Human Genetics Chapter 5

Front 1

Marfan syndrome can be caused by mutations in either of two genes, but mutations in one of the genes blocks activity of the other. This means that Marfan syndrome demonstrates

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genetic heterogeneity and epistasis.

Front 2

Pat Wright has alkaptonuria. Her symptoms include back spasms, dark urine, arthritis, hearing loss, gallstones, heart valve damage, and darkened ear tips. This condition is therefore

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pleiotropic

Front 3

Which of the following is an illness caused by a mutation in mitochondrial DNA?

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Leber optic atrophy

Front 4

When the cystic fibrosis gene was discovered in 1989, only one mutation was described, and a diagnostic test developed to detect it. Over the years, as more mutations were discovered, they were added to the test panel. Today, most pregnant women are offered a CF test that detects 100 or so alleles, although more than 1600 mutations have been discovered. These facts illustrate which of the following?

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multiple alleles.

Front 5

A mitochondrial trait passes from

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mothers to all children.

Front 6

Morris is a big, healthy bull. One spring, he fathers ten calves, but five of them are stillborn, their muscles very rigid in a phenotype called "contracture." Three of the dams (mothers of the stillborns) are also Morris' daughters, and the fourth is his half-sister. Farm animals have little chance for romance and so often mate with their close relatives. The large number of stillborns is most likely due to

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multiple lethal alleles that Morris shares with his mates.

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Different alleles that are both expressed in a heterozygote are

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codominant

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Brittany and Jarod have been trying to have a second child for 10 years. They have one healthy child, but Brittany has had four early spontaneous abortions. The most likely explanation for the many pregnancy losses is that Brittany and Jarod

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are each heterozygous for lethal alleles of the same gene.

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---Hairlessness in dogs is inherited from a single dominant allele. Inheriting two dominant alleles is lethal for the embryo. Ralph and Penelope are both hairless dogs. What is the probability that Penelope will give birth to a hairy puppy?

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1/4

Front 10

***Types of genetic markers include

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places in the genome where a base varies among individuals in a population.

Front 11

***Familial hypercholesterolemia illustrates incomplete dominance in humans because

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heterozygotes have an intermediate number of LDL receptors on their liver cells.

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---Epistasis and multiple alleles differ in that

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heterozygotes have an intermediate number of LDL receptors on their liver cells.

Front 13

---The Addams family has an autosomal dominant condition in which webbing attaches the second toe to the third toe and the second toe is longer than the big toe. Only some of the Addams' who inherit the mutant allele have a second toe longer than the big toe. In addition, the extent of webbing varies. This phenotype is

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variably expressive and incompletely penetrant.

Front 14

***A gene may have many alleles, but a person has only two alleles for a gene, because

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a gene can be altered in many ways, but a person has only two copies of any gene.

Front 15

***In many species, lens crystalline protein aggregates in the eyes to form lenses, yet in other cell types, functions as an enzyme. The gene that encodes this protein is therefore

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pleiotropic

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***Multiple alleles are common because

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a gene sequence can vary in different ways and still encode a functional protein.

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Several different alleles for the same mitochondrial gene is called

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heteroplasmy.

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---When researchers analyzed the remains of Tsar Nicholas II of Russia and his family, they found that the suspected Tsar and his living great-grandniece Xenia differed at nucleotide position 16169 in the mitochondrial DNA. Re-testing the remains showed that for this site in the mitochondrial genome, the purported Tsar had thymine (T) in some samples and cytosine (C) in others. The most likely cause of this finding is

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heteroplasmy

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A man who has normal hearing and has a recessive deafness allele on chromosome 17 marries a woman who also has normal hearing and also has a recessive deafness allele, but this allele is on chromosome 3. The probability that their children will be deaf due to either of these mutated genes is closest to

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0%

Front 20

A trait caused by an environmental influence that appears to be inherited is

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a phenocopy.

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Some combinations of recessive alleles cause problems so severe that the fetus ceases to develop. Such lethal allele combinations appear to alter Mendelian ratios because

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homozygotes for the lethal allele do not appear as a progeny class.

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***Genetic heterogeneity refers to mutations in different genes that cause the same symptoms.

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true

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---LOD (logarithm of the odds) scores are used to indicate linkage between genes. A LOD score of __ or greater is a strong indicator of linkage.

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3

Front 24

The alleles that control the type of blood group antigens that appear on the surfaces of red blood cells are

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codominant

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Zuzu is a white cat. She is genotype Ww for the "white masking gene" that reduces the number of melanocytes (pigment-containing cells). As a result, her body makes pigment, but it cannot get into the cells where it would color the fur. Zuzu's white masking gene is ______ to the melanin pigment gene because it prevents full expression of the pigment.

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epistatic

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---Geneticists construct linkage maps of chromosomes by

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calculating the percent recombination between two genes on the same chromosome.

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Mitochondrial disorders are probably very rare because

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as oocytes formed, those with harmful mitochondrial mutations did not have sufficient energy to survive.

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---Marbles is a Manx cat whose tail is so short that she appears not to have one. The genotype for short tail in Manx cats is heterozygous (Mm). Cats that are genotype mmhave normal length tails, but cats that are genotype MM die before birth due to very abnormal brains and spinal cords. Marbles has one copy of a recessive _____ allele.

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lethal

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The mitochondrial genome consists of ___ genes.

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37

Front 30

Hairlessness in dogs is inherited from a single dominant allele. Inheriting two dominant alleles is lethal for the embryo. Penelope, a hairless dog, is wildly attracted to a hairy dog, Arnold. They mate. The probability that Arnold's sperm fertilizing Penelope's oocyte conceives a pup that would be hairless like Penelope is

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1/2

Front 31

In common English, "linkage" refers to one event that tends to occur when another does. In genetics, linkage has a precise meaning. It refers to

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two genes on the same chromosome.

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A widely-used research technique used to associate patterns of genetic variation with phenotypes that is based on the concept of linkage, but considering all of the chromosomes at once, is

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genome-wide association studies.

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Mitochondrial disorders tend to cause great fatigue because

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muscle cells are normally filled with mitochondria.

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Several members of the Fitzsimmons family have polydactyly, and they differ in their numbers of extra fingers and toes. Yet certain relatives who should have extra digits don't, such as Megan Fitzsimmons. She has two children, a son with two extra fingers and a daughter with an extra toe, and her father has an extra digit on each hand and foot. Polydactyly in this family is

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variably expressive and incompletely penetrant.

Front 35

***Of nearly 200 forms of hereditary deafness, 132 are autosomal recessive, 64 autosomal dominant and 4 X-linked recessive. Hereditary deafness is therefore genetically

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heterogenic

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If allele T (long tongue) exhibits incomplete dominance over the recessive allele t (short tongue), a heterozygote for this gene would most likely have

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it is poorer at DNA repair.

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***Can a woman with blood type A have a child with a phenotype of blood type O with a man who is AB?

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a tongue of intermediate length.

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Can a woman with blood type A have a child with a phenotype of blood type O with a man who is AB?

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Yes, because of epistasis between the I and the H genes.

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---Mendel's results and conclusions were accurate, but are seen differently today, because we now realize that

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single-gene traits are usually influenced by the environmental and other genes.