5.1
Meiosis
Meiosis is a type of cell division that produces gametes (sex cells: sperm and eggs). It reduces the chromosome number by half, from diploid (2n) to haploid (n).
Mitosis vs. Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Produce gametes for sexual reproduction |
| Divisions | 1 division | 2 divisions (Meiosis I & II) |
| Daughter Cells | 2 identical cells | 4 genetically unique cells |
| Chromosome Number | 2n → 2n (stays diploid) | 2n → n (becomes haploid) |
| Genetic Variation | None (clones) | Yes (crossing over, independent assortment) |
Meiosis consists of two divisions: Meiosis I separates homologous chromosomes, and Meiosis II separates sister chromatids. The result is 4 haploid cells.
Stages of Meiosis I
Prophase I
- Chromosomes condense
- Synapsis: Homologs pair up
- Crossing over occurs
- Nuclear envelope breaks
Metaphase I
- Homologous pairs line up at metaphase plate
- Independent assortment
- Random orientation
Anaphase I
- Homologs separate
- Sister chromatids stay together
- Pulled to opposite poles
Telophase I
- Nuclear envelopes may reform
- Cytokinesis occurs
- 2 haploid cells formed
Key Distinction: In Meiosis I, homologous chromosomes separate (reduction division). In Meiosis II, sister chromatids separate (like mitosis). This is a common source of confusion on exams!
AP Exam Tip: Meiosis I is the "reduction" division because it reduces chromosome number from 2n to n. Meiosis II is similar to mitosis — it just separates sister chromatids.
5.2
Meiosis and Genetic Diversity
Meiosis generates genetic diversity through three main mechanisms. This variation is the raw material for natural selection and evolution.
Three Sources of Genetic Variation in Meiosis
1. Crossing Over
Occurs during Prophase I
Homologous chromosomes exchange segments of DNA at chiasmata
2. Independent Assortment
Occurs during Metaphase I
Random orientation of homologous pairs at the metaphase plate
3. Random Fertilization
Occurs at conception
Any sperm can fertilize any egg — millions of combinations!
Crossing over exchanges genetic material between homologous chromosomes, creating new combinations of alleles that didn't exist in either parent.
Independent Assortment: How Many Combinations?
During metaphase I, each homologous pair can orient in either direction. The formula for possible gamete combinations is:
2n = number of combinations
Where n = haploid number of chromosomes
Humans (n = 23)
2²³ = 8,388,608
possible gamete types
Combined (egg × sperm)
8M × 8M = ~70 trillion
possible zygote combinations
+ Crossing Over
Essentially infinite!
genetic variation
AP Exam Tip: Be able to explain WHY genetic diversity is important for evolution: it provides variation for natural selection to act upon. Without meiosis, all offspring would be genetically identical (clones), limiting adaptation to changing environments.
5.3
Mendelian Genetics
Gregor Mendel discovered the fundamental laws of inheritance by studying pea plants. His work forms the foundation of modern genetics.
Key Vocabulary
| Term | Definition |
|---|---|
| Gene | A unit of heredity; a segment of DNA that codes for a trait |
| Allele | Different versions of a gene (e.g., B and b) |
| Dominant | Allele that is expressed when present (uppercase: B) |
| Recessive | Allele only expressed when homozygous (lowercase: b) |
| Genotype | The genetic makeup (e.g., BB, Bb, bb) |
| Phenotype | The physical expression of the genotype (e.g., brown eyes) |
| Homozygous | Two identical alleles (BB or bb) |
| Heterozygous | Two different alleles (Bb) |
Law of Segregation
The two alleles for each gene separate during gamete formation (meiosis). Each gamete receives only ONE allele for each gene.
Parent Bb → Gametes: B or b
Law of Independent Assortment
Genes for different traits are inherited independently of each other (if on different chromosomes).
AaBb → Gametes: AB, Ab, aB, ab
Monohybrid Cross: Bb × Bb
A cross involving ONE trait. Let's cross two heterozygous parents:
B
b
B
BB
Bb
b
Bb
bb
Genotypic Ratio: 1 BB : 2 Bb : 1 bb
Phenotypic Ratio: 3 dominant : 1 recessive
Dihybrid Cross: AaBb × AaBb
A cross involving TWO traits. Each parent makes 4 types of gametes (AB, Ab, aB, ab):
| AB | Ab | aB | ab | |
|---|---|---|---|---|
| AB | AABB | AABb | AaBB | AaBb |
| Ab | AABb | AAbb | AaBb | Aabb |
| aB | AaBB | AaBb | aaBB | aaBb |
| ab | AaBb | Aabb | aaBb | aabb |
Phenotypic Ratio:
9 A_B_ 3 A_bb 3 aaB_ 1 aabb
Test Cross: Finding Unknown Genotypes
To determine if an individual showing the dominant phenotype is homozygous (BB) or heterozygous (Bb), cross it with a homozygous recessive (bb) individual:
If BB × bb:
All offspring Bb (all dominant)
If Bb × bb:
50% Bb, 50% bb (1:1 ratio)
AP Exam Tip: Memorize these ratios! Monohybrid Bb × Bb = 3:1. Dihybrid AaBb × AaBb = 9:3:3:1. Test cross with recessive = 1:1 if heterozygous.
