Unit 6: Chromosome Mutations and Aneuploidy
What happens when whole chromosome segments or whole chromosomes go missing, duplicate, flip, move, or multiply, and how that plays out at meiosis.
7 topics · 25 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Chromosomal Deletions
- How it worksHow does a deletion loop form when a chromosome missing a segment pairs with its intact homolog during meiosis?
- Worked exampleTrace how a deletion spanning three genes on one homolog changes the expected dominant phenotype in a heterozygote.
- Common mistakeWhy can't a deletion ever be repaired by pairing with the homologous chromosome's matching sequence?
- Going furtherWhat happens to a fly's phenotype when a deletion on one X chromosome uncovers a recessive lethal allele sitting inside the deleted region?
Your turn1 of 2
A chromosome missing a segment pairs with its intact homolog during meiosis, forming a visible deletion loop where the intact homolog bulges out with no partner. Why can't this pairing let the deletion simply get repaired using the homolog's matching sequence?Chromosomal Duplications
- How it worksHow does unequal crossing over produce one chromosome with a duplication and one with a deletion?
- Worked exampleTrace the Bar-eye phenotype in fruit flies to a tandem duplication on the X chromosome.
- Going furtherWhat happens over evolutionary time to one copy of a gene that has been duplicated?
Your turn1 of 2
A gene family began as a single copy but has undergone 6 independent duplication events over evolutionary time, each one doubling the number of copies present. How many copies of the gene exist in the genome today?Chromosomal Inversions and the Inversion Loop
- How it worksHow does an inversion loop let an inverted chromosome pair with its normal homolog during meiosis?
- Worked exampleTrace the crossover products inside an inversion loop to show why one recombinant chromosome ends up with two centromeres.
- Common mistakeWhy doesn't an inversion change which genes a chromosome carries, only their order?
- Going furtherWhat happens to the viable offspring rate when crossing over occurs inside a pericentric inversion loop instead of outside it?
- Worked exampleTrace which chromatid becomes an acentric fragment and which becomes dicentric after crossing over inside a paracentric inversion loop.
Your turn1 of 2
An inversion flips a chromosome segment end over end, so the genes inside it now read in the opposite order. Why doesn't this change WHICH genes the chromosome carries, only their order?Translocations and Their Meiotic Pairing Consequences
- How it worksHow does a cross-shaped pairing structure form when two non-homologous chromosomes carrying a translocation try to pair?
- Worked exampleTrace how alternate versus adjacent segregation of a translocation quadrivalent produces balanced or unbalanced gametes.
- Common mistakeWhy doesn't carrying a balanced translocation cause any problem for the carrier themselves?
Your turn1 of 2
A person carries a reciprocal translocation between two non-homologous chromosomes but has no health problems themselves. Why doesn't carrying a balanced translocation cause any problem for the carrier?Aneuploidy: Trisomy and Monosomy
- How it worksHow does nondisjunction during meiosis produce a gamete with an extra or missing chromosome?
- Worked exampleFind the chromosome count in a person with trisomy 21 compared to the normal human diploid number.
- Common mistakeWhy isn't every trisomy survivable to birth the way trisomy 21 is?
- Worked exampleFind the karyotype description for a person with monosomy X compared to a person with trisomy 21.
Your turn1 of 2
A person has trisomy for one chromosome (three copies instead of two), while every other chromosome pair is normal. If the species' normal diploid chromosome number is 39, what is this person's total chromosome count?Polyploidy in Plants
- How it worksHow does a failure of meiosis to reduce chromosome number produce a tetraploid plant from two diploid parents?
- Worked exampleFind the chromosome number of an allotetraploid produced from two parent species with 14 and 18 chromosomes each.
- Going furtherWhat happens to fertility when a triploid plant tries to undergo meiosis?
Your turn1 of 2
An allotetraploid plant forms when two parent species, with diploid chromosome numbers of 20 and 19, hybridize and then the hybrid's chromosome number doubles. What is the resulting allotetraploid's total chromosome number?Position Effects of Chromosomal Rearrangements
- How it worksHow does moving a gene next to heterochromatin near a chromosome breakpoint silence its expression?
- Worked exampleTrace the variegated eye color pattern in Drosophila caused by the white gene relocating next to heterochromatin.
- Ties togetherHow does a position effect connect chromosome structure to the gene regulation seen in eukaryotic chromatin?
Your turn1 of 2
A chromosomal inversion in Drosophila relocates the white eye-color gene next to a block of heterochromatin. The fly's eyes turn out variegated, with random red and white patches instead of a uniform color, even though the white gene's DNA sequence itself was never altered. What explains this?