Know the difference between autosomes and sex chromosomes understand the terms heterogametic sex and homogametic sex and know the difference between the XY and the ZW sex determination systems 22 Autosomes 1 pair of sex chromosomes X and Y do not carry same genes for same female is the heterogametic in the ZW sex determining systems Know the evidence which indicates that the Y chromosome determines sex in mammals existence of Turner and Klinefelter individuals know what the genetic makeup is of people with Turner and Klinefelter Syndromes and what the characteristics phenotypes of these syndromes are Klinefelter traits Heterogametic XY Homogametic XX ZW Male ZZ Female Genotype XXY Tall long arms and legs large hands and feet poor beard growth testicular atrophy intelligence below normal range usually Some female development minor breast development rounded hips Male genitalia and internal ducts but testes are rudimentary and do not produce sperm Turner Syndrome Genotype X Female external genitalia and internal ducts but ovaries are rudamentary Underdevelopment of secondary sex organs Shorter than average skin flaps on the back of the neck and underdeveloped breasts Intelligence usually normal Both defects due to nondisjunction the failure of chromosomes to segregate properly during meiosis Evidence that indicates that the Y chromosome determines sex Presence or absence of Y determines sex Two X chromosomes exist in a klinefelter person but still male In the absence of a Y the person is female and no masculinization occurs Know the difference in the genes that are carried by the X and the Y chromosomes know what the PAR region is on the Y chromosome and why it is important Contains about 2 000 genes Few if any genes on the X chromosome have anything to do directly with sex Many genes on the X chromosome associated with the development of X Y determination intelligence Contains about 75 genes SRY gene Codes for TDT testis determining factor PAR region Pseudoautosomal region X and Y have a small region that is homologous and synapse and recombine with it during meiosis SRY is near the boundary XY females have a deletion of part of the Y and XX males have a translocation from the Y to the X This is evidence for the importance of the SRY gene Understand how the SRY gene controls sex determination in mammals initiates male development through regulation of testosterone and antimullerian hormone production during early development and what the evidence is for the importance of this gene XY females and XX males Humans have bipotential gonads which are ridges that can form either ovaries or testes The presence of absence of a Y chromosome determines which develop In the absence of the Y chromosome no male develop occurs and the Mullerian ducts form fallopian tubes uterus and other female genitalia Adjacent to the PAR region is the SRY sex determining region Y SRY encodes the TDF testes determining factor protein that initiates testis formation Cells in testis secrete anti Mullerian hormone which blocks Mullerian ducts from developing into uterus and ovaries and testosterone which stimulates the formation of internal and external male structures Understand the concept of dosage compensation and why it is important on the X chromosome and how dosage compensation is achieved in mammals X inactivation and in fruit flies increase of activity of genes on the X in males The X chromosome has way more genes on it Females have two X s Twice as many genes Genetic imbalance Dosage compensation necessary Hypothesis X inactivation in human females The Lyon Hypothesis One of the two X s is turned off in development This is why heterozygous calico coat colors in cats can exist Barr body Inactivated X One X chromosome is inactivated in each cell in females during early development N 1 where N is total number of X chromosomes present Fruit flies Activity of all genes on X in males is doubled Turns genes on rather than off Know what a Barr body is and how inactivation of the X chromosome occurs during early development in mammalian females and be able to state how many Barr bodies would be present in a cell that has 1 2 3 or 4 X chromosomes Understand the term genetic chimera genetic mosaic and be able to explain the pattern of inheritance of calico and tortoiseshell coat colors in cats Barr body The inactivated X One X chromosome is inactivated in each cell in females during early Always one less than total number of X chromosomes X N 1 XXX 2 Barr bodies one left on all rest shut off In each cell one X is turned off Not same X in every cell One could be from mom off and other could be off from dad Genetic chimera genetic mosaic females are genetic mosaics Random choice in each cell of which X is inactivated Calico and tortoiseshell coats result of genetic chimera X linked gene with two different alleles Turned on and off at different areas Heterozygous One X from mom one allele and one X from dad another allele Depends on which of two X s turned off Example of epigenetics Only females have calico or tortoise shell Human females can have this with sweat glands Lack of sweat glands in some place Know what the Xist gene does and where it is located Xist Gene responsible for X inactivation X inactivation specific transcript When this gene comes on it turns off the X chromosome it s located in transcribed RNA not translated into protein DNA of inactive X gets methylated turns off interaction between two X s because the other one not turn off when one is turned off X inactivation begins at XIC Be able to provide an explanation of why individuals with Turner and Klinefelter syndromes are not phenotypically normal 10 of genes on X actually not turned off in inactive X Origin of phenotypes due to the genes not turned off in the X no genetic balance you need to have Why does X linked inheritance still exist then Maybe in the case of red green color blindness in heterozygous females the brain averages everything up In hemophilia maybe if heterozygous some cells make clotting and some don t All cells still get benefit if some make it Chromosome mutations or chromosome aberrations vocab Nondisjunction Leads to changes in chromosome number Error in meiosis in which homologous chromosomes fail to separate Aneuploidy monosomy trisomy Aneuploidy 1 or more extra or missing 1 or more Unusual chromosome number Monosomy Trisomy Single copy of a chromosome Three copies of a chromosome Downs syndrome Trisomy on chromosome 21
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