Genetics Exam 2 Study Guide Sex Determination and Sex Chromosomes Chapter 5 Autosomes humans have 22 pairs Code for many genes but do not determine sex Sex Chromosomes X and Y chromosomes in humans determine the sex of an individual Heterogametic sex has 2 different sex chromosomes XY males in most mammals and insects females in birds and reptiles Homogametic sex matching pair of sex chromosomes XX females in humans ZW system used in reptiles birds some amphibians and insects ZZ is male ZW is female Klinefelter and Turner Syndrome Klinefelter syndrome o 47 XXY o tall stature testicular atrophy poor beard growth minor breast development female pubic hair pattern o Y chromosome determines sex to be male Turner Syndrome o 45 X o short stature webbed neck shield chest underdeveloped breasts and widely shaped nipples rudimentary ovaries brown nevi o female Genes that are carried by the X and the Y chromosomes the PAR region The TDF testes determining factor is coded by the SRY gene located on the Y chromosome o What makes you male PAR pseudo autosomal region o Small region on X and Y chromosome that is homologous o Located very close to SRY gene on Y X chromosome is much larger than Y and encodes for more genes SRY gene SRY gene controls sex development initiates male development through regulation of testosterone and antimullerian hormone production mullerian ducts develop into uterus and ovaries during early development prevents development of female reproductive structures SRY gene located on short arm of Y chromosome XX males have a translocation from Y to the X XY females have a deletion of part of the Y SRY gene acts as transcription factor attaches binds to specific regions of DNA and helps control the activity of particular genes Dosage compensation and why it is important on the X chromosome Dosage Compensation Barr body Barr body Expression of genes unbalanced between sexes females have 2 X chromosomes X inactivation in females The Lyon Hypothesis i 1 chromosome is inactivated in each cell in females during early development late blastula stage Drosophila gene dosage compensation ii Activity of X is doubled in males compared with females The inactivated X chromosome Klinefelters individuals will have 1 barr body Turner s will have zero Number of Barr bodies will always be Number of X 1 Random choice in each cell which X chromosome will be Genetic chimeras inactivated Some genes such as fur color are X linked Females will be genetic mosaics of certain traits i Calico and Tortoiseshell cats only female ii Females display different coat color alleles in different areas of their bodies iii Human females are mosaics for lack of sweat glands Xist gene XIST gene Gene responsible for X inactivation mediates the specific silencing of the X chromosome XIST is transcribed but not translated Inactive X chromosome is coated by XIST RNA where Xa activated X is not XIC X inactivation center is where XIST is located Why individuals with Turner and Klinefelter syndromes are not phenotypically normal Genes located on the PAR of X chromosomes have corresponding genes on their Y chromosomes that are capable of being expressed in Klinefelter individuals Chromosome Mutations Chapter 6 Chromosome mutations or chromosome aberrations missing extra or irregular portion of chromosomal DNA can be from atypical number or structural abnormality Nondisjunction failure of homologous chromosomes or sister chromatids to separate properly during meiosis or mitosis Failure of homologous chromosomes to separate occurs in Meiosis 1 Failure of sister chromatids to separate occurs in Meiosis 2 Aneuploidy monosomy trisomy Aneuploidy o Presence of an abnormal number of chromosomes in a cell o Caused by mistakes in mitosis or meiosis Monosomy Trisomy o 1 copy of a chromosome o Turner s syndrome o Monosomy of autosomes does not occur in humans o 3 copies of one chromosome o Trisomy 21 is Down syndrome Occurs more frequently as mother ages Amniocentesis medical procedure used in prenatal diagnosis of chromosomal abnormalities fetal infections and sex determination Small amount of amniotic fluid from amniotic sac surrounding a developing fetus is sampled and examined for genetic abnormalities Polyploidy Triploid tetraploid and polyploidy Multiple chromosome sets Very rare in humans Common in plants important for speciation o Wheat is hexaploid formed by allopolyploidization o Cavendish banana 3n 33 o Solanum tuberosum potato 4n 48 o Crossing two different species can cause chromosome doubling and results in new species Types of chromosomal mutations 1 Chromosome deletion material becomes lost during cell division Results from breakage of a chromosome in which genetic 2 Duplication extra copies of genes are generated on chromosomes 3 Inversions into chromosome 4 Translocations chromosome on a different chromosome Familial Downs Syndrome broken chromosome segment is reversed and reinserted back joining of a fragmented chromosome to a non homologous Piece of 1 chromosome detaches and moves to a new position human translocation involving chromosomes 14 and 21 accounts for 5 of down syndrome cases Trisomy 21 involved copy of chromosome 21 attached to chromosome 14 Monosomy of chromosome 21 is lethal results in miscarriage How nondisjunction in meiosis leads to monosomy and trisomy Non disjunction in meiosis o Most monosomies and trisomies in humans are lethal with exception to chromosomes X Y and 21 o Single Y chromosome without X is lethal however Linkage and Mapping Chapter 7 Linkage and Crossing over Genes on the same chromosomes are linked Crossing over occurs during meiosis and involves the exchange of genetic material between homologous chromosomes Parental or non crossover gametes Occur when there is complete linkage For homologous chromosomes AABB and aabb 2 types of gametes AB and ab Parental gametes are most frequent and will never be less than 50 for offspring Recombinant or crossover gametes chromatids that cross over during Meiosis I can never be more than 50 because the chromosomes that actually cross over need to be touching chromosomes on the outside parentals do not cross over genes on the same chromosome that are more than 50 map units apart undergo recombination so frequently that they assort independently Single crossover Double crossover crossover of 1 gene single crossover for AB and ab results in Ab and aB 2 genes crossover a double crossover for ABC and abc results in AbC and aBc least frequent
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