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DETERMINATION OF THE GENOTYPE OF A DOMINANT PHENOTYPE

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

See also  DEVELOPMENT OF NEW ORGANISMS

 

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:

See also  IMPORTANCE OF VACCINATION

 

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

arteries

FACTORS AFFECTING TRANSPIRATION

BIOLOGY SSS 2 SCHEME OF WORK

Scheme of work

DENTAL DISEASES

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