BIOL 2030: Module 4

Linkage and Mapping in Eukaryotes

Independent assortment

• haploid set of 23 genes from mother + haploid set of 23 genes from father

• DNA synthesis ‘scrambles’ these genes resulting in a combination of a new set of gametes, come containing exact matches of the parental generation, and some that do not match the parental haploid set

• genes on different chromosomes assort into haploid gametes independent of eachother 

Linkage

• a group of genes that do not assort independently into gametes because they are located on the same chromosome 

• not all linkage groups are the same 

  • strong linkage: physically close loci

  • weak linkage: physically distant loci 

  • independent assortment: loci too far apart for linkage 

• degree of linkage varies from Mendelian principles 

Degree of linkage

• if you have 2 genes on the same chromosome, they are linked 

  • they move to the next generation together 

Crossing over 

• farther apart genes = higher likelihood for crossover event

  • for a single crossover event;

    • ½ chromatids are non-recombinant

    • ½ chromatids are recombinant

Recombinant

• assortment of alleles into non-parental combinations

• less common 

• never on the outside

Inter-chromosomal recombination 

• on different chromosomes for different genes 

• assortment of alleles for genes on different chromosomes into novel combinations 

• on different chromosomes for different genes 

Intra-chromosomal recombination

• assortment of alleles for genes located on the same chromosome into novel combinations

• within same chromosome

Non-recombinant 

• parental 

• more common 

• always on the outside 

2 genes very close together on the same chromosome

• crossing over will not occur 

2 genes close on the same chromosome

• because they are slightly far away, there is a small chance of recombination between genes 

  • more opportunities for crossover to produce Ab and aB

2 genes very far apart on the same chromosome

• if far enough apart, crossover occurs in every meiosis event 

• equal 25% chance for all potential gametes 

Genes are independent (unlinked) if…

• they are located on different chromosomes

• located very far apart on the same chromosome 

Complete linkage 

• only parental gamete types are produced

• no recombinant gametes

• crossing over never occurs

• genes must be very close

Independence 

• parental and recombinant gametes are produced with equal frequencies

• crossing over occurs in every meiosis

• genes must be very far apart

Incomplete linkage 

• parental gametes are produced with greater frequency than recombinant gametes

• crossing over occurs in some meioses 

• frequency of crossing over depends on distance 

Coupling

• same dominance with same dominance 

Repulsion 

• mixed up dominance with recessive 

Recombination frequency

• # of recombinant progeny/total # of progeny 

Recombination/map unit

• 1% of recombination = 1 m.u.