When gregor mendel crossed true-breeding tall plants with true-breeding short plants, all the offspring were tall because a. the allele for tall plants is recessive. b. the allele for short plants is dominant. c. the allele for tall plants is dominant. d. they were true-breeding like their parents.

C (The allele for tall plant is dominant)

Explanation:

Gregor Mendel was an Austrian monk born on 20th July, 1822 and died in 1884. He discovered the principles governing heredity. One of his principles is the Law of dominance, which states that in an heterozygous state, one trait will conceal the presence of another trait for the same characteristics.

Relating to this law, Mendel discovered that a gene exists in more than one form called ALLELE, inherited from each parent.

He worked with height in pea plants where he crossed a truebreeding tall plant (TT) and a truebreeding short plant (tt).

N.B: A truebreeding plant is a plant that has identical alleles for a specified trait. A truebred is regarded as homozygous i.e. same allele.

He noticed that all the F1 offsprings were tall (Tt). He concluded that one allele has the ability to mask the expression (phenotype) of another allele in a gene. He called the allele that masks or is expressed the DOMINANT allele and the allele that is covered up the RECESSIVE allele. Hence, in an heterozygote i.e. different allele, the dominant allele (for tallness in this case) will be expressed over the recessive allele (for shortness).

layer 3 is younger than layer 6.