Using D for tall plants and d for dwarf plants, a 1:1 ratio is obtained in which of the following crosses? These are my choices:
Dd X dd
DD X dd
DD X Dd
DD X DD
Dd X Dd
To determine which of the given crosses results in a 1:1 ratio of tall plants (D) to dwarf plants (d), we need to understand the basic principles of Mendelian genetics.
Mendel's Law of Segregation states that during the formation of gametes (sperm and egg), the alleles (alternative forms of a gene) segregate so that each gamete receives only one allele for each trait. In the case of the D and d alleles for plant height, the D allele represents tall plants, while the d allele represents dwarf plants.
Let's analyze each of the options:
1) Dd X dd - In this case, one parent plant is heterozygous (Dd) for the plant height gene, while the other parent plant is homozygous recessive (dd). When we cross Dd with dd, we expect a 50% chance of tall (Dd) and a 50% chance of dwarf (dd) offspring. Therefore, this cross does not result in a 1:1 ratio.
2) DD X dd - Here, one parent plant is homozygous dominant (DD) for the plant height gene, while the other parent plant is homozygous recessive (dd). When we cross DD with dd, all the offspring will be heterozygous (Dd) for the plant height gene. As a result, this cross does not produce a 1:1 ratio.
3) DD X Dd - In this case, both parent plants are either homozygous dominant (DD) or heterozygous (Dd) for the plant height gene. Crossing these plants will result in offspring with either the D or d allele. As a result, this cross does not give a 1:1 ratio.
4) DD X DD - Here, both parent plants are homozygous dominant (DD) for the plant height gene. When we cross DD with DD, all the offspring will be homozygous dominant (DD). Therefore, this cross does not result in a 1:1 ratio.
5) Dd X Dd - In this case, both parent plants are heterozygous (Dd) for the plant height gene. When we cross Dd with Dd, we expect a 25% chance of DD (tall plants), 50% chance of Dd (tall plants but carriers of the dwarf allele), and 25% chance of dd (dwarf plants). This cross results in a 1:2:1 ratio of DD:Dd:dd, which can be simplified to a 1:1 ratio of tall (D) to dwarf (d) plants.
Therefore, the correct answer is: Dd X Dd.