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Genetics & Evolution

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Genetics & Evolution - Complete Interactive Lesson

Part 1: Mendelian Genetics

Genetics & Evolution for the MCAT

Part 1 of 7 — Mendelian Genetics

Mendel's Laws

Law of Segregation: Two alleles for each gene separate during gamete formation
Law of Independent Assortment: Genes on different chromosomes sort independently

Key Terminology

Term	Definition
Genotype	Genetic makeup (e.g., Aa)
Phenotype	Physical expression
Homozygous	Same alleles (AA or aa)
Heterozygous	Different alleles (Aa)
Dominant	Expressed in heterozygote
Recessive	Only expressed when homozygous

Cross Types

Monohybrid cross (Aa $\times$ Aa):

Genotype ratio: 1 AA : 2 Aa : 1 aa
Phenotype ratio: 3 dominant : 1 recessive

Test cross: Cross unknown genotype with homozygous recessive (aa)

If all offspring dominant → parent was AA
If 50% dominant, 50% recessive → parent was Aa

MCAT Punnett Square Strategy

Always set up the cross systematically. For dihybrid crosses (AaBb $\times$ AaBb), use the 4 $\times$ 4 Punnett square or the shortcut: $\text{9 A_B_} : \text{3 A_bb} : \text{3 aaB_} : \text{1 aabb}$

Mendelian Genetics 🎯

Key Takeaways — Part 1

Mendelian ratios: Monohybrid 3:1, Dihybrid 9:3:3:1
Test cross with homozygous recessive reveals unknown genotype
Law of Segregation: alleles separate. Independent Assortment: genes on different chromosomes sort independently
The 3:1 ratio is PHENOTYPIC; genotypic ratio is 1:2:1

Part 2: Non-Mendelian Inheritance

Genetics & Evolution for the MCAT

Part 2 of 7 — Non-Mendelian Inheritance

Extensions to Mendel

Pattern	Description	Example
Incomplete dominance	Heterozygote = intermediate phenotype	Red $\times$ White → Pink flowers
Codominance	Both alleles fully expressed	Blood type AB (both A and B antigens)
Multiple alleles	>2 alleles exist in population	ABO blood type ( $I^A$ , , i)

Part 3: Population Genetics

Genetics & Evolution for the MCAT

Part 3 of 7 — Sex-Linked Inheritance & Pedigrees

X-Linked Inheritance

Males (XY) only have ONE X chromosome → hemizygous
X-linked recessive diseases affect males more (no second X to compensate)
Carrier females ( $X^AX^a$ ) pass the trait to ~50% of sons

Common X-Linked Recessive Diseases

Color blindness
Hemophilia A and B
Duchenne muscular dystrophy
G6PD deficiency

Pedigree Analysis

Key patterns to recognize:

Autosomal Dominant: Affected in every generation, males and females equally affected, unaffected parents don't transmit

Autosomal Recessive: Can skip generations, often appears in consanguineous (related) parents, 25% of carrier carrier offspring affected

Part 4: Natural Selection

Genetics & Evolution for the MCAT

Part 4 of 7 — Population Genetics

Hardy-Weinberg Equilibrium

For a population NOT evolving:

$p + q = 1$ $p^2 + 2pq + q^2 = 1$

Part 5: Speciation & Phylogeny

Genetics & Evolution for the MCAT

Part 5 of 7 — Evolution & Natural Selection

Mechanisms of Evolution

Mechanism	Description	Direction
Natural selection	Differential survival/reproduction based on fitness	Adaptive
Genetic drift	Random changes in allele frequency	Random
Gene flow	Migration between populations	Reduces differences
Mutation	New alleles introduced	Random, raw material

Types of Natural Selection

Type	Effect on Distribution	Example
Stabilizing	Narrows distribution (favors average)	Birth weight in humans
Directional	Shifts mean one way	Antibiotic resistance

Part 6: Immune System

Genetics & Evolution for the MCAT

Part 6 of 7 — Speciation & Phylogenetics

Speciation

Biological species concept: Species = organisms that can interbreed and produce fertile offspring

Type	Barrier	Example
Allopatric	Geographic isolation	River divides population
Sympatric	Reproductive isolation (same location)	Polyploidy in plants

Reproductive Barriers

Prezygotic (prevent mating/fertilization):

Temporal isolation (different mating seasons)
Behavioral isolation (different courtship rituals)
Habitat isolation (different microhabitats)
Mechanical isolation (incompatible anatomy)
Gametic isolation (gametes can't fuse)

Postzygotic (hybrid problems):

Hybrid inviability (embryo doesn't survive)
Hybrid sterility (mule = horse $\times$ donkey)
Hybrid breakdown (F2 generation problems)

Phylogenetics

Part 7: Review & MCAT Practice

Genetics & Evolution for the MCAT

Part 7 of 7 — Genetic Diseases & Chromosomal Abnormalities

Autosomal Dominant Diseases

Disease	Gene/Feature
Huntington's disease	HTT gene, trinucleotide repeat (CAG)
Marfan syndrome	Fibrillin-1, connective tissue
Familial hypercholesterolemia	LDL receptor deficiency
Achondroplasia	FGFR3 mutation, dwarfism

