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(a) Draw the pairing arrangement of these chromosomes in prophase I of meiosis.

(b) Diagram the alternate, adjacent-1, and adjacent-2 segregation patterns in anaphase I of meiosis.

(c) Give the products that result from alternate, adjacent-1, and adjacent-2 segregation.

30. Red-green color blindness is a human X-linked recessive disorder. A young man with a 47,XXY karyotype (Klinefelter syndrome) is color-blind. His 46,XY brother also is colorblind. Both parents have normal color vision. Where did the nondisjunction occur that gave rise to the young man with Klinefelter syndrome?

31. Some people with Turner syndrome are 45,X/46,XY mosaics. Explain how this mosaicism could arise.

*32. Bill and Betty have had two children with Down syndrome. Bill's brother has Down syndrome and his sister has two children with Down syndrome. On the basis of these observations, which of the following statements is most likely correct? Explain your reasoning.

(a) Bill has 47 chromosomes.

(b) Betty has 47 chromosomes.

(c) Bill and Betty's children each have 47 chromosomes.

(d) Bill's sister has 45 chromosomes.

(e) Bill has 46 chromosomes.

(f) Betty has 45 chromosomes.

(g) Bill's brother has 45 chromosomes.

*33. Tay-Sachs disease is an autosomal recessive disease that causes blindness, deafness, brain enlargement, and premature death in children. It is possible to identify carriers for Tay-Sachs disease by means of a blood test. Mike and Sue have both been tested for the Tay-Sachs gene; Mike is a heterozygous carrier for Tay-Sachs, but Sue is homozygous for the normal allele. Mike and Sue's baby boy is completely normal at birth, but at age 2 develops Tay-Sachs disease. Assuming that a new mutation has not occurred, how could Mike and Sue's baby have inherited Tay-Sach's disease? 34. In mammals, sex-chromosome aneuploids are more common than autosomal aneuploids but, in fishes, sex-chromosome aneuploids and autosomal aneuploids are found with equal frequency. Offer an explanation for these differences in mammals and fishes.

*35. A young couple is planning to have children. Knowing that there have been a substantial number of stillbirths, miscarriages, and fertility problems on the husband's side of the family, they see a genetic counselor. A chromosome analysis reveals that, whereas the woman has a normal karyotype, the man possesses only 45 chromosomes and is a carrier for a Robertsonian translocation between chromosomes 22 and 13.

(a) List all the different types of gametes that might be produced by the man.

(b) What types of zygotes will develop when each of gametes produced by the man fuses with a normal gamete from the woman?

(c) If trisomies and monosomies entailing chromosome 13 and 22 are lethal, what proportion of the surviving offspring will be carriers of the translocation?

^CHALLENGE QUESTION^_

36. Red-green color blindness is a human X-linked recessive disorder. Jill has normal color vision, but her father is colorblind. Jill marries Tom, who also has normal color vision. Jill and Tom have a daughter who has Turner syndrome and is color-blind.

(a) How did the daughter inherit color blindness?

(b) Did the daughter inherit her X chromosome from Jill or from Tom?

SUGGESTED READINGS]_

Boue, A. 1985. Cytogenetics of pregnancy wastage. Advances in Human Genetics 14:1-58.

A study showing that many human spontaneously aborted fetuses contain chromosome mutations.

Brewer, C., S. Holloway, P. Zawalnyski, A. Schinzel, and D. FitzPatrick. 1998. A chromosomal deletion map of human malformations. American Journal of Human Genetics 63:1153-1159.

A study of human malformations associated with specific chromosome deletions. Epstein, C. J. 1988. Mechanisms of the effects of aneuploidy in mammals. Annual Review of Genetics 22:51-75. A review of how aneuploidy produces phenotypic effects in mammals.

Feldman, M., and E. R. Sears. 1981. The wild resources of wheat. Scientific American 244, (1):98.

An account of how polyploidy has led to the evolution of modern wheat.

Gardner, R. J. M., and G. R. Sunderland. 1996. Chromosome Abnormalities and Genetic Counseling. Oxford: Oxford University Press.

A guide to chromosome abnormalies for genetic counselors. Goodman, R. M., and R. J. Gorlin. 1983. The Malformed Infant and Child: An Illustrated Guide. New York: Oxford University Press.

A pictorial compendium of genetic and chromosomal syndromes in humans. Hall, J. C. 1988. Review and hypothesis: somatic mosaicism — observations related to clinical genetics. American Journal of Human Genetics 43:355-363.

A review of the significance of mosaicism in human genetics.

Hieter, P., and T. Griffiths. 1999. Polyploidy: more is more or less. Science 285:210-211.

Discusses current research that shows that there is some unbalanced gene expression in polyploid cells.

Patterson, D. 1987. The causes of Down syndrome. Scientific American 257(2):52 — 60.

An excellent review of research concerning the genes on chromosome 21 that cause Down syndrome.

Rabbitts, T. H. 1994. Chromosomal translocations in human cancers. Nature 372:143—149.

Reviews the association of some chromosomal translocations with specific human cancers. Rowley, J. D. 1998. The critical role of chromosome translocations in human leukemias. Annual Review of Genetics 32:495 — 519.

A review of molecular analyses of chromosome translocations in leukemias.

Ryder, O. A., L. G. Chemnick, A. T. Bowling, and K. Benirschke. 1985. Male mule foal qualifies as the offspring of a female mule and jack donkey. Journal of Heredity 76:379 — 381. A study of a male foal (Blue Moon) born to a mule, which was discussed at the beginning of the chapter.

Sánchez-García, I. 1997. Consequences of chromosome abnormalities in tumor development. Annual Review of Genetics 31:429 — 453.

Reviews the nature of fusion proteins produced by chromosome translocations that play a role in tumor development.

Schulz-Schaeffer, J. 1980. Cytogenetics: Plants, Animals, Humans. New York: Springer Verlag. A detailed treatment of chromosomal variation.

100 Pregnancy Tips

100 Pregnancy Tips

Prior to planning pregnancy, you should learn more about the things involved in getting pregnant. It involves carrying a baby inside you for nine months, caring for a child for a number of years, and many more. Consider these things, so that you can properly assess if you are ready for pregnancy. Get all these very important tips about pregnancy that you need to know.

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