Principles Of Inheritance And Variation


 

Principles Of Inheritance And Variation logoSubject: Biology    Exam: AIIMS

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Important Points

Pedigree analysis is a diagrammatic illustration depicting the passage of a certain trait through successive generations in a family lineage. It helps in genetic counseling.
Genic interaction manifests in modified ratios of phenotypic segregation in the F2-genertion of dihybrid crosses. Notable modified dihybrid ratios include 9 : 7 (for complementary genes), 13:3/12:3:1 (for epistasis , also referred to as dominant epistasis; please note that the concept of recessive epistasis that could produce a ratio of 9:3:4, like that for supplementary genes, has been proved absurd/wrong since long), 15 : 1 (for duplicate genes), and 9 : 3 : 4 (for supplementary genes).
Test cross need not be a back cross.
XY-type of sex determination occurs in humans and in fruit-fly (Drosophila melanogaster). In this category, the (XY) males are heterogametic while (XX) females are isogametic/homogamtic. Since human females produce only one king of ovum ( X-type), the contribution form the male progenitor (whether X-sperm, or Y-sperm) is the sole determinant of the gender/sex of the progeny.
In case of mutation resulting in lethal gene that causes death in homozygous condition, the modified ratio of phenotypic segregation in the F2-generation of monohybrid cross in respect to this lethal gene would be 2 : 1 (instead of the normal ratio of 3 : 1)
 
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