Milestones in Biology 4 Test Your Knowledge
You have a tall pea plant.
a. What are its possible genotypes?
b. What crosses could you do to determine if it is true breeding or not? For each cross, give the expected phenotypes of the offspring.
a: It could be homozygous dominant (TT) or heterozygous (Tt). b: You would cross it with a homozygous recessive (true-breeding short) plant (tt). If the tall plant is true-breeding (homozygous), then 100% of the offspring will be Tt (tall). If the tall plant is heterozygous, 50% of the offspring will be Tt (tall) and 50% will be tt (short).
In crossing pea plants:
a. If a true-breeding tall pea plant with purple flowers is crossed with another true-breeding tall pea plant with purple flowers, what will the offspring pea plants look like?
b. Will the offspring be true-breeding? Explain your answer.
a: If both parents are true breeding for both traits, then 100% of the offspring will be tall with purple flowers. b: In this case, all the offspring will be homozygous dominant (TTPP) (as both their parents were homozygous dominant). If all the plants are homozygous dominant, then they will be true breeding.
Half of the gametes of a heterozygous parent will carry the dominant allele, and half will carry the recessive allele. Which of Mendel’s laws explains this?
This is explained by Mendel’s law of segregation. The two alleles will segregate from one another at meiosis I.
Consider Mendel’s laws:
a. Which of Mendel’s laws would be violated if the offspring of two heterozygous tall, purple-flowered pea plants (denoted as TtPp) were only tall, purple-flowered plants or short, white-flowered plants?
b. What would such a violation suggest about the two genes?
a: Mendel’s law of independent assortment would be violated, as the alleles of each gene do not appear to be assorting independently of each other. b: These results would suggest that the T and the P alleles are closely linked on one homologue, and that the t and the p alleles are closely linked on the other homologue.