Question

Geneticists love to solve detective stories. Phenotypic ratios in progeny are one of their most valuable clues.

There are a series of

diagnostic

ratios

that help to define

the genotypes of the partners in a cross. The ratios tell

us how many genetic loci are involved, and the dominance relationships among alleles at a single locus or a

series of loci. What does each of the following progeny phenotypic ratios tell you abou

t the two parents: (1)

3:1 ; (2) 1:3 ; (3) 1:2:1 ; (4) 9:3:3:1 ; (5) 1:3:3:9 ; (6) 1:1 ; (7) 1:1:1:1 ; (8) 27:9:9:9:3:3:3:1 ?

Answer 1

1. if phenotypic ratio is 3:1 then it is monohybrid cross,one parent is homozygous for one allele and other parent is homozygous for other allele
2. 1:3 ratoi represent the pseudodominance monohybrid cross
3. if ratio is 1:2:1 then it is a incomplete dominance,both the parents are heterozygous having Aa allele,in this ratio 1 represent dominant phenotype 2 represent intermediate phenotype and 1 represent recessive phenotype
4. if phenotypic ratio is 9:3:3:1 then it is dihybrid cross,both parents contain heterozygous gametes that is AaBb
5. 1:3:3:9 represent pseudodominance dihybrid cross
6. if ratio is 1:1 then it is monohybrid test cross,one parent having dominant heterozygous allele that is Aa and other parent contain recessive allele that is aa
7. if ratio is 1:1:1:1 then it is dihybrid test cross then one parents contain heteozygous gametes that is AaBb and other parents contains homozygous recessive gametes aabb
8. if ratio is 27:9:9:9:3:3:3:1 then it is trihybrid cross