Null mutations are valuable genetic resources becausethey allow a researcher to determine what happens to anorganism in the complete absence of a particular protein. However, it is often not a trivial matter to determinewhether a mutation represents the null state of the gene.a. Geneticists sometimes use the following test forthe nullness of an allele in a diploid organism: If theabnormal phenotype seen in a homozygote for theallele is identical to that seen in a heterozygote(where one chromosome carries the allele in question and the homologous chromosome is known tobe completely deleted for the gene) then the alleleis null. What is the underlying rationale for thistest? What limitations might there be in interpreting such a result?b. Can you think of other methods to determinewhether an allele represents the null state of a particular gene?

Null mutations are valuable genetic resources becausethey allow a researcher to determine what happens to anorganism in the complete absence of a particular protein. However, it is often not a trivial matter to determinewhether a mutation represents the null state of the gene.a. Geneticists sometimes use the following test forthe nullness of an allele in a diploid organism: If theabnormal phenotype seen in a homozygote for theallele is identical to that seen in a heterozygote(where one chromosome carries the allele in question and the homologous chromosome is known tobe completely deleted for the gene) then the alleleis null. What is the underlying rationale for thistest? What limitations might there be in interpreting such a result?b. Can you think of other methods to determinewhether an allele represents the null state of a particular gene?

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Human Heredity: Principles and Issues (MindTap Course List)

11th Edition

ISBN:9781305251052

Author:Michael Cummings

Publisher:Michael Cummings

Chapter11: Genome Alterations: Mutation And Epigenetics

Section: Chapter Questions

Problem 16QP: Familial retinoblastoma, a rare autosomal dominant defect, arose in a large family that had no prior...

