Gene interaction
and lethality
CHAPTER 11
, INTRODUCTION
•Although Mendel did not give descriptive names to the factors involved in his crosses but deno
by alphabetical letters, later geneticists were not so reticent. Soon after the discovery of Mendel
geneticists began naming each hereditary factor according to the name of the character it affecte
tall, green, early flowering, etc. This served the purpose of identifying genes by a symbol other
letter or number.
•Despite this advantage, however, the succession of names that soon appeared seemed to imply
was a succession of gene effects, each individually distinct from the other. To many people,
development seemed to be a “mosaic” of small, non overlapping individual effects, fitte
organism like pieces of stained-glass window.
•It was however, soon discovered by researchers , including Bateson and Punnet, that gene
merely separate elements producing distinct individual effects, but they could interact with ea
give completely novel phenotypes. For example, although the allele A could be charac
phenotype A and allele B by phenotype B, an organism having both alleles A and B at the
might show an entirely new phenotype C.
,Cont…
•The occurrence of such interactions mean that the phenotype we observe is not present
themselves but arises from a complicated developmental process. Thus, there are no gen
individually create specific organ structures or organisms which have such structures “preforme
the individual gene. In classical terms, the doctrine of preformationism is therefore replaced
modern ‘’epigenetic” concepts, in which development occurs through a complex network of rea
interaction influenced by genes.
•Since genetic analysis is tied in with an understanding of the phenotypic ratios that result from
crosses, we may then ask: what effects do interactions have upon observed ratios?
•When two gene pairs each having two alleles, ,i.e., A, a and B, b are segregating independen
organism, the genetic consequences are easily predictable. A cross between heterozygotes for
AaBb×AaBb, will give offspring carrying genotypes in the following ratios;
1/16 AABB 2/16 AABb 2/16 AaBB 4/16 AaBb 1/16 Aabb
2/16Aabb 1/16 aaBB 2/16 aaBb 1/16aabb
, Cont…
•As long as the assortment at each gene pair is independent of the other, and as long as the exis
each of these genotype is not impaired, the previously mentioned genotypic ratios will hold tru
such crosses. Genotypic ratios however are only inferred from phenotypic ratios since it is only
that we directly observe and not the former. Because some of the different genotypes have
phenotypes, the phenotypic ratios need not always reflect the genotypic ratios.
•The following table (table 1) presents some of the different relations between two gene pairs
consequent phenotypic ratios. In the table the genotypic combinations that are from a cross betw
heterozygotes, AaBb×AaBb are numbered from 1 to 16.
and lethality
CHAPTER 11
, INTRODUCTION
•Although Mendel did not give descriptive names to the factors involved in his crosses but deno
by alphabetical letters, later geneticists were not so reticent. Soon after the discovery of Mendel
geneticists began naming each hereditary factor according to the name of the character it affecte
tall, green, early flowering, etc. This served the purpose of identifying genes by a symbol other
letter or number.
•Despite this advantage, however, the succession of names that soon appeared seemed to imply
was a succession of gene effects, each individually distinct from the other. To many people,
development seemed to be a “mosaic” of small, non overlapping individual effects, fitte
organism like pieces of stained-glass window.
•It was however, soon discovered by researchers , including Bateson and Punnet, that gene
merely separate elements producing distinct individual effects, but they could interact with ea
give completely novel phenotypes. For example, although the allele A could be charac
phenotype A and allele B by phenotype B, an organism having both alleles A and B at the
might show an entirely new phenotype C.
,Cont…
•The occurrence of such interactions mean that the phenotype we observe is not present
themselves but arises from a complicated developmental process. Thus, there are no gen
individually create specific organ structures or organisms which have such structures “preforme
the individual gene. In classical terms, the doctrine of preformationism is therefore replaced
modern ‘’epigenetic” concepts, in which development occurs through a complex network of rea
interaction influenced by genes.
•Since genetic analysis is tied in with an understanding of the phenotypic ratios that result from
crosses, we may then ask: what effects do interactions have upon observed ratios?
•When two gene pairs each having two alleles, ,i.e., A, a and B, b are segregating independen
organism, the genetic consequences are easily predictable. A cross between heterozygotes for
AaBb×AaBb, will give offspring carrying genotypes in the following ratios;
1/16 AABB 2/16 AABb 2/16 AaBB 4/16 AaBb 1/16 Aabb
2/16Aabb 1/16 aaBB 2/16 aaBb 1/16aabb
, Cont…
•As long as the assortment at each gene pair is independent of the other, and as long as the exis
each of these genotype is not impaired, the previously mentioned genotypic ratios will hold tru
such crosses. Genotypic ratios however are only inferred from phenotypic ratios since it is only
that we directly observe and not the former. Because some of the different genotypes have
phenotypes, the phenotypic ratios need not always reflect the genotypic ratios.
•The following table (table 1) presents some of the different relations between two gene pairs
consequent phenotypic ratios. In the table the genotypic combinations that are from a cross betw
heterozygotes, AaBb×AaBb are numbered from 1 to 16.
