Corn is a man-made monstrosity, biologically speaking. If an ear of corn is left to its fate in nature, it will give rise to a group of seedlings so densely clustered that none is likely to survive. Thus all known living corn is dependent on man for survival. — George Beadle, Nobel Prize recipient

George Beadle, "The Mystery of Maize," Field Museum of Natural History Bulletin, Nov. 1972.

Until the mid-20th century, the origins of corn were in dispute. Corn is very different from any other wild grass species and produces so many kernels on each cob that humans must physically separate the kernels in order for the seeds to grow. In the early 1930s, a geneticist named George Beadle began experiments to show that the closest wild relative to corn is a native Mexican and Central American grass called teosinte.

One of the major differences between corn and teosinte is in the shape and casing of the seed. In teosinte plants, the seeds are small and separately encased in a dark, hard jacket. In corn plants, the seeds are much larger and naked.

The shape and casing of the seed is an example of a trait/ character in corn and teosinte plants.

Teosinte and corn have 2n = 20 chromosomes. When cross-fertilized, the hybrid offspring have an intermediate phenotype overall.

Assume that only one gene controls all of the differences between corn and teosinte; call this gene D. All corn plants have the corn allele of gene D, denoted DCDC, and all teosinte plants have the teosinte allele of gene D, denoted DTDT.

A true-breeding corn plant will have the genotype DCDC/DTDT/DCDT If you cross a true-breeding corn plant with a true-breeding teosinte plant, the F₁ generation will be heterozygous. If you allow the F₁ generation to self-pollinate, then you would expect 1/2, 3/4, 1/4 of the F₂ offspring to look like corn. If you draw the Punnett square for this cross, it would resemble that of a monohybrid/ dihybrid cross.

Now, assume that corn differs from teosinte by two genes. That is, one gene controls some differences (such as seed properties) and the other gene controls the other differences (such as stalk properties); call these two genes D and E. The corresponding corn alleles of these genes are DCDC and ECEC, while the teosinte alleles are DTDT and ETET.

If you cross a true-breeding corn plant with a true-breeding teosinte plant, the genotype of the F₁ generation will be DCET/ DCDTECET/ DTDTECEC. If you then allow the F₁ generation to self-pollinate, you would expect 1/2, 1/16, and 1/8 of the F₂ offspring to look like corn. If you draw the Punnett square for this cross, it would resemble that of a monohybrid/ dihybrid cross.

Over the decades, Dr. Beadle carried out these crosses and eventually examined around 50,000 F₂ offspring. About 1500 of them resembled corn and about 1500 of them resembled teosinte. About how many genes account for the differences between teosinte and corn?

Answer rating: 100% (QA)

Based on the information provided it appears that Dr Beadles experiments and examination of around 5... View the full answer