Sex determination in human beings Archives

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 arteries FACTORS AFFECTING TRANSPIRATION BIOLOGY SSS 2 SCHEME OF WORK Scheme of work DENTAL DISEASES