INDEPENDENT ASSORTMENT

16 Jan 2008, 23 Jan 08

GWLC 9^th, pp102-119

Independent assortment of two gene pairs is due to them being on different chromosomes. During meiosis, random chance determines which cell the heterozygous pairs segregate.

See illustration on p 104.

Note that an individual with genotype Aa/Bb produces four different genotypes of gametes (AB, Ab, aB, ab) each with equal frequency (1/4th each)

This variation is set up at metaphase I, and fixed at anaphase I.

A test cross is used to demonstrate this frequency or to test for heterozygosity:

            (Aa/Bb x aa/bb) or (A?/B? x aa/bb) (page109)

Note that the recombinant frequency (i.e., different that the parents) is 50 %.

Polygenic inheritance: more than one set of genes contribute to the phenotype, leading to continuous variation:

R₁R₁/R₂R₂ (deep red) x r₁r₁/r₂r₂ (white)

F1: R₁r₁/R₂r₂ (Pink)

F2: range from having contributing to redness

            genotype                                phenotype      frequency

             four genes(R1R1/R2R2)         Deep red         1/16

            Three                                      Dark pink        4/16

            Two                                        Pink                6/16

            One                                         Light pink       4/16

            None                                       White              1/16

Note bell shaped curve on p 111.

Skin color in humans is an example, siblings can have continuous variation of darkness

ORGANELLE INHERITANCE IS INDEPENDENT OF NUCLEUS

Best example is mitochondrial inheritance in humans:

Maternal inheritance because mitochondria have their own DNA, only mitochondria in eggs are inherited.

P 117 maps mtDNA mutations, inherited through maternal line. (Pedigree on p 118)