Overview Locating Genes Along Chromosomes Mendel s hereditary factors were genes though this was n t known at the time 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 Concept 15 1 Mendelian inheritance has its physical basis in the behavior of chromosomes The chromosome theory of inheritance states Mendelian genes have specific loci positions on chromosomes Chromosomes undergo segregation and independent assortment The behavior of chromosomes during meiosis was said to account for Mendel s laws of segregation and independent assortment Fig 15 2 P Generation Yellow round seeds YYRR Y R Y r R Green wrinkled seeds yyrr y r y Meiosis Fertilization R Y Gametes y r All F1 plants produce yellow round seeds YyRr F1 Generation R R y r Y Y LAW OF SEGREGATION The two alleles for each gene separate during gamete formation y r LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation Meiosis R r Y y r R Y y Metaphase I 1 1 R r Y y r R Y y Anaphase I R r Y y Metaphase II r R Y y 2 2 Y Y Gametes R R 1 F2 Generation 4 YR y r r r 1 4 Y Y y yr r 1 4 Yr y y R R 1 4 yR An F1 F1 cross fertilization 3 3 9 3 3 1 Morgan s Choice of Experimental Organism The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan an embryologist 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 They breed at a high rate A generation can be bred every two weeks They have only four pairs of chromosomes Fig 15 3 Morgan noted wild type or normal phenotypes that were common in the fly populations red eye color Traits alternative to the wild type are called mutant phenotypes white eye color Correlating Behavior of a Gene s Alleles with Behavior of a Chromosome Pair The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan an embryologist Morgan s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel s heritable factors In one experiment Morgan mated male flies with white eyes mutant with female flies with red eyes wild type The F1 generation all had red eyes The F2 generation showed the 3 1 red white eye ratio but only males had white eye Fig 15 4a Morgan determined that the white eyed mutant allele must be located on the X chromosome Morgan s finding supported the chromosome theory of inheritance EXPERIMENT P Generation F1 Generation RESULTS F2 Generation All offspring had red eyes Fig 15 4c CONCLUSION P Generation w X X w X Y w Eggs F1 Generation Sperm w w w w w Eggs F2 Generation w w Sperm w w w w w w The Chromosomal Basis of Sex In humans and other mammals there are two varieties of sex chromosomes a larger X chromosome and a smaller Y chromosome Females are XX and males are XY 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 Each ovum contains an X chromosome while a sperm may contain either an X or a Y chromosome Other animals have different methods of sex determination Inheritance of Sex Linked Genes The sex chromosomes have genes for many characters unrelated to sex A gene located on either sex chromosome is called a sex linked gene In humans sex linked usually refers to a gene on the larger X chromosome For a recessive sex linked trait to be expressed A female needs two copies of the allele A male needs only one copy of the allele Sex linked recessive disorders are much more common in males than in females Some disorders caused by recessive alleles on the X chromosome in humans Color blindness Duchenne muscular dystrophy 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 Fig 15 8 X chromosomes Early embryo Two cell populations in adult cat Active X Allele for orange fur Allele for black fur Cell division and X chromosome inactivation Active X Inactive X Black fur Orange fur Concept 15 3 Linked genes tend to be inherited together because they are located near each other on the same chromosome Each chromosome has hundreds or thousands of genes Genes located on the same chromosome that tend to be inherited together are called linked genes Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters Morgan crossed flies that differed in traits of body color and wing size Fig 15 9 4 EXPERIMENT P Generation homozygous Wild type gray body normal wings Double mutant black body vestigial wings b b vg vg b b vg vg F1 dihybrid wild type Double mutant TESTCROSS b b vg vg b b vg vg Testcross offspring b vg b vg b vg Wild type gray normal Blackvestigial Grayvestigial Blacknormal b b vg vg b b vg vg Eggs b vg b vg Sperm b b vg vg b b vg vg PREDICTED RATIOS If genes are located on different chromosomes 1 1 1 1 If genes are located on the same chromosome and parental alleles are always inherited together 1 1 0 0 965 944 206 185 RESULTS Morgan found that body color and wing size are usually inherited together in specific combinations parental phenotypes He noted that these genes do not assort independently and reasoned that they were on the same chromosome However nonparental phenotypes were also produced Understanding this result involves exploring genetic recombination the production of offspring with combinations of traits differing from either parent The genetic findings of Mendel and Morgan relate to the chromosomal basis of recombination Recombination of Linked Genes Crossing Over Offspring with a phenotype matching one of the parental phenotypes are called parental types Offspring with nonparental phenotypes new combinations of traits are called recombinant types or recombinants Morgan discovered that genes can be linked but the linkage was incomplete as evident from recombinant phenotypes Morgan proposed that some process must sometimes break the physical connection between genes on the same chromosome That mechanism was the crossing over of