Chromosomes and Inheritance
The pattern of chromosomal inheritance also correlated with Mendelian genetics.
The Chromosomal theory of inheritance came about in 1902.
Two important laws for chromosomal inheritance are:
The law of segregation
The law of independent assortment
The law of segregation says that a diploid organism has two of every allele and when gametes are made only one of each of those alleles is placed into a gamete. This means that the two alleles are "segregated" when gametes are formed.
The law of Independent Assortment says that alleles on different chromosomes sort independent of each other.
-All of the pea traits Mendel was studying happened to sort independently which is why he was able to make his dihybrid crosses
It was not until Thomas Hunt Morgan's research on fruit flies (Drosophila melanogaster) in 1907 that scientists obtained proof of the chromosomal theory of inheritance.
This species of fruit fly was perfect to test chromosomal inheritance on since the fly only had four pairs of chromosomes, three pairs of autosomes and one pair of sex chromosomes. Like humans the females had an XX and the male had an XY.
Within two weeks, one pair of fruit flies could have hundreds of offspring. Morgan breed his flies for two years searching for a mutation. Finally he found a male with white eyes. The wild type is red eyes.
-We use the term "wild type" to refer to the original allele, variations of the wild type are mutant alleles.
Now that Morgan had a mutation he could breed that male with white eyes and hopefully find the chromosome eye color was located on. It turned out that the white eyes was located on the X chromosome and Morgan discovered sex-linked genes.
Fruit fly eye color works the same way human color blindness or hemophilia does.
Morgan then discovered two more mutations in his fruit flies; black body color and short vestigial wings.
When he breed a homozygous dominant for long wings and gray body with a homozygous recessive for vestigial wings and black body he expected to get an F1 generation full of heterozygous flies. He then breed the F1 generation expecting the F2 generation to create a 1/4 of each genotype as a dihybrid cross would indicate. But to Morgan's surprise he did not get a 1:1:1:1 ratio.
Morgan discovered linked genes
The closer the locus of the two alleles the less likely the alleles are to separate.
Mutations
-A mutation is a change in the sequence of DNA bases
-There are repair enzymes that proof read the new daughter strands to check for errors
-When an error is found it is repaired
-But sometimes these errors are not repaired in the right way and this leads to a mutation
-There are many types of mutations
-Nucleotide Substitutions are when a single base pair is substituted
-For example an A-T base pair is substituted for a T-A
-This is also known as a point mutation because only one base pair is affected
-Insertion Mutations are when extra bases are added to the DNA
-It could be one base pair or it could be many
-Deletion Mutations are when base pairs are deleted
-It could be one base pair or many
-A deletion mutation or an insertion mutation can cause what we call a frame shift mutation
-A frame shift mutation is when all of the codons are moved over
-Inversion is when a segment of DNA gets cut out and then reinserted backwards
-Translocation is when a segment of DNA is cut out and then inserted into a different chromosome
We will talk more about mutations during the next section on gene expression and see how they affect the organisms
