W, Z chromosomes
Wolman disease a lysosomal storage disease frist described by M. Wolman and three colleagues in 1961. The condition is inherited as an autosomal re- cessive, and it is due to a deficiency of an acid lipase which leads to progressive accumulation of triglycer- ides and cholesterol esters in lysosomes of homozy- gotes.
The mutations responsible are in a gene sym- bolized LIPA (for lipase A), and it is located between bands 24 and 25 on the long arm of chro- mosome 10. working hypothesis a hypothesis that serves as the basis for future experimentation. woolly monkey Lagothrix lagothricha, a species in- habiting the rain forests of the Amazon basin.
The source of the simian sarcoma virus (q.v.). Wright’s equilibrium law an expression of the zy- gotic proportions expected in a population experi- encing a certain amount of inbreeding.
For a pair of alleles A and a with frequencies p and q, respec- tively, the zygotic proportions are expected to be AA = p2 + Fpq : Aa = 2pq (1 − F) : aa = q2 + Fpq, where F is Wright’s inbreeding coefficient The Hardy-Weinberg Law (q.v.) is a special case of Wright’s equilibrium law in which F = 0. Wright’s inbreeding coefficient (F) the probabil- ity that two allelic genes united in a zygote are both descended from a gene found in an ancestor com- mon to both parents.
Also, the proportion of loci at which an individual is homozygous. See Appendix C, 1968, Wright. Wright’s polygene estimate in the case of quanti- tative inheritance (q.v.), the number of segregating pairs of polygenes (n) can be estimated from the variances computed for the F1 and F2 populations. If these are symbolized sF1 and sF2, respectively, and the means for the high and low P1 strains are Xh and X1, respectively, then
See Appendix C, 1968, Wright; quantitative inheri- tance. writhing number the number of times the axis of a DNA molecule crosses itself by supercoiling. wt wild type (q.v.). W, Z chromosomes the sex chromosomes of an animal in which the female is the heterogametic sex (Bombyx mori, for example). In such cases the W chromosome is female-determining and the male is ZZ. See Bkm sequences.
x See basic number. X “crossed with” or “mated to” (as in A O × B P). X2 the offspring of an F1 test cross. X:A ratio the ratio of the number of X chromo- somes to the number of autosomal sets. A diploid organism with two X chromosomes (i.e., 2X:2A) has an X:A ratio of 1:1, or 1. With one X chromo- some (i.e., 1X:2A) this ratio is 1:2, or 0.50. See genic balance, sex determination. xantha any of many chloroplast mutations in vari- ous cereal species.
The xantha 3 mutant of barley, for example, is characterized by chloroplasts that ac- cumulate excessive numbers of pigment granules and never develop orderly arrays of grana. xanthommatin See Drosophila eye pigments.
X chromosome the sex chromosome found in double dose in the homogametic sex and in single dose in the heterogametic sex. See human X chromo- some. X-chromosome inactivation in mammalian de- velopment, the repression of one of the two X chro- mosomes in the somatic cells of females as a method of dosage compensation. At an early embryonic stage in the normal female, one of the two X chro- mosomes undergoes inactivation, apparently at ran- dom. From this point on, all descendant cells will be clonal in that they will have the same X chromo- some inactivated as the cell from which they arose.
Thus, the mammalian female is a mosaic composed of two types of cells—one that expresses only the paternal X chromosome, and another that expresses only the maternal X chromosome. In some cells and tissues, the inactivated X chromosome can be seen as a dense body in the nucleus (referred to as a Barr body or sex chromatin). In abnormal cases where more than two X chromosomes are present, only one X remains active and the others are inactivated. In marsupials, the paternal X is selectively inacti- vated during female development.
In the somatic cells of human females the “inactivated” X chromo- some is not completely silenced. Fifteen percent of all X-linked genes escape inactivation, and another 10% vary in their expression.
