FA2011lecture4 - Mendel s Laws: •  Law of segregation...

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Unformatted text preview: Mendel s Laws: •  Law of segregation: when gametes are formed, the paired hereditary determinants (alleles) segregate so that each gamete receives only one; these determinants are re-united in random combinations (one from each parent) at fertilization. •  Law of independent assortment: during gamete formation, different pairs of hereditary determinants (alleles) segregate independently of each other (if the genes are on different chromosomes!) Both are explained by behavior of chromsomes at meiosis. Message of the day: it takes two to tango at meiosis! Two equally likely arrangements of chromosomes on spindle at metaphase of meiosis I: anaphase ½ receive R and Y ½ receive r and y ½ receive R and y ½ receive r and Y Overall... ¼ of gametes receive R and Y ¼ receive r and y Explains Mendel’s Law of ¼ receive R and y Independent Assortment! ¼ receive r and Y When things go wrong in meiosis… NON-DISJUNCTION “aneuploid” gametes “euploid” gametes Down’s Syndrome (Trisomy 21 or “47,21+”) is the most common condition in humans resulting from aneuploidy FIGURE 17–1 a. A child with Down syndrome. b. The karyotype of a male with Down syndrome. The arrows in part b point to the three copies of chromosome 21. Down’s Syndrome becomes more common as maternal age increases In humans, all oocytes produced before birth: •  arrested at metaphase of meiosis I until ovulation (40 yrs is a long wait!) •  arrested again in meiosis II until fertilization fe n latio ovu on zati rtili •  Down’s Syndrome is common because it’s the only autosomal aneuploid condiGon that is fully viable •  Miscarried fetuses have a chromosomal abnormality in 30 ­50% of cases •  90% of chromosomal abnormaliGes result in miscarriage •  Only two other autosomal trisomies survive to term in humans: trisomy 13 and 18 (causing Patau and Edwards Syndromes, respecGvely – severe abnormaliGes) •  No autosomal monosomies survive to term •  X and Y chromosomal aneuploidy is a special case, e.g. XO and XXX females, XXY and XYY males are relaGvely normal Polyploidy: more than one pair of each chromsome (normal condition for many species, especially plants) >2 copies of each chromosome, all originaGng from same species 4 copies of each chromosome: 2 from one species and 2 from another Allopolyploid* arises from fusion of gametes from different species followed by chromosomal doubling to give each chromosome a partner to pair with at meiosis *called an amphidiploid if both parental species are known CoYon! Karyotype of a female mule (hybrid between horse and donkey) with chromsomes of horse origin labeled “H” and those of donkey origin labeled “D”… many H and D chromosomes are too different to pair at meiosis! Autopolyploidy results from fusion of gametes that were diploid instead of haploid because homologous chromosomes failed to segregate apart at meiosis, either due to natural causes or treatment with colchicine to destroy the meioGc spindle Wheat! ...
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