This, again, requires us to look at offspring resulting from a particular cross. Sometimes information is given in a genetic pedigree. Figure below shows a genetic pedigree of a family over three generations for the trait of ‘widow’s peak’. In this pedigree:
• squares show males, circles females
• shaded symbols are ‘affected’ (have widow’s peak)
• W – dominant allele for widow’s peak
• w – recessive allele for no widow’s peak
• a horizontal line between two individuals represents a marriage
• vertical lines show parents/children
In this example, all the genotypes are shown, but it is possible to work out genotypes from a pedigree. Look at the sisters in the third generation. Thier parents both had widow’s peak, but one of the sisters doesn’t. Even if you weren’t told which was dominant, you could use what we learned in the previous section to work it out. Th only way two parents showing one feature (the sisters’ parents) can have children showing the alternate feature (the sister with no widow’s peak) is if the parents are heterozygous. So they are heterozygous – they have both alleles. But they have widow’s peak. This must mean that the widow’s peak allele is dominant.
What patterns do we get if we consider the inheritance of two genes at the same time?
Ths is known as dihybrid inheritance. When Mendel considered the inheritance of two characteristics at the same time, he followed essentially the same procedure as with his monohybrid experiments.
In one investigation, he bred plants that were homozygous for round, yellow seeds with plants homozygous for wrinkled, green seeds.
All the F 1 plants had round seeds which were yellow, showing that these alleles were dominant over the wrinkled and green alleles.
He then allowed these plants to self-fertilise themselves. In the F 2 generation, the other two features re-appeared, but in new combinations.
Th four phenotypes that appeared in the F2 and their proportions were:
• round and yellow 9
• round and green 3
• wrinkled and yellow 3
• wrinkled and green 1
Th behavior of the alleles in gamete formation illustrates Mendel’s law of independent assortment.
Th F1 plants produce gametes containing an allele for each feature. But they are not linked in any way. When a gamete is formed containing an R allele, there is a 50% chance that it will also contain a Y or a y. This results in the four different types of gametes formed by the heterozygotes and the 16 possible combinations (some of them the same) in the Punnett square, giving the 9:3:3:1 ratio. Remember this ratio; it is always found in a dihybrid cross where the parents are heterozygous for both traits. Also, if you have to construct a Punnett square for a dihybrid cross, always write both sets of gamete genotypes in the sequence (let’s use alleles A and a with B and b) AB, Ab, aB, ab. This will then place the different genotypes in the same location in the square every time. Just think in triangles!
Th nine individuals with both dominant alleles form a big triangle and are represented in blue. Th three with the fist dominant allele only (in our case, A) form a triangle represented in green. These with the second dominant allele only (B) form a small triangle, shown in pink, and, finally, those with two recessive alleles are tucked away in the bottom right-hand corner.
Is there a dihybrid test cross procedure?
There most certainly is! To discover the genotype of a plant whose phenotype shows both dominant features (we will stick with round seeds and yellow seed colur as our example), again a breeding experiment must be carried out. There are more possibilities than in monohybrid inheritance. Th plant could be:
• homozygous for both features – RRYY
• heterozygous for both features – RrYy
• heterozygous for one feature but not the other – RRYy or RrYY Following breeding with a double recessive type (rryy) there are the following possible outcomes:
• If the plants produced show all four possible phenotypes, the original was heterozygous for both features.
• If the plants produced all had round, yellow seeds, the original was homozygous for both features.
• If the plants produced all had rounds seeds but some had green and some had yellow seeds, the original was heterozygous for seed color only.
• If the plants produced all have yellow seeds but some are round and some wrinkled, the original was heterozygous for seed shape only.
Are alleles always simply dominant or recessive?
Th short answer to this is no. Sometimes alleles are codominant; the two alleles of a gene are both equally dominant and so, in the heterozygote, both are expressed. An example of this is found in the flwer color of snapdragons. This is controlled by a single gene with two alleles:
• R – determines red-colored petals
• r – determines white-colored petals Th possible genotypes are:
• RR – plants with red flowers
• rr – plants with white flowers
• Rr – plants with pink flowers; both alleles still express themselves and some red pigment and some white pigment is produced, resulting in pink flowers
If a red-flowered plant and a white-flowered plant are cross-bred, all the offspring will be heterozygous and have pink flowers. If two heterozygotes are crossed, the same genotype ratio (1:2:1) is obtained as with any monohybrid cross, but instead of a 3:1 phenotype ratio, this is also 1:2:1 (1 red:2 pink:1 white).