Chapter 15

Chapter 15 - Chapter 15 Chromosome theory of inheritance...

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Chapter 15 Chromosome theory of inheritance Mendelian genes have specific loci along chromosomes and it is the chromosomes that under go segregation and independent assortment The red eyes are the wild type, the most commonly observed in natural population,w+ The white eyes are called the Mutatant type, mutations from the red type allele, w Females- xx Males-xy Eye color is only on x chromosome, so females cannot have white eyes A female develops from two x chromosomes A male develops from a zygote containing x and y Short segments at either end of the y chromosome are the only regions that are homologous with corresponding regions of x SRY is on the y chromosome determines male Sexlinked gene-a gene located on either sex chromosome The term hemizygous describes the males x linked gene because they only have one locus when receiving the recessive allele In women one of the x’s protein is inactive, therefore we get the same does of preotein as men The inactive x is each cell of a female condenses into a compact object called a barr body which lies along the indside of the nuclear envelope Females consist of two types of cells Active x from father, active x from mother Female heterozygous half of cells will express one allele while the other express the other Ex: female humans who are heterozygous for a recessive sweat trait will have patches of normal skin and patches of skin lacking sweat glands How inactivation of x chromosome occurs: Each x chromosome has xist (x inactive specific transcript) Its becomes active only on the chromosome that will become the barr body Multiple copies of RNA product of this gene attach to the x chromosome on which they are made eventually covering it
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This interaction initiates x inactivation Linked Genes Linked genes- genes located near each other on the same chromosome tend to be inherited together in genetic crosses such genes are said to be genetically linked Genes for body color and wing size are genetically linked Genetic recombination- the production of offspring with combinations of traits that differ from those found in either parent Recombination of Unlinked Genes: Independent assortment of Chromosomes Parental types- offspring expected to inherit a phenotype that matches either of the parent phenotype Recombiant types- are the off spring that are not from the p generation The physical basis of recombination between unlinked genes is the random orientation of homologous chromosomes at metaphase I of meiosis, which leads to the independent assortment of the two unlinked genes Recombination of Linked Genes: Crossing Over Crossing over- accounts for the recombingation of linked genes Meiosis I: Crossing over between b and vg loci produces new allele combinations Meisosis II: Separation of chromatids produces recombinant gametes with the new allele combinations Mapping the Distance Between Genes Using Recombination Datea: Scientific Inquiry
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Chapter 15 - Chapter 15 Chromosome theory of inheritance...

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