Welcome to Genetics:Welcome to Genetics:Unit 4 Seminar!Unit 4 Seminar!

Please feel free to chat with your Please feel free to chat with your classmates!classmates!

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Agenda

• Brief Review

• Self Assessment Questions

• Question

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4Gene Linkage and Genetic Mapping

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But what happens when the genes But what happens when the genes are located on the same are located on the same chromosome?chromosome?

The stages of meiosis I and II

Interphase

Prophase I Metaphase I

Anaphase I

Telophase I

Prophase II

Metaphase II

Anaphase II

Telophase II

Let’s first

label each

stage.

(and cytokinesis)

(and cytokinesis)

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Mendel’s Laws: Chromosomes• Locus = physical location of a gene on a chromosome• Homologous pairs of chromosomes often contain

alternative forms of a given gene = alleles• Different alleles of the same gene segregate at meiosis I• Alleles of different genes assort independently in

gametes

• Genes on the same chromosome exhibit linkage: inherited together

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Gene Mapping• Gene mapping determines the order of genes and

the relative distances between them in map units– Scientist use gene mapping to determine the order of

genes on the same chromosome

• 1 map unit = 1 cM (centimorgan)

In double heterozyote:• Cis configuration = mutant alleles of both genes are

on the same chromosome = ab/AB

• Trans configuration = mutant alleles are on different homologues of the same chromosome = Ab/aB

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Gene Mapping• Gene mapping determines the order of genes and the relative

distances between them in map units

• Gene mapping methods use recombination frequencies between alleles in order to determine the relative distances between them

• Recombination frequencies between genes are inversely proportional to their distance apart

• Distance measurement: 1 map unit = 1 percent recombination (true for short distances)

• 1 map unit = 1 cM (centimorgan)

“Crossing Over” During Meiosis

During ________, each pair of chromatids lines up next to its _________.

prophase I

homologue

homologous chromosomes

This pairing of homologous chromosomes produces _______.tetrads

tetrad

A tetrad consists of ___________.4 chromatids

It is possible for the chromatids within a homologous pair to twist around one another. Portions of the chromatids may break off and attach to adjacent chromatids.

This process is called “____________”

crossing over

“Crossing over” is the exchange of genetic information (genes) between segments of homologous chromosomes during meiosis.

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Gene Mapping• Genes with recombination frequencies less than 50 percent

are on the same chromosome = linked)

• Linkage group = all known genes on a chromosome

• Two genes that undergo independent assortment have recombination frequency of 50 percent and are located on nonhomologous chromosomes or far apart on the same chromosome = unlinked

Fig. 4.7

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Recombination• Recombination between linked genes occurs at

the same frequency whether alleles are in cis or trans configuration

• Recombination frequency is specific for a particular pair of genes

• Recombination frequency increases with increasing distances between genes

• No matter how far apart two genes may be, the maximum frequency of recombination between any two genes is 50 percent.

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Gene Mapping

• Recombination results from crossing-over between linked alleles.

• Recombination changes the allelic arrangement on homologous chromosomes

Fig. 4.4

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Genetic Mapping• The map distance (cM) between two genes equals

one half the average number of crossovers in that region per meiotic cell

• The recombination frequency between two genes indicates how much recombination is actually observed in a particular experiment; it is a measure of recombination

• Over an interval so short that multiple crossovers are precluded (~ 10 percent recombination or less), the map distance equals the recombination frequency because all crossovers result in recombinant gametes.

• Genetic map = linkage map = chromosome map

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