GENDER 2005-2008 James Bier Objectives 1. State the method of determining gender in several genetic systems. 2. List the three regions of the Y chromosome. 3. Describe the events that promote sexual development in an embryo. 4. Describe how developmental disorders explain the genetics of sexual development. 5. Define sex linkage and illustrate the inheritance of sex-linked genes using a Punnett Square. 6. Define X inactivation and describe some of its effects. 7. Define sex-limited traits, sex-influenced traits, and genomic imprinting Outline A. Sex Determination 1. Human Sex Chromosomes 2. Human Sexual Development 3. Sexual Developmental Disorders 4. Homosexuality B. Sex Linkage C. X Inactivation D. Gender Differences 1. Sex-Limited Traits 2. Sex-Influenced Traits E. Genomic Imprinting A. Sex Determination Sex Chromosome Systems X-Y system XX Female, XY Male homogametic heterogametic 1
Z-W system ZZ Male, ZW Female X-O system O absence of chromosome XO Male, XX Female Haplodiploidy n Male, 2n Female Environmental Sex Determination 1. Human Sex Chromosomes X chromosome 1000-1500 genes 2
Y chromosome 85-230 genes pseudoautosomal genes (5%) PAR1 & PAR2 regions X-Y homologs gene sequences similar to X Genes unique to Y SRY gene 2. Human Sexual Development Embryos undifferentiated Gonads unspecialized Müllerian ducts female Wolffian ducts male 3
SRY gene Activated in sixth week Produces transcription factor Signals sustentacular cells Produce anti-müllerian hormone Signals interstitial cells Produce testosterone, dihydrotestosterone 3. Sexual Developmental Disorders XY females Y chromosome has lost SRY gene Androgen Insensitivity Syndrome Absence of testosterone receptors Mutation on X chromosome 4
Pseudohermaphroditism Fetal testosterone not produced Female characteristics suppressed Male characteristics don t develop until puberty XX males carry SRY gene on one X chromosome Congenital Adrenal Hyperplasia Excess testosterone production 4. Homosexuality Partially inherited Twin Studies Brain Anatomy X chromosome sequence correlated with male homosexuality 5
B. Sex Linkage Genes associated with gender X-linked Many critical genes on X but not on Y Females diploid Males hemizygous X chromosome derived from mother 6
X-linked recessive red-green colorblindness, hemophilia A Female Homozygous recessive ill Heterozygote carrier Male Recessive trait always expressed illness more common in males 7
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X-linked dominant Rare Female Trait expressed Partly balanced by normal allele Male Trait expressed more severely No second allele to counter effect 9
Y-linked Rare Few genes on Y Some genes are pseudoautosomal C. X Inactivation Genes on one X are repressed Only in individuals with two X s X chromosome forms Barr Body Inactivation is random 10
XIST gene produces blocking RNA Adds CH 3 to DNA Prevents transcription Mosaic displayed of heterozygous X-linked genes Explains weaker effect of X-linked dominant alleles in females Manifesting Phenotype e.g., tortoiseshell & calico cats 11
D. Gender Differences 1. Sex-Limited Traits Phenotype expressed by only one sex Autosomal or sex-linked Genes carried by both sexes e.g., beard growth, breast size e.g., preeclampsia?? 2. Sex-Influenced Traits Allele dominant in one sex, recessive in other e.g., male pattern baldness allele dominant in males recessive in females 12
E. Genomic Imprinting Epigenetic alteration Parent-of-Origin Effect Allele from one parent silenced Method (How) uncertain Methylation DNA-binding proteins DNA contortion Function uncertain Regulate development e.g., agouti gene 13
e.g., Prader-Willi and Angelman Prader-Willi Syndrome deletion paternal chromosome 15 only maternal genes expressed Angelman Syndrome deletion maternal chromosome 15 paternal gene expressed e.g., callipyge in sheep Large hindquarter passed only by male Silencing passed by mitosis Removed during meiosis Reset after fertilization 14