Genetics Flashcards
What is chromosomal aneuploidy
Cells have abnormal number of chromosomes (loss or gain of one or a few chromosomes relative to the diploid)
What is a karyotype and how is it done
An ordered, visual representation of the chromosomes in a cell
Done by taking a blood sample and treating it with mitogen, then colchicine, then stain
What is the purpose of using colchicine in making a karyotype
To stop the cell cycle at metaphase when the chromosomes are clearly visible
How is karyotype numbered
In order of decreasing size ie. 1 = largest chromosome
What are the 4 major types of chromosomes
Metacentric
Sub-metacentric
Acrocentric
Telocentric
Define metacentric chromosomes and give examples
Have centromeres in the centre of the chromosome, such that both sections have equal length
Chromosome 1,3 in group A
Define sub-metacentric chromosomes and give examples
Have centromeres located towards one end of chromosome resulting in asymmetry
Chromosomes 4-12, groups B,C
Define acrocentric chromosomes and give examples
Have centromeres located severely towards one end of chromosome leading to one very one and one very short section
Chromosomes 13,15 (group D), 21, 22 (group G)
Define telocentric chromosomes and give examples
Have centromeres located at the very end of the chromosome
Humans do not posses telocentric chromosomes but are found in other species such as mice
Define locus
location where a particular gene is found
Define allele
Alternative forms of a gene found at locus on a chromosome
Define gene
Sequence of DNA that produces a functional product found at a particular locus
Examples of aneuploidy in humans
Down syndrome Klinefelter syndrome Turner syndrome Patau syndrome Edwards syndrome
Down syndrome (Trisomy 21) - (4)
- Individual has 3 copies of chromosome 21 (1 extra copy)
- most common chromosomal defect
- occurs 1/750 live births
- can occur in other organisms
Major features of down syndrome (there is variability in features between individuals) - (5)
- broad flat face
- slanting eyes
- short nose
- deep fold/crease across centre of pam
- mental retardation
What factor contributes to 40% of all Down syndrome cases
mother >45
Klinefelter syndrome - cause and effects (3)
Cause XXY - extra copy of X chromosome Effects - affects male sexual development - slightly feminized physique - Testicular atrophy - breast development
Turner syndrome - cause and effect (5)
Cause XO - 1 normal X chromosome but the other chromosome is either missing or structurally altered Effects - affects female development - short stature - Webbed neck (extra folds in the neck) - poor breast development - low hairline at back of neck - constriction of aorta
Name the 2 common methods of prenatal diagnosis of aueuploidy
Amniocentesis
Chorionic Villus sampling
Amniocentesis - when can it be done and risk of miscarriage
Can be done 16-20 weeks into pregnancy
0.1% risk of miscarriage
Outline the process of Amniocentesis
Sample of Amniotic fluid is taken out of the amniotic cavity. This an be done due to fetal cells floating around in the cavity. The sample cells are grown up by cell culture and a sample of it is taken to look at the karyotype
Chorionic villus sampling - when can it be done and risk of miscarriage
Can be done 10-13 weeks into pregnancy
1% risk of miscarriage
Outline the process of Chorionic sampling
Sample of fetal cell is taken from chorionic villus to visualise karyotype
Accuracy of chorionic villus sampling
very accurate - detects 90% of down syndromes
Define nondisjunction and 2 ways in which it can happen
Failure of homologous chromosomes or sister chromatids to separate properly during cell division
Can happen in meiosis I or meiosis II
Nondisjunction in meiosis I
nondisjunction at first division (anaphase I) results in homologues chromosomes failing to separate into 4 daughter cells
Result of nondisjunction in meiosis I (products)
2 (n+1) daughter cells and 2 (n-1) daughter cells
Total number of chromosomes still correct
Nondisjunction in meiosis II
Nondisjunction at second division (anaphase II) resulting in aneuploidy gametes
Result of nondisjunction in meiosis II (products)
2 (n - normal) daughter cells, 1 (n+1) daughter cell and 1 (n-1) daughter cell
The (n+1) cell will have 2 copies of the same chromosome either from mum or dad and can be confirmed by looking at the centromeres
Difference in (n+1) cells produced due to nondisjunction in meiosis I vs. meiosis II
In meiosis 1, there are 2 (n+1) daughter cells that will have 2 difference chromosomes - 1 maternal and 1 paternal which can be confirmed by looking at the centromere region
In meiosis II, there is only 1 (n+1) daughter cell produced and will have 2 copies of the same chromosome either form mum or dad and can be confirmed by looking at the centromeres as they will be identical
What is a barr body
Inactive X chromosome in a female somatic cell seen as a dark dense spot which lies along the inside of the nuclear envelope
Barr body - lyon hypothesis
Dosage compensation through X inactivation (lyonisation) to ensure that the we have the correct amount of gene products from males with only 1 X chromosome and from females who has 2 X chromosomes
When does X chromosome inactivation occur
Quite early on in development - 16 day morula (500-1000 cell stage)
X chromosome inactivation in humans
Occurs at random - the inactivated X chromosome can come from mum or dad
X chromosome inactivation in women
Mosaic cells - some cells have maternal X chromosome inactivated while others have paternal X chromosome inactivated
Example of X chromosome inactivation - calico cats
Calico cats are always female
1 X chromosome carries an allele for black fur and 1 X allele carries an allele for black fur. Early on in embryonic development, one X chromosome is inactivated in every cell at random. From that point on in development, every descendant cell has the same X inactivated as the original cell.
