Draw the configuration of chromosome 10 in the F1 semisterile corn at metaphase
ID: 91867 • Letter: D
Question
Draw the configuration of chromosome 10 in the F1 semisterile corn at metaphase I including the location of the tp2 and zn1 alleles relative to the chromosomal mutation (include linkage distance). Use different colors to represent different chromosomes and label the centromeres to indicate if they originated from the original semisterile parent or the fully fertile parental line.
The answer is also in the image, but I do not understand how it is that.
10. (20 pts) A corn showing semisterility due to a chromosomal mutation but otherwise normal was crossed to a fully fertile line that was homozygous recessive for both zn1 (zebra necrotic 1) and tp2 (teopod 2). The semisterile F1 progeny were then testcrossed with the fertile zn1 tp2 homozygotes. The resulting testcross progeny phenotypes are as follows: fertility. zebra necrotic 1 teopod 2 of progeny X-over classes 283 semisterile parental 917 fertile sco zn1-break point semisterile sco zn1-tp to fertile 31 DCO zn DCO semisterile 34 fertile 87 SCO Zn1-tp zn 1 to 2 semisterile 100 sco zn1-breakpoint to2 fertile 279 parental total 1000 The linkage map of chromosome 1 fsz tp2 zn 1 116 Breakpoint of translocation Draw the configuration of chromosome 10 in the F1 semisterile corn at metaphase l ncluding the location of the tp2 and zn1 alleles relative to the chromosomal mutation (include linkage distance). Use different colors to represent different chromosomes and label the centromeres to indicate if they originated fromthe original semisterile parent or the fully fertile parental line. The translocation breakpoint is 26.2 map units away from zn1 locus and 50 map units from tp2 and thus is located approximately at map position 84.Explanation / Answer
Let wild type of zebra necrotic be ZN1 and its recessive allele be zn1
Let wild type of teopod 2 be TP2 and its recessive be tp2.
We do not know if the semisterile mutant is recessive or dominant over wild type. However, we know that test cross involves homozygous recessive pureline. F1 progeny were test crossed with fertilezn1 tp2 homozygotes. Therefore, we can assume that fertile (ss) (no semisterile) is recessive. Semisterile (SS) is dominant.
Semisterile but otherwise normal parent is “SS SS ZN1 ZN1 TP2 TP2”
Fully fertile line homozygous recessive for zn1 and tp2 is “ss ss zn1 zn1 tp2 tp2”
The cross between the above parents results in F1 progeny with genotype is “SSssZN1zn1TP2tp2”
This F1 is testcrossed with sssszn1zn1tp2tp2.
sssszn1zn1tp2tp2 gives a single type of gamete with all recessive alleles. So phenotype of the progeny is nnot dependent upon this parent.
SSssZN1zn1TP2tp2 give 8 different types of gametes.
Those are
SSZN1TP2 = 283 (Parental)
SSZN1tp2 = 89 (Single crossovers between ZN1 and TP2 loci)
SSzn1TP2 = 34 (double crossovers)
SSzn1tp2 = 100 (Single crossovers between SS and ZN1 loci)
ssZN1TP2 = 97 (Single crossovers between SS and ZN1 loci)
ssZN1tp2 = 31 (double crossovers)
sszn1TP2 = 87 (Single crossovers between ZN1 and TP2 loci)
sszn1tp2 = 279 (Parental)
Highest two in numbers among the 8 progeny are parental combinations.
Lowest two in numbers among the 8 progeny are double crossovers (DCO).
The difference between parental and DCOs are the middle gene.
Parents are SSZN1TP2 and sszn1tp2. DCOs are SSzn1TP2 and ssZN1tp2.
ZN1 is the only gene altered between parentals and DCOs. So, it is the middle gene.
The remaining 4 are single crossovers.
Map distance between SS and ZN1 = (100 + 97 + 34 + 31)/1000 = 262/1000 = 0.262 = 26.2% = 26.2 map units
Map distance between ZN1 and TP2 = (89 + 87 + 34 + 31)/1000 = 241/1000 = 0.241 = 24.1% = 24.1 map units
Map distance between SS and TP2 = 26.2% + 24.1 = 50.3% = 50.3 map units
In the F1 progeny, SSZN1TP2 alleles are on one chromosome (it came from SS SS ZN1 ZN1 TP2 TP2 parent) and the other chromosome has sszn1tp2 alleles (it came from ss ss zn1 zn1 tp2 tp2 parent).
You wrote “The answer is also in the image, but I do not understand how it is that”. The image is given. So, I am not drawing it again.
In that image tp2 is teopod 2, zn1 is zebra necrotic 1. Y is semisterile locus.
The image also shows two other loci fsz, and d which were not part of the question. I guess those to are part of a preceding question in the text book. You may check this, please.
As per the calculation shown above, y should be at 84.1 not at 82 because distance between ss and zn1 is 26.1.
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