Lecture 29: Mendelian Genetics III
Lecture 29: Mendelian Genetics III BIO 105 Cr.4
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Date Created: 11/23/15
Lecture 29 Mendelian Genetics Ill Many species have distinct sexes which requires the action of distinct genes that control the developmental program for each sex 0 In humans there are two sex chromosomes X and Y that behave somewhat differently from the other 22 autosomes o X chromosome similar to the autosomes encoding quot2000 genes little role in female development 0 Y chromosome severe reduction over the recent millennia and now encodes for less that 100 genes almost all of which are expressed only in the testes male gonads 0 Although the X and Y are very different in size and genecontent they have small regions near the ends that are homologous allowing them to pair during meiosis I Human males Human males contain an X chromosome from their mothers and a Y chromosome from their fathers Because males have only one copy of the Xchromosomes they are hemizygous for the Xencoded genes they only have one cow I Human females Females contain an X chromosome from each parent and without the Yencoded genes stick to the default female developmental pattern Note that like the other 22 autosomes females can be heterozygous for the Xencoded genes whereas males have only one copy of all the Xencoded genes Sexlinked or Xlinked traits involve the genes encoded on the X chromosome the expressed phenotype can vary between males and females 0 Because all females have 2 Xchromosomes I Recessive alleles for Xencoded genes will have no effect on the heterozygotes X N X n I Recessive alleles must be present as homozygotes Xan to have any phenotype 0 However since males only ever have one Xchromosome I Recessive Xencoded gene Xquot will automatically have an effect in the hemizygous males Females silence one copy of their X chromosome so that its gene are NOT expresses this happens in embryonic cells 0 Such silencing is visible since the inactive X chromosome is supercondensed down to an object called the Barr body 0 The choice of which X chromosome is inactivated in females is completely random and each cell in the early embryo chooses differently 0 So in the case of Xlinked genes that are heterozygous each woman is a mosaic of cells that express either the dominant or recessive trait I Tortoiseshell cats are an example of the random nature of Xinactivation where different alleles of a coat pigment gene are expressed in different patches of the skin One of the tools used by geneticists to study inheritance in humans because of the small number of progeny is pedigree analysis 0 When using pedigree analysis different patterns are found that can indicate whether traits are dominant recessive sexlinked or complex I If affected individuals showing a particular trait appear in every generation that trait is most likely dominant I If affected individuals showing a particular trait appear to skip a generation then this is a sign that a trait is likely recessive I If affected individuals are only or predominantly or one sex then it is very likely that the trait is sexlinked recessive Chromosomes have many genes and sometimes two trait loci are on the same chromosome they are linked o Unlinked genes like the flowercolor and seed shape loci lie on different chromosomes you can assort individually during meiosis resulting in the 9331 segregation pattern 0 Linked genes like the flower color and pollen shape loci lie on the same chromosome and they cannot assort individually resulting in something that does not resemble a 9331 pattern instead it s almost closer to a 31 pattern If the genes were on the same chromosome how d we get any purpleround or white long plants These are called recombinant plants 0 Something has to happen to the chromosomes during meiosis to make recombinant gametes that can fuse to create the purpleround or whitelongs o Recombinant chromosomes occur due to how the homologous chromosomes recombine form chiasmata during synapsis in meiosis metaphase I I Recombination between the homologous chromosomes always occur during meiosis but the place recombination occurs is essentially random I Recombination occurs at the same place where chiasmata form and is more simple called crossingover Not all crossing over events on a chromosome will necessarily lead to recombination between two particular linked loci 0 Only those crossing over events that occur between the two loci will result in recombination I Two loci very far apart will experience a crossing over event between them MORE OFTEN recombination is MORE frequent and the F2 will segregate very differently than 9331 Calculating distance on chromosome based on recombination frequency 0 Counting the phenotypic classes in the testcross progeny gives I Parentals 2534 I Recombinants 305 I Total progeny 2839 o The percent age of the progeny that are recombinants the recombination 305 recombinants 0 frequency 2I839wwlprogenygtlt100 1070 o A map unit is equivalent to a recombination frequency of 1 which is equal to a centimorgan CM 0 Widely separated linked genes assort independently Chromosomal mutations that affect inheritance o Deletion I ABC DEFGH I AVD DEGH o Duplication I ABC DEFGH I ABC DEDEFGH o Reciprocaltranslocation I ABC DEFGH 39 K LMN I ABC DEFGN 39 K LMFGH o Inversion I ABC DEFGH I ABC DGFEH 0 Some translocations can lead to cancer The new translocation fuses the gene ABL to the gene BCR o The new gene BCR ABL is an oncogene I The ABL gene is now MUCH more active Nondisjunction can cause whole chromosome abnormalities 0 Trisomy having three copies of chromosome 21 leads to down syndrome 0 Having the diploid amount of chromosomes is crucial for proper development 0
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