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Chromosomes, Mapping, and the Meiosis-Inheritance Connection Chapter 13 Chromosome Theory T.H. Morgan isolated a mutant white-eyed Drosophila red-eyed female X white-eyed male gave a F1 generation of all red eyes Morgan concluded that red eyes are dominant 2 Chromosome Theory Morgan crossed F1 females X F1 males F2 generation contained red and white- eyed flies but all white-eyed flies were male testcross of a F1 female with a white-eyed male showed the viability of white-eyed females Morgan concluded that the eye color gene is linked to the X chromosome 3 4 5 6 Chromosome Theory Exceptions Mitochondria and chloroplasts contain genes. traits controlled by these genes do not follow the chromosomal theory of inheritance genes from mitochondria and chloroplasts are often passed to the offspring by only one parent 7 Chromosome Theory Exceptions Maternal inheritance: uniparental (oneparent) inheritance from the mother the mitochondria in a zygote are from the egg cell; no mitochondria come from the sperm during fertilization in plants, the chloroplasts are often inherited from the mother, although this is species dependent 8 Genetic Mapping Early geneticists realized that they could obtain information about the distance between genes on a chromosome. - this is genetic mapping This type of mapping is based on genetic recombination (crossing over) between genes. 9 10 Genetic Mapping To 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 progeny 11 Genetic Mapping The distance between genes is proportional to the frequency of recombination events. recombination frequency recombinant progeny = total progeny 1% recombination = 1 map unit (m.u.) 1 map unit = 1 centimorgan (cM) 12 13 Genetic Mapping Determining the order of genes can be done with a three-point testcross the frequency of double crossovers is the product of the probabilities of each individual crossover therefore, the classes of offspring with the lowest numbers represent the double crossovers and allow the gene order to be determined 14 15 Genetic Mapping Mapping genes in humans involves determining the recombination frequency between a gene and an anonymous marker Anonymous markers such as single nucleotide polymorphisms (SNPs) can be detected by molecular techniques. 16 17 Human Genetic Disorders Some human genetic disorders are caused by altered proteins. the altered protein is encoded by a mutated DNA sequence the altered protein does not function correctly, causing a change to the phenotype the protein can be altered at only a single amino acid (e.g. sickle cell anemia) 18 19 Human Genetic Disorders Some 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 chromosomes fertilization of these gametes creates trisomic or monosomic individuals Down syndrome is trisomy of chromosome 21 20 21 Human Genetic Disorders Nondisjunction of sex chromosomes can result in: XXX triple-X females XXY males (Klinefelter syndrome) XO females (Turner syndrome) OY nonviable zygotes XYY males (Jacob syndrome) 22 23 Human Genetic Disorders Genetic 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 examination chorionic villi sampling collects cells from the placenta for examination 24 25 26