Chromosome Abnormalities & Cytoplasmic Inheritance - Lecture 15 Flashcards
What is aneuploidy?
A type of chromosome abnormality where the number of chromosomes is abnormal
How can anueploidy arise?
○ Nondisjuction of homologous chromosomes in meiosis I:
○ Nondisjuction of homologous chromosomes in meiosis II
What is monosomy?
Missing a chromosome
What is trisomy?
One extra chromosome
What is tetrasomy?
Two extra chromosomes
How does aneuploidy affect humans - monosomy?
Lethal if it is autosomal
How does aneuploidy affect humans - trisomy?
○ Down syndrome (trisomy 21)
○ Patau syndrome (trisomy 13)
○ Edwards syndrome (trisomy 18)
How are prenatal screenings for Down syndrome done?
○ CVS and amniocentesis is used to sample cells from the foetus
○ Non-invasive prenatal testing looks at foetal DNA in mother’s blood
How else is Down syndrome caused?
○ Robersonian translocation involving chromosome 21 and chromosome 14:
- llegitimate recombination between rDNA repeats on the short arms of Chr 14 & 21 fuses the long arms of the chromosomes
○ Genetic mosaicism:
- Individuals have a mix of normal and trisomy 21 cells (2% of cases)
○ Arises from embryo with trisomy 21 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
How does aneuploidy affect humans - monosomy in sex chromosomes?
○ Turner syndrome (XO):
- Sterile females
- Most individuals not diagnosed as babies
How does aneuploidy affect humans - trisomy in sex chromosomes?
○ Klinefelter syndrome (XXY):
- Sterile males
○ Triplo-X (XXX):
- Mostly normal but some effect on fertility and intelligence
○ XYY syndrome:
- Fertile males and above average height
What can happen to human oocytes?
○ 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 menstrual cycle
What can happen in older women that leads to aneuploidy?
○ 95% of trisomy 21 is due to maternal non-disjunction:
- Most non-disjunction occurs in meiosis 1
- Loss of cohesion is prophase I
What type 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
What types of genes are found in chloroplast genomes?
○ Genes encoding tRNAs and rRNAs involved in chloroplast translation
○ Structural genes for proteins involved in photosynthesis
