HONR 1034 Chapter Notes - Chapter 8: Down Syndrome, Monosomy, Nondisjunction
Chromosome Mutations: Variation in Number and Arrangement
I. Terminology
A. Aneuploidy: organism gains or loses ONE or MORE chromosomes (2n ± chromosomes)
1. Monosomy: 2n - 1
2. Trisomy: 2n + 1
B. Euploidy: presence of complete SETS of haploid chromosomes
1. Diploid: 2n
2. Triploid: 3n
3. Tetraploid: 4n
4. Polyploidy: 3n, 4n, 5n, etc
C. Autopolyploidy: multiples of the SAME GENOME
D. Allopolyploidy: multiples of CLOSELY RELATED GENOMES
E. All of these variations happen because of random errors during production of gametes →
nondisjunction (homologs fail to disjoin during segregation so both end up in one gamete)
F. Normal disjunction followed by second-division nondisjunction and first-division nondisjunction
followed by normal;
II. Monosomy: 2n - 1
A. Not tolerated in humans and animals → better tolerated in plants
B. For large autosomal chromosomes II and III → monosomy is lethal
C. If just one homolog in monosomic individuals is lethal, unpaired chromosomes condition will kill
the organism
D. Haploinsufficiency: when a single copy of a recessive gene is insufficient in providing adequate
function for sustaining an organism
III. Trisomy: 2n + 1
A. Effects generally parallel monosomy, but organisms become slightly more viable
B. Plant phenotype may be altered
C. Down syndrome/trisomy 21: only trisomy with many cases where individuals will survive
1. Random error in nondisjunction
2. From male or female parent chromosomes, but usually female
3. Individuals exhibit similar characteristics in their face
4. Prone to respiratory disease and heart malformations
D. Down syndrome critical region: chromosome 21 contains genes that are dosage sensitive
E. When it comes to females, meiosis starts when they are still a fetus and its arrested at that state;
the eggs already formed; as the mother is aging they age also
1. They enter meiosis every month
F. Invasive testing
1. Amniocentesis
2. Chorionic villus sampling (CVS) → obtaining fetal cells around the placenta
G. Noninvasive prenatal genetic diagnosis → blood sample to get fetal cells
IV. Patau syndrome (47,13+)
A. Lethal (trisomy of chromosome 18)
B. 20% of all conception terminate spontaneously
1. 30% of spontaneously aborted fetuses demonstrate some form of chromosomal imbalance
V. Polyploidy
A. Autopolyploidy: the addition of one or more extra sets of chromosomes identical to the normal
haploid complement of the same species; multiples of the SAME GENOME
B. Allopolyploidy: the combination of chromosome sets from different species occurring as a
consequence of hybridization; multiples of CLOSELY RELATED GENOMES
C. If A = original set of haploid chromosomes, the A = a1 + a2 + a3 + … + an
VI. Variation in the arrangement of chromosomes
A. Deletion, duplication, or inversion
B. Usually, changes are due to one or more breaks along the axis of the chromosome
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