Slide 6.2

Lecture 6 - Inheritance and change in DNA - Part II -

Meiotic Recombination

A. Evolution of Sex

•Evolution selected for DNA that itself was capable ofchange from generation to generation.

•Inheritance (between generations) is typically NOTthrough mitosis which would make clones of a parent.•Instead, most organisms have sex. Why do we need sex?

•In a species, each gene can have many variations (usuallysmall mutations) = alleles

•Higher life has 2 copies of each gene. If two copies arethe same, then that locus/gene is homozygous. If the two

copies are different, then the gene is heterozygous.

•Theoretically, we only need one copy. Why do we have 2copies then?•Hypothesis:

The second copy allows allelic experimentation withoutkilling the organismThe second copy buffers the organism from deleterious

mutations.

Slide 6.3

Dominance and recessiveness - both alleles may encode proteins; example - eye colour-the gene that encodes the protein that "does the job" is dominant.

-most alleles are codominant (both proteins contribute to function)-in agricultural breeding, many useful alleles are dominant, causedby quantitative gain-of-function alleles (eg. disease resistance)

Because evolution favored having two alleles per gene, evolution created sexes (male and female), each parent giving one copy to the progeny (parent = P; progeny = F1)

Allele A Functional protein

No protein or little proteinAllele B

Sex brings these two sets of alleles together.

Problem: To maintain 2 copies/gene in the progeny, each parent must only contribute one copy of the two it has = meiosis

Fig 18.24

Fig 5.1Introduction to Genetic Analysis (6th ed)A.J. Griffiths et al. WH Freeman and Co. Publishers, NY, 1996

B. Meiosis and Meiotic Recombinationmeiosis = the process of producing gametes that involves reducing the allele number from two to one per gene to facilitate inheritance via sex (demo showing duplication/reduction)

Meiosis

Mitosis

Introduction to Genetic Analysis (6th ed)A.J. Griffiths et al. Figure 5.2WH Freeman and Co. Publishers, NY, 1996

Slide 6.4

Meiosis permits the independent assortment of genes because of the existence of multiple chromosomes to allow the progeny to try out new combinations of alleles. This is useful because many genes are involved in producing a trait such as seed yield.

Independent assortment - for each chromosome pair, each gamete can contribute the maternal or the paternal chromosome.

If two genes in a genome do not assort independently after meiosis (from a sample of many meiosis), then the two genes are likely on thesame chromosome. In such a case, the two genes are said to be linked

In addition to mixing and matching entire chromosomes, evolution selected for segments within chromosomes to be mixed and matched = meiotic recombination demo

Why is a genome divided into multiple chromosomes instead of one large chromosome?

(demo of advantage of >2 chromos vs 1 chromosome)

Fig 19.1

Fig 19.3

Introduction to Genetic Analysis (6th ed)A.J. Griffiths et al. Chapter 19WH Freeman and Co. Publishers, NY, 1996 Slide 6.5

Chiasmata

For meiotic recombination to occur, the two chromosome pairs must be aligned perfectly = chromosome pairing. This pairing is mediated by precise recognition proteins that recognize homologous DNA sequences and align them. Fig 3.27 Fig 3.28

SynaptonemalComplexProteins “zip”the pairstogether

•The result of meiotic recombination is that blocks of genes (chromosomal segments) are inherited intact from each parent, NOT entire chromosomes. •In the discipline of Genetics, it has been observed that the physical distance between alleles of two linked genes ("map units" or CentiMorgans, cM) is directly proportional to the number of meiotic recombination events that occur in the gametes.

Fig 5.9

Introduction to Genetic Analysis (6th ed)A.J. Griffiths et al. Chapters 3 and 5WH Freeman and Co. Publishers, NY, 1996 Slide 6.6

Fig 5.7

Therefore, the percentage of "recombinants" that are recovered equals the relative, physical map distance between the two genes. This method (called "linkage mapping") is used to determine the order and physical distances of genes on each chromosome. Once a gene has been assigned a relative location on the chromosome (relative to other genes), the gene is said to be "mapped". Once a gene is mapped, it accelerates breeding and accelerates being able to isolate ("clone") that gene.

Fig 5.15C

Slide 6.7

A tomato linkagemap

Map distancesbetween linkedbenes (m.u. = map unit)

Fig 5.10

Introduction to Genetic Analysis (6th ed)A.J. Griffiths et al Chapter 5WH Freeman and Co. Publishers, NY, 1996

Meiotic recombination allows new allelic block combinations to be generated (demo with lethal flags)

Independent assortment and meiotic recombination are only usefulif different alleles exist.

Extremely different alleles can be found if the parents were previously geographically isolated from each other and hence developed independent sets of mutations.

Meiotic recombination allows blocks of alleles to be exchanged between parental chromosomes -- this is the basis of breeding.

C. BreedingSex between these two parents ("breeding") can bring the two sets of alleles together. Sometimes, the F1 progeny resulting from the cross between two geographically-diverse parents results in hybrid vigor, which can increase yields (corn, rice, etc.) by >15%.

