Linkage and Recombination Between Two Genes

Genes on the same chromosome are like passengers on a charter bus: they travel together and ultimately arrive at the r

Meiosis I

Late Prophase Metaphase

Crossing over !

vxxy

Anaphase

Recombinant chromosomes

Recombinant chromosomes r\

Meiosis II

Genes may switch from one homologous chromosome to another by crossing over in meiosis I.

Metaphase

Anaphase

Gametes

In meiosis II, genes that are normally linked...

...will then assort independently...

...and end up in different gametes.

In meiosis II, genes that are normally linked...

...will then assort independently...

...and end up in different gametes.

I 7.4 Crossing over takes place in meiosis and is responsible for recombination.

same destination. However, genes occasionally switch from one homologous chromosome to another through the process of crossing over (Chapter 2) ( FIGURE 7.4). Crossing over produces recombination—it breaks up the associations of genes imposed by linkage. As will be discussed later, genes located on the same chromosome can exhibit independent assortment if they are far enough apart. In summary, linkage adds a further complication to interpretations of the results of genetic crosses. With an understanding of how linkage affects heredity, we can analyze crosses for linked genes and successfully predict the types of progeny that will be produced.

Notation for Crosses with Linkage

In analyzing crosses with linked genes, we must know not only the genotypes of the individuals crossed, but also the arrangement of the genes on the chromosomes. To keep track of this arrangement, we will introduce a new system of notation for presenting crosses with linked genes. Consider a cross between an individual homozygous for dominant alleles at two linked loci and another individual homozygous for recessive alleles at those loci. Previously, we would have written these genotypes as:

AABB X aabb

For linked genes, however, it's necessary to write out the specific alleles as they are arranged on each of the homologous chromosomes:

In this notation, each line represents one of the two homologous chromosomes. In the first parent of the cross, each homologous chromosome contains A and B alleles; in the second parent, each homologous chromosome contains a and b alleles. Inheriting one chromosome from each parent, the F1 progeny will have the following genotype:

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