Genetics Study Guide Exam 2Chapter 4Hemizygous: XYHomozygous: XXY Chromosome: Male specific genes are found on it, lacks copies of genes found on the X chromosomeSex-Linked genes are found on one of the two sex chromosomes, not both● X-linked: most likely to affect males● Y-linked: Relatively few genes, referred to as holandric genes, transmitted only from father to sonPseudoautosomal inheritance refers to the very few genes found on both X and Y chromosomesX Linked ConditionsColor Blindness (deutan type) Insensitivity to Green LightColor Blindness (Protan type) Insensitivity to Red LightFabry’s disease Deficiency in galactosidase A, Heart and kidney defects, early deathG-6-PD deficiency Severe anemic reactions following intake of primarques in drugs and foods (fava beans)Hemophilia A Classic form of clotting deficiency; Clotting factor VIIIHemophilia B Christmas diseases; deficiency of clotting factor IXHunter syndrome Mucopolysaccharide storage disease; Short stature, claw-like fingers, coarse facial features, slow mental deterioration and deafnessIchthyosis Scaly dry skin, on extremitiesLesch-Nyhan syndrome HPRT deficiency. Motor and mental retardation, self-mutilation and early deathDuchenne muscular dystrophy Progressive, life shortening disorder. Muscle degeneration and weakness, mental retardationLethal X-Linked recessive disorders: Only in males, females can only be heterozygous carriersEx. Duchenne muscular dystrophyA man with Duchenne muscular dystrophy must have inherited it from his mother● Bc he only has one X chromosome which he gets from mom, he would only passit to his daughtersSex-influenced traits● Traits where an allele is dominant in one sex but recessive in the opposite sex (heterozygote phenomenon● Sex-influenced does not mean sex-linked. Sex-influenced are autosomalSex-Limited inheritance● Traits that only occur in one of the two sexes● Genes are controlled by sex hormones or the sexual development pathway○ Ex. In humans, ovary development is limited to females○ Ex. In birds, males have more ornate plumageMale pattern baldness● Allele B behaves dominant in males and recessive in females● In BB genotype in females, phenotype is less pronouncedHomogametic● Producing like chromosomes● Zygotes with two x chromosomes● Results in female offspring in humansHeterogametic● Producing unlike chromosomes● Zygotes with one X and one Y chromosome● Results in male offspring in humans● The presence of a Y chromosome determines maleness in humansHeterogametic Females● ZZ/ZW sex determination● Females are heterogametic (ZW) sex● Males are homogametic (ZZ) sex● In chickens X and Y are not homologous● Pseudoautosomal regions refer to the very few genes found on both● Y Chromosome=50 genes and X chromosome=100 genes● PARS-Present on both ends of Y, shares homology w/ regions on X, synapse and recombine with X during meiosisMSY:Male specific region of the Y● Non Recombining region of YSRY: Sex-determining region Y● Located adjacent to PAR of the short arm of Y chromosomes● Controls Male developmentTDF: Testis-determining factor● At 6-8 weeks of development SRY gene becomes active in XY embryos● Encodes protein that triggers testes formation[PAR][SRY][MAY][PAR]Haplotype: The set of alleles at two or more loci present in a particular chromosomeSex Chromosome Variation (75% go undiagnosed) XXX (Trisomy X): Females● Sometimes perfectly normal● Sometimes sterile or mental retardationXXY (Klinefelter syndrome): Nondisjunction of XX (Turner Syndrome): Female- Nondisjunction of XXYY● Males are over 6ft tall● Subnormal intelligence● Personality disorders● 1/1000 newborn boysSex ratio ● Actual proportion of male to female offspringPrimary sex ratio● Reflects proportion of males to females conceived in populationSecondary sex ratio● Reflects proportion of each sex bornIt is speculated slightly more males are convinced because Y-bearing sperm are more motile than X-bearing sperm Although none of these hypotheses is strongly supported by experimentalevidence, Y-bearing sperm have less mass than X-bearing sperm and are presumably more motile. Dosage CompensationDrosophila: The level of expression of genes on the male X chromosome is increased two foldHumans: One of the females X chromosomes is deactivated (Barr Body)Marsupials: Females paternal X chromosome is deactivatedElegans: Level of gene expression on both X’s are decreased by 50%Susumu Ohno correctly proposed that the Barr body is a highly condensed X chromosomeMary Lyon proposed that dosage compensation in mammals occurs by theinactivation of a single X chromosome in femalesXic: X inactivation centerXist: On Xic, causes inactivationTsix: On Xic, prevents inactivationIt does not encode a protein● It codes for a long RNA, which coats the inactive X chromosome● Other proteins will then bind and promote chromosomal compaction into a Barr bodySex determination in Drosophila● Ratio of X chromosomes to number of haploidsets of autosomes (Y does not determine sex)○ 1>=Female○ 1<x<.5=Intersex○ <.5=MaleCh. 5Alleles: Alternative forms of a geneMutation: Ultimate source of alleles● New phenotypes result from changes in functional activity of gene product○ Eliminating enzyme function○ Changing relative enzyme efficiency○ Changing overall enzyme functionWild-type (wt) allele: Occurs most frequently in nature and is usually, but not always, dominant● “Mutant” is how we refer to the recessive oneGene interaction: single phenotype is affected by more than one set of genesLoss-of-function mutations● New phenotype results from change in activity● Mutation causes loss of wildtype function● Result generally in recessive mutationsGain-of-function mutations● Mutation enhances function of wild type● Quantity of gene product increases● Result generally in dominant mutationsRecessive Human diseasesDisease Protein that is produced by normal geneDescriptionPhenylketonuria Phenylalanine Hydroxylase Inability to metabolize phenylalanine.Albinism Tyrosinase Lack of pigment in skin, eyes,and hairTay-Sachs disease Hexosaminidase A Defect in lipid metabolism. Leads to paralysis, blindness and early deathSandhoff disease Hexosaminidase B Defect in lipid metabolism. Muscle weakess, blindess, and deteriorationCystic Fibrosis Chloride transporter Inability to regulate ion balance across epithelial cells. Lung infections Lesch-Nyhan syndrome