Ch15 The Chromosomal Basic of Inheritance 1 Chapter 15 The Chromosomal Basis of inheritance Locating Genes along chromosomes o We know that genes Mendel s factors are located along chromosomes We can see the location of a particular gene by tagging chromosomes with a fluorescent dye that highlights that gene Concept 15 1 Mendelian inheritance has its physical basis in the behavior of chromosomes Cytology and genetics converged when biologists began to see parallels between the behavior of chromosomes and the o o o behavior of Mendel s proposed hereditary factors during sexual life cycles Chromosomes and genes are both present in pairs in diploid cells Homologous chromosomes separate and alleles segregate during the process of meiosis Fertilization restores the prepared condition for both chromosomes and genes The Chromosome theory of inheritance according to this theory Mendelian genes have specific loci positions along chromosomes and it is the chromosomes that undergo segregation and independent assortment The behavior of non homologous chromosomes can account for the independent assortment of the alleles for two or more genes located on different chromosomes o Morgan s Experimental Evidence Scientific inquiry Mendel chose the garden pea because a number of distinct varieties were available Morgan selected a species of fruit flies a common insect that feeds on the fungi growing on fruit Fruit flies are prolific breeders a single mating will produce hundreds of offspring The advantage of the fruit fly is that it only has 4 chromosomes 1 sex chromosome and 3 pairs of autosomes Morgan discovered a single male fly with white eyes instead of the usual red The phenotype for a character most commonly observed in natural populations such as red eyes in the fruit flies is called the wild type w Traits that are alternatives to the wild type are called mutant phenotypes because they are due to alleles assumed to have originated as changes or mutations in the wild type alleles When Morgan bred the white eyed male fly with a red eyed female the offspring had red eyes He observed the classical 3 1 phenotypic ratio among the F2 offspring The white eye trait showed up only in males All the F2 females had red yes while half the males had red and the other half had white Morgan s findings of the correlation between a particular trait and an individual s sex provided support for the chromosome theory of inheritance namely that a specific gene is carried on a specific chromosome He correlated 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 Morgan s work indicated that genes located on a sex chromosome exhibit unique inheritance patterns 15 1 Summary The chromosome theory of inheritance states that genes are located on chromosomes and that the behavior of chromosomes during meiosis accounts for Mendel s laws of segregation and independent assortment Morgan s discovery that transmission of the X chromosome in Drosophila correlates with inheritance of an eye color trait was the first solid evidence indicating that a specific gene is associated with a specific chromosomes Concept 15 2 Sex linked genes exhibits unique patterns of inheritance o The chromosomal Basis of sex In humans and other mammals there are two varieties of sex chromosomes designated X and Y The Y chromosomes is much smaller than the X chromosome In mammalian testes and ovaries the two sex chromosomes segregate during meiosis and each gamete receives one Sex determination is a matter of chance 50 50 Sex determining region of Y SRY a gene on the Y chromosome required for the development of testes In the Ch15 The Chromosomal Basic of Inheritance 2 absence of SRY the gonads develop into ovaries SRY codes for a protein that regulates other genes Researchers have sequenced the human Y chromosome and have identified 78 genes that code for about 25 A gene located on either sex chromosome is called a sex linked gene those located on the Y chromosome are The Y chromosome is passed virtually intact from a father to his sons Because there are so few Y Linked genes very few disorders are transferred from father to son on the Y proteins called Y linked genes chromosome The human X chromosome contains approximately 1 1000 genes which are called X linked genes The fact that males and females inherit a different number of X chromosomes leads to a pattern of inheritance different from that produced by genes located on autosomes o Inheritance of X linked genes While most Y linked genes help determine sex the X chromosomes have genes for many characters unrelated to sex X linked genes in humans follow the same pattern of inheritance that Morgan had observed for the fruit flies that allele Fathers pass X linked alleles to all of their daughters but to none of their sons Mothers can pass X linked alleles to both sons and daughters If an X linked trait is due to a recessive allele a female will express the phenotype only if she is homozygous for Hemizygous term used to describe male genes since males have only one locus Any male receiving the recessive allele from his mother will express the trait For this reason far more males than females have X linked recessive disorder and loss of coordination Duchenne muscular dystrophy is a disease that is characterized by a progressive weakening of the muscles Researchers have traced the disorder to the absence of a key muscle protein called dystrophin and have mapped the gene for this protein to a specific locus on the X chromosome Hemophilia is an X linked recessive disorder defined by the absence of one or more of the proteins required for When a person with hemophilia is inured bleeding is prolonged because a firm clot is slow to form Female mammals including humans inherit two X chromosomes twice the number inherited by males Most of the X chromosomes in each cell in female mammals become inactivated during early embryonic The cells of females and males have the same effective dose one copy of most x linked genes The inactive X in each cell of a female condenses into a compact object called a Barr body which lies along the In the ovaries Barr body chromosomes are reactivated in the cells that give rise to eggs so every inside of the nuclear envelope female gamete has an active X Females consist of a mosaic of two types of cells Those with the active X derived from the father and from the mother o X