Bài giảng Biology - Chapter 13: Chromosomes, Mapping, and the Meiosis-Inheritance Connection

Chromosomes, Mapping, and the Meiosis-Inheritance Connection Chapter 131Chromosome TheoryChromosomal theory of inheritance- developed in 1902 by Walter Sutton- proposed that genes are present on chromosomes- based on observations that homologous chromosomes pair with each other during meiosis- supporting evidence was provided by work with fruit flies2Chromosome TheoryT.H. Morgan isolated a mutant white-eyed Drosophilared-eyed female X white-eyed male gave a F1 generation of all red eyesMorgan concluded that red eyes are dominant3Chromosome TheoryMorgan crossed F1 females X F1 malesF2 generation contained red and white- eyed flies but all white-eyed flies were maletestcross of a F1 female with a white-eyed male showed the viability of white-eyed femalesMorgan concluded that the eye color gene is linked to the X chromosome45Sex ChromosomesSex determination in Drosophila is based on the number of X chromosomes2 X chromosomes = female1 X and 1 Y chromosome = maleSex determination in humans is based on the presence of a Y chromosome2 X chromosomes = femalehaving a Y chromosome (XY) = male6Sex ChromosomesIn many organisms, the Y chromosome is greatly reduced or inactive.genes on the X chromosome are present in only 1 copy in malessex-linked traits: controlled by genes present on the X chromosomeSex-linked traits show inheritance patterns different than those of genes on autosomes.78Sex ChromosomesDosage compensation ensures an equal expression of genes from the sex chromosomes even though females have 2 X chromosomes and males have only 1.In each female cell, 1 X chromosome is inactivated and is highly condensed into a Barr body.Females heterozygous for genes on the X chromosome are genetic mosaics.910Chromosome Theory ExceptionsMitochondria and chloroplasts contain genes.traits controlled by these genes do not follow the chromosomal theory of inheritancegenes from mitochondria and chloroplasts are often passed to the offspring by only one parent11Chromosome Theory ExceptionsMaternal inheritance: uniparental (one-parent) inheritance from the motherthe mitochondria in a zygote are from the egg cell; no mitochondria come from the sperm during fertilizationin plants, the chloroplasts are often inherited from the mother, although this is species dependent12Genetic MappingEarly geneticists realized that they could obtain information about the distance between genes on a chromosome.- this is genetic mappingThis type of mapping is based on genetic recombination (crossing over) between genes.1314Genetic MappingTo determine the distance between genes:- dihybrid organisms are testcrossed- offspring resembling the dihybrid parent result from homologues that were not involved in the crossover- offspring resulting from a crossover are called recombinant progeny15Genetic MappingThe distance between genes is proportional to the frequency of recombination events.recombination recombinant progeny frequency total progeny1% recombination = 1 map unit (m.u.)1 map unit = 1 centimorgan (cM)=1617Genetic MappingMultiple crossovers between 2 genes can reduce the perceived genetic distance progeny resulting from an even number of crossovers look like parental offspring 1819Genetic MappingDetermining the order of genes can be done with a three-point testcrossthe frequency of double crossovers is the product of the probabilities of each individual crossovertherefore, the classes of offspring with the lowest numbers represent the double crossovers and allow the gene order to be determined2021Genetic MappingMapping genes in humans involves determining the recombination frequency between a gene and an anonymous markerAnonymous markers such as single nucleotide polymorphisms (SNPs) can be detected by molecular techniques.2223Human Genetic DisordersSome human genetic disorders are caused by altered proteins.the altered protein is encoded by a mutated DNA sequencethe altered protein does not function correctly, causing a change to the phenotypethe protein can be altered at only a single amino acid (e.g. sickle cell anemia)2425Human Genetic DisordersSome genetic disorders are caused by a change in the number of chromosomes.nondisjunction during meiosis can create gametes having one too many or one too few chromosomesfertilization of these gametes creates trisomic or monosomic individualsDown syndrome is trisomy of chromosome 212627Human Genetic DisordersNondisjunction of sex chromosomes can result in:XXX triple-X femalesXXY males (Klinefelter syndrome)XO females (Turner syndrome)OY nonviable zygotesXYY males (Jacob syndrome)2829Human Genetic Disordersgenomic imprinting occurs when the phenotype exhibited by a particular allele depends on which parent contributed the allele to the offspringa specific partial deletion of chromosome 15 results in: Prader-Willi syndrome if the chromosome is from the father Angelman syndrome if it’s from the mother30Human Genetic DisordersGenetic counseling can use pedigree analysis to determine the probability of genetic disorders in the offspring.Some genetic disorders can be diagnosed during pregnancy.amniocentesis collects fetal cells from the amniotic fluid for examinationchorionic villi sampling collects cells from the placenta for examination313233