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Dr. Lawrence Ramsden Department of Botany

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1 Dr. Lawrence Ramsden Department of Botany
BIOL 1106 GENETICS Lecture 3 Dr. Lawrence Ramsden Department of Botany

2 The Complications Multiple alleles Incomplete dominance Codominance
Epistasis Lethal alleles Expression Continuous variation Environmental effects

3 Multiple Alleles A gene may have many alleles not just 2
But, a single diploid individual can only carry 2 alleles

4 Dominance Where there is complete dominance the phenotype of the heterozygote is the same as that of the homozygote dominant. Incomplete or partial dominance can lead to the expression of intermediate phenotypes. Andalusian blue, colour produced by CB/CW heterozygote from Black x White chickens.


6 Codominance In codominance both phenotypes are expressed in the heterozygote, not an intermediate type as in incomplete dominance In ABO blood group system heterozygous individuals produce both A & B antigens.


8 Gene Interactions No gene acts by itself in determining phenotype.
Genes can interact to produce new phenotypes. or, Genes can interact to hide the expression of phenotypes,- Epistasis.

9 Chicken Combs Phenotype determined by interaction of alleles of 2 genes, R & P 4 comb types; Rose R/- p/p Walnut R/- P/- Pea r/r P/- Single r/r p/p True breeding if the alleles are homozygous



12 F2 9:6:1 ratio in Squash 2 genes A & B can interact 3 phenotypes
Spherical A/- b/b or a/a B/- Long a/a b/b Disc A/- B/-


14 Epistasis Genes interact to mask another genes phenotype.
No new phenotypes are produced. Can be dominant or recessive.

15 Agouti 9:3:4 ratio Rodent coat colour and pattern.
Agouti, albino or black. Albino lacks any coat colour. Black lacks yellow colour in agouti pattern. But, you can’t have a coat pattern without any colour! so even A/- c/c will be albino, and the recessive c/c masks the A allele


17 Flower colour, 9:7 ratio White variety crosses sometimes produce purple flowered F1 Purple F1 x F1 gives 9:7 purple:white Due to interaction between dominant alleles Purple C/- P/- Duplicate recessive white


19 Lethal Genes Yellow coat colour in mice is an essential gene.
Yellow heterozygotes bred to non-yellow mice give 2:1 yellow non-yellow ratio. Yellow AY/AY homozygotes are aborted. Yellow dominant for coat colour. Recessive with respect to lethality.


21 Environmental Effects
Fur colour in Himalayan rabbits; above 30°C all white at 25°C normal pattern with dark extremeties. Cooled below 25°C, more dark patches.

22 Neurofibromatosis Penetrance, the phenotype does not always reflect the genotype. Expressivity, the degree to which the phenotype is expressed for a given genotype.

23 Continuous Variation Many phenotype characters are the product of the action of a large number of genes. So the phenotype is the product of independent assortment in the individual genes. In a population we will see gradual differences, or continuous variation.

24 Linkage Non-independent assortment of genes
Linked genes, located on same chromosome Genetic recombination, where progeny show new combinations of alleles not present in the parents


26 Total flies = 2,441 Total parental phenotypes = 1,541 Total recombinant phenotypes = 900 % recombinants = 36.9 If there was independent assortment we would predict 50% recombinants Conclusion, the genes are linked

27 Crossing Over

28 Linkage Maps Use linked genes to determine physical location on chromosome. The greater the separation between 2 genes the higher the probability that crossover will occur. 1% recombination = 1 map unit (mu or cM) Multiple cross-overs can also occur


30 Avoiding multiple cross-overs
Use closely-linked genes (within 10 mu) Use a three-point test cross including a third allele. The products of a double crossover will be the least frequent. Can help establish the order of the genes on the chromosome. Interference can happen between cross-overs.

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