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SCI 355B Lecture 5

Insect Orders (cont.)Contributions of Insect Research to Our

Understanding of Genetic Processes

Pterygota: NeopteraHemimetabolous: gradual

development

Last time:Apterygota

Pterygota: Paleoptera

(Holometabolous)- Complete Metamorphosis

Order Neuroptera-”lacewings & others”Order Coleoptera- “beetles”Order Mecoptera-”scorpionflies”Order Siphonoptera-”fleas”Order Diptera- “true flies”Order Trichoptera- “caddisflies”Order Lepidoptera- “moths & butterflies”Order Hymenoptera- “ants, bees, & wasps”

Entognathous, Pterygota, NeopteraRefer to slides from previous handout

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Insects & Their Contributionsto Genetics

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Genetics“Branch of biology that deals with heredity

and the expression of inherited traits”• Heredity - Transmission of traits from one

generation to another• Trait - any detectable phenotypic

(observable properties of organism)variation of a particular inherited character– Discrete: presence or absence; color– Continuous: height, weight, color

Meiosis

• Cell division which results in halpoid “sex” cells(i.e., egg and sperm)

• One replication of the genetic material (DNA)during interphase, but two nuclear divisions(meiosis I and meiosis II).

• Results in haploid (N) cells (= gametes in animals)from an initial diploid (2N) cell

• Very similar to mitosis except that the cellsproduced are not genetically identical.

Chromosome Structure

sister chromatids

sister chromatids

centromere: region of thechromosome wherechromatids attach.

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Alleles Alternativeforms of thesame gene.

Alleles occur at the same locations (loci) on homologous chromosomes.

A- a-

B- b-

C- C-

Heterozygous

Homozygous

DNA Replication during Interphase

A- a-

B- b-

-A -a

-b-Brepresents gene A,from one parent. represents the

same gene fromthe other parent,codes for the~same protein,may have aslightly differentDNA sequence.Homologous Chromosomes

Meiosis Prophase I

… Synapsis: the highly specific parallelalignment of homologous chomosomesduring the first division of meiosis,

A A a a

B B b b…tetrad: the two homologous

chromosomes become attachedalong their length in a structuretermed a tetrad.

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Meiosis Prophase Icont.

Crossing Over: rearranges the genes from each parent.

A aA a

B B b b

a A

Meiosis Metaphase I -- Telophase I

A aA a

B B b b

a A

AA

B B

a aa

b b

A

Meiosis Prophase II -- Telophase II

aA

BB

Aa

bb

A A

B B b b

aa

no DNA synthesis

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Meiosis is critical for sexualreproduction in all diploid

organisms(1) meiosis leads to the formation of gametes.

– gametes (one from each parent) conjugate to form azygote.

(2) meiosis is the basis for extensive variationamong members of a population.

Johann Gregor Mendel(1822-1884)

• Experiments in PlantHybridization (1865,1866)

• Had read Darwin’s Origin ofSpecies (1859) with interest

• Saw no immediateconnection

• Nor did anyone else fordecades

Who was that robed man?

• A monk• Monastery of St. Thomas

in Czech Republic• 1884: “I am convinced

that it [my scientificwork] will be appreciatedbefore long by the wholeworld”.

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Mendel’s Contributions

• Identified that genes exist….he called them “unitfactors”

• Indicated that pea plants contain two copies ofeach unit factor/gene….we call these alleles.

• Described dominance and recessiveness• Indicated that the unit factors/alleles segregate

independently. In other words, during gamateformation (during meiosis) the alleles “go into”gametes independently of one another.

Monohybrid Cross

GenerationParental (P)

First Filial (F1)

Second Filial (F2)

yellow pea green pea (pollen) (eggs)

x

all yellow

grow plants, crosspollinate

grow, allow to self-fertilize

6022 yellow : 2001 green

3 : 1

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Monohybrid CrossP: GG x gg

F1: Gg (GgxGg)

Gametes: G g G g1/2 1/2 1/2 1/2

F2: GG1/2 x 1/2

1/4

Gg1/2 x 1/2

1/4

gG1/2 x 1/2

1/4

gg1/2 x 1/2

1/4

Dominant vs. Recessive Traits

x P

F1

The trait that appears in the F1 generation is the DOMINANT trait.

The trait that disappears in the F1 generation is termed RECESSIVE.

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Dominance

Dominance - the interaction of genes (alleles)at the same locus.

Three Kinds of Dominance(1) Complete Dominance(2) Incomplete/Partial Dominance(3) Codominance

Sex/Gender DeterminationSex-Linked Traits

Sex Determination• History:

– Medieval Europe (500 - 1500 AD)• Hammer = boy• Scissors = girl

– Ancient Greece• Semen from right testicle

– Rural China and India (present day)• Female infanticide/sex specific abortion

– Western Societies (present day)• PGS - preconception gender selection - separate sperm bearing x

and y chromosomes. Usually x-bearing sperm contain ~3%more DNA than y-bearing sperm

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Sex Chromosomes• Heteromorphic sex chromosomes - different between the

sexes.• All mammals have the XY-system

Females: XX = homogametic sexMales: XY = heterogametic sex

Human Male: 46, XYHuman Female: 46, XX

• Karyotype - the chromosome complement of a cell or anindividual. Often used to refer to the arrangement ofchromosomes in a sequence according to size.

Sex Determination• In mammals, the male sperm determine the sex of

the offspring (X-bearing sperm vs. Y-bearingsperm).

• Female gametes: 23, X• Male gametes: 23, X or Y

Other sex determination systems

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ZW System

• Some insects• Some birds (chickens)• Some fish• Same as XY except female is heterogametic• The use of ZW is arbitrary and for distinction

between XY only, but is commonly used intextbooks

• ZZ = male• ZW = female

XO Method of Sex Determination“most insects”

• Some insects (no sex specific chromosome• Females: XX• Males: X0 (where 0 indicates lack of 2nd X)

Haplodiploidy (# of whole chromosome sets)

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In Crepidula fornicata, the common slipper limpet, sex is determined by an environmental factor: the limpet’s position in a stack of limpets.

Sex in the isopod Armadillidium vulgare is usually determined by sexchromosomes, but genetic males may be converted into functionalfemales by the presence of infecting bacteria.

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Drosophila melanogaster

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Morgan’s Experiment

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The Double Helix

• On the last day ofFebruary 1953,according to JamesWatson

• Francis Crickannounced to thepatrons of the Eaglepub in Cambridge

• "We have discoveredthe secret of life."

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Gene

• a region of DNA thatcontrols a hereditarycharacteristic.

• It usually correspondsto a sequence used inthe production of aspecific protein

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Protein Function is Directly Related to theStructure of the Molecule

• Enzymes (chem. reaction rate)• Transport (movement of molecules, e.g.,

hemoglobin and myoglobin)• Structural (e.g., collagen and keratin)• Binding (histones, hormones)• Contractile (actin, myosin)• Immunity (immunoglobulins)• Etc, Etc, Etc, Etc, Etc, Etc, Etc, Etc, Etc, Etc, Etc,

The key to development lies in the regulation of gene expression. This early fruit-fly embryo illustrates the localized production of proteins from two genes that determine the development of body segments.

Drosophila: genetics and development

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Insect Genomes