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Chromosomal theory of inheritance: chromosomes are the carriers of genetic material. Independent Assortment alleles for different characters sort independently of each other during gamete formation Law of Segregation two alleles for a given character separate during gamete formation, and go to different gametes The chromosomal makeup of gametes depends on the arrangement of homologous chromosomes during metaphase I Page 2
Thomas Hunt Morgan conducted experiments with fruit flies (Drosophila melanogaster) that provided the first evidence for inherited genes being located on chromosomes. Wild type the phenotype for most organisms (what you might find in the wild ) Wild type flies have red eyes. Only the male flies in Morgan s experiments showed the mutation for white eyes. Sex-linked gene gene located on a sex chromosome The gene for eye color is linked to the sex of the fly. Sex chromosomes chromosomes that contain genes that determine the sex of the organism, X/Y in humans There are few genes located on the Y chromosome, so there are few Y-linked genes. The X chromosome contains many genes, so there are many X-linked genes. Page 3
Women have two X chromosomes, but men have only one, so the expression of genes on the X chromosome must be balanced between the sexes. This is referred to as dosage compensation. X-inactivation one X chromosome must be inactivated in women Barr bodies the highly condensed, inactive X chromosome Which X chromosome will inactivate is randomly determined in each cell of the embryo during X-inactivation. Mosaicism when an organism has cells with more than one genotype Page 4
Linked genes genes that are located near each other on a chromosome, and tend to be inherited together. Parental type offspring with the same allele combinations seen in the parents Recombinant offspring with different allele combinations than those of the parents Genetic recombination is due to crossing over during prophase I of meiosis Genetic map an ordered list of gene loci along a chromosome Recombination frequency the frequency with which a cross over between two genes on a chromosome will take place The further apart two genes are the more likely a crossover will occur between them Genes far enough apart behave as if unlinked, because cross over is nearly assured Linkage map a genetic map based on recombination frequency Page 5
CONCEPT: CHROMOSOMAL ABNORMALITIES Nondisjunction when sister chromatids, or homologs fail to separate during cell division Aneuploidy the wrong number of chromosomes, can be as a result of nondisjunction Monosomy one chromosome instead of the normal two Trisomy three chromosomes instead of the normal two Polyploidy an organism with more than two complete sets of chromosomes Deletion loss of a chromosome fragment Duplication can be cause by attachment of extra chromosome fragment Inversion chromosome fragment attaches in reverse orientation Translocation chromosome fragment attaches to a nonhomologous chromosome Page 6
CONCEPT: NON-MENDELIAN INHERITANCE When Mendel performed his test crosses he saw that, in the characters studied, it didn t matter which parent gave the allele. Genomic imprinting when phenotype is determined by the parent that gave the allele During gamete formation alleles for particular genes get silenced so that the offspring only expresses one allele. Imprints are carried in the somatic cells of offspring, but removed from germ cells. Mitochondria and chloroplasts contain their own DNA, and exhibit non-mendelian inheritance. Mitochondria are inherited from the mother. Page 7