Genetics Final Exam Study Guide Meiosis stage of cell cycle where one diploid cell gives rise to four haploid cells each having 1 set of chromosomes Homologous chromosomes are separated in the first division and crossing over occurs in prophase 1 Sister chromatids are separated in the 2nd round of meiosis Homologous chromosomes contain the same genes but may contain different alleles Sister chromatids are exact genetic replicas of each other diploid 2n X haploid n x 2 copies of each chromosome humans 2n 46 1 copy of each chromosome Crosses and inheritance Dihybrid Cross Common ratios Partial Dominance 9 3 3 1 Both Parents Heterozygous for both traits comes from a test cross between 2 homozyogous parents Flower example cross of red and whites will produce pink flowers heterozygotes are partially dominant phenotype Codominance AB blood type example both alleles are expressed simultaneously X linkage trait is carried on the X chromosome May be dominant or recessive color blindness hemophilia Duchenne Muscular dystrophy Be able to analyze human pedigrees to determine whether a trait is inherited as an autosomal recessive autosomal dominant X linked recessive or X linked dominant Human Pedigrees Squares Male Circles Female Darkened shapes have the trait Dominant traits a fully dominant trait typically appears in each generation because the allele usually is expressed even in heterozygotes it will therefore not skip generations and will therefore often appear to be relatively common in a pedigree reversing the typical Mendelian logic remember that a dominant trait will not occur in an individual unless it also appears in at least one of the parents unaffected parents will have only unaffected offspring Recessive traits a recessive trait commonly skips one or more generations recessive conditions are frequently found in pedigrees that include marriage between close relatives in a marriage of two affected individuals all of the offspring will be affected if both parents are heterozygous any child of theirs will have a 50 chance of being a heterozygous carrier and a 25 chance of being homozygous recessive and affected Autosomal inheritance an autosomal trait can be passed on from a father to his son especially for a recessive autosomal trait approximately the same number of males and females will be affected Sex linked inheritance an X linked trait can never be passed from a father to his son therefore a single example of father to son transmission is sufficient proof that the trait is not sex linked a X linked recessive both males and females can be affected but the trait is much more common in males affected males transmit the allele to all of their daughters but not to any sons though daughters of all sons of a female who expresses the trait will also be affected affected males are usually heterozygous and therefore not affected b X linked dominant if the trait is dominant it will be expected to occur slightly more often in females all daughters of an affected male will be affected can not be passed on from father to son Genetic map genetic maps are constructed using recombination frequencies Steps to create a genetic map o 1 Identify progeny with parental phenotypes single and double crossover events o 2 Determine the gene order determine which gene has moved to convert the parent types into double crossover types THE GENE IN THE MIDDLE identify gene moved to make single crossover types o 3 Add up the crossover frequencies to make the map the farther apart two genes are the more likely crossing over occurs between them Sex chromosomes XY Sex Determination individual Sex Chromosomes X and Y chromosomes in humans determine the sex of an 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 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 Be able to provide an explanation for the existence of XX males and XY females Dosage compensation and why it is important on the X chromosome Dosage Compensation chromosomes Expression of genes unbalanced between sexes females have 2 X X inactivation in females The Lyon Hypothesis i One 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 Barr body Barr body 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 Genetic chimeras inactivated Random choice in each cell which X chromosome will be 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 XX males have a translocation from the Y to the X XY females have a deletion of part of the Y Chromosome mutations 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
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