Chapter 7 – Linkage, Recombination, and Eukaryotic Gene Mapping
Mar 19, 2016
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