BIOL202 TA Problem Set 7
1. Imagine the progeny of a cross between two related plant species of identical ploidy (2n=36). Cytogenetic analysis indicates that three of the chromosome pairs form bivalents at meiosis while the others do not. How would you describe the ploidy of this plant and what do you predict about its fertility? What would need to take place for these hybrids to be able to intercross, what is the diploid number of chromosomes in that plant?
2. You are a member of the International Chicken Genome Sequencing Consortium and you are hard at work assembling the genome sequence of the red junglefowl Gallus gallus, the ancestor of the domestic chicken (Nature 432 p 695­716, 2004). Using a BAC library prepared from the DNA of a female junglefowl, you assemble contigs and fill in gaps until you end up with a total 40 contigs. What is the number of chromosomes (n) in the junglefowl?
3.
Chromosomes are always numbered in order of size (in humans 1 being largest, 22 being smallest). Several lines of evidence show that humans shared a common ancestor with chimpanzees between 5­6 million years ago, and that we shared a common ancestor with gorillas between 7­8 million years ago.
1) What is the name of the chromosomal re­arrangement that distinguishes humans from chimps and gorillas? Based solely on this image, what would you expect the diploid chromosome number to be in gorillas and chimpanzees? 2) What is the most likely range of time in which this major change occurred? How could you increase your confidence in this?
BIOL202 TA Problem Set 7
3) What would we expect to find at position A in the human chromosome 2?
4) You discover that at position B, the segment in gorillas has been inverted and the orientation flipped around…what is this called and what is the range of time in which it most likely occurred?
5) Based on the dates mentioned above since a shared common ancestor, are chimpanzees more closely related to humans or gorillas?
4.
Part A
If meiotic non­disjunction in females occurs at first division, how many aneuploid gametes present after the second division? In addition, if normal meiotic division occurs in the first division, but non­disjunction occurs during second division how many aneuploid gametes are produced?
1) Two and Three
2) Three and Two
3) Four and Two
4) One and Two
Part B
A male individual that is XYY mates with a female (XX), what possible gametes are produced?
6) 2 males (XY) and 2 females (XX)
7) 2 males (XYY) and 2 females (XX)
8) 3 males (XYY) and 1 female (XX)
9) 4 males (XXY)
Part C
A female individual (XXX), mates with a male (XY), list all possible genotypes for male and female gametes. BIOL202 TA Problem Set 7
Answers:
1.
The new hybrid plant is an aneuploid monosomic, there are 30 univalents during meiosis in this individual leading to its sterility. For fertility to be restored there would need to be duplication at those 30 chromosomes that do not pair, but not at those that do. If this happened the progeny from, say, a self of this plant would be 2n=66.
2.
39 chromosomes. In birds, the female is heterogametic and has sex chromosomes ZW whereas the male is homogametic and has sex chromosomes ZZ. Therefore a genome sequenced from a female with 40 contigs means that the number of chromosomes is 39 (38 autosomes + the sex chromosomes Z and W). 3.
5) Based on the banding patterns we can see that the human chromosome 2 is the result of a fusion between two chromosomes. Therefore we can expect gorillas and chimps to have an extra chromosome, in the diploid state then, they would have 46 autosomes as well their sex chromosomes…48 (which is their actual number)
6) Given that only humans seem to have this chromosome fusion, it is overwhelmingly more likely that this occurred once in the human lineage (orangutans also don’t have this fusion) rather than three chromosome breaks in gorilla, chimp and orangutan lineages. Therefore it would have happened after the split between humans and chimps.
7) If it was a fusion rather than two separate deletions, there would be a vestigial centromere here in humans (which is actually found there in reality)
8) It is called an inversion, if it is only found in the gorilla, then it most likely occurred once in the gorilla lineage after it split with the ancestor that it shared with humans and chimps 7­8 million years ago
9) Humans and chimpanzees shared a common ancestor more recently than humans and gorillas as well as chimps and gorillas which also would have shared a common ancestor between 7­8 million years ago. Therefore, gorillas split from the lineage that led to humans and chimpanzees first. Although chimps and gorillas are more similar than chimps and humans in terms of this chromosome fusion, chimps share more genomic characteristics with humans on the whole then BIOL202 TA Problem Set 7
they do with gorillas.
4.
Part A – Non­disjunction at first division leads to two (n­1) and two (n+1) chromosomes and at second division it leads to one (n­1) and one (n+1) chromosome. The answer is C. Part B – The second Y chromosome from the male does not pair and is not transmitted to gametes. The answer is A. Part C – The third X chromosome from the female also does not pair, so possible gametes are females, XX and males, XY.