ABO blood grouping of humans is an example of co-dominance. Homozygotes (IAIA and IBIB) express either the A or the B phenotype.
Heterozygotes (IAIB) express both phenotypes equally and the IAIB individual has blood type AB. Blood type O is the result of the genotype IOIO: this blood type is considered the recessive trait and IO is the recessive allele for blood type. Imagine a genetic cross between two people, blood type AB. What is the probability that these people will have offspring with type B blood? A) 0% B) 10% C) 25% D) 50%
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Answer: The genotype ratio of offsprings of a cross between one homozygous tall plant and one heterozygous tall plant is 2TT:2Tt
The phenotype ratio is that all offsprings will be tall.
The genotype ratio of a cross between two heterozygous red flowers is 1RR:2Rr:2rr. The phenotype ratio is 3 Red flowers: 1 white flower.
Explanation: From the information given, tall is dominant and short is recessive while red is dominant and white is recessive.
Let T represent the allele for tall height, t represent the allele for short height, R represent allele for red flowers and r represent allele for white flowers.
The genotype of a homozygous tall plant is TT, the genotype of a heterozygous tall plant is Tt, the genotype of a heterozygous red flowered plant is Rr.
The possible genotypes of offsprings of a cross between one homozygous tall plant and one heterozygous tall plant are TT and Tt.
Tt will manifest outwardly as tall because T is dominant over t. Therefore, all the offsprings will be tall.
The possible genotypes of a cross between two heterozygous red flowers are RR, Rr and rr. RR will manifest as red, Rr will also manifest as red because R is dominant to r while rr will manifest as white because white allele (r) is recessive. Therefore 3 plants will be red-flowered while 1 plant will be white flowered.
See the attached punnet squares for details of the cross.
