Passage
A 6-day-old, genotypically normal (46,XX) female patient shows symptoms of hypoglycemia (low blood glucose) and hypotonia (decreased muscle tone). Physical examination reveals abnormal development of the external genitalia, and laboratory studies show elevated urinary sodium content, increased serum levels of potassium, and abnormal serum concentration of several steroid hormones. The patient is diagnosed with 21-hydroxylase deficiency, the most common type of congenital adrenal hyperplasia (CAH).CAH refers to a group of autosomal recessive diseases, each associated with defective production of one or more steroid hormones by the adrenal glands, two crescent-shaped glands located above each kidney in the abdominal cavity. It has been reported that unequal crossing over involving chromosome 6 can result in CAH. More than 90% of CAH diagnoses are associated with a deficiency of 21-hydroxylase, an enzyme encoded by the CYP21A2 gene on chromosome 6.Normally, 21-hydroxylase catalyzes chemical reactions involved in the synthesis of aldosterone and cortisol, the principal mineralocorticoid and glucocorticoid hormones, respectively. As a result, patients with 21-hydroxylase deficiency exhibit diminished serum levels of aldosterone and cortisol. However, synthesis and secretion of the sex hormone testosterone is excessive. Glucocorticoid deficiency disrupts the negative feedback loop linking the brain to the adrenal gland, leading to hyperplasia (increased cell proliferation) of adrenal tissue.The patient's 21-hydroxylase deficiency is treated with regular administration of synthetic steroid hormones. Her condition improves after receiving these medications.
-Given that the fusion of male and female gametes results in fertilization, which statement best explains how unequal crossing over during gametogenesis can lead to 21-hydroxylase deficiency in the fertilized embryo? One of the parental gametes:

Question

Passage
A 6-day-old, genotypically normal (46,XX) female patient shows symptoms of hypoglycemia (low blood glucose) and hypotonia (decreased muscle tone).  Physical examination reveals abnormal development of the external genitalia, and laboratory studies show elevated urinary sodium content, increased serum levels of potassium, and abnormal serum concentration of several steroid hormones.  The patient is diagnosed with 21-hydroxylase deficiency, the most common type of congenital adrenal hyperplasia (CAH).CAH refers to a group of autosomal recessive diseases, each associated with defective production of one or more steroid hormones by the adrenal glands, two crescent-shaped glands located above each kidney in the abdominal cavity.  It has been reported that unequal crossing over involving chromosome 6 can result in CAH.  More than 90% of CAH diagnoses are associated with a deficiency of 21-hydroxylase, an enzyme encoded by the CYP21A2 gene on chromosome 6.Normally, 21-hydroxylase catalyzes chemical reactions involved in the synthesis of aldosterone and cortisol, the principal mineralocorticoid and glucocorticoid hormones, respectively.  As a result, patients with 21-hydroxylase deficiency exhibit diminished serum levels of aldosterone and cortisol.  However, synthesis and secretion of the sex hormone testosterone is excessive.  Glucocorticoid deficiency disrupts the negative feedback loop linking the brain to the adrenal gland, leading to hyperplasia (increased cell proliferation) of adrenal tissue.The patient's 21-hydroxylase deficiency is treated with regular administration of synthetic steroid hormones.  Her condition improves after receiving these medications.
-Given that the fusion of male and female gametes results in fertilization, which statement best explains how unequal crossing over during gametogenesis can lead to 21-hydroxylase deficiency in the fertilized embryo?  One of the parental gametes:

Quizplus · Accepted Answer

11ecba2d_0fcf_4874_a3bf_7517a3d90972_MD0008_00 In eukaryotes, reproductive cells (gametes) are produced via meiosis, during which a parent cell (2n) divides to produce four genetically distinct daughter cells containing half the original number of chromosomes (n). Crossing over which occurs during prophase I of meiosis, is the process by which homologous chromosomes swap genetic material through the breakage and exchange of corresponding DNA segments between non-sister chromatids. As a result, crossover events give rise to daughter cells with chromosomes containing allele combinations that differ from those in the parent cell, leading to genetic recombination and increased genetic diversity. When homologous chromosomes fail to align properly, unequal crossing over occurs. Unequal crossing over is the exchange of unequal segments of DNA between non-sister chromatids, yielding one chromosome that contains extra copies of one or more genes (ie, gene duplication) and another chromosome that lacks those genes entirely (ie, gene deletion). According to the passage, 21-hydroxylase deficiency occurs due to insufficient activity of 21-hydroxylase, the enzyme encoded by the CYP21A2 gene on chromosome 6. If unequal crossing over involving chromosome 6 occurs, it may result in a daughter cell that contains a chromosome lacking CYP21A2, which would contribute to the development of 21-hydroxylase deficiency in offspring. (Choices A and B) Nondisjunction is defined as the failure of homologous chromosomes to separate in anaphase of meiosis I or the failure of sister chromatids to separate in anaphase of meiosis II. Consequently, nondisjunction produces gametes that either lack a copy of a chromosome or contain an extra copy of a chromosome. Unequal crossing over of genetic material between non-sister chromatids gives rise to gametes that lack or contain an extra copy of individual genes, not entire chromosomes. (Choice C) 21-hydroxylase deficiency occurs due to insufficient 21-hydroxylase activity, which can be caused by reduced CYP21A2 expression or by loss of function of the 21-hydroxylase enzyme itself. Although it can result from unequal crossing over, duplication of CYP21A2 would cause enhanced 21-hydroxylase activity (not deficiency) due to greater expression of the gene. Educational objective: Crossing over is a form of genetic recombination involving the exchange of genetic material between non-sister chromatids of homologous chromosomes. Unequal crossing over can result in offspring that lack important genes.

Passage a 6-Day-Old, Genotypically Normal (46,XX) Female Patient Shows Symptoms of of Hypoglycemia