Question 1

Which clinical feature in a newborn baby girl suggests the possibility of Turner syndrome?

Accepted Answer

Girls with Turner syndrome are often identified at birth because of the presence of "classic" phenotypic features. These include a smaller-than-expected size at full-term pregnancy, neck webbing, pedal edema, and cardiac abnormalities. These children have a normal early neurologic response, including Babinski reflex. Although epicanthal folds often occur in Turner syndrome, they are not unique to this syndrome. Aniridia is associated with WGAR syndrome and is not found in Turner syndrome.

Question 2

At what specific location on an X chromosome is a break most likely to occur in fragile X syndrome?

Accepted Answer

The term fragile X comes from the fact that when chromosomal analysis is performed under certain laboratory conditions, the X chromosome often has a small break or gap at the end of the long arm.

Question 3

Which process can cause a person with an XY genotype to have a female phenotype?

Accepted Answer

The situation resulting in a person with an XY genotype having a naturally occurring female phenotype is complete androgen insensitivity or androgen insensitivity syndrome (AIS). In this condition, all tissues, including the masculine tissues (mesonephric ductal tissues), are missing androgen receptors. The presence of the Y chromosome starts the organization of the bipotential gonad into a testis with the secretion of testosterone; however, the testosterone has no developmental influence on the mesonephric ductal tissues because they lack the receptors for binding and allowing the testosterone to change the gene activity of these cells. Thus, the mesonephric ducts regress, and the paramesonephric ducts undergo partial growth. At birth, the child has very clear female external genitalia, and this phenotype continues throughout life. The girl does go through her adolescent growth spurt and starts to develop hip curves and breasts but does not begin menstruation. She cannot become pregnant but is female in every other sense. The position of the bipotential gonad does not influence its function. Klinefelter men and Turner women are infertile. The X chromosome does not inactivate the Y chromosome.

Question 4

Which of the following mechanisms in fragile X syndrome (FXS) leads to expression of the disorder?

Accepted Answer

In FXS, the FMR1 gene-coding regions are normal, but the gene's expression is silenced by large numbers of the trinucleotide sequence of CGG being repeated within the noncoding regions of the gene.

Question 5

Which trait or characteristic in a 10-year-old-girl would lead her pediatrician to consider the possibility of androgen insensitivity syndrome?

Accepted Answer

The gene for color vision is on the X chromosome and is recessive. Thus, color-blindness is an X-linked-recessive disorder resulting from a mutation in the gene. If a male inherits this gene from his mother, he will express color-blindness. If a girl inherits this gene from her mother, she will only express color-blindness if her father is color-blind.

Question 6

What is the function of the FMR1 gene?

Accepted Answer

The FMR1 gene, which is located on the X chromosome, codes for a protein that is critical in brain development and maintenance of neural synapses. If this protein is not produced in sufficient amounts or is abnormal, reduced cognition occurs, especially in males. FMX1 stands for fragile X mental retardation 1. Mutations are not responsible for the amyloid plaques and neuronal tangles that develop in Alzheimer's disease.

Question 7

Which of the following definitions accurately represents the concept of expansion?

Accepted Answer

Expansion is an increase, usually of trinucleotide repeat sequences within a gene. The definition of response A is that of anticipation. The definition of response C is amplification. Response D is just a mutation that would result in either an inappropriate protein or no protein produced.

Question 8

What is the risk for a man with a 47,XYY karyotype to produce a child with a 47,XYY karyotype?

Accepted Answer

Men with a 47,XYY karyotype are fertile. Because the extra Y chromosome is likely to be present in approximately 50% of mature spermatozoa, the risk for a man with this karyotype to produce a child who is also 47,XYY is also 50%.

Question 9

On which chromosome is the androgen receptor (AR) gene located?

Accepted Answer

Even though men have more androgen receptors than do women, the AR gene is located on the X chromosome, not the Y. Women do produce small amounts of androgens in the adrenal gland, which need to bind to the AR receptors in order to produce their effects.

Question 10

Which condition or health problem is more common in women who have an FMR1 mutation?

Accepted Answer

Women with FMR1 mutations or trinucleotide repeat sequences that categorizes them as premutation carriers can develop fragile X-associated primary ovarian insufficiency syndrome (FXPOI), which is an early onset of ovarian failure causing the onset of menopause in women who are premutation carriers at ages under 40 years.

Question 11

Which feature is common among people who have Klinefelter syndrome (47,XXY) or a karyotype with 47,XXX but not among people who have Down syndrome or Edward syndrome?

Accepted Answer

In childhood, the only phenotypic manifestation of Klinefelter syndrome is that the boy usually has long legs and is taller than siblings at the same ages. Often, his final height is taller than that of either parent. The most outstanding and consistent feature is that 47,XXX women are taller than average and are taller than most family members. Although 47,XXY males are infertile, women with 47,XXX are not. Neither is associated with severely reduced cognition or cleft palate. Both Down syndrome and Edward syndrome result in shorter-than-average stature.

