Page 71 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

3.2 Extensions to Mendel for Two-Gene Inheritance 63

Figure 3.19 Redundant genes result in a 15:1 phenotypic ratio. (a) Normal maize leaves (AA BB) and a leaf lacking both
dominant alleles A and B (aa bb). (b) In maize, either dominant allele A or B is sufficient for normal leaf development. Only the absence
of both dominant alleles (aa bb) results in malformed, thin leaves. The result is a 15:1 ratio in the dihybrid cross F 2 .
a: © Dr. Michael J. Scanlon, Cornell University
(a) Normal maize leaves and mutants lacking (b) A and B are redundant for maize leaf development
two redundant gene functions
P AA bb aa BB

Gametes A b a B

(all identical)
F 1 Aa Bb Aa Bb

F 2
A B A b a B a b

A B AA BB AA Bb Aa BB Aa Bb
(9) A– B–
(3) A– bb (normal)
15
(3) aa B– A b AA Bb AA bb Aa Bb Aa bb
1 aa bb (skinny)
a B Aa BB Aa Bb aa BB aa Bb

a b Aa Bb Aa bb aa Bb aa bb
AA BB aa bb

examples (squash and chicken colors) yield different malformed only in the absence of both A and B (aa bb),
F 2 phenotypic ratios because of differences in the biochem- the F 2 phenotypic ratio signifying redundant gene action
ical pathways involved. is 15:1 (Fig. 3.19b).
The proteins (A and B) encoded by the dominant al-
∙ Recessive epistasis usually indicates that the domi-
nant alleles of the two genes function in the same leles act in parallel, redundant pathways that recruit precur-
sor cells to become part of the leaf (Fig. 3.20). That is, if
pathway to achieve a common outcome. In the either pathway functions, the leaves will develop their nor-
Labrador retriever example, B and E both function mal broad shape.
to generate black hairs.
∙ Dominant epistasis usually indicates that the domi- Often, as in this case, redundant genes specify nearly
nant alleles of the two genes have antagonistic func- identical proteins that perform the same function. Why
does the organism have two genes that do the same thing?
tions. Both in the cases of squash and chicken color, One answer is that redundant genes often arise by chance
the dominant allele of gene B prevents deposition of a evolutionary processes that duplicate genes, as will be
pigment whose synthesis depends on the dominant explained in Chapter 10.
allele of gene A.

Summary: A Variety of Different
Redundancy: One or More Genes in a Biochemical Pathways Can Produce
Pathway Are Superfluous Any Given Altered Mendelian Ratio
In maize, two genes, A and B, control leaf development. So far we have seen that when two independently assorting
Normal broad leaves develop as long as the plant has ei- genes interact to determine a trait, the 9:3:3:1 ratio of the
ther a dominant A allele or a dominant B allele (A− B−, four Mendelian genotypic classes in the F 2 generation can
A− bb, or aa B–). However, the leaves of plants that have produce a variety of phenotypic ratios, depending on the
neither dominant allele (aa bb) are skinny because they nature of the gene interactions. The result may be four, three,
contain too few cells (Fig. 3.19a). Given that leaves are or two phenotypes, composed of different combinations of