The mutant FMR-1 allele that causes fragile X syndrome is considered to be X-linked dominant withincomplete penetrance and variable expressivity.Why do most females heterozygous for one mutantand one normal allele have at least some symptomsof the disease?

The mutant FMR-1 allele that causes fragile X syndrome is considered to be X-linked dominant withincomplete penetrance and variable expressivity.Why do most females heterozygous for one mutantand one normal allele have at least some symptomsof the disease?

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Description: The mutant FMR-1 allele that causes fragile X syndrome is considered to be X-linked dominant withincomplete penetrance and variable expressivity.Why do most females heterozygous for one mutantand one normal allele have at least some symptomsof the di

Human Heredity: Principles and Issues (MindTap Course List)

11th Edition

ISBN:9781305251052

Author:Michael Cummings

Publisher:Michael Cummings

Chapter15: Genomes And Genomics

Section: Chapter Questions

Problem 1QP: The gene controlling ABO blood type and the gene underlying nail-patella syndrome are said to show...

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Duchenne muscular dystrophy is a recessive disorder caused by a rare,loss-of-function allele that is located on the X chromosome in humans. Anunaffected woman (i.e., without disease symptoms) who is heterozygousfor the X-linked allele causing Duchenne muscular dystrophy has childrenwith a man with a functional (non-disease-causing) allele. What is theprobability that this couple will have an unaffected son?
. In mice, the dominant allele Gs of the X-linked geneGreasy produces shiny fur, while the recessive wildtype Gs+ allele determines normal fur. The dominantallele Bhd of the X-linked Broadhead gene causesskeletal abnormalities including broad heads andsnouts, while the recessive wild-type Bhd+ alleleyields normal skeletons. Female mice heterozygousfor the two alleles of both genes were mated withwild-type males. Among 100 male progeny of thiscross, 49 had shiny fur, 48 had skeletal abnormalities,2 had shiny fur and skeletal abnormalities, and 1 waswild type.a. Diagram the cross described and calculate thedistance between the two genes.b. What would have been the results if you hadcounted 100 female progeny of the cross?
In 1952, an article in the British Medical Journalreported interesting differences in the behavior ofblood plasma obtained from several people who suffered from X-linked recessive hemophilia. Whenmixed together, the cell-free blood plasma from certain combinations of individuals could form clots inthe test tube. For example, the following table showswhether clots could form (+) or not (−) in variouscombinations of plasma from four people withhemophilia:1 and 1 − 2 and 3 +1 and 2 − 2 and 4 +1 and 3 + 3 and 3 −1 and 4 + 3 and 4 −2 and 2 − 4 and 4 −What do these data tell you about the inheritance ofhemophilia in these individuals? Do these data allowyou to exclude any models for the biochemical pathway governing blood clotting?
Let’s suppose a recessive allele encodes a completely defective protein. If the functional allele is dominant, what does that tell youabout the amount of the functional protein that is sufficient to causethe phenotype? What if the allele shows incomplete dominance?
Many genetic disorders exhibit locus heterogeneity. Define andgive two examples of locus heterogeneity. How does locus heterogeneityconfound a pedigree analysis?
Female fruit flies homozygous for the X-linked white-eye alleleare crossed to males with red eyes. On very rare occasions, an offspringof such a cross is a male with red eyes. Assuming these rareoffspring are not due to a new mutation in one of the mother’s Xchromosomes that converted the white-eye allele into a red-eyeallele, explain how a red-eyed male arises.
Duchenne muscular dystrophy is sex linked and usuallyaffects only males. Victims of the disease become progressively weaker, starting early in life.a. What is the probability that a woman whose brotherhas Duchenne’s disease will have an affected child?b. If your mother’s brother (your uncle) had Duchenne’sdisease, what is the probability that you have receivedthe allele?c. If your father’s brother had the disease, what is theprobability that you have received the allele?
The recessive, X-linked z1mutation of the Drosophilagene zeste (z) can produce a yellow (zeste) eye coloronly in flies that have two or more copies of the wildtype white (w) gene. Using this property, tandem duplications of the w+ gene called w+Rwere identified.Males with the genotype y+ z1w+R spl+ / Y thus havezeste eyes. These males were crossed to females withthe genotype y z1 w+R spl / y+ z1 w+R spl+. (These fourgenes are closely linked on the X chromosome, in theorder given in the genotype, with the centromere tothe right of all these genes: y = yellow bodies; y+ =tan bodies; spl = split bristles; spl+ = normal bristles.) Out of 81,540 male progeny of these females,the following exceptions were found:Class A 2430 yellow bodies, zeste eyes, wild-type bristlesClass B 2394 tan bodies, zeste eyes, split bristlesClass C 23 yellow bodies, wild-type eyes, wild-type bristlesClass D 22 tan bodies, wild-type eyes, split bristlesa. What were the phenotypes of the remainder of the81,540 males…
The Mic2 gene in humans is present on both the X and Y chromosome. Let’s suppose the Mic2 gene exists in a dominant Mic2 allele, which results in normal surface antigen production, and arecessive mic2 allele, which results in defective surface antigenproduction. Using molecular techniques, it is possible to distinguish homozygous and heterozygous individuals. By followingthe transmission of the Mic2 and mic2 alleles in a large human pedigree, would it be possible to distinguish between pseudoautosomalinheritance and autosomal inheritance? Explain your answer
What is a reciprocal cross? Suppose that a gene is found as a wildtype (functional) allele and a recessive mutant (nonfunctional)allele. What would be the expected outcomes of reciprocal crossesif a true-breeding normal individual was crossed to a true-breedingindividual carrying the mutant allele? What would be the results ifthe gene is maternally inherited?
A woman who sought genetic counseling is found to be heterozygousfor a chromosomal rearrangement between the second andthird chromosomes. Her chromosomes, compared to those in anormal karyotype, are diagrammed on the next page:(a) This woman is phenotypically normal. Does thissurprise you? Why or why not? Under what circumstancesmight you expect a phenotypic effect of such arearrangement?
In 1995, doctors reported a Chinese family in whichretinitis pigmentosa (progressive degeneration of theretina leading to blindness) affected only males. Allsix sons of affected males were affected, but all of thefive daughters of affected males (and all of thechildren of these daughters) were unaffected.a. What is the likelihood that this form of retinitispigmentosa is due to an autosomal mutationshowing complete dominance?b. What other possibilities could explain the inheritance of retinitis pigmentosa in this family? Whichof these possibilities do you think is most likely?