COMPLETE AND INCOMPLETE DOMINANCE

Biology

COMPLETE AND INCOMPLETE DOMINANCE

Complete 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.
See also  KINGDOM PLANTAE

Incomplete Dominance

  • 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.
See also  ASEXUAL REPRODUCTION

The ABO Blood Group System

Rhesus factor

  • 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.
See also  STRUCTURE OF A SEED

See also:

STRUCTURE OF DNA

STRUCTURE AND PROPERTIES OF CHROMOSOMES

GENETICS

Growth and Development in Animals

ROLE OF GROWTH & HORMONES IN PLANTS

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