Autosomal Recessive Diseases

Disease	Feature	Population
Cystic fibrosis	CFTR chloride channel (thick mucus)	European descent
Sickle cell anemia	HbS (Glu→Val), pleiotropic	African descent
Phenylketonuria (PKU)	Can't metabolize phenylalanine	Newborn screening
Tay-Sachs	Hexosaminidase A deficiency	Ashkenazi Jewish

Chromosomal Abnormalities

ABO Blood Type (MCAT FAVORITE)

Genotype	Blood Type	Antigens	Antibodies
$I^AI^A$ or $I^Ai$	A	A antigen	Anti-B
$I^BI^B$ or $I^Bi$	B	B antigen	Anti-A
$I^AI^B$	AB	Both A and B	Neither
ii	O	Neither	Anti-A and Anti-B

$I^A$ and $I^B$ are codominant to each other
Both are dominant over i
Type O = universal donor (no antigens)
Type AB = universal recipient (no antibodies)

Non-Mendelian 🎯

Key Takeaways — Part 2

Incomplete dominance: blending. Codominance: both expressed (AB blood).
ABO: $I^A$ and $I^B$ codominant, both dominant over i
Pleiotropy = one gene, many effects. Epistasis = one gene masks another.
Polygenic traits show continuous variation (bell curve)

X-Linked Recessive: Mostly males affected, carrier mother → 50% sons affected, no male-to-male transmission

X-Linked Dominant: Affected fathers pass to ALL daughters (never sons), more females affected

MCAT Pedigree Strategy

Check for male-to-male transmission → if yes, NOT X-linked
Check if trait skips generations → if yes, likely recessive
Count affected males vs. females → more males = X-linked recessive

Key Takeaways — Part 3

X-linked recessive: mainly males affected, no male-to-male transmission, carrier females
Autosomal recessive: can skip generations, 25% risk from carrier parents
Autosomal dominant: every generation, 50% chance if one parent affected
Pedigree strategy: check male-to-male, skipping, and sex ratios

Where: $p$ = frequency of dominant allele, $q$ = frequency of recessive allele

Term	Represents
$p^2$	Frequency of homozygous dominant
$2pq$	Frequency of heterozygous (carriers)
$q^2$	Frequency of homozygous recessive

Conditions for Hardy-Weinberg (no evolution)

No mutations
No migration (gene flow)
No natural selection
Large population (no genetic drift)
Random mating

If ANY condition is violated → population is evolving!

Usually given: " $q^2$ = frequency of affected individuals (recessive phenotype)"

Calculate $q = \sqrt{q^2}$
Then $p = 1 - q$
Carrier frequency = $2pq$

Hardy-Weinberg 🎯

Key Takeaways — Part 4

Hardy-Weinberg: $p^2 + 2pq + q^2 = 1$ (only if 5 conditions met)
Start with $q^2$ (recessive phenotype frequency), then work backward
Carrier frequency ( $2pq$ ) is always much higher than disease frequency ( $q^2$ )
Any violation of the 5 conditions = evolution occurring

Bottleneck effect: Disaster reduces population → random alleles lost
Founder effect: Small group colonizes new area → reduced genetic diversity
Both are RANDOM (unlike natural selection, which is adaptive)

$\text{Fitness} = \text{Reproductive success (# of viable offspring)}$

It's NOT about being strongest — it's about who reproduces most successfully.

Key Takeaways — Part 5

Natural selection is adaptive; genetic drift is random
Stabilizing: favors average. Directional: shifts mean. Disruptive: favors extremes.
Bottleneck and founder effects reduce genetic diversity randomly
Fitness = reproductive success, not strength or survival alone

Homologous structures: Same Origin, different function (human arm vs. whale flipper) → common ancestor
Analogous structures: Different origin, same function (bird wing vs. insect wing) → convergent evolution
Vestigial structures: Reduced/nonfunctional remnants (human appendix, whale hip bones)

Key Takeaways — Part 6

Allopatric: geographic separation. Sympatric: same location, different mechanism.
Prezygotic barriers prevent mating; postzygotic barriers reduce hybrid fitness
Homologous = same origin = common ancestor. Analogous = convergent evolution.
Species concept: can interbreed + produce FERTILE offspring

Condition	Karyotype	Features
Down syndrome	Trisomy 21	Most common viable trisomy
Turner syndrome	45,X (monosomy X)	Female, short, infertile
Klinefelter syndrome	47,XXY	Male, tall, infertile

Heterozygote Advantage

Sickle cell carriers (HbAS) are resistant to malaria → explains high frequency of sickle cell allele in malaria-endemic regions. This is balancing selection.

Genetic Diseases 🎯

Genetics & Evolution — Complete! ✅

From Mendel to Hardy-Weinberg to natural selection to genetic diseases — genetics and evolution are heavily tested on the MCAT. Master Punnett squares, pedigree analysis, and population genetics calculations for test day.

Genetics & Evolution

ABO Blood Type (MCAT FAVORITE)

Key Takeaways — Part 2

MCAT Pedigree Strategy

Key Takeaways — Part 3

Conditions for Hardy-Weinberg (no evolution)

MCAT Shortcut

Key Takeaways — Part 4

Genetic Drift

Fitness

Key Takeaways — Part 5

Key Takeaways — Part 6

Heterozygote Advantage

Genetics & Evolution — Complete! ✅