Chapter 7 DEVELOPMENT AND SEX DETERMINATION Chapter Summary The male and female reproductive systems produce the sperm and eggs, and promote their meeting and fusion, which results in a fertilized egg. Genes control the development of the reproductive systems, and genes influenced your entire development from the moment of conception. In this chapter, you are able to review the structure and function of the reproductive systems, as well as the events of fertilization and human embryonic development. When you read about the main developmental features of each three month period (trimester) of uterine existence, remember that specific genes must function at exactly the right time to produce a phenotypically normal infant. Teratogens are environmental factors that interfere with normal fetal development, and that range from drugs to infectious agents like viruses. Although the sex of an embryo is determined at the time of conception, the phenotype associated with this sex only appears as embryonic development proceeds. During sex differentiation, internal as well as external features specific to one sex or the other start to develop. The Y chromosome is instrumental in the development of testes, which then produce the balance of hormones that elicit the male phenotype. An alternative developmental pathway induces the formation of ovaries and the female sexual phenotype. Phenotypic sex characteristics that normally accompany a particular gonadal sex may be altered by single gene traits. Because females have two X chromosomes, they do not necessarily produce twice as much of an X-linked gene product as do males. Dosage compensation results when one of the two Xs forms a Barr body, but this can produce some interesting phenotypic mosaicism if the female is heterozygous at X-linked loci.
54 Chapter 7 Sex-influenced traits are caused by genes that are not sex linked, but which interact with the sex phenotype to produce different numbers of affected males and females. Sexlimited traits are also single gene traits that are only able to be expressed in members of one sex or the other. Imprinting of some alleles during gamete production can cause differences in gene expression, depending on whether the allele was inherited from an individual s mother or father. Learning Objectives After studying this chapter, you should be able to: a. give the major events in human embryological development. b. list the common teratogens. c. explain the difference between chromosomal sex, gonadal sex and phenotypic sex. d. discuss the implications of two X chromosomes in females vs. one X in males. e. explain the difference between sex linked, sex influenced and sex limited genetic traits. Key Terms zygote sperm oocyte gametes gonads testes ovaries scrotum seminiferous tubules spermatocytes spermatogenesis epididymis vas deferens ejaculatory duct urethra seminal vesicles prostaglandins Multiple Choice prostate gland bulbourethral glands semen follicles ovulation oviduct uterus endometrium cervix vagina oogenesis oogonia fertilization blastocyst inner cell mass trophoblast chorion teratogen fetal alcohol syndrome sex ratio SRY testosterone Müllerian inhibiting hormone (MIH) androgen insensitivity pseudohermaphroditism dosage compensation Barr body Lyon hypothesis X-inactivation center sex-influenced traits pattern baldness sex-limited genes imprinting
Development and Sex Determination 55 Circle the letter of the best answer. 1. In humans, fertilization normally occurs a. in the oviduct b. in the vagina c. in the uterus d. in the ovary 2. The human female reproductive cell that is fertilized is known as a. an oogonium b. a primary oocyte c. a secondary oocyte d. a polar body 3. Formation of organ systems in a human embryo a. is completed by the end of the third week b. does not begin until the second trimester c. occurs in sequence, so that nervous system development does not begin until the circulatory system is complete d. begins by the end of the third week 4. A developing human is called a fetus a. from the moment of conception b. after seven weeks of existence c. after the first trimester d. after the second trimester 5. A zygote is another name for a. a primary spermatocyte b. an ootid c. a fertilized egg d. a week-old embryo 6. Of the following structures, the one that is not part of the male reproductive system is a. the prostate gland b. the cervix c. the vas deferens d. the epididymis
56 Chapter 7 7. Viruses such as herpes have their greatest teratogenic effects a. as the baby passes through the birth canal b. during embryonic and fetal development c. if the baby receives viral antibodies in the mother s milk d. in the weeks following birth, before the baby s immune system begins to function 8. An individual with a Y chromosome a. is always phenotypically male, no matter how many Xs are present b. is only male if no more than two Xs are present c. can be identified as male from the third week of development on d. cannot have a second Y chromosome 9. Cheryl has breasts and female external genitalia. These are parts of Cheryl s a. gonadal sex b. chromosomal sex c. phenotypic sex d. heterogametic sex 10. An example of dosage compensation is a. Susan has two active alleles of an X-linked gene; David has only one. b. inactivation of a part of Susan s maternal chromosome 15 produces Prader-Willi syndrome. c. Susan has one allele of an X-linked gene producing twice as much product as the same allele in David. d. Susan has one allele of an X-linked gene inactivated; David does not. 11. Several dominant traits carried by genes on the X chromosome, such as incontinentia pigmenti, are seen only in females because a. males have no X chromosome. b. two copies of the dominant allele are necessary for the trait to appear. c. affected males die without reproducing. d. males with one dominant allele die before birth. Fill-ins Supply the word or words to complete the statement. 1. Sex-limited genes are different from other genes because they. 2. Following fertilization, the embryo is in a critical period in its life. It must in the if it is to obtain a food supply for continued growth.
