Let us discuss some terminology and notation. If you collect fruit flies or Drosophila melanogaster in the wild, they will always have red eyes. And so phenotypes that occur naturally in the wild are called wild type phenotypes.
However, you can find alleles or mutants that do not show the phenotype that you see in the wild. For example, there are flies that have purple eyes instead of red eyes and these mutants can arise either naturally so they could be spontaneously occurring in in the wild very rarely or you can create mutants by treating flies with a mutagen so you could either expose them to x-rays or feed them this chemical called ethyl methanosulfate EMS that causes mutations. Now a word about notation so sometimes although it's not always the case you denote wild type alleles with the plus symbol.
So the plus symbol doesn't mean that it's dominant or recessive. It doesn't tell you anything about the phenotype except for the fact that this allele or this phenotype tends to occur in the wild or is the wild type phenotype. So now let's move on to Thomas Hunt Morgan's experiment where he was tracking the inheritance of two traits.
One is the eye color and there are two alleles, the wild type allele PR plus that gives red eyes which is the most common allele that you would find in flies from the wild or PR which gives purple eyes. The mutant allele is recessive and therefore you have to be PR over PR in order to have purple eyes. The other trait that Morgan was tracking was vestigial and the wild type allele VG plus you have normal looking wings however in VG over VG flies so vestigial homozygous recessive flies you have such vestigial or small wings. Morgan carried out a series of crosses where he took a pure breeding wild type line and that means they were homozygous for the plus alleles of both purple and vestigial and he crossed this to a fly that was mutant homozygous recessive for the mutant allele for both purple and vestigial and such an individual is sometimes called a double mutant and evidently the wild type individual will produce only one type of gamete which is purple plus VG plus and also since the double mutant is homozygous for the mutant alleles of purple and vestigial they will also produce only one type of gamete purple vestigial and fertilizing these two gametes gives us a dihybrid individual who is heterozygous for both purple and vestigial and next Morgan carried out a test cross, crossed them to an individual who was homozygous for the recessive alleles for both the traits and Then we can work out the genotypes of the gametes this dihybrid individual will produce, as well as the genotypes and the proportion of those genotypes in the progeny of this test cross. And the gametes this dihybrid individual will produce are purple plus, vestigial plus, purple, Vestigial, purple plus, vestigial.
purple vestigial plus. The tester individual will only produce one type of gamete which is purple vestigial. And that's the benefit of doing test crosses since the progeny will follow the phenotype of whatever alleles they received from the dihybrid parent. And therefore, This first progeny from the fertilization of the purple plus vestigial plus gamete with the purple vestigial gamete will be a wild type for both the traits that means they will have red eyes and normal wings. The second type of progeny have both the mutant alleles in the gamete from the dihybrid parent and therefore they will have purple eyes and vestigial wings or let's say to be consistent small wings.
The third type of progeny will have red eyes and small wings. And the last type of progeny will have purple eyes and normal wings. these traits assort independently according to Mendel's law of independent assortment we also expect that 25% or a quarter will be purple plus and vestigial plus 25% will be purple and vestigial and so on furthermore We also know that the parents in the parental generation had the genotypes purple plus and vestigial plus and the other parent was purple and vestigial and therefore these are the parental genotypes and These guys, purple plus vestigial along with purple and vestigial plus are new combinations that have been created as a result of independent assortment and therefore they are the recombinant genotypes. Now these phenotypic proportions are predictions or are expectations if the genes, the traits were assorting independently. However, when Morgan performed the experiment, the phenotypic proportions he got are shown in this table and he got very different results.
than what is predicted by independent assortment. The total number of individuals was around 3000 and therefore for red eyes and normal wings, the parental genotype red eyes and normal wings, Morgan obtained 1339 individuals or about 45%. The other parental genotype, purple eyes and small wings, Morgan obtained about the same 1200 individuals, so around 45%.
For the recombinant genotypes, red eyes and small wings. he obtained 151 individuals, which is about 5%. And for the last genotype, the secondary competent genotype, purple eyes and normal wings, Morgan again obtained 154, or about 5% individuals. And in these data, it's clear that recombinant genotypes are much rarer than what is predicted by independent assortment. And in some sense, purple plus likes to be with vestigial plus and purple prefers to be with vestigial.
In other words, these two genes are linked since the purple plus vestigial plus combination is more abundant than would be expected by independent assortment. And the reason these two genes are linked as it happens is that they happen to lie on the same chromosome. And so one parent has purple plus and vestigial plus and has homozygous.
for those alleles while the other parent has purple and vestigial alleles, the mutant alleles and is homozygous for them and this is the parental generation and when this parent or these parents undergo meiosis. They're going to produce after these chromosomes segregate only one type of gamete carrying the chromosome with purple plus and vestigial plus or the other parent will create gametes that have the chromosome with the purple and vestigial mutant alleles. And when these two gametes undergo fertilization, they will produce the dihybrid individual who will look like this. They will have one chromosome with purple plus and vestigial plus alleles and they will have a second chromosome which they received from the other parent which will have the purple and vestigial mutant alleles.
And just to be clear, these are the gametes produced by the parental generation and this is the dihybrid individual produced in the F1 generation. And when this F1 individual undergoes meiosis to produce their gametes. One type of gamete will have the wild type alleles and the other type of gamete will have the mutant alleles and these will be present in roughly half the gametes and these will also be present in roughly half the gametes and in Morgan's data we saw that about 45 percent were the first genotype and another 45 percent were the second genotype and therefore the fact that purple and vestigial genes lie on the same chromosome, that is they are linked, explains why instead of obtaining a one is to one is to one is to one ratio of gametic genotypes as would be expected by independent assortment, we instead obtain progeny in ratios that favor the parental genotypes overwhelmingly.