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Lecture 17

HMB265H1 Lecture Notes - Lecture 17: Down Syndrome, Karyotype, Aneuploidy

Human Biology
Course Code
Maria Papaconstantinou

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HMB265: Lecture 17 - Changes in Chromosome Number
- So far looking at consequences of mutations
- Can have whole chromosomes missing
Lecture Outline
2 classes of chromosomal changes:
[1] Euploidy: Change in number of sets of chromosomes
- EX: A normally diploid organism has 2 sets of chromosomes
- If individual has more or less than 2 sets
[2] Aneuploidy: Correct number of sets of chromosomes, but missing or extra single
Two main themes underlying the observations on chromosomal changes
Main observation when look at karyotypes w/i spp
[1] Find that karyotypes highly conserved within spp
- If compare 2 individuals, karyotypes look similar
- B/c genetic imbalances usually have selective disadvantage = few differences
[2] Related species usually have different karyotypes
- This is why hybrids are often sterile
- Chromosomes not similar enough to pair
- Closely related spp differ by few rearrangements
- Distantly-related spp differ by many rearrangements
- Correlation between karyotypic rearrangements and speciation
- Must be mechanisms that preserve genetic material from one generation to
Changes in chromosome number
- Cases where end up with more or less sets of chromosome
- Monoploidy = 1 set of chromosomes instead of 2
- Lethal in humans
- Polyploidy = More than 2 sets of chromosomes
- Lethal in humans
- Case where have one missing/extra chromosome
- Monosomy = only have one copy of particular chromosome instead of 2
- Usually lethal unless it’s on chromosome 21 (die shortly after birth)
- Trisomy = 3 copies of particular chromosome instead of 2
- Down syndrome = trisomy 21
- Deleterious effects often shorter lifespan
- Only have 1 set of chromosomes
- In male bees, wasps, ants
- Develop from unfertilized eggs
- Generate viable gametes b/c gametes generated via mitosis (not meiosis)
- Other than these insects, finding monoploid organisms really rare
- Rare b/c they unmask recessive lethals
- In normal situation WT can dominate over recessive allele
- If only have 1 set of chromosomes, rely on single copy of gene
- = recessive lethal mutation always expressed
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- If these individuals do survive to adulthood, don’t generate any gametes
Monoploidy can be produced experimentally
Monoploid organisms can be really useful
- Can use them to uncover mutations
- If mutation is recessive, always have to wait for F2 generation
- Even then only a quarter of them have mutation
- Bypass this if generate plants that have a single set of chromosome
EX: Try to identify mutants with phenotype of interest
- Harvest pollen from plants
- Grow them in plates and add mutagen
- Select for plant with phenotype of interest
- But this plant will be sterile
- When end up with plant we selected for, and want it to propagate
- Need to make sure meiosis can occur
- Inhibit formation of spindle and separation of 2 sets
- = cell with 2 sets of chromosomes
- Turn monoploid with diploid plant that can propagate
- Can select for plants that are herbicide resistant
- Very common in plants
- Sometimes get new species
- Used a lot in agriculture to generate different species with desirable traits
- If have extra sets of chromosomes = corresponds with bigger size, greater vigour
- Strawberries are tetraploid (have 8 sets of chromosomes)
Polyploidy: Autopolyploids
Want to generate polyploid
- In normal diploid organism, gametes = haploid
- To generate triploid organism (zygote):
- Need fusion of one diploid gamete and one haploid gamete
- Diploid gamete can only come from tetraploid parent or from nondisjunction
- (Haploid gamete can come from normal diploid parent)
Autotriploids are sterile
Triploid gametes are sterile
- Have 3 homologues per chromosome
- The third homologue has no one to pair with
- 3rd can’t = can’t form balanced gametes
- = Whenever have odd number of sets of chromosomes, progeny is sterile
- EX: Bananas
Generate tetraploids
- Can achieve via spontaneous doubling
- Have to come from 2 diploid gametes
- Use drug called colchicine
- Drug prevents spindle from forming
- = in new cell just goes from being diploid to tetraploid
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