Which phoenomenon gives 2 : 1 ratio instead of 3 : 1 ratio? Describe with graphical representation.
A pea plant homozygous for yellow round seed is crossed with its recessive parents. Calculate the phenotypic and genotypic ratio with the help of checker board.
In Pleiotropy the ratio is 2 : 1 instead of 3 : 1.
For example, the disease, sickle-cell anaemia, is caused by a gene HbS. Normal or healthy gene HbA, is dominant. The carries (heterozygotes – HbA/HbS) show signs of mild anaemia as their RBCs become sickleshaped (half-moon-shaped) in oxygen deficiency. They are said to have sickle-cell trait and are normal in normal conditions. The homozygotes with recessive gene HbS, however, die of fatal anaemia. Thus the gene for sickle-cell anaemia is lethal in homozygous condition and produces sickle-cell trait in heterozygous carrier. Two different expressions are produced by a single gene and this is example of pleiotropy. (A gene which causes death of the bearer is called lethal gene) A marriage between two carrier will produce normal, carriers and sickle-cell anaemia children in 1 : 2 : 1 ratio.
Sickle – cel anaemics die leaving carriers and normals in the ratio 2 : 1.
Mendel started with pure lines. Therefore, genotype of parent with yellow round seeds in YYRR and that of parent with green wrinkled seeds is yyrr. Both the parents are homozygous. Therefore, they would produce only one type of gamete i.e. YR and yr respectively. All F1 dihybrid seeds resulting from the cross will be heterozygous for both the traits and with genotype YyRr. Due to dominance, all the seeds of F1 generation will be yellow round.
Mendel allowed selfing of F1 dihybrids. During gamete formation by the dihybrids, the alleles in both the pairs separate (law of segregation). Each gamete will receive only one allele from each pair. A gamete that receives ‘Y’ for colour may receive ‘R’ for shape or ‘r’ for shape. This would result in the formation of YR and yr types of gametes. Similarly, a gamete that receives ‘y’ for colour may receive ‘R’ or ‘r’ for shape. This would result in formation of yR and yr types of gametes (Independent assortment). Thus F1 dihybrid would produce four different types of gametes in equal proportion. (Each type will be 25%).
There would be four types of male gametes and four types of female gametes and random fusion will take place during selfing. Due to this chance fusion (4 x 4 = 16) sixteen combinations which fall into nine categories as shown in Punnett square are possible.
The nine different genotype are ; YYRR, YYRr, YyRR, YyRr, Yyrr, yyRR, yyRr and yyrr.
The genotypic ratio is 1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1. Due to dominance there are only four phenotypes. The phenotypic ratio 9 : 3 : 3 : 1, is called dihybrid ratio.