Page 37 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

2.2 Genetic Analysis According to Mendel 29

Figure 2.20 Molecular explanations of Mendel’s pea shape and pea color genes. The R allele of the pea shape gene
specifies the enzyme Sbe1, which converts unbranched starch (amylose) to branched starch (amylopectin). The r allele does not produce
Sbe1. The buildup of unbranched starch in rr peas ultimately causes seed wrinkling. The Y allele of the pea color gene specifies the enzyme
Sgr, which functions in a pathway to break down chlorophyll during pea maturation, resulting in yellow peas. The y allele does not produce
Sgr. Chlorophyll is not broken down in yy peas, and they remain green.
(a) Biochemical function of Mendel’s pea shape gene
R: Sbe1
r: no Sbe1 CH 2 OH CH 2 OH
RR, Rr H H O H H H O H
OH H OH H
CH 2 OH CH 2 OH CH 2 OH CH 2 OH CH 2 OH O O O
H O H H O H H O H H O H H O H
H H H H H Sbe1 H OH H OH
OH H O OH H O OH H O OH H O OH H O CH 2 OH CH 2 OH CH 2 CH 2 OH CH 2 OH
O O O O O
H OH H OH H OH H OH H OH H H H H H H H H H H H H H H H
OH H O OH H O OH H O OH H O OH H O
Amylose H OH H OH H OH H OH H OH
Amylopectin

CH 2 OH CH 2 OH CH 2 OH CH 2 OH CH 2 OH
O O O O O
H H H H H H H H H H
H H H H H
OH H OH H OH H OH H OH H
O O O O O
H OH H OH H OH H OH H OH
rr
(b) Biochemical function of Mendel’s pea color gene
Y: Sgr
y: no Sgr
YY, Yy Me
Me O O
N HN NH HN
OMe Sgr O OMe
H
NH N O O NH N O
OH OH

O O

Chlorophyll (green pigment) Breakdown product (colorless)

Me O
N HN
OMe
NH N O
OH
yy
O

color. The pea shape gene specifies an enzyme known as converted into starch. The excess sucrose modifies the os-
Sbe1 (for Starch-branching enzyme 1). Sbe1 catalyzes the motic pressure, causing water to enter the young seeds. As
conversion of amylose, an unbranched linear molecule of the seeds mature, they lose the water, shrink, and wrinkle.
starch, to amylopectin, a starch molecule composed of sev- The single dominant allele in Rr heterozygotes apparently
eral branching chains (Fig. 2.20a). The dominant R allele specifies enough of the normal Sbe1 enzyme to prevent
of the pea shape gene determines a normal, functioning wrinkling.
Sbe1 enzyme. In contrast, the recessive allele r specifies The pea color gene determines an enzyme called Sgr (for
no Sbe1 enzyme. As a result, RR homozygotes contain a Stay green). Sgr performs one step in the pathway leading
high proportion of branched starch molecules, which al- to the breakdown of the green pigment chlorophyll, a
lows the peas to maintain a rounded shape. In homozygous process that occurs naturally in peas as they mature
recessive rr peas, sucrose builds up because less of it is (Fig. 2.20b). The dominant Y allele specifies Sgr, and the