The recessive gene s produces shrunken endosperm in corn kernals and
its dominant allele S produces full, plump kernals. The recessive gene c
produces colorless endosperm and its dominant allele C produces
colored endosperm. Two homozygous plants are crossed, producing an
F1 which are all phenotypically plump and colored. The F1 plants are
a) What were the phenotypes and genotypes of the original parents?
b) How are the genes linked in the F
c) Calculate the map distance between the two gene loci.
In corn, the gene R for red color is dominant over the gene r for green
color. The gene N for normal seeds is dominant over the gene n for
tassel seed. A fully heterozygous red plant with normal seeds was
crossed with a green plant with tassel seeds, and the following ratios
were obtained in the offspring:
124 red, normal
77 red, tassel
126 green, tassel
72 green, normal.
Does this indicate linkage? If so, what is the map distance between the
Suppose genes A and B are 14.5 map units apart. Another gene, C,
linked to these, is found to cross with gene B, 7 percent of the time. Are
these data sufficient to determine the exact order of the three genes? If
not, what other information is needed to order the genes?
The cross-over percentage between linked genes J and M is 20%, J and
L 35%, J and N 70%, L and K 15%. M and N 50%, M and K 30%, and
M and L 15%. Thus , the sequence of genes on this chromosome is
One of your unicorns gives birth after you and she toured the Ukraine.
does not look like its parents or your other
unicorns. While your animals have a straight horn and a green coat; this
one has a twisted horn and a blue coat. You mate “Old Blue” and keep
only the blue offspring, after successive generations you have pure-
breeding progeny of “Old Blue." Now when you mate animals from
your two populations: all the progeny have straight horns and green
coats. These hybrid offspring are then crossed to unicorns with twisted
horns—blue coats. This cross produces offspring in the following