AP Bio Exam Review

Meiosis

• Definition: Meiosis is a process used by sexually reproducing eukaryotes (animals, plants, fungi, protists) to transmit genes from one generation to the next.
• Importance:
  • Creates variation between parents and offspring.
  • Creates variation among the offspring.
• Life Cycle:
  • Adults have specialized tissues (testes and ovaries) for creating gametes (sperm and egg cells) through meiosis.
  • Sperm fertilizes the egg, producing a zygote (fertilized egg).
  • The zygote divides and develops, with tissues differentiating to produce an adult organism.
• Haploid vs. Diploid Cells:
  • Parents have two sets of chromosomes in their body cells (diploid), except for gametes.
  • Chromosomes are paired (e.g., two chromosome ones, two chromosome twos).
  • One chromosome from each pair is inherited from each parent.
  • These pairs are homologous.
  • Meiosis halves the number of chromosomes.
  • Haploid cells have half the number of chromosomes found in diploid cells.
• Homologous Chromosomes:
  • Matching chromosomes inherited from parents.
  • Example: Chromosome three, one from mom, one from dad; chromosome four, and so on.
  • Not identical; chromosomes from parents differ.
  • Same genes in the same order, but alleles (specific code at gene locations) may differ.
  • Analogy: Gene as a recipe; mom's tomato sauce recipe has more garlic, dad's has more basil.
  • If $c$ refers to a specific protein, DNA coding for amino acids may differ, even changing the amino acid sequence.

Meiosis Genetics Vocabulary

• Germ Cells:
  • Diploid cells in testes and ovaries that undergo meiosis.
  • Produce gametes.
• Gametes:
  • Haploid sperm and egg cells produced after meiosis.
  • Human diploid number is 46 (23 pairs of chromosomes).
  • Haploid gametes have 23 unpaired chromosomes.
  • Sperm fertilizes the egg, forming a zygote.
• Somatic Cells:
  • Diploid cells that make up body tissues.
  • Formed after the zygote divides and cells differentiate.
• Summary:
  • Somatic cells: diploid.
  • Germ cells: diploid.
  • Gametes: haploid.

Process of Meiosis

• Reduction Division: Meiosis reduces chromosome number, going from diploid (two sets of chromosomes) to haploid (one set).
• Steps:
  • DNA replication: Creates double chromosomes consisting of two sister chromatids.
  • Meiosis I: Separates homologous pairs.
    • Each resulting cell has one member of each homologous pair (haploid).
    • Each chromosome is still doubled.
  • Meiosis II: Separates the sister chromatids.
    • Result: Four unique haploid gametes.

Mitosis vs. Meiosis

• Mitosis:
  • One round of cell division separating sister chromatids.
  • Cells begin and end as diploid.
  • Daughter cells are clones of the parent cell.
  • Used for growth and repair.
• Meiosis:
  • Two cell divisions.
  • Meiosis I separates homologous pairs.
  • Meiosis II separates sister chromatids.
  • Goes from diploid to haploid.
  • Used to create gametes for reproduction.
  • Introduces variation; daughter cells are unique.

How Meiosis Creates Variation

• Two main ways meiosis generates diversity:
  • Independent assortment.
  • Crossing over and genetic recombination.
• Independent Assortment:
  • Phases of mitosis and meiosis have the same names, but meiosis has two cell divisions, so designations like prophase I, metaphase I, etc., are used.
  • Independent assortment occurs between prophase I and metaphase I.
  • Homologous pairs pair up during prophase I.
• Process:
  • Mother's and father's chromosome number one find one another and embrace, and the same for chromosomes two, three, etc.
  • During metaphase I, spindle fibers pull the pairs to the cell equator.
  • The way each pair is dragged to the middle is independent of every other pair.
  • In a simplified system, paternal chromosomes might be on the left, maternal on the right, or vice versa.
  • Random like flipping a coin, creates diversity.
• Mathematical Possibilities:
  • Two homologous pairs: Four different chromosome arrangements possible ($2^2$).
  • Three homologous pairs: Eight possible arrangements ($2^3$).
  • 23 pairs (humans): $2^{23}$ possible arrangements (8,388,608 combinations).
• Probability Example:
  • Chance of you and a sibling having the same chromosomal inheritance:
  • Same egg: $1/2^{23}$
  • Same sperm: $1/2^{23}$
  • Combined probability: $(1/2^{23}) * (1/2^{23}) = 1/70,000,000,000,000$
• Crossing Over:
  • Homologous pairs not only embrace but exchange parts during prophase I (synapsis).
  • At a point called a chiasma, DNA segments move from one homolog to the other.
  • Array of four sister chromatids is called a tetrad.
  • Crossing over creates recombinant chromosomes with unique DNA sequences.
• Diversity in Sexual Reproduction:
  • Independent assortment randomly arrays combinations of chromosomes in gametes.
  • Crossing over and genetic recombination create uniquely recombinant chromosomes.
  • Fertilization combines sperm and egg from different individuals.

