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GeneticLINKAGE

APresented by :Ayush Jain (Alm 3008)

University of agricultural science,banglore

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Chromosomal theory of inheritanceSutton and Boveri proposed the chromosomal theory of inheritance.(1902-1903)According to chromosome theory of inheritance, it is well established that many genes are located in each chromosome in a linear fashion. And all genes located in same chromosome would move to same pole during cell division. As a consequence, such genes will fail to show independent segregation and would tend to be inherited together.

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What is genetic linkage???Genetic linkage is the tendency of DNA sequences that are close together on a 

chromosome to be inherited together during the meiosis phase of sexual reproduction.

OR “ The Tendency of genes to remain together in their original combination

during inheritance is called linkage”  The Two genes that are physically near to each other are unlikely to be separated onto different chromatids during chromosomal crossover, and are therefore said to be more linked than genes that are far apart. In other words, the nearer two genes are on a chromosome, the lower the chance of recombination between them, and

the more likely they are to be inherited together. Genes on different chromosomes are perfectly unlinked.

Term linkage was given by T H Morgan 

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Why genetic linkage ??? “Because in organisms genes are far more in number than the

chromosomes, which implies that each chromosome contains many loci.” example: humans

Genome size

No. of genes

3,234.83 Mb (Mega-basepairs) per haploid genome6,469.66 Mb total (diploid).

19000-20000 genes are present

Number of chromosomes 23 pairs (46 chromosomes)

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When and who discovered???

• Shortly after Mendel's work was rediscovered, exceptions to this rule were found. In 1906, the British geneticists William Bateson and Reginald Punnett discovered the phenomenon of linkage in sweet pea (lathyrus odoratus).

• Genetic linkage is the most prominent exception to Gregor Mendel's Law of Independent Assortment.

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How it was discovered???The researchers crossed varieties of sweet peas that differed in two traits, flower colour and pollen length. Plants with red flowers and long pollen grains were crossed to plants with white flowers and

short pollen grains. All the F1 plants had red flowers and long pollen grains indicating that the alleles for these two phenotypes

were dominant. When the F1 plants where self-fertilized, Bateson and punnet observed a peculiar distribution of phenotypes among

the offspring. Instead of 9:3:3:1 ratio expected for two independent assorting genes, they obtained a ratio of 24.3:1.1:1:7

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Can linked genes ever separate???

Yes they can , through recombination

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WHAT IS RECOMBINATION ???• Recombination is the process through which genes present on the

same chromosomes can be separated and new combination of genes could be formed.

• Recombination is caused by a physical exchange between paired homologous chromosomes early in prophase of the first meiotic division after chromosomes have duplicated(crossing over).

• At any one point along a chromosome, the process of exchange(crossing over) involves only two of four chromatids in a meiotic tetrad.

• Late in prophase I, crossovers become visible as chiasmata (chisamatype theory – janessens 1909).

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Who provided the evidence that crossing over causes recombination and how???

• In 1931 Harriet Creighton and Barbara McClintock obtained evidence that genetic recombination where associated with a material exchange between chromosomes.

• They studied homologous chromosomes in maize that where morphologically distinguishable.

• Two forms of chromosome 9 was available for analysis, one was normal, and other had cytological aberrations at each end --- a heterochromatic knob at one end and a piece of of a different chromosome at other.

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What is recombination frequency???• The frequency of the recombinant progeny produced by the

heterozygous F1 plants is called recombination frequency. It can be use to measure the intensity of linkage between genes.

• Genes that are tightly linked seldom recombine, that is there recombination frequency in low.

• To calculate frequency of recombinants = (total no. of recombinants observed/total no. plants observed) in a test cross*100

• Recombination frequency never exceeds above 50%. Why ???

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What are the Factors affecting recombination frequency???

• Distance between the genes• sex: heterogametic sex shows relatively lower recombinations (males) than homogametic

sex (females). Drosophila males present the extreme example as they show no recombination between linked genes.

• age: progressive decline with age• Temperature: lowest in drosophila 220C • Nutrition• Chemicals: antinomycin D promotes recombination• Radiation: X-Rays and gamma rays increases recombination• Chromosomal abbreviations: paracentric inversions, translocation, deletions • Distance from centromere: centromere tends to suppress recombination while

recombination rates are much higher near telomeric regions.

