Which of the following conditions will produce more than one Barr body in cells of affected patients?
D. In testicular feminization syndrome, the karyotype is usually 46 XY. Due to insensitivity of androgen receptors, female sexual characteristics develop in such individuals. They will not have Barr bodies. In those with Kleinfelter’s syndrome, the karyotype is usually 47 XXY. Here, the individuals will have one Barr body in spite of being phenotypical males. Patients with Turner’s syndrome have no Barr bodies as they have only one X chromosome, in spite of being phenotypical females. Patients with triple-X syndrome show two Barr bodies in each cell. These individuals are also called metafemales. In fragile-X syndrome the number of Barr bodies will not be altered.
Reference:
Mrs Smith is a 32-year-old woman with normal IQ scores whose son has been recently diagnosed to have fragile-X syndrome. There is no family history of fragile-X syndrome in her husband’s lineage, but Mrs Smith’s maternal uncle had mental retardation, suspected to be fragile X retrospectively.
Which of the following best describes Mrs Smith’s genotype?
A. Premutation is a term used in trinucleotide repeat diseases to suggest that someone is harbouring the trinucleotide expansion but the expansion is not long enough to produce the disease. But premutants will produce further expansion of the loci during gametogenesis and thus their children will express the mutation if inherited. In this question the mother has no phenotypic expression, which is rare to occur after age 32. Her genotype cannot be normal as her uncle and son are both affected by fragile-X syndrome. Fragile-X syndrome has nearly complete penetration falsifying the fourth option.
If a mother has alleles ‘pp’ while a father has alleles ‘Pp’ at the same locus, then which of the following distributions can be expected in the next generation?
A. The mother has genotype pp. Her gametes can both have p only. The father has Pp. His gametes may be either p or P. If these gametes combine in the children four possible combinations—pp, pp, pP, and pP—will be produced. Hence there will be 1/2 pp and 1/2 Pp variants in the children.
Which of the following genetic abnormalities is associated with rocker bottom feet, protrusion of bowel through the umbilical cord, and low-set ears in a male child, newly born to both healthy parents with no history of genetic disorders in the family?
E. This question refers to Edwards’ syndrome, which is 18 trisomy. This is an aneuploidy. Euploidy refers to the presence of chromosomal numbers in multiples of 23. Haploid refers to the presence of 23 chromosomes, as normally seen in gametes. Most somatic cells are diploid, possessing 46 chromosomes. Aneuploidy refers to any aberrations in chromosomal numbers, for example monosomy, trisomy, etc. Edward’s syndrome is characterized by 47XX +18 or 47XY +18 constitutions. It is seen in around 1 in 6000 live births; 90% of infants die in the first year of life. The common clinical features are small size, small mouth and low-set ears, clenched fi st with overlapping fingers, congenital heart defects, and omphalocele.
Which of the following genetic mechanisms can explain the occurrence of Angelman’s syndrome?
B. Angelman’s syndrome is an example of genomic imprinting. Deletion of maternally inherited 15q11-13 (70%) or uniparental disomy where both 15q11-13 come from the father (4%) leads to Angelman’s syndrome. This is because certain genetic loci in 15q11-13 are selectively imprinted (that is inactivated via methylation) according to the parent of origin. When the maternally derived chromosome is absent due to deletion or paternal disomy this produces the phenotype. Similarly, maternally derived disomy or deletion of the paternally derived chromosome can produce Prader–Willi syndrome at the same locus.
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