Meiosis: The Complete Process

• Interphase:
  • Replicates chromosomes and duplicates DNA like mitosis.
  • Each chromosome consists of two sister chromatids by prophase I.
• Prophase I:
  • Homologous pairs pair up and embrace.
  • Synapsis and crossing over occur.
• Metaphase I:
  • Spindle fibers pull homologous pairs to the center of the cell.
  • Independent assortment occurs (main source of variation).
  • Number of chromosomal arrangements: $2^{\text{number of pairs}}$
• Anaphase I:
  • Homologous pairs are pulled apart.
• Telophase I:
  • New nucleus forms.
  • Cytokinesis I occurs.
  • Interphase II (not always shown).
• Prophase II:
  • Chromosomes condense again.
  • Transition from diploid to haploid.
• Metaphase II:
  • Doubled chromosomes are pulled to the cell equator.
• Anaphase II:
  • Sister chromatids are pulled apart.
• Telophase II:
  • New nuclear membrane forms.
  • Another cytokinesis occurs.
• Result: Four haploid gametes, each genetically unique, with single chromosomes.

Sex Determination

• Mammals:
  • Chromosomes 1-22 are autosomes (homologous pairs, same in males and females).
  • Sex chromosomes determine sex.
  • Females: Two X chromosomes.
  • Males: One X and one Y chromosome.
  • X and Y chromosomes are different; do not cross over.
  • X chromosome: Normal, with alleles for non-sex functions (immune, vision, clotting).
  • Y chromosome: Contains SRY region, initiates testes development, and later testosterone production.
  • Sperm determines zygote's sex.
  • Males pass on either X or Y chromosome.
  • Eggs always have an X chromosome.
  • X-carrying sperm + egg = female (XX).
  • Y-carrying sperm + egg = male (XY).
  • Ratio of males to females at birth is approximately 50:50.
• Birds:
  • Females have a Z and a W chromosome.
  • Males have two Z chromosomes.
  • Egg determines sex: Z or W.
  • Z-carrying egg + sperm = male (ZZ).
  • W-carrying egg + sperm = female (ZW).
  • Ratio of males to females at birth is approximately 50:50.
• Reptiles:
  • Sex determined by temperature during embryonic development.
  • Eggs in a nest in the sand; warmer at the top, cooler below.
  • Pivot point ($T_{PIV}$) determines sex.
    • Sea turtles: Above $T<em>{PIV}$ = female, below = male, at $T</em>{PIV}$= random (50:50)
    • Tuatara: Above $T_{PIV}$ = male, below = female.
    • Crocodiles: Two pivot points; coolest and warmest = females, intermediate = males.
• Ants, Bees, and Wasps:
  • Haplo-diploid sex determination (haplo-diploidy).
  • Males are haploid, from unfertilized eggs.
  • Females (queen and workers) are diploid, from fertilized eggs.
  • Queen undergoes normal meiosis.
  • Father (drone) is haploid, passes on 100% of chromosomes in sperm.
  • Worker bees are more closely related to each other (75%) than to their own offspring.

Nondisjunction and Chromosomal Variation

• Nondisjunction:
  • Failure of homologous pairs or sister chromatids to separate during meiosis.
  • Variations:
    • Meiosis I: Homologs don't separate; the result is 50% of gametes are n+1 (haploid plus one extra), and 50% are n-1 (haploid missing a chromosome).
    • Meiosis II: Sister chromatids don't separate; the result is that 25% of gametes are n+1, 25% are n-1, and 50% are normal.
• Consequences:
  • Eggs are n+1: Zygote has an extra chromosome (trisomy).
    • Example: Down syndrome (trisomy 21).
  • Eggs are n-1: Zygote is missing a chromosome (monosomy).
    • Example: Turner Syndrome (females with one X chromosome).
  • Variations in sex chromosomes (men with extra X or X and two Y chromosomes).

