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Chapter 15 The Chromosomal Basis of Inheritance.docx

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Life Science
LIFE 102

Chapter 15 The Chromosomal Basis of Inheritance • Mendel’s heredity factors were genes. Today we can show that genes are located on chromosomes. • The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene. • The Chromosome theory of Inheritance states: a. Mendelian genes have specific loci (positions) on chromosomes b. Chromosomes undergo segregation and independent assortment • The behavior of chromosomes during meiosis was said to account for Mendel’s laws • Law of segregation: The two alleles for each gene separate during gamete formation. • Law of independent assortment: Alleles of genes on nonhomologous chromosomes assort independently during gamete information • Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors. • Several characteristics make fruit flies a convenient organism for genetic studies: a. They breed at a high rate b. A generation can be bred every two weeks c. They have only four pairs of chromosomes • Morgan noted wild type, or normal phenotypes that were common in the fly populations • Traits alternative to the wild type are called mutant phenotypes • In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) a. The F 1eneration all had red eyes b. The F 2eneration showed the 3:1 red: white eye ratio, but only males had white eyes. • Morgan determined that the white eyed mutant allele must be located on the X chromosome • Morgan’s finding supported the chromosome theory of inheritance • In humans and other mammals, there are two varieties of sex chromosomes: a large X chromosome and a smaller Y chromosome • Only the ends of the Y chromosome have regions that are homologous with the X chromosome • The SRY gene on the Y chromosome codes for the development of testes. • Sex-linked genes follow specific patterns of inheritance • For a recessive sex-linked trait to be expressed a. A female needs two copies of the allele b. A male needs only one copy of the allele • Sex- linked recessive disorders are much more common in makes than in females • Some disorders caused by recessive alleles on the X chromosome in humans: a. Color Blindness b. Duchene muscular dystrophy c. Hemophilia • In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development • The inactive X condenses into a Barr Body • If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character • Genes located on the same chromosome that tend to be inherited together are called Linked Genes • Genetic Recombination, the production of offspring with combinations of traits differing from either parent • Offspring with a phenotype matching one of the parental phenotypes are called Parental Phenotypes. • Offspring with nonparental phenotypes (new combinatio
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