7. In humans there are noautosomal trisomies, but individuals with XXX, XXY, XXX

Question

7. In humans there are noautosomal trisomies, but individuals with XXX, XXY, XXXY, etc. havebeen found. The XXX and XXXY individuals have had two Barr bodiesin their somatic cells. The XXY individual had one Barr body. Whatexplanation might you make about why autosomal trisomies cannotsurvive, but individuals with 3 or more sex chromosomes maysurvive. a. The sex chromosomes haverelatively few genes. Variation in their number is not important tosurvival. b. The Y chromosome has relativelyfew genes. It does not cause death when duplicated orabsent. c. The amount of the X chromosomegene products is adjusted when more than one X is present. Once oneactive X is present all other Xs are transcriptionallysilenced. d. The amount of the autosomalproducts is not adjusted in trisomics. All genes are available tobe transcribed and that can lead to the production of 1.5 X theamount of RNAs than expected in the normal individual. e. Abnormal amounts of specific mRNAscan lead to the production of abnormal amounts of protein. Theamount of specific proteins can influence developmental events. Ifthere is too much or too little protein, the organism maydie. f. All of the above aretrue. g. Answers B through E are accurateand relevant to this question.

Explanation / Answer

c.The amount of the X chromosome gene products is adjustedwhen more than one X is present. Once one active X is present allother Xs are transcriptionally silenced. Lyon hypothesis refers directly to a Barr body. It wasproposed by Mary Lyon (1925-) in 1961 that a Barr body is actuallyan inactivated X chromosome. According to this hypothesis, femalemammals sequester one X chromosome in each of their cells duringthe early stages of development. This folded chromosome becomes thedark body of Barr's observation. This means that both males andfemales rely on the information from only a single X chromosome.Therefore, it is only one X chromosome that provides geneticinformation in both males and females. The random and fixed inactivation (in the form of sex chromatin) ofall X chromosomes in excess of one in mammalian cells at an earlystage of embryogenesis, leading to mosaicism for X-linked genes inthe female, since the paternal X chromosome is inactivated in somecells and the maternal one in the remainder.

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