Genetics Final

Regulated development

Cell fate determined by interactions with other cells (mammals).

Autonomous development

Cell fate determined by inherited cytoplasmic determinants (flies).

Which depends most on egg structure

Autonomous development.

Totipotent cells

Cells that can form all cell types including extraembryonic tissues (up to 8-cell stage).

Pluripotent cells

Cells that form all body cell types but not extraembryonic tissues.

3 stages of cell differentiation

Totipotent → Pluripotent → Differentiated.

Epiblast

Structure that gives rise to all cells in the embryo.

Primitive streak

Structure that forms mesoderm during development.

Somites

Segments that form vertebrae and muscle.

Development timeline mammals

Day 1 fertilization → Day 5 epiblast → Day 6.5–8.5 primitive streak → Day 7–10 somites/Hox → Day 9.5 AER → Day 11.5 ZPA.

Retinoic acid

Morphogen that alters chromatin and regulates gene expression.

Wnt signaling

Pathway controlling cell fate (wingless in flies).

FGF (Fibroblast Growth Factor)

Controls cell division and growth.

AER (Apical Ectodermal Ridge)

Controls limb outgrowth.

ZPA (Zone of Polarizing Activity)

Controls digit identity via Sonic Hedgehog.

Development axes

A/P and D/V axes determine positional identity.

Development grid concept

A/P and D/V axes combine to specify organ positions.

Hox genes

Determine identity of body segments.

Hox gene mutation (fly)

Causes homeotic transformations (wing → haltere).

Hox gene mutation (mouse)

Alters vertebrae identity.

Gap genes

Form broad early developmental regions.

Gap gene function

Respond to morphogen concentration thresholds.

Pair-rule genes

Define repeating segments.

Segment polarity genes

Refine segment boundaries.

Field specification genes

Determine where structures form.

Field compartment genes

Divide structures using Wnt and Dpp signaling.

Cell-type specification genes

Control final differentiation.

Gene reuse

Same gene used at multiple developmental stages.

Bicoid

Anterior morphogen activating head genes.

Nanos

Posterior morphogen repressing anterior genes.

mRNA localization

Placement of maternal mRNA via 3’ UTR.

Why mRNA localization matters

Creates spatial protein gradients before fertilization.

Spaetzle/Dorsal pathway

Controls dorsal/ventral axis.

Pax6 misexpression

Causes ectopic eye formation.

Why Pax6 matters

Master regulator of eye development.

Pole cells

Give rise to germline cells.

FosB gene

Maternal care gene affecting offspring survival.

Fly vs mammal development

Fly = autonomous; Mammal = regulated.

Recombination frequency (RF)

RF = recombinants / total offspring.

RF calculation formula

RF = (# recombinants / total) × 100.

Map units (centimorgans)

1% recombination = 1 cM.

Linked genes

RF < 50%.

Linked vs unlinked threshold

Less than 50% linked; about 50% unlinked.

Distance vs recombination

Greater distance increases recombination.

How to identify recombinants

Less frequent offspring.

How to identify parental types

Most frequent offspring.

Test cross

Cross with homozygous recessive.

Test cross result interpretation

1:1 ratio indicates heterozygote.

Independent assortment

Random segregation in meiosis I.

Crossing over

Exchange between homologous chromosomes.

Epistasis

One gene masks another.

Epistasis mechanism

Interaction between genes affecting phenotype.

9:3:4 ratio

Recessive epistasis.

1:2:1 ratio

Incomplete dominance.

Lethal allele

Allele causing death.

Lethal allele ratio

2:1 ratio indicates lethal genotype.

X-linked lethal

Skewed sex ratio due to lethal allele.

X-linked lethal explanation

One sex dies leading to 2:1 ratio.

Complementation test

Determines if mutations are same gene.

Complementation meaning

Wildtype = different genes.

Non-complementation meaning

Mutant = same gene.

Heterozygote complementation

Indicates mutations in different genes.

Polygenic trait

Controlled by many genes.

Multifactorial trait

Genes + environment.

Genetic background

Other genes influence phenotype.

Norm of reaction

Range of phenotypes from one genotype.

Norm of reaction definition

Phenotype varies with environment.

Pedigree rare recessive

Appears in offspring of carriers.

Consanguinity risk

Increases recessive disease probability.

DNA to RNA transcription

Complementary base pairing (A→U, T→A, C→G, G→C).

DNA → RNA rule

A→U, T→A, C→G, G→C

Codon

3-base mRNA sequence.

Anticodon

tRNA complementary sequence.

mRNA → amino acid

Codons translated via genetic code.

DNA gyrase

Relieves supercoiling.

RNA primase

Adds RNA primer.

Leading strand

Synthesized continuously.

Lagging strand

Synthesized in Okazaki fragments.

Leading vs lagging strand

Continuous vs discontinuous synthesis.

Why Okazaki fragments form

DNA polymerase works 5’→3’ only.

Hfr cell

Donor with integrated F factor.

Endogenote

Recipient genome.

Interrupted mating

Maps gene order by timing.

Gene order in conjugation

First genes closest to origin.

Gene order (conjugation)

Entry order determines gene order.

Auxotroph

Requires nutrient supplement.

Auxotroph definition

Cannot synthesize required nutrient.

How to detect auxotroph

Grows only on supplemented media.

Lac operon basics

Repressor blocks transcription without lactose.

Lac operon (no lactose)

Repressor bound → OFF.

Lac operon (with lactose)

Repressor removed → ON.

Oc mutation

Operator always ON.

Oc mutation effect

Repressor cannot bind → always ON.

Is mutation

Super-repressor always OFF.

Is mutation effect

Cannot be inactivated → always OFF.

P- mutation

Promoter inactive → no transcription.

P- mutation effect

RNA polymerase cannot bind.

Penetrance

Percent showing phenotype.

Penetrance definition

Proportion expressing phenotype.