Patterns of inheritance 2
Joanna George-Johnson, Contributor

Norman Manley High School players hoist Kenneth Walters after their 24-8 victory over Jose Marti Technical in the final of the U-19 Schoolboy Rugby League at G.C. Foster College. Walters was voted Most Valuable Player for the final with his two second-half tries. - Photo by Anthony Minott

Last time, we looked at the terms that are normally used when studying patterns of inheritance. It is my hope that they are now well understood. And, with this hope, we will now look at how inheritance takes place.

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

Traits

Every trait or characteristic that an organism possesses is controlled by what we call genes. Therefore, as organisms, all of our traits are controlled by genes. As we learned last time, genes are a section of our DNA, and DNA is the major component of our chromosomes in our cells.

We also have learnt that we have 46 chromosomes in most of our cells or 23 pairs. However, in our gametes (sperm/egg) we have 23 chromosomes. (N.B. Not pairs, but 23 single chromosomes.)

• For each human to be conceived, one sperm must fertilise one egg.
  
  
• When this happens a zygote is formed.
  
  
• This zygote contains the combination of all of the chromosomes from each of the original cells. (sperm and egg)
  
  
• This is the explanation for the 46 chromosomes in most body cells. (23 from the sperm and 23 from the egg)
  
• Therefore, all traits are controlled by at least one pair of genes.
  
  
• On each chromosome, there are a number of different genes.
  
  
• These genes are the same on that particular chromosome throughout the species. For example, if chromosome number 5 contains the gene for eye colour, then chromosome number 5 will contain the gene for eye colour in all humans, no matter the race to which they belong. Also, the position of the gene for eye colour will always be the same throughout the species.

Genes and alleles

• Each gene has alleles (alternate forms of the gene). Some have two and others may have more.
  
  
• At this level, we only look at genes with two alleles.
  
  
• Since students tend to get confused here, I will attempt to clarify this for you. If, for example, there were a cellphone gene, two possible alleles could be 'flips' and 'sliders' as you may understand they are both cellphones, but are of different styles. So it is with genes and alleles.
  
  
• However, there is another feature to comprehend and it is that of either being dominant or recessive.
  
  
• Some alleles are dominant over others.
  
  
• This means that if a cell contains even one copy of the dominant allele, then that particular version of the gene is what will be portrayed.

For example, if brown eyes are dominant over blue, and an individual's cell contains the allele for brown on one of their chromosome '#5' and the allele from blue on the other chromosome '#5' that individual's eyes will be brown.

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 simple types of inheritance.

Insides determine outsides

Let B represent the allele for brown

Let b represent the allele for blue

• Remember that there are two copies of every gene in a typical cell.
  
  
• Therefore, an individual with brown eyes (phenotype) could have one of the following genetic compositions for eye colour (genotype), 'BB' or 'Bb'.
  
  
• But if an individual had blue eyes (phenotype), then the only possible genetic composition will have to be 'bb'.

Points to remember

• As long as the dominant allele is a part of the genotype, then that is the trait that will be expressed (seen).
  
  
• The only way for a recessive trait to be expressed (seen) is if both chromosomes with the gene carry the recessive allele.

Questions:

1. In terms of genotype, describe when a recessive phenotype can be expressed?

2. In genetic terms, name the two types of homozygous situations that can be found.

3. In a typical dominant/recessive situation, describe the phenotype of the heterozygote. Illustrate further with an example.

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