Mendelian Genetics

• Genes: Basic unit of heredity passed from parents to offspring, determining traits.
  • Molecular perspective: sequence of DNA nucleotides coding for RNA and protein.
• Mendel's Principle of Segregation:
  • Individuals have two copies of each gene on homologous chromosomes.
  • Alleles are alternative versions of genes with different DNA sequences.
  • Homozygous: Two identical alleles (e.g., AA).
  • Heterozygous: Two different alleles (e.g., Aa).
  • During gamete formation, individuals pass on only one allele per gene (segregation).
• Dominant and Recessive Alleles:
  • Dominant alleles are always observed in the phenotype (represented by a capital letter).
  • Recessive allele only shows up in a homozygote (lowercase letter).
• Genotype vs. Phenotype:
  • Phenotype: Observable characteristics (e.g., brown eyes).
  • Genotype: Underlying DNA, the inherited genes (e.g., Bb).
• Monohybrid Cross:
  • Cross between two heterozygotes (e.g., Pp x Pp).
  • Uses Punnett square to predict offspring.
  • Offspring ratio: 3:1 (dominant to recessive).
• P, F1, and F2 Generations:
  • P generation: Parental generation, true-breeding homozygotes.
  • F1 generation: First filial generation, offspring of P generation (heterozygotes).
  • F2 generation: Second filial generation, offspring of F1 generation.
• Dihybrid Cross:
  • Cross between two double heterozygotes (hybrid for two characteristics).
  • Example: Cross F1s (big T little t, big P little p).
• Mendel’s Principle of Independent Assortment:
  • Genes for different traits are segregated independently.
  • In a dihybrid organism (big T little t, big P little p), the T gene pair passes on independently of the P gene pair.
  • Four unique gametes can be created: big T big P, big T little p, little t big p, little t little p.
  • Use FOIL algorithm (First, Outside, Inside, Last) to deduce gametes.
  • Limited to genes on different chromosomes.
• Dihybrid Cross:
  • Cross between two double heterozygotes.
  • Uses FOIL to figure out gametes.
  • Results in a 9:3:3:1 ratio in the offspring.
• Connection Between Mendel's Laws and Meiosis:
  • Segregation: Parents have two alleles for each trait, but pass on only one.
  • In meiosis, diploid parent produces haploid gametes.
  • Independent assortment: What happens to one gene pair is independent of others.
  • Chromosomes assort independently during metaphase one of meiosis.
• Rule of Multiplication:
  • Probability of independent events occurring together is the product of their individual probabilities.
  • Used to predict probability of genotype from a trihybrid cross (e.g., aa bb cc from Aa Bb Cc x Aa Bb Cc).
  • Calculated as three independent Punnett squares.
  • Example: Probability of aa is 1/4, bb is 1/4, cc is 1/4; combined probability is (1/4) * (1/4) * (1/4) = 1/64.

Non-Mendelian Genetics and Environment Phenotype Interaction

• Linked Genes:
  • Genes located on the same chromosome.
  • Inherited together, unlike independent assortment.
  • Example: Genes for bristled appendages, body color, eye color, wing length in fruit flies.
  • Do not follow the Mendelian rule of independent assortment.
  • Notation: Plus sign (+) indicates wild type or dominant allele; symbols without + indicate recessive allele.
• Crosses Involving Linked Genes:
  • Test Cross: Dihybrid (B+B, VG+VG) crossed with a double recessive (bb, vgvg).
  • If genes were perfectly linked, half the offspring would have normal body and normal wings, and the other half would have a black body and vestigial wings.
  • Numbers won't always be the same, but the general concepts apply.
  • Majority of offspring have parental phenotypes, but some have recombinant phenotypes.
• Recombinant Phenotypes:
  • Combine phenotypes of the parents (e.g., gray body with vestigial wings).
  • Caused by recombination and crossing over during meiosis.
  • Linked genes can separate due to this process.
  • Some sister chromatids are recombinant, some are not.
  • The closer the alleles are, the less they'll tend to cross over.
• Recombination and Distance Between Genes:
  • The further apart genes are on the chromosome, the higher the percentage of recombinant gametes.
  • Genes A and E will recombine the most because they're the furthest apart.
  • Genes B and C will recombine the least because they are the closest together.
• Chromosome Mapping:
  • The percentage of recombination can be used to calculate the map distance between two alleles.
  • Columbia University researchers in the 1900s created chromosome maps by doing breeding experiments with fruit flies.
• Sex-Linked Genes:
  • Located on the X chromosome.
  • Males can't be heterozygous; they either have the allele or they don't.
  • Females can be heterozygous or homozygous.
• Inheritance of Recessive Sex-Linked Trait (Hemophilia):
  • The hemophilia allele is on the X chromosome, making it more common in males.
  • Sons inherit X-linked alleles from their mothers.
  • Mom is a heterozygote (carrier) or homozygous recessive.
  • Pedigree shows a cross between a heterozygous female and a normal male.
  • The mom passed on her defective X chromosome with the hemophilia allele.
  • Examples of X-linked recessive conditions: Hemophilia, red-green color blindness.
• Female Inheriting Recessive Sex-Linked Trait:
  • Absolutely, but it's uncommon.
  • The male parent must have the sex-linked recessive trait.
  • The female must be a heterozygote or have the trait.
  • 50% of the offspring are carriers.
• Non-Nuclear Inheritance:
  • Inheritance of genes that are not on a nuclear chromosome but on a mitochondrion or chloroplast.
  • Genes on mitochondria or chloroplasts are only passed on to the offspring through the female gamete.
  • Sperm's mitochondria are left outside the egg membrane during fertilization.
  • Female line inheritance.