The mechanistic basis of hybrid vigor is not understood and is verycontroversial. Any hypotheses?-each parent has different recessive, harmful mutations-by crossing them, the progeny inherit a dominant, functional allelefor each locus (gene)

Slide 6.8

Small plant A x small plant B = large plant

Introduction to Genetic Analysis (6th ed)A.J. Griffiths et al. Figure 5.21WH Freeman and Co. Publishers, NY, 1996

Slide 6.9

Meiotic recombination allows small chromosomal segmentscontaining useful genes (eg. disease resistance) to be bred into locally adapted lines

Goal:

ProblemLocal x wild progeny = F150% = wild

1 2 3 4Local variety

1 2 3 4Wild variety

diseaseresistanceallele

SolutionAllow meiosis to recombine Disease R gene onto “local” chromosome 2 background

Backcross recombined chromosome 2 to Local lines for5-6 generations (BC1-BC6) to regenerate a nearly-pure Localvariety genetic background.

source: M.Raizada

Independent assortment and meiotic recombination are only useful if different alleles ("genetic diversity") exist; otherwise new combinations of alleles cannot be mixed up for breeding.

These alleles can come from diverse geographic populations (such as from seedbanks) or they can be generated artificially using chemicals or high energy radiation ("mutagens"). This is called mutation breeding, which has been practiced since ~World War II:

Fig. 15.30

Table 15.5

Introduction to Genetic Analysis (6th ed)A.J. Griffiths et al. Chapter 15WH Freeman and Co. Publishers, NY, 1996

Slide 6.10

D. Evolution of genomes and the speciation of cropsMistakes during meiosis contribute to large changes in genome evolution and speciation, and have been critical to the evolution of crop species. meiotic recombination involves breaking and pasting of chromosome segments such that all genes are conserved. However, due to mistakes in chromosome alignment or DNA repair, a block of a chromosome can randomly break and reattach to itself or to other chromosomes resulting: (demonstrations of)

These mistakes allow for genes to becoming duplicated (and initially redundant), but then they mutate to create related, but novel genes (through gradual spontaneous mutations). This is a way for gene numbers to rapidly increase and create more complexity = gene families.

Introduction to Genetic Analysis (6th ed)A.J. Griffiths et al. Figure 17.1AWH Freeman and Co. Publishers, NY, 1996

Slide 6.11

What are each of these?•tandem duplication•deletion •inversion•translocation•loss or gain of an entire chromosome

Change - Merging Entire Genomes at FertilizationMistakes in reproduction that permit the chromosomes from the pollen and eggs from different species to unite can result in the production of new species:1 set of chromosomes = haploid2 sets of chromosomes = diploid4 sets of chromosomes = tetraploid6 sets of chromosomes = hexaploid

bread wheat = hexapoloid - recent fusion of 3 speciespasta wheat = tetraploid - recent fusion of 2 speciesmodern corn = tetraploid - ancient fusion of 2 species

-because ancient, genes have diverged, so appeared to be a diploid

Introduction to Genetic Analysis (6th ed)A.J. Griffiths et al. Figure 18.12WH Freeman and Co. Publishers, NY, 1996

Figure 6.12

It appears as if most crop species have undergone multiple genome fusions during the last 100 million years. This appears to be a common way to rapidly recombine large numbers of different alleles together, perhaps during periods of rapid or extreme environmental change.

Slide 6.13

Introduction to Genetic Analysis (6th ed)A.J. Griffiths et al. Figure 18.11WH Freeman and Co. Publishers, NY, 1996

***The result is that genomes are "a mess", an ancient record of the mistakes in recombination, the fusion of genomes, the gain and loss of blocks of chromosomes, and many, many rearrangements*****

eg. Corn vs rice, highly related, though overall synteny, has 15,000 local chromosomal rearrangements

QuickTime™ and aPhoto - JPEG decompressor

are needed to see this picture.

The rearrangements, duplications in the 5 chromosomesof Arabidopsis thaliana (related to canola)

TAGI (2000) Nature 408, 796-814

Nature Publishing Group, U.K. Slide 6.14

E. Lecture 6 - Summary of ConceptsTherefore, how did different crop plants and evolution of higher plants evolve?

--New alleles for natural selection and breeding selection from:-Point mutations-Jumping Genes-Segmental gene duplications-inheritance or loss of entire chromosomes or entire genomes-All of these recombine, mix and match, due to meiosis (independent assortment of chromosomes) and meiotic recombination (exchange of blocks within each chromosome)

•meiotic recombination with sex drives the mixing of blocks of chromosomal DNAto generate new combinations of alleles -- this is the basis of evolution and breeding

•alleles are inherited as chromosomal blocks

•the genome is not ordered, but messy, the result of mistakes in DNA recombination, resulting in new genes

Questionnaire please

Slide 6.15