Chromosomal Inheritance AP Biology Mrs. Ogden
So Mendel was right after all… Around 1900, cytologists and geneticists began to see parallels between the behavior of chromosomes and the behavior of Mendel’s factors. Chromosomes and genes are both present in pairs in diploid cells. Homologous chromosomes separate and alleles segregate during meiosis. Fertilization restores the paired condition for both chromosomes and genes. • Around 1902, Walter Sutton, Theodor Boveri, and others noted these parallels and a chromosome theory of inheritance began to take form.
Fruit Flies Thomas Hunt Morgan was the first to associate a specific gene with a specific chromosome in the early 20th century. Morgan choose Drosophila melanogaster , a fruit fly species that eats fungi on fruit. Fruit flies are prolific breeders and have a generation time of two weeks. Fruit flies have three pairs of autosomes and a pair of sex chromosomes (XX in females, XY in males).
Fruit Flies Morgan spent a year looking for variant individuals among the flies he was breeding. He discovered a single male fly with white eyes instead of the usual red. The normal character phenotype is the wild type . Alternative traits are mutant phenotypes .
Mating Fruit Flies When Morgan crossed his white-eyed male with a red-eyed female, all the F1 offspring had red eyes, The red allele appeared dominant to the white allele. Crosses between the F1 offspring produced the classic 3:1 phenotypic ratio in the F2 offspring. Surprisingly, the white-eyed trait appeared only in males. All the females and half the males had red eyes. Morgan concluded that a fly’s eye color was linked to its sex.
Discovery of Sex Linkage Morgan deduced that the gene with the white-eyed mutation is on the X chromosome alone, a sex- linked gene. Females (XX) may have two red-eyed alleles and have red eyes or may be heterozygous and have red eyes. Males (XY) have only a single allele and will be red eyed if they have a red-eyed allele or white-eyed if they have a white-eyed allele.
Exception to Independent Assortment Each chromosome has hundreds or thousands of genes. Genes located on the same chromosome, linked genes , tend to be inherited together because the chromosome is passed along as a unit. Results of crosses with linked genes deviate from those expected according to independent assortment.
Linked Genes Morgan observed this linkage and its deviations when he followed the inheritance of characters for body color and wing size. The wild-type body color is gray (b+) and the mutant black (b). The wild-type wing size is normal (vg+) and the mutant has vestigial wings (vg). Morgan crossed F1 heterozygous females (b+bvg+vg) with homozygous recessive males (bbvgvg). According to independent assortment, this should produce 4 phenotypes in a 1:1:1:1 ratio.
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- Math, Factors, Thomas Hunt Morgan