COMPLETE AND INCOMPLETE DOMINANCE
Mendel happened to choose characters that showed complete dominance, the dominant trait completely masked the recessive one in the F1 generation. In man, certain characters are inherited in the same way e.g. colour of the skin; normal colour is dominant to albinism (lack of skin pigment).
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The children are all normal but have the gene for albinism. Such individuals are referred to as carriers.
Other characters that show complete dominance in humans are:
- Ability to roll the tongue.
- Polydactyly (having more than 5 digits in one limb).
- Brachydactyly – having short fingers.
- Achondroplasia – dwarf with bow legs.
In this kind of inheritance there is no dominant or recessive gene but the two are expressed equally in the offspring, resulting in blending of the characters.
The gene for red colour (R) in cattle and the gene for white colour (W) show incomplete dominance or co-¬dominance. The offspring are neither red nor white but are intermediate between the two.
They are said to be roan.In humans, the sickle cell gene and the normal gene are co-dominant.
Inheritance of ABO blood groups in humans
- Blood groups in human are determined by three alleles, A, B, and O.
- An individual can have only two of these genes.
- Genes A and Bare co-dominant, while gene 0 is recessive to A and B.
- These are referred to as multiple alleles.
The ABO Blood Group System
- The Rhesus factor is responsible for the presence of a protein (Antigen D) in the red blood cells.
- If blood from a Rhesus positive (Rh+) person is transferred into a person without the Rhesus factor (Rh-);
- The recipients’ body produces antibodies against the Rhesus factor.
- This causes agglutination of red blood cells which can be fatal if subsequent transfusion with Rh+ blood is done.
Sex Determination in Humans
- XY type e.g. human male
- In males, two types of sperms are produced.
- Half of then containing X chromosomes and half Y chromosomes.
- During fertilisation only one sperm fuses with the egg.
- If it is an X-carrying sperm then a female zygote is formed;
- If it is a Y-carrying sperm then a male zygote is formed.
- It follows then that the chances of getting a boy or girl are half or fifty-¬fifty.
- Note also that it is essentially the type of sperm that fertilises the egg that determines the sex.