DETERMINATION OF THE GENOTYPE OF A DOMINANT PHENOTYPE
A dominant phenotype has the genotypic patterns RR, Rr. The genotype is determined using test cross or back cross
Test cross is the crossing of an organisms with an homologous recessive organism
Back cross is the crossing of an organism with an homologous recessive organism from the original parental generation
PRINCIPLE OF INCOMPLETE DOMINANCE
This deal with the ability of two contrasting alleles to interact and produce a heterozygous phenotype that is different from the two homologous phenotypes: Examples of organisms exhibiting incomplete dominance include: Mirabilisjalapa,4 o’clock plant, Audlausian fowl. This principle opposes Mendel’s principle of complete dominance.
Diagrammatically:
Parent genotypes: RR X rr
Phenotypes: (red flower) (white flower
Gametes: R R r r
Rr Rr Rr Rr (All pink flowers)
Self pollinating: Rr x Rr
R r R r
RR Rr Rr rr
Red Pink White
PR = GR = 1: 2: 1
CO – DOMINANCE
In co-dominance both alleles in the heterozygous individuals are fully expressed. The effect of one is not modified by the presence of the other. Therefore, three distinct phenotypes are produced e.g Inheritance of human “ABO” blood group
| Genotype | Phenotype (Blood group) |
| 1A1A or | Group A |
| 1A 1O | |
| 1B1Bor | Group B |
| 1B 1O | |
| 1A 1B | Group AB |
| 1O1O | Group O |
Allele 1A implies the addition of antigen A to the cell surfaces of red blood cells resulting in a person with group A blood. Likewise Allele 1B implies the addition of antigen B to the cell surfaces of red blood cells resulting in a person with Group B.
In a heterozygous individual, (1A 1B) both antigens A and B are added to the cell surfaces of red blood cells. So the individual has blood group AB.
NOTE: 1A and 1B are co-dominants while 1Ois recessive.
Multiple alleles
Genes that have more than two alleles in the population are said to have multiple alleles e.g. the human ‘ABO’ blood grouping
Sex determination in human beings
Each body cell of human beings has 23 pairs (46) of chromosomes, 22 of which are autosomes and a pair is sex chromosome. In male the two sex chromosomes in each body cell are X and Y chromosomes, therefore, each male gamete carries either X or Y chromosome. In the female, all egg cells of the body contain two X chromosomes. Therefore all egg cells contain one X chromosomes each. At fertilization, the combinations of an egg with a sperm carrying either X or Y chromosome occurs by chance. The formation of a male or female offspring has equal chances as shown below parents:
XY x XX
Gametes: X Y X X
XX XX XY XY
50: 50
GR = PR = 1: 1
Genotype Phenotype
(1) 2XX Girls
(2) 2XY Boys.
See also