Development and Sex Determination 57 3. Calcium for fetal bone development is obtained from. 4. Meiosis in females requires a minimum time of from start to finish. 5. Alcohol may produce the symptoms of fetal alcohol syndrome because it to the fetus. 6. The presence of the Y chromosome with its associated SRY gene causes the sex neutral embryo to develop the structures known as. 7. Barr bodies seen in nuclei of mammalian cells are actually X chromosomes which,. 8. In order for a cell to have a Barr body in its nucleus, there must be X chromosome(s) in the nucleus. 9. A phenotypic male who has cells with one Barr body must have as his sex chromosomes. A phenotypic female with two Barr bodies has X chromosomes. 10. An option for infertile couples that involves introducing a single sperm into an egg is called. 11. The expression of most of your genes is not affected by being inherited from either your mother or your father. When maternal or paternal inheritance makes a difference, the gene is said to be. Editing If the statement is true, label it T. If the statement is false, change the underlined word or phrase to make it true. Merely adding no or not is not an acceptable change. 1. In the case of the sex-influenced gene producing pattern baldness, the gene acts as a recessive in males. 2. Due to sex-influenced expression of the pattern baldness gene, males with either one or two baldness alleles will be bald. 3. Non-Mendelian expression at the pattern baldness locus is due to an interaction between the genotype and the place where the individual lives.
58 Chapter 7 4. If Theresa is heterozygous for the X-linked gene that produces the enzyme G6PD, approximately half her cells in which the enzyme is normally produced should contain G6PD. 5. We can make the statement about Theresa in question #4 because an X chromosome is deleted in every cell. 6. Males and females produce about the same amount of each X-linked gene product because the X-linked genes in males work twice as hard. 7. Individuals who exhibit androgen insensitivity are XY but have ovaries. 8. Androgen insensitivity occurs in individuals who had very domineering mothers. 9. If some cells in a person s body are XX and others are XY, the individual is known as a mosaic. 10. The SRY gene is thought to be responsible for the production of testosterone. 11. It is recommended that pregnant women limit themselves to two alcoholic drinks per day. 12. Females with Duchenne muscular dystrophy are rare because having two recessive alleles is lethal. Short Discussion Questions/Problems 1. Sandy is a sprinter who is invited to compete for a place on the women s team at the next World Cup trials. Officials are sent information from a jealous competitor that Sandy is actually XY with testicular feminization and should compete as a male. How would you react to this news if you were competing against Sandy? How would you defend yourself if you were Sandy? What decision would you make as the official? 2. Arnold has abdominal surgery, and the surgeon discovers a poorly developed uterus and tubes in Arnold s abdomen. A karyotype shows Arnold to be XY. Explain the cause of this unusual anatomical finding. Would you expect Arnold to have transsexual tendencies? Would you expect Arnold to think differently about his sexuality? 3. The chapter material shows that males and females start out as embryos with the same genetic structures and indifferent gonads. What determines whether this sex-neutral embryo will become male or female?
Development and Sex Determination 59 4. Given all that is known about human sexual development, it is possible in the near future that choosing the sex of your child will become easier. How available should this technology (when it exists) be made to the general public? Would more people choose to have boys or girls? What are possible consequences of having your choice in this matter? 5. Andy has fetal alcohol syndrome, and has been befriended by a lawyer who wants Andy to sue his mother for negligence and abuse. On what grounds is the lawyer basing his case? Should a child be allowed to sue his parents for actions taken while pregnant? Would your answer be different if Andy had a birth defect caused by anesthesia that his mother had before she knew she was pregnant? If the birth defect was due to German measles, which his mother contracted in the first trimester? 6. Explain the differences between chromosomal sex, gonadal sex, and phenotypic sex. Under what conditions might Janaan s phenotypic sex (female) not match her karyotypic sex? Her gonadal sex? 7. An SRY gene is accidentally transferred from a Y chromosome to an X during male meiosis. This altered X gets into a sperm, and the sperm fertilizes an egg. Discuss the effects this will have on the individual who results from this conception. 8. John knew that Perry, his father, had a full head of hair well into his 60s. John is therefore surprised when he begins losing his hair in his late 20s. How would you explain the inheritance of pattern baldness to John? 9. Would a person with Klinefelter s syndrome have testes? Ovaries? A Wolffian duct system? A Müllerian duct system? Would the person produce MIH? Now answer the same questions for an XY individual with testicular feminization.