COMPLETE AND INCOMPLETE DOMINANCE
- Mendel happened to choose characters that showed complete dominance,
- e. the dominant trait completely masked the recessive one in the F1 generation.
- In man, certain characters are inherited in the same way
- g. colour of the skin; normal colour is dominant to albinism (lack of skin pigment).
- 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.