HUMAN GENETICS OUTLINE Exam 2 CHAPTER SIX Sex Chromosomes I Sex Chromosomes determine Gender Human males heterogametic sex with different sex chromosomes XY Human females homogenetic sex with like sex chromosomes XX This is different in some other species Birds and Snakes Males homogametic ZZ Females heterogametic ZW II Anatomy of Y chromosome 2 Pseudoautosomal regions PAR1 and PAR2 Regions on either tip of the Y chromosome which contain 63 pesudoautosomal genes which have counterparts on the X chromosome in which they can cross over with together comprise for only 5 of the chromosome Male Specific Region MSY lies between the 2 pseudoautosomal regions and consists of three DNA sequences 1 10 15 of MSY consists of X transposed sequences that are 99 identical to counterparts on the X chromosome 2 20 of MSY consists of X degenerate DNA sequences that are somewhat similar to X chromosome sequence 3 65 75 of MSY includes palindrome ridden regions called amplicons that are unique to the Y chromosome Sex Ratio Primary sex ration at conception 1 0 Secondary sex ratio at birth Tertiary sex ratio at maturity Environment specific sex specific health problems can alter sex ratio environmental factors endocrine disruptors cultural factors Y linked Traits Genes on the Y chromosome Very rare Transmitted male to male no affected females X linked Traits act different in males and females possible genotypes 1 Hemizygous wild type male normal 2 Hemizygous mutant male 3 Homozygous wild type female normal dominant 4 Heterozygous female carrier 5 Homozygous mutant female recessive III IV V Most X linked diseases are recessive VI X linked Recessive Traits Always expressed in hemizygous males Female homozygotes show the trait Female heterozygotes do not show the trait Affected males inherited from affected or heterozygous mother inherited from affected father or heterozygous mother Affected females Clotting Factors AHF Factor Clotting Factor 8 problem Hemophilia A Christmas Factor Clotting Factor 9 problem Hemophilia B VII X linked Dominant Inheritance Expressed with one copy both sexes Males are often more severely affected than females Typically associated with miscarriages or lethally in males Passed from father to all of his daughters but non of his sons Congenital Generalized Hypertrichosis excessive hair growth VIII Sex limited Traits traits that affect a structure or function occurring only on one gender May be autosomal or X linked Examples Beard Growth Milk Production Pregnancy Phenotypes such as preeclampsia Sperm production levels IX Sex Influenced Traits traits in which the phenotype expressed by a heterozygote is influenced by gender Example allele appears dominant in one gender and recessive in the other Pattern Baldness is a sex influenced trait X X Inactivation Females have 2 alleles for X chromosome genes but males only have 1 In mammals X inactivation balances this inequality and 1 chromosome is randomly inactivated in each cell The XIST gene encodes an RNA that binds to and inactivated the X chromosome Inactivated x Chromosome forms a Barr Body a dark staining X chromosome in the nucleus found in females Barr Body not found in males nucleus because his one X chromosome is still active Manifesting Heterozygotes carriers of x linked traits who express the phenotype due to x inactivation If homozygous for some X linked trait which one is inactivated is not important phenotypically because no matter which one is shut off you still get the same phenotype If heterozygous for some X linked trait which one is inactivated is important phenotypically normal X inactivation get disease mutant X inactivation don t get disease Cats are heterozygous for the coat color gene XI Genomic Imprinting silencing expression from one parent Function unknown may play a role in development Genes silenced by an epigenetic event DNA methylation Teratoma developing from fertilized egg as a result of altering imprinting XII Figure 6 16 imprints maintained in mitotic divisions but lost in meiosis imprints erased during meiosis then reinstituted according to the sex of the new individual approximately 30 disorders result from mistakes in genetic imprinting XIII Figure 6 17 Chromosome 15 Paternal Deletion Mom s imprinted non functional and Dad s functional deleted Prader Willi Syndrome small at birth around age 1 3 obsession Maternal Deletion Dad s imprinted non functional and Mom s with eating functional deleted Angelman Syndrome craniofacial deformities mental retardation poor coordination I CHAPTER SEVEN Multifactorial Traits Genes or Environment Genes rarely act completely alone Environmental factors and other genes may modify expression Traits can be describes as 1 Mandelian primarily due to single genes 2 Polygenic primarily due to multiple genes 3 Multifactorial due to an interaction between genes and the environment Trait can be polygenic and multifactorial Trait can be Mandelian and Multifactorial Bee Diagram Discontinuous variation Mendelian polymorphism e g winged vs wingless fruit flies mutation e g normal vs vestigial wings Continuous distribution polygenic you get a normal curve e g amount of pigment height II Polygenic Traits Effect of genes is additive variation is continuous NOT discrete individual genes follow Mendel s laws also called quantitative trait loci QTL Genes can have major minor impacts Examples height hair color body weight cholesterol levels III Multifactorial Traits Are influenced by interaction of genes and by the environment What the genes do in light of a certain environmental conditions Examples Fingerprints IV fingerprints polygenic many genes and prenatal contact height polygenic many genes and nutrition skin color polygenic many genes and UV exposure number of ridges polygenic identical and monozygotic twins have the same finger prints except the possibility of prenatal contact differing the two finger prints V VI continuously varying nature nutrition influence along with genes Inheritance of Height Eye Color ONLY polygenic NOT multifactorial 2 genes 2 alleles 5 basic phenotypes 1 blue green crystals made in cells reflection blue green light 2 brown quantity of melanin 1 Pure blue aabb 1 16 1 Dark blue 4 16 2 Lightest Brown 6 16 3 Brown 4 16 4 Dark brown AABB 1 16 VII Skin Color Melanin Production hemoglobin red pink another pigment makes yellow tone skin pigmentation protects skin from UV radiation melanocytes pigment producing cells melanosomes balloons inside melanocytes