Chromosome Abnormalities/ Cytoplasmic Inheritance Flashcards
What is aneuploidy?
Aneuploidy is a type of chromosome abnormality where the number of chromosomes is abnormal (but NOT loss or gain of a complete chromosome set).
What is the normal human karyotype for males and females?
46, XY for males and 46, XX for females.
What process can give rise to aneuploid gametes?
Abnormal chromosome segregation during meiosis, specifically non-disjunction.
What percentage of human conceptions are aneuploid?
20-50% of human conceptions are aneuploid, but <0.5% of live births are aneuploid.
What is Down syndrome associated with?
Trisomy 21, characterized by facial features, short stature, learning disabilities, and higher risk of heart defects.
What is the risk of Down syndrome at birth?
Approximately 1/1000 births.
What prenatal tests are offered to pregnant women in the UK to identify risk of aneuploidy?
The combined test (ultrasound + blood test) and the quadruple test (blood test).
What is chorionic villus sampling (CVS)?
A procedure used to sample cells from the fetus to determine the karyotype.
What is the most common cause of Down syndrome?
Maternal non-disjunction, accounting for 95% of cases.
What is Turner syndrome?
A condition characterized by monosomy (2n - 1) where individuals have 45 chromosomes (XO), leading to sterile females and short stature.
Most individuals not diagnosed as babies
What is Klinefelter syndrome?
A condition involving trisomy (2n + 1) where males have an extra X chromosome (47, XXY), leading to sterility and slower muscle growth.
What is the significance of maternal age in relation to trisomy?
The incidence of trisomy increases with maternal age, largely due to non-disjunction during meiosis.
What are mitochondrial and chloroplast genomes characterized by?
They have circular double-stranded DNA genomes and many prokaryotic features.
What types of genes are found in mitochondrial genomes?
- Components required for translation (e.g., tRNAs and rRNAs)
- Structural genes for proteins involved in oxidative phosphorylation
Many genes have transferred from mtDNA to the nuclear genome
How is mitochondrial DNA inherited in mammals?
Maternal inheritance; paternal mitochondria are destroyed after fertilization.
What is heteroplasmy?
The presence of more than one type of mitochondrial DNA within a cell producing variation
Due to passive segregation of mitochondria during cell division (divides randomly)
What is mitochondrial replacement therapy?
A technique where the mother’s egg is repaired before fertilization by transplanting the metaphase II spindle and associated chromosomes to a donor egg.
Fill in the blank: The normal human karyotype for females is _______.
46, XX
True or False: Most individuals with Klinefelter syndrome are diagnosed at birth.
False
What is the typical size difference between human eggs and sperm?
Egg diameter ~100 mm, sperm head ~5 mm.
What is genetic mosaicism?
Where 2 chromosomes are mixed
Causes Down syndrome
- individuals have a mix of normal and trisomy 21 cells
- arises from embryo with trisomy 21 followed
- followed by loss of the extra chromosome 21 by mitotic non-disjunction in early cleavage divisions
- or by a euploid embryo with mitotic non-disjunction
What is the significance of human oocytes?
- Human oocytes are arrested at late meiotic prophase I with paired, replicated chromosomes
- Meiotic arrest begins prior to birth and is maintained for decades until the egg matures during the menstrual cycle
How does loss of cohesion in Prophase I lead to aneuploidy in older women?
Cohesion keeps homologous chromosomes together
Premature loss of cohesion gives 2 univalents (instead of bivalents) that segregate independently giving aneuploidy (n+1) gametes
What are the features of extra nuclear genomes?
Mitochondria and chloroplasts have prokaryotic features
- circular dsDNA genomes
- ribosomes of these are different from those in cytoplasm and are sensitive to bacterial antibiotics
What types of genes are found in chloroplast genomes?
Bigger than the mitochondrial genome
- genes encoding tRNAs and rRNAs involved in chloroplast translation
- structural genes for proteins involved in photosynthesis
How are extranuclear genomes transmitted?
Mitochondrial and chloroplast genomes are often transmitted from one generation to the next in a uniparental pattern - usually maternal inheritance
- Females transmit the trait to all of their children
- Males never transmit the trait
