Incomplete dominance
AVERY good day to you all! How are you? What a wonderful time we are going to have togther! Guess what? If you waste it, you will never get another chance to make up for the wasted hours, so use them wisely.
Genetics is fun if you understand what is being done and if you practise as you go along. It can be easy to learn but also easy to forget, so stay connected!
This week, we will look at the phenomenon known as incomplete dominance. So far we have looked at two types of dominance, complete and co-dominance. Do you remember what these are? Of course you do! In complete dominance, one allele is dominant over the other and crossing the heterozygotes results in a phenotypic ratio of 3:1. In co-dominance, two dominant alleles come together and both are expressed in the phenotype, for example, the blood group AB. In incomplete dominance, this does not happen. Whenever the dominant and the recessive alleles come together, a third phenotype is produced. This will result in a phenotypic ratio of 1:2:1. Let us see how this happens.
This phenomenon is shown in the flower colour in the plant, Impatiens. The flower colour is expressed in 3 phenotypes, red, pink and white. There is an allele that represents red colour and one that represents white. We can use the symbol R to represent the red allele and we can use the symbol r to represent the white allele. Let us look at what happens when a red flower is crossed with a white. (See Figure 1) Genotypic ratio: All heterozygotes - Rr
Phenotypic ratio: Here is where the surprise comes, these flowers instead, of being red, are pink! So our phenotypic ratio would be: All pink. Let us see what happens when we cross the F1, the heterozygote. (See Figure 2)
What is our genotypic ratio? Is your answer 1:2:1? If it is, then you are correct! What is the phenotypic ratio? If this were a normal case, then it would be 3:1 or three red to one white, but this is not the normal case. Remember the phenotype of the parents? Yes, you do. It was pink! So, knowing this, can you work out the phenotypic ratio? Of course you can. It is like the genotypic ratio, 1:2:1 or 1 red (RR): 2 pink (Rr):1white (rr). If you are given these results in an exam and are asked to state the condition, then you would be able to name it as incomplete dominance. That completes our study on dominance; not too bad, was it? We switch now to another type of diagram used in genetics. This is the pedigree chart. What is a pedigree chart? A pedigree chart is used to show how a trait is passed on or transmitted in a given family. It can be used to show how the trait is passed on and also to predict the probability of the genotype of future offspring. In constructing a pedigree chart, certain symbols and conventions are used: Symbols are used to represent people and lines are used to represent relationships. Males are represented by squares and females are represented by circles. Horizontal lines connecting a male and female represent mating. Vertical lines leading downwards from the horizontal line represent the offspring.
The symbols at the top of the chart represent the oldest individuals of the generations. Subsequent generations are written underneath the first parental generation.
When analysing the pattern of inheritance of a particular trait, it is customary to shade in the symbol of all individuals that possess this trait. Below is a very simple example of a pedigree chart. Notice the following: There are two offspring produced in the F1, one male (square) and one female (circle). In the F1, the male has inherited the trait. The male with the trait has mated with a normal female There are four offspring in the F2, three males and one female.
Three of the offspring, two males and one female do not have the trait while one male has the trait.
Pedigree charts are sometimes given in the exams, so learn the conventions. Can you construct one for your family? See you next week!