Chapter 7 – Linkage, Recombination, and Eukaryotic Gene Mapping

Chapter 7 – Linkage, Recombination, and Eukaryotic

Gene Mapping

Genetic Principles

• Principle of Segregation– Diploid organisms have 2 alleles for each

gene• Separate during meiosis – only one gamete enters

each gamete

• Principle of Independent Assortment– 2 alleles of a gene separate independently

from alleles at other loci/other genes

Chromosomes

• Chromosomes follow independent assortment IF:– Genes are located of different chromosomes

BUT:

– If genes are on the same chromosome, they tend to travel together

• Linked genes – close together on the same chromosome

Sweet peas – dihybrid cross

• P generation purple, long x red, round

• F1 generation – all purple,long

• Prediction for F2 generation – ratio of 9:3:3:1

Sweet pea – dihybrid cross cont• Expected F2 phenotype

ratios is not observed

• Conclusion – genes for flower color and pollen shape must be located close together on the same chromosome

• Why are any recombinant progeny seen?

Crossing over• If 2 genes are on the same chromosome, but far apart,

crossing over can allow for recombination of gametes

• Genes very far apart on the same chromosome will always be separated by crossing over, and are not considered to be linked

Notation for linked genes

• Horizontal lines indicate actual chromosomeA_________Ba b*individual heterozygous for 2 different genes where both dominant alleles are on one chromosome, and both recessive alleles are on its homologous chromosome

• Can be abbreviated by AB/ab

Testcross for linkage

• For determination if two genes are linked (close together on the same chromosome) or not

• Set-up:– One individual heterozygous for both traits x

individual homozygous recessive for both traits

Testcross for linkage cont• MmDd x mmdd

• If not closely linked, alleles will assort independently – MmDd individual can

form 4 different types of gametes

– 50% recombinant offspring/50% non-recombinant offspring

Testcross for linkage cont

• MD/md x md/md

• If closely linked, 2 alleles will always travel together– all offspring are non-

recombinant

Testcross for linkage cont

• Can be separated by crossing over

– Small number of recombinant progeny/chromosomes is seen

Crossing over• Single cross over produces 50%

nonrecombinant chromosomes (same configuration as parental chromosome) and 50% recombinant chromosomes (new allelic combination)

Recombination frequency • = number of recombinant progeny x 100

total number of progeny

Values from slide #118 + 7 1555+53+8+7 = 123 = 12.2% or .122

• Smaller the recombination frequency = more closely linked

Coupling and Repulsion • For heterozygous individuals

• Cis configuration/coupling– Both wildtype alleles are on one chromosome;

both mutant alleles are on the homologous chromosome

• Trans configuration/repulsion– Each chromosome has one wildtype allele and

one mutant allele

Recombination• Interchromosomal

– Between genes on different chromosomes– Independent assortment/random segregation during

Metaphase/Anaphase I – Produces 50% recombinant/50% non-recombinant

gametes

• Intrachromosomal – Between genes on same chromosome– Crossing over during Prophase I– Usually produces recombinant gametes less than 50%

• Unless very far apart on the same chromosome

Genetic mapping

• Relative position of different genes based on recombination rates

• Does NOT state actual chromosome, or position (locus)

• Distance measured in map units or centimorgans (cM)– 1 m.u. (or cM) = 1% recombination

Genetic mapping example• A and B = 5 m.u.• A and C = 15 m.u.• B and C = 10 m.u.

• A and D = 8 m.u.• B and D = 13 m.u.• C and D = 23 m.u.

• Any genes with 50% recombination are either on different chromosomes, or very far apart on the same chromosome (crossing over always separates them)

Physical mapping

• Locates gene to a specific chromosome/region of chromosome

• Deletion mapping – Chromosome deletion studies – how phenotype is

affected/what genes may be missing– Duchenne m.s.

• X linked disease – but where on X?• Some affected males have small deletions – common

deleted area must be where gene is located

Somatic cell hybridization• Fusion of 2 cell types (altered

by viruses or tumor cells to allow cell lines – uninhibited growth)

– Somatic cells

• Heterokaryon – 2 distinct nuclei

– Eventually fuse

• Most chromosomes are lost (differentially from one type)

– Human chromosomes usually lost, only a few remain

– Human genes expressed in hybrid cell lines must be located on retained chromosomes

• deletion studies can give more specific location on chromosome

Molecular Analysis• Fluorescence In Situ

Hybridization (FISH)– Probe complementary to

gene sequence will bind to DNA

• Gene sequence/partial sequence must be known

• DNA sequencing– Yields base pair distance

between two genes