Morgan’s Experimental Evidence • Thomas Hunt Morgan • Experimental embryologist at Columbia University in the 20th century • Provided evidence that chromosomes are the location of Mendle’s heritable factors.
Morgan’s Choice of Experimental Organism Morgan selected a species of fruit fly They are prolific breeders which made it a convenient organism for the study. Soon his lab room became known as “the fly room.” Fruit flies only have four pairs of chromosomes- 3 pairs autosomes and 1 pair of sex chromosome.
Morgan was rewarded with discovering of a single male fly with white eyes instead of red. The normal phenotype for character is called the wild type. Wile type- an individual with the normal (most common) phenotype.
Morgan and students invented notation for symbolizing alleles in Drosophilia. • The gene takes its symbol from the first mutant discovered. Thus, the allele for white eyes in Drosophilia is symbolized by w. • + shows the allele for the wild type allele.
Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair After mating the white eye male to a red eye female, all the f1 offspring's had red eyes. After breeding f1 flies to each other, he observed classical 3-1 ratio. The white eye trait showed up in the males only. Morgan concluded that somehow the eye color was related to its sex
The correlation between the white eyes and males suggested to Morgan that the gene affected that was affected was located in the X chromosome. For males a single copy of the mutant allele would confer white eyes. Females could have white eyes onlly if both her X chromosomes carried the recessive mutan allele (w). Morgan’s work indicated that genes located on a sex chromosome exhibit unique inheritance patterns.
Linked genes tend to be inherited together because they are located near each other on the same chromosome
LINKAGE • 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.
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 (bb vgvg).
According to independent assortment, this should produce 4 phenotypes in a 1:1:1:1 ratio. • Surprisingly, Morgan observed a large number of wild-type (gray-normal) and double-mutant • (black-vestigial) flies among the offspring. • These phenotypes correspond to those of the parents.
Linkage Strength • Degree of strength related to how close the traits are on the chromosome. • Weak - farther apart • Strong - closer together
LINKAGE MAPS Sturtevant used the test cross design to map the relative position of three fruit fly genes, body color (b), wing size (vg), and eye color (cn). The recombination frequency between cn and b is 9%. The recombination frequency between cn and vg is 9.5%. The recombination frequency between b and vg is 17%. The only possible arrangement of these three genes places the eye color gene between the other two.
LINKAGE MAPS • A linkage map provides an imperfect picture of a • chromosome. • Map units indicate relative distance and order, not • precise locations of genes. • The frequency of crossing over is not actually uniform • over the length of a chromosome. • Combined with other methods like chromosomal • banding, geneticists can develop cytological maps. • These indicated the positions of genes with respect to • chromosomal features. • More recent techniques show the absolute distances • between gene loci in DNA nucleotides.
Sex-linked genes exhibit unique patterns of inheritance By: Shakil Chowdhury and Nazmun Nahar
X and Y Chromosomes XX= Female XY= Males ** Besides the mammalian X–Y system, three other chromosomal systems for determining sex exist.
Inheritance of Sex–Linked Genes • Sex chromosomes have genes for many characters unrelated to sex. A gene located on either sex chromosome is called a sex–linked gene. • Duchenne muscular dystrophy
Inheritance of Sex–Linked Genes • Hemophilia
X Inactivation in Female Mammals • The inactive X in each cell of a female condenses into a compact object called a Barr body, which lies along the inside of the nuclear envelope.