: 1 X and 1 Y (XY)
• Females: 2 X’s (XX)
• Can be distinguished on karyotype b/c Y
chromosomes is smaller than an X
How to Determine Nondisjunction from Karyotyping
• If a chromosome is missing or if there's an
extra one by looking at the number of
chromosomes.
• Should only be two of each chromosome.
• Each 23 chromosomes should have a pair
resulting in 46 chromosomes in total.
Activity
• Goal: Arrange and pair chromosomes
together to create a karyotype. Identify if
your karyotype is male or female and if it
has undergone nondisjunction.
Works Cited
• http://www.southtexascollege.edu/nilsson/4_GB_Lecture_figs_f/4_GB_
10_Genetics_Fig_f/13_2_NonDisjunction.GIF
• http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001992/
• http://www.biostudio.com/d_%20Meiotic%20Nondisjunction%20Meiosi
s%20I.htm
• http://faculty.stcc.edu/BIOL102/Lectures/lesson3/punnett.htm
• http://www.hematogenix.com/DirectoryofServices/Cytogenetics/tabid/
70/Default.aspx
• http://www.angleseybonesetters.co.uk/bones_DNA.html
• http://www.exitmundi.nl/giggle.htm