Therefore over 15% of the genes on the X are transcribed at higher levels in women than in men. See Appendix C, 1949, Barr and Bertram; 1961, Lyon and Russell; 1962, Beutler et al.; 1963, Russell; 1998, Lyon; Cattanach translo- cation, human X chromosome, XG, XIST. xenia referring to the situation in which the geno- type of the pollen influences the developing embryo or the maternal tissue of the fruit so as to produce a phenotypically demonstratable effect upon the seed. xenogeneic transplantation xenoplastic trans- plantation (q.v.). xenograft a tissue from an animal that is trans- ferred to another species.
The classical experiments of Spemann and Mangold involved xenografts be- tween gastrulas of one newt species and another.
The embryos of Triton taenitus were pigmented, while those of T. cristatus had little or no pigment. Therefore the source of the cells in the induced sec- ondary structures in the graft hybrid could be ascer- tained.
See Appendix C, 1924, Spemann and Man- gold. xenoplastic transplantation the transplantation between individuals of different genera or widely distant species. See Sus scrofa. Xenopus a genus of aquatic anurans found in sub- Saharan Africa. They are commonly called South Af- rican clawed frogs.
The 16 species in the genus have genome sizes that range from 3.5 × 109 bp to 1.6 × 1010 bp. These size differences are probably the re- sult of a series of chromosome doublings that oc- curred within the past 40 million years.
The ances- tral chromosome number for the genus appears to have been 18, but there are now species with 36, 72, and 108 chromosomes. X. laevis and X. borealis are favorites for research in molecular genetics. Studies on the nucleolar mutants of X. laevis have shown that the nucleolus contains about 450 rRNA genes. This frog has one class of 5S rRNA genes tran- scribed in the oocyte and another in somatic cells.
In Xenopus lampbrush chromosomes there are about 20,000 copies of the oocyte 5S rRNA genes per hap- loid genome, and these are distributed among large chromomeres that terminate the long arms of 15 of the 18 bivalents. There are also 1,300 copies of the somatic 5S rRNA genes, and these are distributed at nonterminal sites along the chromosomes. Since this species has a diploid chromosome number of 36, it can be designated a tetraploid from an evolutionary
standpoint. See Appendix A, Chordata, Amphibia, Anura; Appendix C, 1966, Wallace and Birnsteil; 1967, Birnsteil; 1968, Davidson, Crippa, and Mir- sky; 1973, Ford and Southern; Appendix E, Species Web Site Addresses; Cajal body, concerted evolution, polyploidy, ribosomal RNA genes. xeroderma pigmentosum a group of hereditary diseases inherited as autosomal recessives in which the skin is extremely sensitive to sunlight or ultra- violet light, and death is usually due to skin cancer. Normal skin cells can repair UV damage to DNA by cut-and-patch repair (q.v.).
Skin cells from patients with xeroderma pigmentosum contain mutations in genes functioning in this process. For example, the XPA gene at 9q encodes a DNA-binding protein that recognizes damaged regions. Helicases encoded by XPB (2q) and XPD (19q) unwind the double he- lix and expose these damaged segments. Endonucle- ases encoded by XPG (13q) then cut out the defec- tive segments.
See Appendix C, 1968, Cleaver. http: //www.xps.org. XG the first X-linked gene in humans shown to es- cape X-chromosome inactivation. XG resides on Xp between MIC2 (q.v.) and the centromere. It spans the pseudoautosomal boundary and has its first three exons in the pseudoautosomal region and the rest in the X-specific region. The Xg blood group antigen is encoded by XG. The gene product is a protein that shows a 50% sequence identity to CD99, the product of the MIC2 gene. It therefore appears that the two genes share a common ancestor. X-inactivation See X chromosome inactivation.
Xiphophorus maculatus the platyfish, and X. hel- leri, the swordtail. Pigment cell genetics and the ge- netics of sex determination have been intensively studied in laboratory strains of these freshwater species. See Appendix A, Chordata, Osteichythes, Neopterygii, Antheriniformes. XIST a gene located in the X chromosome inacti- vation center of humans at Xq13. The homologous gene in the mouse is symbolized Xist.
XIST is the acronym for X-Inactive Specific Transcript, and the major transcript is an RNA molecule about 19 kb long. Shorter transcripts are also generated by alter- native splicing (q.v.). The XIST gene is 232,103 base pairs long.
