Cross a constricted plant with a heterozygous plant. What fraction of the offspring will be homozygous?

To determine the fraction of the offspring that will be homozygous, we need to first understand the principles of plant genetics and how alleles are inherited.

In this case, we have a constricted (recessive) plant crossed with a heterozygous (dominant/recessive) plant. Let's represent the alleles for the constricted trait with "c" (recessive) and "C" (dominant).

When a heterozygous plant (with alleles "Cc") is crossed with a constricted plant (with alleles "cc"), the Punnett square can be used to analyze the possible genotypes of the offspring:

| C | c
----------------------------
C | CC | Cc
----------------------------
c | Cc | cc

From the Punnett square, we can see that there are four possible combinations of alleles for the offspring: CC, Cc, Cc, and cc.

Out of the four possible combinations, only one combines both recessive alleles (cc), resulting in homozygous offspring for the constricted trait. Therefore, the fraction of offspring that will be homozygous is 1/4, or 25%.

It's important to note that this is a basic example and assumes that there is no variance due to other genetic factors or the presence of any modifiers. In the real world, genetics is often more complex, and additional factors may influence the inheritance pattern.