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Six months pregnant, an expectant mother had a routineultrasound that showed that the limbs of the fetus wereunusually short. Her physician suspected that the babymight have a genetic form of dwarfism called achondroplasia,an autosomal dominant trait occurring with a frequency of about1 in 27,000 births. The parents were directed to a genetic counselorto discuss this diagnosis. In the conference, they learnedthat achondroplasia is caused by a mutant allele. Sometimes itis passed from one generation to another, but in 80 percent ofall cases it is the result of a spontaneous mutation that arisesin a gamete of one of the parents. They also learned that mostchildren with achondroplasia have normal intelligence and a normallife span. What information would be most relevant to concluding whichof the two mutation origins, inherited or new, most likelypertains in this case? How does this conclusion impact on thiscouple’s decision to have more children?
Six months pregnant, an expectant mother had a routineultrasound that showed that the limbs of the fetus wereunusually short. Her physician suspected that the babymight have a genetic form of dwarfism called achondroplasia,an autosomal dominant trait occurring with a frequency of about1 in 27,000 births. The parents were directed to a genetic counselorto discuss this diagnosis. In the conference, they learnedthat achondroplasia is caused by a mutant allele. Sometimes itis passed from one generation to another, but in 80 percent ofall cases it is the result of a spontaneous mutation that arisesin a gamete of one of the parents. They also learned that mostchildren with achondroplasia have normal intelligence and a normallife span. It has been suggested that prenatal genetic testing for achondroplasiabe made available and offered to all women. Wouldyou agree with this initiative? What ethical considerationswould you consider when evaluating the medical and societalconsequences of offering…
In retinoblastoma, a mutation in one allele of the RB1 tumorsuppressorgene can be inherited from the germ line, causingan autosomal dominant predisposition to the developmentof eye tumors. To develop tumors, a somatic mutation in thesecond copy of the RB1 gene is necessary, indicating that themutation itself acts as a recessive trait. Given that the firstmutation can be inherited, in what ways can a second mutationalevent occur?
Galactosemia is a recessive human disease that istreatable by restricting lactose and glucose in the diet.Susan Smithers and her husband are both heterozygous for the galactosemia gene.a. Susan is pregnant with twins. If she hasfraternal (nonidentical) twins, what is theprobability both of the twins will be girls whohave galactosemia?b. If the twins are identical, what is the probabilitythat both will be girls and have galactosemia?For parts (c–g), assume that none of the children isa twin.c. If Susan and her husband have four children, whatis the probability that none of the four will havegalactosemia?d. If the couple has four children, what is the probability that at least one child will have galactosemia?e. If the couple has four children, what is the probability that the first two will have galactosemia andthe second two will not?f. If the couple has three children, what is the probability that two of the children will have galactosemia and one will not, regardless of order?g. If…
A DNA variant has been found linked to a rare autosomal dominant disease in humans and can thus beused as a marker to follow inheritance of the diseaseallele. In an informative family (in which one parentis heterozygous for both the disease allele and the DNA marker in a known chromosomal arrangementof alleles, and his or her mate does not have the samealleles of the DNA variant), the reliability of such amarker as a predictor of the disease in a fetus is related to the map distance between the DNA markerand the gene causing the disease.Imagine that a man affected with the disease(genotype Dd) is heterozygous for the V1and V2forms of the DNA variant, with form V1on the samechromosome as the D allele and form V2on the samechromosome as d. His wife is V3V3dd, where V3isanother allele of the DNA marker. Typing of the fetusby amniocentesis reveals that the fetus has the V2andV3variants of the DNA marker. How likely is it thatthe fetus has inherited the disease allele D if thedistance…
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?
A 30 - year - old woman was undergoing therapy for b-thalassemia,a recessive trait caused by absence of or reduced synthesis ofthe hemoglobin b chain, a subunit of the oxygen-carrying moleculein red blood cells. In this condition, red blood cells are rapidlydestroyed, freeing a large amount of iron, which is deposited in tissuesand organs. The blood transfusions the patient had received every twoor three weeks since the age of 7 to stave off anemia were furtheraggravating iron buildup. Her major organs were showing damage, andshe was in danger of death from cardiac disease. Her physician suggestedthat she consider undergoing a hematopoietic (bone marrow)stem cell transplant (HSCT). Since these stem cells give rise to redblood cells, such a transplant could potentially restore her health. Whilethis might seem like an easy decision, it is not. Advanced cases havea high risk (almost 30 percent) for transplantation-related death. At thispoint, the woman is faced with a difficult and…
A 30 - year - old woman was undergoing therapy for b-thalassemia,a recessive trait caused by absence of or reduced synthesis ofthe hemoglobin b chain, a subunit of the oxygen-carrying moleculein red blood cells. In this condition, red blood cells are rapidlydestroyed, freeing a large amount of iron, which is deposited in tissuesand organs. The blood transfusions the patient had received every twoor three weeks since the age of 7 to stave off anemia were furtheraggravating iron buildup. Her major organs were showing damage, andshe was in danger of death from cardiac disease. Her physician suggestedthat she consider undergoing a hematopoietic (bone marrow)stem cell transplant (HSCT). Since these stem cells give rise to redblood cells, such a transplant could potentially restore her health. Whilethis might seem like an easy decision, it is not. Advanced cases havea high risk (almost 30 percent) for transplantation-related death. At thispoint, the woman is faced with a difficult and…
Imagine that you caught a female albino mouse inyour kitchen and decided to keep it for a pet. A fewmonths later, while vacationing in Guam, you caughta male albino mouse and decided to take it home forsome interesting genetic experiments. You wonderwhether the two mice are both albino due to mutations in the same gene. What could you do to find outthe answer to this question? Assume that both mutations are recessive.
A researcher sequences the whole exome of a patientsuffering from Usher syndrome, a rare autosomal recessive condition that is nonetheless the leading causefor simultaneous deafness and blindness. The exomesequence does not show homozygosity for any polymorphisms different from the human RefSeq.a. How could the researcher examine the data alreadygathered to try to find the disease gene, assumingthe sequence is accurate?b. If the attempt described in part (a) was unsuccessful, the researcher might contemplate sequencingthe patient’s whole genome. What are the potentialpitfalls of this strategy?
. In 1932, H. J. Muller suggested a genetic test to determine whether a particular mutation whose phenotypiceffects are recessive to wild type is a null (amorphic)allele or is instead a hypomorphic allele of a gene.Muller’s test was to compare the phenotype of homozygotes for the recessive mutant alleles to the phenotype of a heterozygote in which one chromosomecarries the recessive mutation in question and thehomologous chromosome carries a deletion for aregion including the gene.In a study using Muller’s test, investigators examined two recessive, loss-of-function mutant alleles ofrugose named rg41 and rgγ3. The eye morphologiesdisplayed by flies of several genotypes are indicated inthe following table. Df(1)JC70 is a large deletion thatremoves rugose and several genes to either side of it.Genotype Eye surface Cone cells per ommatidiumwild type smooth 4rg41/rg41 mildly rough 2–3rg41/Df(1)JC70 moderately rough 1–2rg γ3/rg γ3 very rough 0–1rg γ3/Df(1)JC70 very rough 0–1a. Which allele…
The maternal-effect mutation bicoid (bcd) is recessive. Inthe absence of the bicoid protein product, embryogenesis isnot completed. Consider a cross between a female heterozygousfor the bicoid mutation (bcd+/ bcd-) and a homozygousmale(bcd-/ bcd-). How is it possible for a male homozygous for the mutationto exist?