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Types of linkage• Complete linkage: when only parental character combinations are recovered

in test cross progeny, it is called complete linkage ex: male drosophila• incomplete linkage: when recombinant types are also recovered with

parental types in test cross progeny it is called incomplete linkage• A case of incomplete linkage is tight linkage in when genes are so closely

located that they may show very low frequency of recombination.• Linkage is also classified as coupling phase (dominant alleles of linked genes

are present together) and repulsion phase linkage( dominant allele of gene is linked with recessive allele of other gene).

• Sex linkage : linkage in genes present in sex chromosomes.

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What is a linkage group ??? • All genes that are linked together form a linkage group,• The no. of different linkage groups in a species is, as a rule, equal to

its gametic chromosome no. • Drosophila has 5 linkage group and human beings have 24 linkage

groups.

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What is a chromosome map how it is created???

• A straight line drawing(supposing chromosome are linear), depicting the linked genes and the distances (recombination frequencies) between them is known as chromosome map.

• The procedure for chromosomal mapping was invented by Alfred H. Sturtvent in 1911.

• Geneticists construct chromosome maps by counting the number of crossovers that occur during meiosis. Since crossing over event can’t be seen directly. So they must estimate the no. of crossovers have taken place by counting either chiasmata(cytological analysis) or recombinant chromosomes(genetic analysis).

• The distance between two points on genetic map of a chromosome is the average number of crossovers between them.

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Recombinant mapping with a two point testcross

• A two point test cross uses 2 loci to determine the distance between them using the amount of average recombinants obtained from the test cross.

How it is done ???

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This simple analysis indicates that, on an average 17 out of 100 chromosomes recovered from meiosis had a crossover between vg and b.

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What are limitations of a two point cross??

• We cant determine the relative order on the chromosome of the loci.• As well as we cannot analyse the effect of multiple crossovers as two

crossovers between two loci can cause the chromosome to look as if no cross overs took place, causing to underestimate map distances.

So what to do we do now???Its simple, We take a third locus, between the first two, to

detect multiple cross over events.

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Recombination mapping with a three point cross

• analysis of three loci, each segregating two alleles, is referred to as a three-point cross.

• We can determine order of loci or genes in the chromosome.• We can determine relative distance between the genes• But we cannot distinguish right and left hand of chromosome.

How it is done ???

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What is interference & Coefficient of Coincidence ???

As we know 3 point cross has an important advantage, that is it allows the detection of double crossovers, permitting us to determine if exchanges in adjacent regions are independent of each other or does one cross over inhibit the occurrence of another nearby??If one crossover inhibit the occurrence of another nearby, the phenomenon is known as interference.And the extent of the interference is customarily measured by the coefficient of coincidence(c), which is the ratio of observed frequency of double cross over to the expected frequency.The level of interference(I) is calculated as I =1-C.Coefficient of coincidence 1 would imply no interference at all. But negative interference means that the occurrence of a crossing over promotes other. (go to slide no. 21 and then 20)

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Does chromosome map always tell true distances between

genes??No, this method works well as long as genes are fairly close together.

However, when they are far apart, the frequency of recombination may not reflect the true map distance

For example: go to slide no. 22

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How are linkage analysed in humans???

• To detect and analyse linkage in humans, pedigree are used.• A pedigree is a family tree tracing an trait of significance.Various kinds traits include sex linked dominant or recessive sex limited ( secondary sexual characters in humans) sex influenced ( baldness) autosomal dominant or recessive• pedigree analysis also provides estimates of recombination

frequencies to map genes on human chromosome.

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Is there a Genetic control for recombination ??

yes, there is• Recombination involves product of many genes, some of which play

role in chromosome pairing, others catalyse the process of exchange, and still others help to rejoin broken chromatid segments.

• Inversions supress recombination as it supress crossing over• Recombination can bring favourable mutation together.

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EXPERIMENT OF T H MORGAN• In Drosophila, Both the white eye gene (w) and a

gene for miniature wings (m) are on the X chromosome.

• Morgan (1911) crossed a female white miniature (w m/w m) with a wild-type male (w+ m+/ Y).

• In the F1, all males were white-eyed with miniature wings (w m/Y), and all females were wild-type for eye color and wing size (w+ m+/w m).

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MORGAN’S EXPERIMENTAL CROSSES OF WHITE EYE AND MINATURE WING VARIANTS Of Dorsophilia Melanogaster

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In F2, the most frequent phenotypes for both sexes were the phenotypes of the parents in the original cross (white eyes with miniature wings, and red eyes with normal wings). Non-parental phenotypes (white eyes with normal wings or red eyes with miniature wings) occurred in about 37% of the F2 flies. Well below the 50% predicted for independent assortment, this indicates that non-parental flies result from recombination of linked genes.