Patterns of inheritance 3
Joanna George-Johnson, Contributor

Members of the Campion College team compete against Jamaica College in the 2005 Schools' Debating competition at CPTC, Arnold Road, on April 14, 2005. - Ian Allen/Staff Photographer

The last lesson focused on traits and how they are expressed. This week, we will be looking at how they are passed on to the next generation. Please note that inheritance and genetics are a very complex area of study. Generally, for our purposes we tend to simplify things for ease of understanding. Therefore, I do not want you coming away from this lesson feeling that what you have learnt can be used to determine paternity, etc. Paternity testing is much more complicated than what you will ever learn at CSEC level biology.

How are genes passed on to the next generation?

Recently, we learned that there are two copies for each gene in a somatic cell. Let's suppose that eye colour is controlled by one gene with one pair of alleles (blue and black), where black is dominant over blue. Let's see what the possibilities for eye colour would be for the children of two individuals who were heterozygous for black eyes.

• A Punett square is used to visualise all the possible combination of alleles from the parents.
  
  
• Dominant alleles are symbolised by capital letters and the recessive ones will be symbolised by common letters.

With this in mind, let us do the cross for the aforementioned individuals.

This cross suggests that these parents have a one in four chance of having a baby with blue eyes, but a three in four chance of having a black-eyed baby.

Let's do a next one:

Let's suppose that eye colour is controlled by one gene with one pair of alleles (blue and black), where black is dominant over blue. Let us see what the possibilities are for the child of two individuals, where one is heterozygous for black eyes, and the other has blue eyes.

(Remember, since blue is recessive, a person who has blue eyes must be homozygous, that is, bb.)

Let: B represents black
b represents blue

Parental phenotypes heterozygous X heterozygous

Black Blue

Parental genotypes Bb bb
(2n genes in a somatic cell)

This cross suggests that these parents have a 50 per cent (or 1 in 1) chance of having a baby with blue eyes or black eyes.

* Eye colour inheritance in humans is much more complicated. As we all know, humans have many more eye colours. The above is just an explanation for a simple type of inheritance.

Questions:

1. Long hair in dogs is dominant over short hair. If you are a dog breeder, and you were sold a long- haired dog that you were told is pure breeding, what could you cross it with to determine whether this is true? Give the result if your dog was of pure breed and give the result if your dog was not of pure breed.

2. The following Punett square shows the possible combinations of alleles for fur colour in rabbits. Black fur, B, is dominant over white fur, b. 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.