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

http://video.about.com/pregnancy/Amniocentesis.htm

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/