Rebecca DavenportBIOL 140611/29/16Ch 15 Blueprint1. The Chromosomal Basis of Inheritancea. Morgan showed that Mendelian inheritance has its physical basis in the behavior of chromosomes.i. Thomas Hunt Morgan provided evidence associating a specific gene with a specific chromosome - Drosophila melanogasterii. Several characteristics make fruit flies a convenient organism for genetic studies1. They produce many offspring 2. A generation can be bred every two weeks3. They have only four pairs of chromosomesiii. Morgan noted wild type, or normal, phenotypes denoted as “w+”iv. Traits alternative to the wild type are called mutant phenotypes denoted as “w”.b. Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pairi. In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type)1.2.3. The F1 generation all had red eyes4. The F2 generation showed a 3:1 red to white eye ratio, but only males had white eyes.ii. Morgan determined that the white-eyed mutant allele must be located on the X chromosomeiii. Morgan’s finding supported the chromosome theory of inheritance2. Chromosomal Basis of Sexa. In humans and other mammals, there are two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosomeb. A person with two X chromosomes develops as a female, while a male develops from a zygote with one X and one Yc. Only the ends of the Y chromosome have regions that are homologous with corresponding regions of the X chromosome.d.e. Sex linked genes:i. A gene that is located on either sex chromosome is called a sex-linked geneii. Genes on the Y chromosome are called Y-linked genes; there are few of theseiii. Genes on the X chromosome are called X-linked genesiv. X chromosomes have genes for many characters unrelated to sex, whereas most Y-linked genes are related to sex determinationf. X-linked genes follow specific patterns of inheritance:i. For a recessive X-linked trait to be expressed1. A female needs two copies of the allele (homozygous)2. A male needs only one copy of the allele (hemizygous)ii. X-linked genes:1. Fathers pass X-linked alleles to all daughters but none of their sons2. Mothers pass X-linked to both sons and daughters.iii. X-linked recessive disorders are much more common in males than in femalesiv. X inactivation in female mammals-1. In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development 2. The inactive X condenses into a Barr body; genes are not expressed because it is squashed up. **BARR BODIES ARE ONLY SEEN IN WOMEN**a.3. Selection of which X chromosome will form Barr body occurs randomly and independently in each cell4. If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that charactera. Half cells express one allele while other express the alternate allelev. Linked genes tend to be inherited together:1. Each chromosome has hundreds or thousands of genes (except the Y chromosome)2. Genes located on the same chromosome that tend to be inherited together are called linked genes3. Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters4. Morgan crossed flies that differed in traits of body color and wing size5. Morgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes) 6. He noted that these genes do not assort independently, and reasoned that they were on the same chromosomevi. Abnormal chromosome number:1. Nondisjunction- pairs of homologous chromosomes do not separate normally during meiosis. a.2. As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy3. Aneuploidy- results from one of the gametes in which nondisjunction occurred gets fertilized. a. ONLY ONE chromosome is out of wack.4. Offspring with this condition have an abnormal number of a particular chromosome5. A monosomic zygote has only one copy of a particular chromosome6. A trisomic zygote has three copies of a particular chromosome7. Polyploidy is a condition in which an organism has more than two complete sets of chromosomes.i. ENTIRE EXTRA set of chromosomes!b. Triploidy (3n) is three sets of chromosomesc. Tetraploidy (4n) is four sets of chromosomes8. Polyploidy is common in plants, but not animals.9. Polyploids are more normal in appearance than aneuploids.a. Its better to have too much (Polyploid) than too little (Aneuploids). vii. Alterations of Chromosome Structure:1. Breakage of a chromosome can lead to four types of changes in chromosome structurea. Deletion removes a chromosomal segmentb. Duplication repeats a segmentc. Inversion reverses orientation of a segment within a chromosomed. Translocation moves a segment from one chromosome to anothere.2. Human disorders due to Chromosomal Alterations:a. Alterations of chromosome number and structure are associated with some serious disordersb. Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyondc. These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidyd. Down syndrome (Trisomy 21) is an aneuploid - three copies of chromosome 21i.viii. Aneuploidy of Sex Chromosomes1. Nondisjunction of sex chromosomes produces a variety of aneuploid conditions.2. XXX females are healthy, with no unusual physical features3. Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals4. Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans ix. Disorders caused by Structurally altered chromosomes 1. The syndrome cri du chat (“cry of the cat”), results from a specific deletion in chromosome 52. A child born with this syndrome is severely intellectually disabled and has a catlike cry; individuals usually die in infancy or early childhood.3.4. Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of
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