Methods in Anopheles Research - MR4

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Chapter 2 : Anopheles Biology Laboratory and Culture2.5 Basic Anopheles Mendelian GeneticsPage 1 of 82.5 Basic Anopheles Mendelian GeneticsMark BenedictIntroductionIt is difficult to culture stocks on a long-term basis without some knowledge of their genetics. After all,what ultimately distinguishes stocks and species is their genetic constitution, not the name that we assignto them. The following are aspects of Anopheles genetics that are very relevant to understanding the dayto-daystability and integrity of stocks. Exceptions can be applied to every generalization made here.However, the following will ground you in Anopheles genetics.As a basis for the modern discussions of anopheline phylogenetics and genomics, the existing reviewsare still very useful (Kitzmiller and Mason 1967; Kitzmiller 1976).In this chapter, we provide the minimum background to understand anopheline genetics and exercisesusing anopheline characters to help make the information relevant. Answers are provided after thequestions. If you have no understanding of Mendelian genetics, study the basic genetic principles ofdiploid organisms in a basic text before proceeding. You should particularly understand Punnett squaresand basic probability.Genetic GlossaryAllele: Variant forms of genesCoupling: Also called ‘cis.’ Two alleles being referred to are on the same homolog. In contrast, see‘repulsion..Diploid: Having two sets of chromosomes.Expressivity: The degree or Extent of expression of the phenotype. This addresses the issue e.g. not ofwhat proportion of individuals have freckles, but how many or how large the freckles are. Or foranophelines not, do they have a red stripe, but how intense and clear is it? In contrast, see ‘penetrance.’Gene: The basic unit determining heritable expression. (There are other kinds of heritable expression, butgene covers 99.9% of them.)Genotype: The allelic constitution of an individual, but sometimes applied to tissues or cells.Haploid: Having one set of chromosomes.Hemizygote: Usually refers to the state of having only one copy of a gene located on the X chromosomein males (which have only one X).Heterozygote: The condition in which a diploid individual has two different alleles of a gene.Homolog: One member of a pair of chromosomes.Homozygote: The condition in which a diploid individual has two apparently identical alleles of a gene.Linkage group: May temporarily be the genes or loci that are experimentally identified as not segregatingindependently from one another, but ultimately refers to the collection of everything located on onechromosome. The number of linkage groups identified experimentally may be greater than the actualnumber of chromosomes. (Think of contigs in a DNA sequence. One usually starts with several, but asthe sequence in the gaps is obtained, multiple contigs coalesce into one.Locus: A general term for a place on the chromosomes. It may be a region, a base-pair, or functionallydefined.
Chapter 2 : Anopheles Biology Laboratory and Culture2.5 Basic Anopheles Mendelian GeneticsPage 2 of 8Paracentric inversion: A rearrangement of the chromosome in which a portion of a homolog is flipped andthe centromere is NOT included in the flipped region. Important because they often occur naturally andareuseful as phylogenetic and population genetic tools.Penetrance: A qualitative (low vs. high) or numerical value that refers to the Proportion of individuals in apopulation that express the phenotype that definitively identifies a particular genotype when observed.For example, in An. gambiae, c+ / c females have a red stripe on the larval dorsum generally. Somethinglike 5% of c+ / c females do not appear to have a red stripe. So one could say that red stripe is about95% penetrant. In contrast, see ‘expressivity.’Pericentric inversion: A rearrangement of the chromosome in which a portion of a homolog is flipped andthe centromere is included in the flipped region. Virtually all inversions of this sort in anophelines areinduced by irradiation.Phenotype: The expressed manifestation of a genotype.Quantitative vs. discrete traits: Traits whose expression varies primarily in degree. For example: plantheight, intensity of flower color, malaria parasite encapsulation. Discrete traits can be classified intoclasses e.g. white eye vs. wild eye, collarless vs. collarless+, ebony vs. ebony+.Repulsion: Also called ‘trans’. Two alleles being referred to are on the opposite homologs. In contrast,see ‘coupling.’Trait: Rough term meaning the same as a ‘character’. (Sounds more scientific than ‘a thingie!’) It is afunctional description of some distinct behavior, form, color etc. Used the same as the way we would useit in common speech.Allelic relationshipsComplete dominant: In a heterozygote, only the dominant allele is expressed, and the recessive allele isnot. For example: c+ is completely dominant over c. Stripe+ is dominant over st in An. albimanus.Codominant: In a heterozygote, the phenotypes associated with both of two alleles present are observed.For example: microsatellite alleles are usually referred to as codominant markers because the repeat sizeof both alleles in a heterozygote can be observed. Another example would be enzyme electromorphs.Epistasis: The phenotype of the expression of one gene eliminates the expression of another gene i.e.the phenotype of gene A prevents expression of gene B. white anophelines have no pigment associatedwith the stripe and collarless genes.Gene interaction: The phenotype that is observed is different from that associated with gene A or gene B,but is a result of their combined effect. For example: An. gambiae that have pink-eye mutations generallyhave white or pink eyes. An. gambiae that have red-eye mutations have red eyes. When they have bothpink-eye and red-eye mutations, the have ‘pumpkin’ colored eyes. Neither gene alone can produce thiseffect.Partially dominant or semi-dominant: In a heterozygote, the dominant allele is expressed to a lesserdegree than in a homozyous dominant individual. For example: A. albimanus ebony heterozygotes areintermediate in darkness to either homozygoteThe Facts of LifeFemale Monogamy & Male PolygamyTo generalize the observations of numerous studies, Anopheles females mate only once. The sperm withwhich the male inseminates the female are stored in the spermatheca and are sufficient to fertilize severalbatches of eggs. Under typical insectary conditions, the sperm in the spermathecae are never depleted.Males, on the other hand, will mate several females if given the opportunity. One male can fertilizeapprox. 6-10 females. But again, males are seldom depleted of sperm in typical insectaries due to thelimited number of virgin females and the presence of competing males. The genetic significance of these
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