Let’s stick with the gene for earlobes. It has two alleles – free earlobes and attached earlobes. Everyone has two alleles for this feature in all their cells (except the sex cells). A person could have:
• two attached earlobe alleles per cell (homozygous for the recessive allele)
• two free earlobe alleles per cell (homozygous for the dominant allele)
• one attached allele and one free earlobe allele per cell (heterozygous)
We can represent this process diagrammatically. If we use symbols for the different alleles: E to represent the dominant allele for free earlobes e to represent the recessive allele for attached earlobes. Suppose a man homozygous for free earlobes and a woman homozygous for attached earlobes have a child. Th possible offspring are shown in table below
We would expect three out of four children to have free earlobes and one out of four to have attached earlobes – a ratio of 3:1. Put another way there is a 75% or ¾ probability that any particular child will have free earlobes and a 25% or ¼ probability that the child will have attached earlobes.
Remember this ratio … It occurs in all organisms where two heterozygotes cross-breed.
So now we can answer our question ‘how can parents showing one version of a feature have children with the other version?’. If both the parents show the feature determined by the dominant allele, but are heterozygous, they can produce children that show the feature determined by the recessive allele.
Remember this too … It is an important idea in solving genetic problems. Back cross/test cross an organism showing the dominant trait is bred with one showing the recessive trait. The results allow determination of whether the original organism showing this dominant trait was homozygous or heterozygous.
But the ratios don’t always work out in the real world? What genetic diagrams show are probabilities that a certain genotype or phenotype will be produced. For instance, in the cross between two heterozygotes we predict that one quarter, 25%, will be homozygous recessive and will show the feature determined by the recessive allele. But a moment’s thought will make you realize that it is only a prediction and may not be realized in any particular situation. You might predict that with any toss of a coin there is a 50% chance of the coin landing head side up and if you tossed a coin ten times you would predict fie heads and fie tails. However, if you actually did it, you could easily get seven heads and three tails or four heads and six tails. But if ten people tossed a coin ten times, it would probably come close to 50 heads and 50 tails and even closer to 500 heads and 500 tails if one hundred people tossed a coin ten times. Predicted ratios from genetic diagrams are only likely to be realized with large numbers of offspring. When the numbers are small, the laws of chance have a disproportionate effect.
How could you find out if an organism showing the trait determined by a dominant allele is homozygous or heterozygous?
After all, all tall pea plants are tall and all unattached earlobes are unattached. It makes no difference to the appearance whether the organism is homozygous or heterozygous. Th only possible way is to carry out a breeding experiment. However, this is not possible with humans, so we must look at any children they may have, or perhaps gain clues from their parents. But fist, let’s look at the breeding experiment. Th particular experiment to fid out if an organism is homozygous or heterozygous for a dominant trait is called the test cross or the back cross. Let’s again use the flower color in pea plants as our example.
If we breed the purple-flowered plant with a plant whose genotype is definitely known, we can make predictions about the outcome. This can only be a white-flowered plant, which must have two recessive white alleles. There are then two possible outcomes.
So, if we were to carry out the cross and find that any of the offspring had white flowers, we could be certain that the original purple parent was heterozygous. We know this because any white-flowered offspring would need to inherit two recessive ‘white’ alleles – one from each parent. If all the offspring were purple-flowered, we could be almost certain that the purple-flowered plant was homozygous. But because predicted ratios aren’t always met in the real world, we couldn’t be absolutely certain.
Remember … When fertilization occurs, any type of male sex cell could fertilize any type of female sex cell; it is a random process. Again, we work out the possible genotypes in the children by drawing a Punnett square.