Patterns of inheritance Pt IV
Joanna George-Johnson, Contributor

Jose Marti Technical's Brian Spence (left) tries to outpace the Norman Manley High duo of Derval Rodney (right) and Zavier Carr (background) during the final of the U-19 Schoolboy Rugby League at GC Foster College. Norman Manley High School won 24-8. - Anthony Minott/Freelance Photographer

Last week's lesson focused on how traits are passed on to the next generation. This week, we are looking at when traits do not follow the dominant recessive rule.

Please note that inheritance and genetics are a very complex and intricate line of study. Generally for our purposes, we tend to simplify things for ease of understanding. Therefore, I do not want anyone coming away from this lesson feeling that what he/she had learnt can be used to determine paternity, etc. Paternity testing is much more complicated than what you will ever learn at CSEC level biology.

Previously, we discussed dominant and recessive alleles. There are times, however, when some alleles do not follow rule of dominant alleles. The two main ways are:

• Incomplete dominance
• Co-dominance

Incomplete dominance

Flower colour in a species of pea plants is controlled by one gene with a pair of alleles. One day, a scientist was investigating the patterns of inheritance of flower colour in these plants. He got some red flower plants and crossed them with white flower plants. The surprising thing was that all the offspring came out to have pink flowers! He wondered if there were a new allele in this plant. So he self-pollinated these pink flower plants and got the following result:

From this he concluded that:

• There were only two alleles for this gene,
  
  
• But when the plant poses the heterozygous genotype (i.e. allele for red and allele for white) a new phenotype shows up.
  
  
• The new phenotype seems to be a partial expression of both alleles.
  
  
• The alleles showed only partial dominance. Hence, the term incomplete dominance.

With this in mind, let's do the cross for the aforementioned plants

Parental phenotypes

Random fertilisation

Possible phenotype RR* All Pink

From this cross, it can be seen that all offspring will be heterozygous and exhibit a pink phenotype.

Let's do the next one:

Parental phenotypes

Random fertilisation

Possible genotype of offspring (2n)

This cross suggests that these plants should produce offspring with all the possible phenotype in the ratio of 1 red to 2 pink to 1 white.

Questions:

1. How would you determine if a particular trait exhibits incomplete dominance?

2. The following punnett square shows the possible combinations of alleles for fur colour in rats. (Black = B, White = B* and Grey = heterozygous) It was found that the alleles exhibited incomplete dominance. Given the combinations shown, (a) What are the genotype of the parents? (b)What are the phenotypes of the parents?

Joanna George-Johnson teaches at Ardenne High School.
Email: Masterbio@gmail.com.