Rebecca Davenport BIOL 1406 11 29 16 Ch 15 Blueprint 1 The Chromosomal Basis of Inheritance a 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 melanogaster ii Several characteristics make fruit flies a convenient organism for genetic studies 1 They produce many offspring 2 A generation can be bred every two weeks 3 They have only four pairs of chromosomes iii 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 Pair i 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 eyes 4 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 chromosome iii Morgan s finding supported the chromosome theory of inheritance 2 Chromosomal Basis of Sex a In humans and other mammals there are two varieties of sex chromosomes a larger X chromosome and a smaller Y chromosome b A person with two X chromosomes develops as a female while a male develops from a zygote with one X and one Y c 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 gene ii Genes on the Y chromosome are called Y linked genes there are few of these iii Genes on the X chromosome are called X linked genes iv X chromosomes have genes for many characters unrelated to sex whereas most Y linked genes are related to sex determination f X linked genes follow specific patterns of inheritance i For a recessive X linked trait to be expressed 1 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 sons 2 Mothers pass X linked to both sons and daughters iii X linked recessive disorders are much more common in males than in females iv X inactivation in female mammals1 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 cell 4 If a female is heterozygous for a particular gene located on the X chromosome she will be a mosaic for that character a Half cells express one allele while other express the alternate allele v 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 genes 3 Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters 4 Morgan crossed flies that differed in traits of body color and wing size 5 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 chromosome vi Abnormal chromosome number 1 Nondisjunction pairs of homologous chromosomes do not separate normally during meiosis 2 3 4 5 6 7 a As a result one gamete receives two of the same type of chromosome and another gamete receives no copy Aneuploidy results from one of the gametes in which nondisjunction occurred gets fertilized a ONLY ONE chromosome is out of wack Offspring with this condition have an abnormal number of a particular chromosome A monosomic zygote has only one copy of a particular chromosome A trisomic zygote has three copies of a particular chromosome 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 chromosomes c Tetraploidy 4n is four sets of chromosomes 8 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 structure a Deletion removes a chromosomal segment b Duplication repeats a segment c Inversion reverses orientation of a segment within a chromosome d Translocation moves a segment from one chromosome to another e 2 Human disorders due to Chromosomal Alterations a Alterations of chromosome number and structure are associated with some serious disorders b Some types of aneuploidy appear to upset the genetic balance less than others resulting in individuals surviving to birth and beyond c These surviving individuals have a set of symptoms or syndrome characteristic of the type of aneuploidy d Down syndrome Trisomy 21 is an aneuploid three copies of chromosome 21 i viii Aneuploidy of Sex Chromosomes 1 Nondisjunction of sex chromosomes produces a variety of aneuploid conditions 2 XXX females are healthy with no unusual physical features 3 Klinefelter syndrome is the result of an extra chromosome in a male producing XXY individuals 4 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 5 2 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 chromosomes
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