Genes - Lecture 20, Chromosome abnormalities and cytoplasmic inheritance Flashcards
What is aneuploidy?
Chromosome abnormality where there’s an abnormal number of chromosomes
What is monosomy?
Loss of a single chromosome
What is trisomy?
One extra chromosome
What is tetrasomy?
An extra pair of chromosomes
What happens during meiosis to produce aneuploid gametes?
Mistakes in chromosome segregation
How many human conceptions are aneuploid?
20-50%
How many live births are aneuploid?
<0.5%
What is the trisomy of Down syndrome and what are its characteristics?
Trisomy 21
Facial features, short, learning disabilities
1 in 1000 births
What is the trisomy of Patau syndrome?
Trisomy 13
What is the trisomy of Edwards syndrome?
Trisomy 18
What are the two types of prenatal screening for down syndrome?
Combined test - ultrasound and blood test
Quadruple test - blood test
How are cells sampled from the foetus?
Chorionic villus sampling
OR
Amniocentesis
What is the process of sampling cells from the foetus?
CVS or amniocentesis is used
The karyotype of foetal cells is determined
What is robertsonian translocation as another cause of down syndrome?
Involves chromosome 21 and 14
There is illegitimate recombination between rDNA repeats on short arms of chromosome 21 and 14
This fuses the chromosomes
What is genetic mosaicism as another cause of down syndrome?
Individuals have a mix of normal and trisomy 21 cells
An embryo with trisomy 21 followed by loss of extra chromosome 21 by mitotic non-disjunction
What is turner syndrome as an example of monosomy?
45 X
Sterile females, short, learning difficulties
What is Klinefelter syndrome as an example of trisomy?
47 XXY
Sterile males, lanky, feminised physique, difficulty speech and reading
What is triplo-X as an example of trisomy?
47 XXX
Normal with some effect on fertility and intelligence
What is XXY syndrome as an example of trisomy?
47 XXY
Fertile males
Above average height
What is the link between maternal age and incidence of trisomy?
As womens age increases the change of trisomy and therefore miscarriage increases
What is meant by human oocytes being paused?
In meiosis I, human oocytes are paused
They are arrested at late meiotic prophase I with paired replicated chromosomes
Occurs before birth and is maintained until egg matures in menstrual cycle
What is the link between cohesion and aneuploidy?
In older women there can be a loss of cohesion in prophase I which can cause aneuploidy
Premature loss of cohesion gives two univalents that segregate independently
This gives aneuploid gametes
What are the main two places DNA can be found outside of the nucleus?
Mitochondria
Chloroplasts
What type of genes are found in mtDNA?
Components needed for translation
Structural genes for proteins needed in oxidative phosphorylation
What types of genes are found in cpDNA?
Genes encoding tRNAs and rRNAs involved in chloroplast translation
Structural genes for proteins needed in photosynthesis
How are the extranuclear genomes inherited?
Through the uniparental pattern of maternal inheritance
How is mitochondrial disease transmitted?
Maternal inheritance
Females transmit the trait to all children and males never transmit the trait
Why are the symptoms of mitochondrial disease variable, even within one family?
Heteroplasmy
What is heteroplasmy?
In terms of mitochondrial disease:
Some oocytes randomly contain more mutant mitochondria
This means in the mature oocyte the symptoms will be more severe than others
What is the treatment for mitochondrial disease?
Maternal spindle transfer to create a three person baby
There is repair done to the egg before fertilisation
Metaphase II spindle and associated chromosomes from mother’s egg are transplanted to the donors egg
The donors egg’s nucleus has been removed
