AP Bio Outline - Chapter 15 - Ginamarie Period 4 Basis of...

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Ginamarie Giacoio October 28, 2016 Period 4 Chapter 15: The Chromosomal Basis of Inheritance Concept 15.1 Morgan showed that Mendelian inheritance has its physical basis in the behavior of chromosomes: Scientific inquiry While Mendel used garden peas for his work, Morgan selected a species of fruit fly known as Drosophila melanogaster. o Fruit flies only have four pairs of chromosomes, three pairs of autosomes and one pair of sex chromosomes. The phenotype for a character most common observed in natural populations, such as red eyes in Drosophila , is called the wild type , while traits that are alternative to the wild type, such as white eyes, are called mutant phenotypes . The correlation between the trait of the white eye color and the male sex of the affected F 2 flies suggested to Morgan that the gene involved in his white-eyed mutant was located exclusively on the X chromosome, with no corresponding allele present on the Y chromosome. o Chromosome theory of inheritance: a specific gene is carried on a specific chromosome. Concept 15.2 Sex-linked genes exhibit unique patterns of inheritance A gene located on either sex chromosome is called a sex-linked gene , those located on the Y chromosomes are called Y-linked genes. The human X chromosome contains approximately 1,100 genes, which are called X- linked genes. While most Y-linked genes help determined sex, the X chromosomes have genes for many characters unrelated to sex. Mutant phenotypes : are due to alleles assumed to have originated as changed, or mutations, in the wild-type allele
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If an X-linked trait is due to a recessive allele, a female will express the phenotype only if she is homozygous for that allele, because males have only one locus, the terms homozygous and heterozygous lack meaning for describing their X- linked genes; the term hemizygous is used in such cases. Many human X-linked disorders are much more serious than color blindness (which is almost always inherited as an X-linked trait), such as Duchenne muscular dystrophy .
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