There are two loci that influence coat colour in horses. The first locus black (B) is dominant to chestnut (b). The second locus possesses a gene that masks all others and is referred to as dominant white (W). Dominant white is lethal during gestation when homozygous. When found in a homozgygus recessive form (ww) colour from the B loci is expressed. What would be the genotypic and phenotypic ratios for the following matings
a. BBww × BbWw
b. Bbww x bbww
c. BbWw x bbWw
Soulution:
First locus- The black 'B' dominant to chetnut 'b'. BB and Bb produces Black coat , bb produces chestnut coat.
Second locus- White 'W' dominant to all other alleles. If 'W' presents no other colour express, and all phenotypes will be white. Only in 'ww' homozygous recessive condition B loci (first locus) is expressed
a.BBww x BbWw
Bw | Bw | Bw | Bw | |
BW | BBWw | BBWw | BBWw | BBWw |
Bw | BBww | BBww | BBww | BBww |
bW | BbWw | BbWw | BbWw | BbWw |
bw | Bbww | Bbww | Bbww | Bbww |
Genotype ratio
BBWw : BBww : BbWw ; Bbww = 3 : 3 : 3 ; 3 i.e 1: 1 : 1 : 1
Phenotype ratio
BBWw - White
BBww - Black
BbWw - White
Bbww - Black
White : Black = 6 : 6 i.e 1 : 1
b. Bbww x bbww
Bw | Bw | bw | bw | |
bw | Bbww | Bbww | bbww | bbww |
bw | Bbww | Bbww | bbww | bbww |
bw | Bbww | Bbww | bbww | bbww |
bw | Bbww | Bbww | bbww | bbww |
Genotype ratio
Bbww : bbww = 8 : 8 i.e 1:1
Phenotype ratio
Bbww - Black
bbww- Chesnut
Blck : Chesnut = 8 : 8 i.e 1 : 1
c. BbWw x bbWw
BW | Bw | bW | bw | |
bW | BbWW | BbWw | bbWW | bbWw |
bW | BbWW | BbWw | bbWW | bbWw |
bw | BbWw | Bbww | bbWw | bbww |
bw | BbWw | Bbww | bbWw | bbww |
Genotype ratio
BbWW : BbWw : Bbww : bbWW : bbWw : bbww = 2 : 4 : 2 : 2 : 4 :2 i.e 1 : 2 : 1 : 1 : 2 : 1
Phenotype ratio
BbWW - White
BbWw - White
bbWW - White
Bbww - Black
bbWw - white
bbww - Chestnut
White : Black : Chestnut = 12 : 2 : 2 i.e 6 : 1 : 1
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