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Chapter 6 Advanced Genetics Section 6A Chromosome and Gene Changes Two types of Gene Changes Chromosomal changes – the # of chromosomes or the arrangement of genes on a chromosome Gene Mutations – a change in the sequence of bases in a gene. Changes affecting # of Chromosomes A genome is a complete haploid set of its chromosomes. A diploid cell has two complete genomes. Review haploid and diploid cells if this is confusing. Diploid organisms, like us, have to go through meiosis to produce haploid gametes (either sperm or eggs). Parthenogenesis There are examples of haploid organisms. These come from the development of unfertilized eggs. This is called Parthenogenesis An example of this in the natural world is worker honey bees. Euploidy A chromosome number that is a multiple of the haploid number (N). For Example, the human haploid number is 23, this is N. Our diploid chromosome number is 46, which is 2N. Organisms can be polyploid, which means they can have three or more genomes. Euploidy (cont.) n – one genome 2n (diploid) – two sets of the same genome 3n (triploid) – three sets of same genome 4n (tetraploid) – four sets of same genome Liver cells in humans are tetraploid. Polyploids Many of the crops that we plant today are good examples of polyploids. Corn, wheat, cotton, seedless watermelons. Polyploids can be produced by using colchicine to stop the formation of the spindle fibers during mitosis. This will from diploid gametes. 2n + 2n = 4n (tetraploid) Aneuploidy Lacking or have extra chromosomes 2n + 1 or trisomy 2n – 1 or monosomy A chromosome will fail to separate during meiosis. This is called nondisjunction. Examples are Downs, Klinefleters, Turners, Trisomy X Mutations A change in the DNA of an organism Think of the Chromosomes as a bead of pearls, like a necklace. This necklace can be broken and repaired. The repair can be out of order and moved, thus causing a rearrangement of the genes. Translocation Remember crossing over in Meiosis. This is one form of Translocation. One piece of a chromosome coming loose and attaching to another chromosome. But this can also happen with nonhomologous chromosomes and cause real problems. Deletion A complete loss of a section of chromosome is called Deletion. The piece that comes loose leaves the nucleus of the cell and disintegrates. This is harmful, if the other homologous chromosome has a “bad” recessive gene and the dominant “normal” gene is the one that is lost. Cri du Chat Inversion Inversion is a segment of the chromosome breaks off and reattaches at the same spot, but it is in reverse order. Gene Mutations When one nucleotide is changed is called a point mutation. Examples are: Substitution Addition deletion Biological Effects of Mutations Spontaneous mutations happen naturally Mutagens can also induce mutations. Mutagens are gases, chemicals, or radiation. Somatic mutations are mutations in somatic cells (non-sex cells) Germ Mutations is a mutation in a cell that forms gametes. Gene Expression Gene expression is the activation or turning on of a gene that results in its transcription and the production of a specific protein. Morphogenesis is a change in the form that an organism undergoes, this is controlled by the expression of genes. Controls of gene expressions Sex-linked characteristics are the results of gene expression. The presence or absence of sex hormones turns on or off the genes that code for sex-limited characteristics. EX. Feathers of birds, body hair, breast development. Controls of gene expressions Environmental Factors, temperature, light, chemical, nutrients. Temperature on the fur of the Himalayan rabbit. Ice pack on the ears turns fur black, while heat turns fur white. Cancer Cells that are not responsive to the normal controls of the cell cycle. Two genes that regulate the cell cycle are: Proto – oncogenes – code for proteins that stimulate cell division. Tumor suppressor genes – Prevent uncontrolled cell growth. There has to be a balance between these two. Chapter 6 Population Genetics A population is a group of individuals of the same species that live in the same area. Gene pool is a sum of all of the alleles that all members of an organism could possess. The genotypic differences between individuals form the same gene pool are called variations.