What is the best explanation of a child having the physical phenotype of Down syndrome but having a totally normal karyotype on chromosomal analysis of blood?
A)Mosaicism of trisomy 21 in various tissues
B)Inaccurate technique for chromosomal analysis
C)Uniparental disomy for somatic cells but not for germ cells
D)Selective chromosome loss during meiosis II of gametogenesis

Question

Quizplus · Accepted Answer

Mosaicism is a condition in which two (or more) different karyotypes are consistently present in one individual or in which the karyotype does not match with the phenotype. Mosaicism can be tissue specific, meaning that some tissues can express a mixture of normal and abnormal karyotypes, and other tissues may express all or nearly all of just one type. In this question, only the normal karyotype is expressed in the person's blood. Because most cytogenetic studies are performed on blood cells or skin cells, when mosaicism is found in these tissues, the percentage of cells with abnormal karyotypes may not represent the actual proportion of cells with the abnormality in other tissues.

What Is the Best Explanation of a Child Having the Physical