Chapter 11 - 11.1 The Chromosomal Basis of Inheritance 1 A...

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11.1 The Chromosomal Basis of Inheritance 1. A gene is a unit of info about a heritable trait. Genes of eukaryotic cells are distributed among a number of chromosomes. Each gene has its own location, or locus, in one type of chromosome. 2. A cell with a diploid chromosome number, 2n, has pairs of homologous chromosomes. All but one pair are normally the same length, shape and order of genes with the exception being non- identical sex chromosomes. Each chromosome becomes aligned with its homologous partner at metaphase I of meiosis. 3. Genes mutate, so a pair on homologous chromosomes may/may not be the same. All of the slightly different molecular forms of a gene that occur among individuals of a population are alleles. 4. A wild-type allele is a gene’s most common form, in either a natural population or in a standardized, laboratory-bred strain of the species. A less common form is a mutant allele. 5. All genes on the same chromosome are physically connected. The farther apart any two genes are along the length of a chromosome, the more vulnerable they are to crossing over. By this event, a chromatid of one chromosome and a chromatid of its homologue swap corresponding segments. Crossing over between non-sister chromatids is a form of genetic recombination that introduces novel combos of alleles in chromosomes. 6. On rare occasions, the structure of a chromosome or parental chromosome number changes in mitosis or meiosis. Such chromosomal abnormalities can have severe phenotypic consequences. Human X and Y chromosomes differ physically; Y is shorter, almost a remnant of the other in appearance - Also differ in which gen es they carry, but still synapse (zipper briefly) in a s m all region, allo the m to interact as ho mologues in m eiosis - SRY is m a st er gen e for m ale sex det er mination (one of 330 on Y chromoso m e) - X chromoso m e carries 2,062 g en es d e aling with traits such as distribution of body fat a nd h a nd blood-clotting functions
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