Asked Oct 17, 2019

What are the causes of unexpected rations and why? (non-dominant nature of traits, several genes affecting the trait, environmental effects, incomplete penetrance, lethal phenotypes, etc)


Expert Answer

Step 1

The Mendelian traits show phenotypic ratio of 9:3:3:1 in case of a dihybrid cross, and 3:1 in case monohybrid cross. There are various conditions which cause unexpected phenotypic ratios. These are gene interaction, incomplete dominance, incomplete penetrance, lethal phenotypes, and environmental effects.

Step 2

Gene interaction occurs when two or more different genes contribute to a particular phenotype. For example, the dominant forms of two different (non-allelic) genes function as enzyme of a pathway leading to the synthesis of purple pigment (anthocyanin). If any of the alleles is present in the recessive form then the pathway will be non-functional and anthocyanin will not be produced. The figure depicting this pathway and unexpected ratio is shown in the white board.


Image Transcriptionclose

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Step 3

The incomplete dominance of genes also produces a ratio that deviates from the normal mendelian ratio. When two incompletely dominant genes are present in a heterozygous condition then the phenotype of the individual is an intermediate of the two extreme phenotypes.  In Mira...

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