fast plants.

What is the aleurone in corn?

A thin outer layer surrounding the endosperm that may be colorless or purple; its color is determined by R alleles

What genotypes produce a purple aleurone in corn?

R/R and R/r (any genotype with at least one dominant R allele)

What genotype produces a colorless (yellow) aleurone in corn?

r/r (homozygous recessive); without purple aleurone, the yellow endosperm color shows through

What gene controls starchy vs. sweet endosperm in corn?

The Su gene; Su/Su or Su/su = starchy endosperm; su/su = sugary (sweet) endosperm

Why do sweet corn kernels wrinkle when dry?

The su/su genotype produces endosperm high in sugar instead of starch; as the corn dries, the sugary endosperm loses water and shrinks, causing wrinkling

On which chromosomes are the R and Su genes located in corn?

R alleles (R2) are on Chromosome 2; Su alleles (Su1) are on Chromosome 4

Why do R and Su genes follow independent assortment?

They are located on different (non-homologous) chromosomes (2 and 4), so they segregate independently during meiosis I

What are the expected F2 phenotype ratios in a corn dihybrid cross (R/r Su/su × R/r Su/su)?

9 Purple Starchy : 3 Purple Sweet : 3 Yellow Starchy : 1 Yellow Sweet

What are transposable elements and how do they affect corn color?

DNA sequences that move around the genome; in corn they can disrupt aleurone color production, causing speckled rather than solid purple kernels

How should you score a speckled corn kernel with even one purple speck?

Score it as purple, because at least one R allele is present

What is the pericarp in a corn kernel?

The outermost layer of the kernel; it is of maternal origin and is always colorless in the corn used in genetics labs

What is the endosperm nucleus and how is it formed?

Formed when one sperm nucleus (n) fuses with the fusion nucleus (2n), producing a triploid (3n) endosperm nucleus

What is the zygote in corn and how does it form?

Formed when the second sperm nucleus (n) fuses with the egg nucleus (n), producing a diploid (2n) zygote that develops into the embryo

In the monohybrid cross R/R × r/r, what are the F1 and F2 phenotype ratios?

F1: all purple (R/r); F2: 3 purple : 1 yellow

In the monohybrid cross Su/Su × su/su, what are the F1 and F2 phenotype ratios?

F1: all starchy (Su/su); F2: 3 starchy : 1 sweet

What is the parental (P) cross genotype for the corn dihybrid cross?

R/R Su/Su (purple starchy) × r/r su/su (yellow sweet)

What is the F1 genotype and phenotype in the corn dihybrid cross?

Genotype: R/r Su/su; Phenotype: Purple Starchy (heterozygous for both traits)

How many possible genotype combinations appear in the F2 dihybrid Punnett square?

16 combinations (4×4 grid); yielding 9 genotypically distinct combinations