The chromosomal basis of inheritance
Mendelian Inheritance has its physical basis in the behavior of chromosomes during sexual life-cycle. 2)
Morgan traced a gene to a specific chromosome.
Linked genes tend to be inherited together because they are located on the same gene. 4)
Independent assortment of chromosomes and crossing over produce genetic recombinants. 5)
Geneticists use recombination data to map a chromosome’s genetic locus.
1900 Biology finally caught up with Mendel.
Independently, Corren, Tschermak, and Vries all found that Mendel had explained the same result 35 years go. Resistance remained about Mendels laws until evidence had supported it.
Chromosoems and genes are both present in pairs of diploid cells. Homologous chromosomes separate and alleles segregate during meiosis. Fertilization restores the paired condition for both chromosomes and genes. Around 1902, the chromosome theory of inheritance began to take form.
Thomas Hunt Morgan was the first t associate a specific gene with a specific chromosome in the early 20th century. Like Mendel, Morgan made an insightful choice as an experimental animal, he chose fruit flies. Advantages of fruit flies: Prolific breeders, generation time of 2 weeks, 3 pairs of autosomes and a pair of sex chromosomes (XX in females, XY in males) Morgan spent a year looking for variants in the fruit flies. Most had red eyes but he found 1 male with white eyes.
Normal phenotypic characteristics are called wild type.
Alternative traits are called mutant phenotypes.
When Morgan crossed his white eyed male with a red eyed female, all the offspring’s had red eyes. Therefore the allele for red eyes is dominant over the one for white eyes. When he crossed the F1 offspring, it produced a 3:1 ratio in the F2 generation. 3 red, 1 white. The white eyes appeared only in males. Half the males and all the females had red eyes. His conclusion: eye color is linked to sex. The gene with the white eye color is only located on the X chromosome therefore it is a sex linked gene. Females may have two red eyed alleles or have 1 red eye and 1 white. Males have a single allele and will have either red or white.
Each chromosome has hundreds or thousands of genes. Genes that are located on the same chromosome are called linked genes and they 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. Meaning that when a cross happens and linked genes are involved, the expected phenotypic outcome is different than what independent assortment expects. Morgan observed this linkage and its deviation when he followed the inheritance of characters for body color and wing size. The wild type body color is gray (b+) and the mutant is black (b). The wild type wing size is normal (vg+) and the mutant has vestigial (small) wings (vg). Morgan crossed the F1 heterozygous females (b+b vg+vg) with homozygous recessive males (bb vgvg). According to independent assortment, this should produce 4 phenotypes in a 1:1:1:1 ratio but 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. Morgan reasoned that body color and wing shape are usually inherited together because their genes are on the same chromosome.
The production of offspring with new combination of traits inherited from 2 parents is genetic recombination. Genetic recombination can result from independent assortment of genes located on non-homologous chromosomes or from crossing over of genes located on homologous chromosomes. Mendels dihybrid cross experiments produced some offspring’s that had a combination of traits that did not match the parents in the P generation. If the P generation consists of a yellow round parent (YYRR) crossed with a green wrinkled parent...
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