Question 12

A male patient is tall and has some gynecomastia along with a low sperm count. During infertility testing, he was found to have a 47,XXY karyotype. Which disorder isconsistent with these findings?

Accepted Answer

This patient has a phenotype and karyotype consistent with Klinefelter syndrome. This is not a normal karyotype, but the patient is male. Turner syndrome karyotype is 45,X. The karyotype for testicular feminization (an old term) is 46,XY.

Question 13

What factor most strongly influences the development of the paramesonephric tissue into female sex organs?

Accepted Answer

The early embryonic tissue capable of developing into male sex organs, including the penis, scrotum, prostate, and the tubular system connecting the testis to the urinary system, is the mesonephric ductal tissue (wolffian glands). The tissue capable of developing into female sex organs is the paramesonephric ductal tissue (müllerian ducts). Both types of tissues are present in embryos that are 46,XX and those that are 46,XY. Under the influence of testosterone and anti-müllerian factor, which are normally secreted in the 46,XY embryo, mesonephric ductal tissue differentiates into anatomic male sex structures, and the paramesonephric ductal tissue regresses and degenerates so that anatomic female sex structures do not develop. Without the presence of Y-chromosome-influenced testosterone and anti-müllerian factor, the mesonephric ducts regress and degenerate. The lack of anti-müllerian factor together with the genetic influence of autosomal gene products causes the development of the paramesonephric ductal tissues into complete anatomic female sex structures. Although the absence of the Y chromosome is not the only reason that female structures develop, it appears to be the strongest.

Question 14

Which health screening activity is most important for girls and women with Turner syndrome?

Accepted Answer

The incidence of osteoarthritis, lung problems, and breast cancer is not increased in this population. Other health problems are common and can be significant for the person who has Turner syndrome, especially long-term hypertension and hyperlipidemia, which may be present even in early childhood.

Question 15

A man and his sister are both affected with a mitochondrial disorder. Because the sister has two children who also have the disorder, the man is concerned about thepossibility of also having children with the disorder. What is the estimated risk for each pregnancy of this man producing a child with the same mitochondrial disorder that he has if his wife does not have a mitochondrial disorder?

Accepted Answer

All mitochondria at conception come from the egg because they are located in the egg's cytoplasm. The sperm has no cytoplasmic mitochondria. Therefore, most mitochondrial disorders can only be inherited from a woman with the disorder and not from a man with the disorder.

Question 16

What is the most likely mechanism for a 45,X karyotype?

Accepted Answer

Inheriting only one chromosome of a pair is monosomy, which most likely occurs as a result of nondisjunction during meiosis for gamete formation. (Recall that with nondisjunction of a chromosome pair, the pair fails to separate during meiosis. Thus, there is no segregation; both chromosomes of the pair move into one gamete, and the other gamete does not receive a chromosome from this pair.) This results in an ovum with two X chromosomes and an ovum with no X chromosome. If the ovum with no X is fertilized and results in a pregnancy, the product of the pregnancy would have Turner syndrome with a karyotype of 45,X. Nondisjunction appears to occur more often with the sex chromosome, particularly the X.

Question 17

Which single physical feature is most often associated with a 47,XXX karyotype?

Accepted Answer

A karyotype of 47,XXX produces a physically normal phenotype, and the individual is fertile. The most outstanding and consistent feature is that these women are taller than average and are taller than most family members. Breast development is not affected by the extra X chromosome, and neither is hair loss.

Question 18

Why is the incidence of an extra Y chromosome in the general population unknown?

Accepted Answer

There are no consistent and specific variations in the phenotype to indicate any type of chromosomal aberration.

Question 19

What would be the expected consequence to two daughter cells after mitosis if the ability to produce mitochondria was lost?

Accepted Answer

Mitochondria are organelles within a cell's cytoplasm that are responsible for most of the generation of a high-energy chemical substance used to power cellular work. Thus, they are the "power plants" of cellular energy production. Without mitochondria in the newly generated cell, ATP production would be profoundly reduced.

Question 20

Which type of problem results from mutations in mitochondrial DNA (mtDNA)?

Accepted Answer

Mitochondria produce most of the adenosine triphosphate (ATP) to use as the energy source to drive cellular actions. Mutations in mitochondrial DNA reduce the amount of ATP produced and affect the function of highly metabolic cells, tissues, and organs. Crossing over and translocations do not occur in mtDNA. Although single-nucleotide polymorphisms can occur in mtDNA, they are not the problem caused by mutated mtDNA.

Quiz 7: Sex Chromosome and Mitochondrial Inheritance and Disorders