4.2.4 – 4.3.1 Non-disjunction, karyotyping, chorionic villus sampling, amniocentesis, theoretical genetics
Tyler and Tahjai
4.2.4Explain that non-disjunction can lead to changes in chromosome number, illustrated by reference to Down syndrome (trisomy 21).
4.2.4Non-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)
4.2.4Synapsis- pairing up of homologous chromosomes during Prophase I
Resulting pairs of homologous chromosomes are called tetrads or bivalents
4.2.4Trisomy 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 parent gamete will carry two copies of chromosome 21
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
4.2.5State that, in karyotyping, chromosomes are arranged in pairs according to their size and structure
4.2.5Karyotyping- 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
4.2.6State that karyotyping is performed using cells collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis of chromosome abnormalities
AmniocentesisChorionic Villus Sampling
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
4.2.6Chorionic 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
4.2.6Amniocentesis
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 DefinitionsGenotype- the alleles of an organismPhenotype- includes all the characteristics of an organismDominant Allele- one which has the same effect on the phenotype whether it is present in the homozygous or heterozygous stateRecessive Allele- one which only has an effect on the phenotype when present in the homozygous stateCodominant Alleles- a pair of alleles that both affect the phenotype when present in a heterozygoteLocus- the particular position on homologous chromosomes of a geneHomozygous- having two identical alleles of a geneHeterozygous- having two different alleles of a geneCarrier- a heterozygous individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for this alleleTest Cross- testing a suspected heterozygote by crossing with a known homozygous recessive
4.3.1Genotype- the alleles of an organism
Phenotype- includes all the characteristics of an organism
4.3.1Dominant 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
4.3.1Codominant 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
4.3.1Homozygous- having two identical alleles of a gene
Heterozygous- having two different alleles of a gene
4.3.1Carrier- 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-Villus-Sampling.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-gulf-war-illness-test-results-video.htm>.
Karyotype game http://learn.genetics.utah.edu/content/begin/tr
aits/karyotype/
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