Can you explain how you can tell that there are 3 good gametes out of this? I only see 2 (the first two arrows on the left). The answer explanation only refers to Figure 1 and no other info.
NS FL2 - Bio/Biochem Question 5
- Thread starter Mr. otcoD
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The male has one chromosome 14, one chromosome 21, and one translocated chromosome 14/21. During meiosis, the 14/21 either pairs with 14 or 21.
If it pairs with 14, 14 and 14/21 will necessarily be in different gametes. 21 can be in either gamete so the possibilities are: 14 (no 21), 14 (with 21), 14/21 (with 21), 14/21 (no 21)
If it pairs with 21, 21 and 14/21 will necessarily be in different gametes. 14 can be in either gamete so the possibilities are: 21 (no 14), 21 (with 14), 14/21 (with 14), 14/21 (no 14)
Of the 8 possibilities, 2 pairs (bolded) are repeats so there are 6 unique combinations (shown in that chart attached). But there is still a 4/8 chance of good gametes (bolded).
TLDR: The chart attached is misleading! I wouldn't sweat this question 🙂
FWIW: I only figured this out after getting super confused on a KA passage awhile back.
If it pairs with 14, 14 and 14/21 will necessarily be in different gametes. 21 can be in either gamete so the possibilities are: 14 (no 21), 14 (with 21), 14/21 (with 21), 14/21 (no 21)
If it pairs with 21, 21 and 14/21 will necessarily be in different gametes. 14 can be in either gamete so the possibilities are: 21 (no 14), 21 (with 14), 14/21 (with 14), 14/21 (no 14)
Of the 8 possibilities, 2 pairs (bolded) are repeats so there are 6 unique combinations (shown in that chart attached). But there is still a 4/8 chance of good gametes (bolded).
TLDR: The chart attached is misleading! I wouldn't sweat this question 🙂
FWIW: I only figured this out after getting super confused on a KA passage awhile back.
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just took this NS2 exam and was wondering the same thing, what a misleading graph and set up!
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I think both KA and NextStep either did not understand the phenomenon or were negligent in writing this question. The answer is indeed 1/2 BUT not because of what they think.
If you actually draw the tetravalent synapse out, you can clearly see that homolog segregations can only result in 3 cases:
"Alternate:" two gametes with a normal 14 and a normal 21, and two gametes with a Robertsonian translocation (14 fused with 21)
"Adjacent -1:" two gametes with a normal 14 and a Robertsonian translocation, and two gametes with a normal 21
"Adjacent -2:" two gametes with a normal 21 and a Robertsonian translocation, and two gametes with a normal 14
So we have 12 possible gametes here. Of the 12, all the gametes produced by "alternate" segregation (1/3) are viable and will produce phenotypically normal offsprings. In "adjacent-2," the 2 gametes with normal 21 and a Robertsonian will produce viable offspring with familial Down syndrome (1/6)
So 1/3+1/6 = 1/2, the answer is 1/2!
Actually, no. The reason the answer is 1/2 is because the "alternate" and "adjacent-1" segregation each has a probability of ~50% while "adjacent-2" segregation is very rare.
1/2 +1%< =~ 1/2
Had the question asked about the probability of getting phenotypically normal gametes or just famility Down gametes, the actually answers would've been ~1/2 and ~1%, respectively.
If you actually draw the tetravalent synapse out, you can clearly see that homolog segregations can only result in 3 cases:
"Alternate:" two gametes with a normal 14 and a normal 21, and two gametes with a Robertsonian translocation (14 fused with 21)
"Adjacent -1:" two gametes with a normal 14 and a Robertsonian translocation, and two gametes with a normal 21
"Adjacent -2:" two gametes with a normal 21 and a Robertsonian translocation, and two gametes with a normal 14
So we have 12 possible gametes here. Of the 12, all the gametes produced by "alternate" segregation (1/3) are viable and will produce phenotypically normal offsprings. In "adjacent-2," the 2 gametes with normal 21 and a Robertsonian will produce viable offspring with familial Down syndrome (1/6)
So 1/3+1/6 = 1/2, the answer is 1/2!
Actually, no. The reason the answer is 1/2 is because the "alternate" and "adjacent-1" segregation each has a probability of ~50% while "adjacent-2" segregation is very rare.
1/2 +1%< =~ 1/2
Had the question asked about the probability of getting phenotypically normal gametes or just famility Down gametes, the actually answers would've been ~1/2 and ~1%, respectively.
