Variation In Chromosome Number Flashcards
1
Q
Variations in chromosome number
A
- aneuploidy: increase or decrease in the number of individual chromosomes
- eg. Trisomy - 3 copies of a chromosome
- polyploidy: increase in the number of sets of chromosome
- eg. Triploid - 3 copies of every chromosome
2
Q
Aneuploidy (common)
A
- the 4 most common types of aneuploidy in diploid individuals
1. Nullisomy: loss of both members of a pair of homologous chromosomes
2. Monosomy: loss of a single chromosome
3. Trisomy: Gain of a single chromosome
4. Tetrasomy: gain of 2 homologous chromosomes
3
Q
Aneuploidy (less common)
A
- double monosomic: loss of 2 non-homologous chromosomes
- double trisomic: gain of 2 non-homologous chromosomes
4
Q
Origins of aneuploidy
A
- Non-disjunction in meiosis or mitosis
- failure of homologous chromosomes or sister chromatids to separate
- Deletion of a centromere leads to chromosome loss
- trisomy may be viable
- monosomy not viable unless sex chromosome
5
Q
Aneuploidy: plants
A
- usually viable
- phenotype maybe altered and increased sterility
6
Q
Aneuploidy: humans
A
- trisomy 13: patau syndrome; 1/16,000 newborns
- trisomy 18: Edwards syndrome; 1/5000 live-born infants
- trisomy 21: Down syndrome: 1/800 newborns
- sex chromosome aneuploidies:
1. Monosomy X (XO): Turner syndrome (1/2500 girls)
2. Extra X copies: Klinefelter syndrome (1/500-1000 males) - chromosomal abnormalities is thought to be the most common cause of spontaneous abortions/miscarriages
- live birth usually have trisomy of smaller chromosomes or sex chromosomes
7
Q
Primary Down syndrome
A
- trisomy 21
- most cases arise from random non disjunction during meiotic division
- mother contributes the extra chromosome in 75% of cases
- incidence of trisomy 21 increases sharply with increasing maternal age
- possibly due to the fact that eggs are formed at birth, and arrest in the stage of meiosis
8
Q
Familial Down syndrome
A
- an extra copy of chromosome 21 is attached to another chromosome (14 or 15)
- occurs in about 3-4% of cases
- arise in offspring of parent who carry a chromosome that underwent robertsonian translocation
- translocation carrier
- 45 chromosome, one of which is a translocation chromosome
- normal phenotype, does not have Down syndrome
9
Q
Polyploidy
A
- for diploid individuals, polyploidy is the presence of more than two sets of chromosomes
- triploids: 3n
- tetrapods: 4n
- pentaploids: 5n
- common in plants, less common in animals
10
Q
Types of polyploidy
A
- Autoploidy: multiples of the same genome
- eg. Autotetraploid: 4n
- Allopolyploid: multiple of closely related genomes
- eg. Allotetraploid: 4n - 2n from species i and 2n from species ii
11
Q
Origins of autoploidy
A
-nondisjunction of all chromosomes during mitosis can produce autotetraploid
- nondisjunction of all chromosomes during meiosis produces diploid gametes
- diploid gamete + normal gamete = autotriploid (3n)
- diploid gamete + diploid gamete = autotetraploid (4n)
12
Q
Effects of autopolyploidy
A
- usually sterile
- most gametes produced are genetically unbalanced
- triploid zygotes can produce many possible gametes
- each gamete can get either one or two copies of each chromosomes in many possible combinations
13
Q
Generating allopolyploid species
A
- gametes of 2 species fuse together
- hybrid is sterile because unbalanced gametes are non-viable
- but if entire genome is doubled by mitotic nondisjunction, the fertility problem is solved
14
Q
Significance of polyploids: agriculture
A
-production of larger fruits (strawberries and grapes)
- production of seedless fruit (sterile)
- eg. Watermelon, bananas, grapes
15
Q
Bananas in trouble!
A
- domestic bananas (3n=33) are derived from 2 wild species: Musa acuminata (A) and Musa Balbisiana (B)
- most of our bananas are AAA (Gros Michel, cavendish)
- most plantains ABB or AAB
- Gros Michel wiped out by panama disease
- replaced by resistant cavendish
- new strain of panama disease spreading around world and Cavendish are not resistant
