REVIEW QUESTIONS

1. Modified from the text a bit – Please do the following: A) DEFINE the factor and then B) EXPLAIN how the factor alters or disrupts EXPECTED PHENOTYPIC RATIOS (this means that you need to include the ratios we would EXPECT under Mendelian inheritance and how the factor CHANGES the ratios/what they are then changed to).
Use examples from the text/my lecture/external websites if that helps you explain. (6 points).

1. Explain how each of the following phenomena can disrupt Mendelian phenotypic ratios.
a. lethal alleles
b. multiple alleles
c. incomplete dominance
d. codominance
e. epistasis
f. incomplete penetrance
g. variable expressivity
h. pleiotropy
i. a phenocopy
j. genetic heterogeneity

APPLIED QUESTIONS

1. Modified from the text a bit - In addition to A) IDENTIFYING the factor(s), B) EXPLAIN why you selected the factor(s) that you did for full credit (5 points).

1. For each of the diseases described in situations (a) through (i), indicate which of the following phenomena (A–H) is at work. More than one may apply.
A. lethal alleles
B. multiple alleles
C. epistasis
D. incomplete penetrance
E. variable expressivity
F. pleiotropy
G. a phenocopy
H. genetic heterogeneity

a. A woman with severe neurofibromatosis type 1 has brown spots on her skin and several large tumors beneath her skin. A genetic test shows that her son has the disease-causing autosomal dominant allele, but he has no symptoms.

b. A man would have a widow’s peak, if he wasn’t bald.

c. A man and woman have six children. They also had two stillbirths—fetuses that stopped developing shortly before birth.

d. Mutations in a gene that encodes a muscle protein called titin cause 22 percent of cases of inherited dilated cardiomyopathy, a form of heart disease. Other single genes cause the other cases. e. A woman with dark brown skin uses a bleaching cream with a chemical that darkens her fingertips and ears, making her look like she has the inherited disease alkaptonuria.

f. In Labrador retrievers, the B allele confers black coat color and the b allele brown coat color. The E gene controls the expression of the B gene. If a dog inherits the E allele, the coat is golden no matter what the B genotype is. A dog of genotype ee expresses the B (black) phenotype.

g. Two parents are heterozygous for genes that cause albinism, but each gene specifies a different enzyme in the biochemical pathway for skin pigment synthesis. Their children thus do not face a 25 percent risk of having albinism.

h. All children with Aicardi syndrome have defects in their retinas that look like white craters (chorioretinal lacunae), but only some of the 4,000 recognized cases also have absence of the corpus callosum (the band of nerve fibers that joins the halves of the brain).

i. Two young children in a family have very decayed teeth. Their parents think it is genetic, but the true cause is a babysitter who puts them to sleep with juice bottles in their mouths.

Chapter 6 (9 points total)

CASE STUDIES AND RESEARCH RESULTS

1. Modified from the text a bit - In addition to A) IDENTIFYING the factor(s), EXPLAIN why you selected the factor(s) that you did for full credit.

1. For each case description, identify the principle at work from the list that follows. More than one answer may apply to a given case.
A. Y-linked inheritance
B. X-linked recessive
C. X-linked dominant inheritance
D. sex-limited inheritance
E. sex-influenced inheritance
F. X inactivation or manifesting heterozygote
G. imprinting abnormality

a. A child with Russell-Silver syndrome displays poor growth, a large head, a characteristic triangular face, and digestive problems. In some cases, a gene on chromosome 11 that is normally methylated in the father is not methylated.

b. Six-year-old LeQuan inherited Fabry disease from his mother, who is a heterozygote for the causative allele. The gene, on the X chromosome, encodes a lysosomal enzyme. LeQuan would die before age 50 of heart failure, kidney failure, or a stroke, but fortunately he can be treated with twice-monthly infusions of the enzyme. His mother, Echinecea, recently began experiencing recurrent fevers, a burning pain in her hands and feet, a rash, and sensitivity to cold. She is experiencing mild Fabry disease.

c. The Chandler family has many male members who have a form of retinitis pigmentosa (RP) in which the cells that capture light energy in the retina degenerate, causing gradual visual loss. Several female members of the family presumed to be carriers because they have affected sons are tested for RP genes on chromosomes 1, 3, 6, and the X, but are found to not carry these genes. Years ago, Rachel married her cousin Ross, who has the family’s form of RP. They had six children. The three sons are all affected, but their daughters all have normal vision.

d. Simon’s mother and his mother’s sister are breast cancer survivors, and their mother died of the disease. Simon’s sister Maureen has a genetic test and learns that she has inherited the BRCA1 mutation that increases risk of developing certain cancers. Simon has two daughters but doesn’t want to be tested because he thinks a man cannot transmit a trait that affects a body part that is more developed in females.

e. Tribbles are extraterrestrial mammals that long ago invaded a starship on the television program Star Trek. A gene called frizzled causes kinky hair in female tribbles who inherit just one allele. However, two mutant alleles must be inherited for a male tribble to have kinky hair. The gene is on an autosome.

f. Devon died at age 16 of Lowe syndrome. He had slight intellectual disability, visual problems (cataracts and glaucoma), seizures, poor muscle tone, and progressive kidney failure, which was ultimately fatal.
His sister Lily is pregnant, and wonders whether she is a carrier of the disease that killed her brother. She remembers a doctor saying that her mother Drucilla was a carrier. Lily’s physician determines that she is a carrier because she has cataracts, which is a clouding of the lenses. It has not yet affected her vision. When a prenatal test reveals that Lily’s fetus is a female, her doctor tells her not to worry about Lowe syndrome.

G. Mating among Texas field crickets depends upon females responding to a male mating call. The sounds must arrive at a particular frequency to excite the females, who do not sing back in response. However, females can pass on an allele that confers the “correct” frequency of singing.

H. When Winthrop was a baby, he was diagnosed with “failure to thrive.” At 14 months of age, he suddenly took an interest in food, and his parents couldn’t feed him fast enough. By age 4, Winthrop was obese, with disturbing behavior. He was so hungry that after he’d eaten his meal and everyone else’s leftovers, he’d hunt through the garbage for more. Finally a psychiatrist who had a background in genetics diagnosed PraderWilli syndrome. Testing showed that the allele for the Prader-Willi gene that Winthrop had inherited from his father was abnormally methylated.

i. Certain breeds of dogs have cryptorchidism, in which the testicles do not descend into the scrotum. The trait is passed through females.

REVIEW QUESTIONS 1. Modified from the text a bit – Please do th...