5.4
Non-Mendelian Genetics
Not all inheritance follows Mendel's simple dominant/recessive patterns. Many traits show more complex inheritance patterns.
Types of Non-Mendelian Inheritance
| Pattern | Description | Example |
|---|---|---|
| Incomplete Dominance | Heterozygote shows intermediate phenotype (blend) | Red × White = Pink snapdragons |
| Codominance | Both alleles fully expressed simultaneously | AB blood type (both A and B antigens present) |
| Multiple Alleles | More than 2 alleles exist in the population | ABO blood types (IA, IB, i) |
| Polygenic Inheritance | Multiple genes affect one trait (continuous variation) | Human height, skin color, eye color |
| Epistasis | One gene affects expression of another gene | Labrador coat color (E gene masks B gene) |
| Pleiotropy | One gene affects multiple phenotypes | Sickle cell gene → anemia, malaria resistance |
| Sex-Linked | Genes located on sex chromosomes (usually X) | Colorblindness, hemophilia |
Sex-Linked Inheritance (X-Linked)
Genes on the X chromosome show different inheritance patterns in males vs females:
Females (XX)
Have TWO X chromosomes
Can be: XHXH (normal), XHXh (carrier), XhXh (affected)
Males (XY)
Have only ONE X chromosome
Can be: XHY (normal) or XhY (affected) — no carriers!
Result: X-linked recessive disorders (like colorblindness, hemophilia) are much more common in males.
X-Linked Cross: Carrier Female × Normal Male
Colorblindness: XC = normal vision, Xc = colorblind
Cross: XCXc (carrier female) × XCY (normal male)
XC
Y
XC
XCXC
♀ Normal
♀ Normal
XCY
♂ Normal
♂ Normal
Xc
XCXc
♀ Carrier
♀ Carrier
XcY
♂ Colorblind!
♂ Colorblind!
Result: 50% of sons will be colorblind; 0% of daughters will be colorblind (but 50% are carriers)
Linked Genes & Recombination
Genes on the same chromosome tend to be inherited together (linked). However, crossing over can separate them.
Recombination frequency = (# recombinant offspring / total offspring) × 100
The farther apart genes are on a chromosome, the MORE likely crossing over will occur between them.
AP Exam Tip: For sex-linked problems, always write out the full genotype including the sex chromosomes (X and Y). Remember: males only need ONE recessive allele to show the trait, while females need TWO.
5.5
Environmental Effects on Phenotype
Phenotype is determined by both genotype AND environment. The same genotype can produce different phenotypes depending on environmental conditions.
The Big Picture: Nature vs. Nurture
Phenotype = Genotype + Environment
Examples of Environmental Effects on Phenotype
| Organism | Environmental Factor | Effect on Phenotype |
|---|---|---|
| Siamese Cats | Temperature | Cooler body parts (ears, nose, paws, tail) have darker fur due to temperature-sensitive enzyme |
| Hydrangeas | Soil pH | Same plant produces blue flowers in acidic soil, pink in basic soil |
| Human Height | Nutrition | Malnutrition can prevent reaching genetic potential for height |
| Himalayan Rabbits | Temperature | Similar to Siamese cats — dark fur on cold extremities |
| Flamingos | Diet | Pink color comes from carotenoids in diet; without them, they're white |
| Arctic Fox | Season (temperature/light) | White coat in winter, brown in summer |
In Siamese cats, the enzyme that produces melanin (dark pigment) is temperature-sensitive. It only works at cooler temperatures, so the cat's extremities (which are cooler) develop dark fur.
Norm of Reaction
The norm of reaction describes the range of phenotypes that a single genotype can produce across different environments.
Narrow Norm of Reaction
Genotype produces similar phenotype regardless of environment
Example: Blood type (environment doesn't change it)
Wide Norm of Reaction
Genotype produces very different phenotypes in different environments
Example: Human height, behavior
Multifactorial Traits
Most traits are influenced by:
- Multiple genes (polygenic)
- Environmental factors
- Interactions between genes and environment
Examples: Heart disease risk, intelligence, athletic ability
Epigenetics
Environmental factors can cause chemical modifications to DNA or histones that affect gene expression without changing the DNA sequence.
Examples:
- DNA methylation (often silences genes)
- Histone modification
Some epigenetic changes can be inherited!
Important Distinction: Environmental effects on phenotype do NOT change the DNA sequence. The genotype remains the same — only the expression changes. This is different from mutations, which DO change the DNA.
AP Exam Tip: Be ready to explain why identical twins (same genotype) can have different phenotypes. Environmental factors like nutrition, exercise, disease exposure, and even random developmental differences can cause phenotypic variation despite identical DNA.
Unit 5 Summary: Key Concepts
Meiosis
2 divisions → 4 haploid gametes
Genetic Variation
Crossing over, independent assortment, random fertilization
Mendelian Ratios
3:1 (monohybrid), 9:3:3:1 (dihybrid)
Non-Mendelian
Incomplete dominance, codominance, sex-linked, polygenic