Exam 17: Large-Scale Chromosomal Changes

In the plant genus Triticum there are many different polyploid species,as well as diploid species.Crosses were made between three different species,and hybrids were obtained.The meiotic pairing was observed in each hybrid and is recorded in the following table.(A bivalent is two homologous chromosomes paired at meiosis,and a univalent is an unpaired chromosome at meiosis.) Species crossed to make hybrid Pairing in hybrid 1. T. turdigum \times T. monococcum 7 bivalents +7 univalents 2. T. aestivum \times T. monococcum 7 bivalents +14 univalents 3. T. aestivum \times T. turdigum 14 bivalents +7 univalents The first cross suggests a diploid and tetraploid individual were involved with one set of 7 chromosomes in common.Based on the other crosses,which species is the likely diploid individual?

In a triploid of genotype B/b/b,what proportion of gametes will be B?

You discover a Drosophila male that is heterozygous for a reciprocal translocation between the second and third chromosomes,each break having occurred near the centromere (which for these chromosomes is near the center). a)Draw a diagram showing how these chromosomes would synapse at meiosis. b)You find that this fly has the recessive genes bw (brown eyes)and eb (ebony body)on the second and third chromosomes,respectively,and wild-type alleles on the translocated ones.The fly is crossed with a female having normal chromosomes and homozygous for bw and eb.What phenotypic classes and in what ratio would be expected in the progeny of this cross? (Hint: Zygotes with an extra chromosome arm or deficient for one do not survive.Also,remember that there is no crossing over in Drosophila males.)

Which of the following diagnostic features is indicative of an inversion having occurred?

A triploid with 5 chromosomes in each set (3n = 15)is discovered.What is the probability of a meiosis in which all univalents pass to the same pole?

In the fungus Neurospora,a standard lab strain carrying the auxotrophic mutation m was crossed to a prototrophic strain isolate from a burned sugar cane field.There were three types of asci as shown below,where the symbol "ab" means an aborted ascospore whose m locus constitution could not be tested. Type I Type 2 Type 3 m ab m m ab m m ab ab m ab ab + ab ab + ab ab + ab + + ab + Explain the origin of each ascus type.

In Neurospora,the genes his2 and leu2 are closely linked.From a cross,his2 × leu2,which is a rare octad of the following type,arose: his2, + his 2. + his 2. + his 2, + abort abort +, leu2 +, lou 2 This octad most likely arose by:

Suppose that in Guinea pigs ABO blood types are determined like in humans,where I^A and I^B are expressed codominantly and i (blood type O)is recessive to both.During a set of routine crosses you notice something unusual: Repeated matings between a certain female with blood type AB and a certain male with blood type A produces F₁ Guinea pigs of all four blood types in the following ratios: 5A : 2B : 4AB : 1O.The genotypes of the parents are most likely:

Below are a list of chromosomal aberrations (a-d)and a list of potential diagnostic features (1-7).On the blank line following each aberration,write the number(s)of all the diagnostic features that may apply to that aberration.Note that each diagnostic feature may be used more than once. List of chromosomal aberrations: a)deletion _____ b)duplication _____ c)inversion _____ d)translocation _____ List of potential diagnostic features: 1)formation of chromosomal "cruciform" structures in meiosis I 2)formation of chromosomal "loop" structures in meiosis I 3)Recombinant frequencies between genes on the same chromosome are (much)lower than expected. 4)reduced fertility (other than semisterility) 5)semisterility 6)pseudodominance 7)pseudolinkage (two genes on separate chromosomes do not segregate independently).

In a lab strain of Drosophila,cinnabar (cn)and brown (bw)are recessive eye color mutations known to be 41 map units apart on chromosome 2.When similar mutant alleles were induced in a strain from nature,the same linkage of cn and bw was observed.However,when a wild-type strain from nature was crossed with a cn bw/cn bw lab strain to create the genotype + +/cn bw,and females of this type were testcrossed to cn bw/cn bw males from the lab strain,the following phenotypic proportions were obtained in the progeny: + + 25,200 cn bw 21,009 cn + 11 + bw 36 a)What is unexpected about these results? b)What is the most likely explanation? (Summarize your model with a chromosome diagram with labelled genes.) c)On your diagram show the precise origin of the cn + and the + bw classes.

In humans,males have only one X chromosome and females have two,yet housekeeping X chromosome genes are expressed in roughly equal amounts in males and females.How can this be explained?

a)Diagram the pairing structure of the inversion heterozygote abcdefg/abfedcg.Assume the centromere is to the left of a.Is this a paracentric or pericentric inversion? b)Diagram the early anaphase configuration of the bivalent in part (a)if a chiasma occurred between d and e.

In corn R is a gene for red aleurone;its recessive allele r determines colorless aleurone.A cross is made between a diploid r/r female and a trisomic R/r/r male.If pollen grains with extra chromosomes are inviable,the ratio of red to colorless kernels expected in the progeny would be:

A triploid of genotype a b/a B/a b is crossed to a triploid of genotype A b/a b/a b.If genes A (a)and B (b)are 20 map units apart,what proportion of the progeny will be phenotypically A B?

The two loci Y (y)and R (r)are normally 20 map units apart.A particular dihybrid individual is heterozygous for an inversion that spans 98% of the region between Y and R,but does not include Y or R.If this dihybrid is testcrossed to an individual with normal chromosomes,the recombinant frequency in progeny will be:

A pure line of plants of genotype a/a;b/b;c/c;d/d;e/e (all recessive to wild type)was crossed to a wild type.One F₁ individual expressed the recessive alleles d and e.This individual arose most likely from:

In yeast,the gene phe1 is on chromosome 3 and lys4 is on chromosome 7.The progeny (spores)from the cross +;+ × phe1;lys4 were: 47\% +;+ 44\% phel; lys4 4\% phel; + 5\% +; lys 4 This results suggests that:

In an animal bearing the heterozygous inversion ABCDE[*]FGHI / ABGF[*]EDCHI,in one meiocyte a crossover occurred between the D and E loci and another crossover occurred between the F and G loci.The two crossovers involved all four chromatids.What will be the proportion of abnormal meiotic products from that meiosis? (Note: [*] = centromere.)

In Drosophila,h determines hairy body (h⁺ = smooth)and se determines sepia eyes (se⁺ = red).The loci are approximately 40 m.u.apart on an autosome.A cross was made: se h⁺/se h⁺ X se⁺h/se⁺h,and the F₁ were all wild type.F₁ females were testcrossed to se h/se h males and the offspring were: 39 sepia eyes. 42 hairy body. a)What progeny and in what proportions were expected? b)Provide an explanation of the observed results if they differ from expectations.