The euchromatic part of the X is six times longer than that of the Y. XIST RNA is tran- scribed in the nuclei of female somatic cells, where it coats the X chromosome and causes its inactiva- tion. The transcribing gene is on the chromosome that is being silenced. Chromosomal regions subject to inactivation are enriched with DNA LINE-1 ele- ments.
These L1 elements may serve as targets to which XIST RNAs bind. See Appendix C, 1996, Penny et al.; 1998, Lyon; repetitious DNA, X-chro- mosome inactivation. X linkage the presence of a gene located on the X chromosome; usually termed “sex linkage” (q.v.). XO the symbolic designation of the situation in some heterogametic organisms in which the X chro- mosome is present and the Y chromosome is absent. XO monosomy See Turner syndrome. x radiation radiations produced when high-speed electrons strike a metallic target.
X-rays have wave- lengths in the range between ultraviolet and gamma radiation and are ionizing radiations. x-ray crystallography the use of the diffraction patterns produced by x-ray scattering from crystals to determine the three-dimensional structure of the atoms or molecules in the crystal. X-ray crystallogra- phy and nuclear magnetic resonance spectroscopy (q.v.) are the only techniques capable of determin- ing the 3D structures of proteins and nucleic acids at atomic resolution. See Appendix C, 1913, Bragg and Bragg; 1949, Hodgkin et al.; 1951, Wilkins and Gosling; 1952, Franklin and Gosling; 1953, Watson and Crick; 1958, Kendrew et al.; 1959, Franklin, Caspar, and Klug; 1960, Perutz et al.; 1976, Finch and Klug; isomorphous replacement, large angle x- ray diffraction, photograph 51, small angle x-ray dif- fraction.
XXY trisomy See Klinefelter syndrome. XYY trisomy a human karyotype observed in about 1 in 1,000 male births. Most adult XYY males are over six feet tall. A few are sterile, and some are mentally retarded or have behavioral disorders. XYY individuals make up a greater-than-average propor- tion of the patients in mental-penal institutions. This may be accounted for in part by their diminished intelligence, which may make it easier for them to be apprehended.
Y the single-letter symbol for pyrimidine. See R3. YAC yeast artificial chromosome (q.v.). Y chromosomal DNA lineages paternal lines of evolutionary descent traced using molecular markers on the Y chromosome. Studies of 1,000 men from 22 geographical areas traced the ancestral Y chromo- some to an African male who lived about 59,000 years ago. In the popular press this male was called Y chromosome Adam.
The observation that this man lived thousands of years later than mitochondrial Eve simply means that at the time Eve’s mtDNA had become fixed in Homo sapiens, males still had a vari- ety of Y chromosomal DNAs. It took tens of thou- sands of years more before one particular version of the Y became fixed in our species. See Appendix C, 2001, Underhill et al.; mitochondrial DNA lineages. Y chromosome the sex chromosome found only in the heterogametic sex. In Drosophila melanogaster (q.v.) the Y chromosome is composed almost en- tirely of heterochromatin (q.v.). In many Drosophila
species, the Y chromosome develops prominent lampbrush loops in the nuclei of spermatocytes. The loops consist of a DNA axis to which fibers of tran- scribed RNA are attached. This RNA is associated with large amounts of protein.
The loop-associated transcripts lack open reading frames. Proteins such as tektins (q.v.) are bound to specific loops. How- ever, tektins are not encoded by Y-linked genes. Therefore, the RNAs associated with the Y loops may bind specific exogenous proteins that are des- tined for assembly into sperm axonemes. In humans the Y is rich in heterochromatin and only contains about 1% as many genes as the X. One of these, SRY (q.v.), is responsible for switching development into the male pathway.
See Appendix C, 1968, Hess and Meyer; 1987, Page et al.; 1993, Pisano et al.; human gene maps, human Y chromosome, Jews, TSPY gene. yeast any fungus that generally exists as single cells and usually reproduces by budding. When “yeast” is mentioned in genetics, Saccharomyces cerevisiae (q.v.), the yeast used by bakers and brewers, is usually the species being referred to. Schizosaccharomyces pombe (q.v.), a fission yeast, is also mentioned in the more recent literature.
Yeast artificial chromosomes