The calico cat displays a mixture of red- and black-based colours, depending on which of its two X chromosomes has been inactivated.
What is polyploidy
Possession of multiple entire sets of chromosomes. This is when an organism has more than 2 homologues chromosome sets in all somatic cells
Explain triploidy (3n) - cause and consequence
Possible cause: fertilization of an abnormal diploid egg (produced by nondisjunction of all its chromosomes) with a haploid sperm cell producing a zygote which has inherited an extra set of chromosomes from one of the parents
Consequence: imbalanced gametes - 3 homologues pairs cannot line up next to each other at the equator so it is sterile
Explain Tetraploidy (4n) - cause and consequence
Possible cause: failure of a 2n zygote to divide after replicating its chromosomes/result of nondisjunction
Consequence: normal mitotic division would then produce a 4n embryo which is fertile
What is autopolyploidy
Polyploids with multiple chromosome sets derived from a single species
What is allopolyploidy
Polyploids with chromosomes derived from different species. Occurs when 2 species produce hybrid offspring. Are fertile when mating with each other but cannot interbreed with either parent species
List examples of polyploids
Bananas
Wheat
Lizards and reptiles
Outline the evolution of wheat (3)
- 2 species of diploid wheat (AB) hybridised to produce an allotetraploid (AA BB)
- The allotetraploid hybridised with another wild diploid wheat
- A feritile allohexaploid (AA BB CC) is produced
Outline some examples of chromosome rearrangements in humans
Through different types of mutations
- Deletion
- Duplication
- Inversion
- Translocation
Deletion (most common)
Removes a chromosomal segment
Duplication (often during DNA replication)
repeating a segment
Inversion (No net gain or loss)
Reverses a segment within a chromosome - same genes but wrong order
Translocation - reciprocal and nonreciprocal
Translocation moves a segment from one chromosome to another between non-homologous chromosomes Reciprocal translocation (most common) - non-homologous chromosomes exchange fragments Nonreciprocal translocation - a chromosome transfers a fragment without receiving a fragment in return
Name the disorders caused by structurally altered chromosomes (5)
Lejeune syndrome (Cri du chat) William - Beuren syndrome Chronic myelogenous Leukemia (CML) Duchenne Muscular Dystrophy (DMD) Familial Down Syndrome
Lejeune syndrome - cause and characteristics (4)
Deletion of the tip of the short arm of chromosome 5 (sub-metacentric chromosome) Characteristics - mental retardation - poor growth - eyes widely separated - makes cat-like sounds
William - Beuren syndrome - cause and characteristics (8)
Due to micro-deletion of chromosome 7 (extend of micro-deletion can vary between individuals) Characteristics - mental retardation - reduced spatial awareness/cognition - autism - ease with strangers - left-handedness - love for music - social - cardiovascular problems
Chronic Myelogenous Leukemia (CML) - causes
Somatic reciprocal translocation between a large portion of chromosome 22 and a small fragment of the tip of chromosome 9 resulting in a much shortened, easily recognised chromosome 22, called the Philadelphia chromosome resulting in cancer at the white cells
What is Philadelphia translocation t(9;22)
Result of crossing over between non-homologous chromosome 9 and 22 causing bone marrow not dividing properly
Can happen during meiosis or during development
Duchenne Muscular Dystrophy (DMD) t(X;21) - cause and characterstics
It is a severe recessive X-linked form of muscular dystrophy where a small portion of the top of an X chromosome is translocated with a portion of the top of chromosome 21
Characteristic - rapid progression of muscle degeneration, eventually leading to loss of walking and death
What is familial down syndrome caused by
Caused by robertsonian translocation where the long arms of chromosome 14 and chromosome 21 fuse (Acrocentric chromosomes)
Possible zygotes formed from familial down syndrome
3 lethal
- 1 missing a copy of chromosome 14
- 1 missing a copy of chromosome 21
- 1 has 2 and a half copies of chromosome 14 and 1 and a half copies of chromosome 21
1 Down syndrome - 2 and a half (fused) copies of chromosome 21 and 1 and a half (fused) copies of chromosome 14
1 Translocation carrier - 1 and a half (fused) copies of each chromosome (14 and 21)
1 normal zygote
how can chromosomal abnormalities occur in the soma cells alone
Error in mitotic division
Error in DNA replication
What chromosomal abnormailities are caused by deletion (2)
Lejeune syndrome and William - Beuren syndrome
What chromosomal abnormalities are caused by translocation
Chronic myelogenous Leukemia (CML), Duchenne Muscular Dystrophy (DMD), Familial Down Syndrome
