Genetics and variation - Part two
Joanna George-Johnson, Contributor

The last time we familiarised ourselves with some of the crucial terms employed in the study of genetics and variation. Now we will be taking a more practical, hands-on approach to the topic. We will examine how traits move from one generation to the next and the mechanism of sex determination in humans.

The characteristics of the offspring were determined by the genotype of both of their parents. You can appreciate that some persons show similar traits to their parents; however, they are never identical. This is because at least two pairs of alleles are involved in determining the characteristics of an individual at all times!

At the Caribbean Secondary Education Certificate level, we deal with the inheritance, that is the result of a single pair of genes, known as monohybrid inheritance. It is possible to determine the possible outcome of an organism if the genotype of the parent is known. In order to do this, however, a Punnet square is done. Do not get frightened by the term, Punnet, it just refers to the scientist who came up with the technique. A Punnet square is a diagram used to predict the outcome of a particular cross or breeding experiment. It shows all the possible gene combinations when one maternal allele comes together with a paternal allele.

Monohybrid inheritance

Let us now examine the monohybrid inheritance. If a man is homozygous dominant for right-handedness and his wife is homozygous recessive for the trait, the chances of him having a left handed child can be shown by doing a Punnet square. Please note that right-handedness is the dominant trait, while left-handedness is the recessive trait.

Remember that you should always choose one letter to work with to represent a single pair of alleles. Therefore, we can let H represent right- handedness and h represent left-handedness.

The man's genotype is HH, while his wife's is hh. Remember that sperm and eggs only have one copy of each chromosome. Therefore, all of the man's sperm will have the H allele. And all the eggs of the wife will have the h allele. The Punnet square can now be constructed.

We are going to put the alleles for the wife in the top horizontal cells and the alleles for the husband in the left vertical cells as shown below.

		Alleles of the eggs
		h	h
Alleles of the sperm	H	Hh	Hh
	H	Hh	Hh

Now you can see that all of the offspring are heterozygous for the trait and remember that in all heterozygous situations, the dominant trait will always be expressed in the phenotype; hence, all their children will be right-handed.

Let us not try another cross. This time, where both of the parents were heterozygous for the trait, handedness. Therefore, the genotypes would be Hh and Hh. The results are shown in the table.

In this example, it is seen that all the possible genotype combinations are produced. That is, the homozygous dominant, homozygous recessive and the heterozygous condition for the trait. Therefore, a child from this union could be right handed or left handed. The ratio of the genotypes, HH : Hh : hh would be 1:2:1, however, the phenotypic ratio is different. From the Punnet square, there were three cases of right handedness and one case of left handedness; hence the phenotype of right handedness to left handedness is 3:1. Therefore, there is a 75 per cent chance that any one of their offspring will turn out right handed and a 25 per cent chance that any one of their offspring will turn out left handed.

		Alleles of the eggs
		HbA	Hb5
Alleles of the sperm	HbA	HbAHbA	HbA Hb5
	Hb5	HbA Hb5	Hb5 Hb5

Now do not get deceived! This does not mean that if the couple had four children that one of them will be left handed. All it means that there is a 25 per cent chance they will have a left handed child each time they conceived. Actually, the parents in the previous example could have produced three offspring that were all left handed!

Sickle cell anaemia

The sickle cell disease is a hereditary disease, it is based on the genotype of our parents. Sickle cell causes the red blood cells to change shape, instead of being flexible and disc-shaped, these cells are more stiff and curved in a shape similar to a crescent moon.

Red blood cells with normal haemoglobin (haemoglobin A, or HbA) move easily through the bloodstream, delivering oxygen to all of the cells of the body. Normal RBCs are shaped like discs or doughnuts with the centres partially scooped out and are soft and flexible. They can easily squeeze through even very small blood vessels.

Sickle cell anaemia occurs because an abnormal form of haemoglobin (HbS) is produced. HbS molecules tend to clump together, making red blood cells sticky, stiff, and more fragile, causing them to form into a curved, sickle shape. The diagram below shows the normal red blood cells in comparison to the sickle cells.

Figure 1- Diagram showing sickle cells and red blood cells.

Now, the disease is only represented when the individual is homozygous recessive for the trait. However, if they are heterozygous, then the person is said to have a trait, or is a carrier. So by letting HbS represent a sickle cell and HbA represent a normal cell, we can do a Punnet square to determine what the genotype and phenotype of having a carrier father and a carrier mother will be.

Mother's Genotype: HbAHbS

Father's Genotype: : HbAHbS

		Alleles of the eggs
		HbA	Hb5
Alleles of the sperm	HbA	HbAHbA	HbA Hb5
	Hb5	HbA Hb5	Hb5 Hb5

Therefore, there will be a 25 per cent chance that the offspring will be normal a 50 per cent chance that the offspring will be a carrier and a 25 per cent chance that the offspring will have the sickle cell disease.

The diagram below is a representation of the passing on of the sickle cell disease.

N.B. Sickle cell disease is a genetic disease that does not follow the usual dominant recessive process we have been studying. A person who is a heterozygous (carrier) for the sickle trait does have symptoms of the disease. Thus this form of inheritance is called incomplete dominance, where the alleles are only partially dominant over each other.

Questions

1) What is meant by the terms

a) Genetic cross?

b) Punnet square?

c) Monohybrid inheritance

2) What are the chances that a male heterozygous tongue roller and a female homozygous non-tongue roller, will produce a non-tongue roller offspring? Illustrate your answer using a Punnet square. Note: The tongue roller is the dominant trait.

3) John's father is right handed, his mother is right handed, however John is left handed. Explain why this may be so.

4) Differentiate in terms of physical characteristics, between a red blood cell and a sickle shaped cell.

5) Why is it not advisable for two parents who have the sickle cell trait to have children?

Kasey Carr uses a compass during a math class at Bridgeport High School, recently. - Anthony Minott/Freelance Photographer	Science teacher Sherline Lee-Booth explains something to this student during a class at Bridgeport High recently. - Anthony Minott/Freelance Photographer

Mr T Campbell, a mathematics teacher at Bridgeport High School, helps Kasey Carr (right), with a geometry problem during a maths class at the school recently. - Anthony Minott/Freelance Photographer

Joanna George-Johnson teaches at Ardenne High School.