Biology1A03: Theme 5 Module 2

11/26/25, 10:03 PM Biology1A03: Theme 5 Module 2 2025. 11. 26. 5:04 Biology 1A03 Unit 1: The inheritance of chromosomes-meiosis Sexual reproduction, gametes & genetic variation Genetic information is transmitted from generation to generation within families Childs sexually reproducing organisms are not identical copies of either parent In contrast, asexual reproduction produces offspring that are exact genetic copies of the parent The only human cells not produced by mitosis are the gametes Gametes are derived from specialized germ cells in the ovaries (females, and testes (males, In sexually reproducing organisms, chromosomes are passed to offspring through the formation of gametes Gametes: Many traits are inherited, yet there is also extensive genetic variation Offspring receive unique combinations of genes from both parents Production of a diploid offspring occurs via gamete formation Parental sex cell precursors undergo ​DNA replication Followed by two successive rounds of meiosis​, reducing chromosome number diploid → haploid Diploid (2n) = Haploid (n) = Diploid (2n) Haploid (n) If gametes were produced by mitosis, they would be diploid, producing offspring with double chromosome number → fatal consequences It is the unique haploid gametes from each parent that pass genetic information to the next generation In humans: Male parent: 1 precursor cell → 4 haploid sperm cells Female parent: 1 precursor cell → 1 large egg (high cytoplasmic volume) + 3 polar bodies Polar bodies serve to reduce chromosome content of the egg Meiosis, chromosome reduction & genetic variation Each sexually reproducing organism has a characteristic diploid and haploid chromosome number Meiosis reduces chromosome number in gametes → produces haploid cells Meiosis allows recombination of parental homologous chromosomes Unique and variable gametes Gametes can then fuse to produce a diploid offspring Many steps of meiosis are similar to mitosis, but there are key differences Before meiosis begins, interphase allows chromosome duplication (same as mitosis) Meiosis involves two consecutive cell divisions No second interphase = DNA Final result: four haploid daughter cells, each with half the chromosome number of the parent cell After chromosome duplication, cells enter Prophase I of Meiosis I Characterized by chromosome condensation Synapsis = pairing & physical connection of homologous chromosomes A synaptonemal complex forms between homologous chromosomes to hold them together 11/26/25, 10:03 PM A duplicated chromosome = two sister chromatids Homologous chromosomes​= one from each parent They carry the same genes but may contain different alleles at specific loci As meiosis progresses: Chromosome number is reduced in each daughter cell Final 4 daughter cells each receive one unique set of chromosomes These cells are genetically different from each other and from the parent cell Prophase I & Crossing Over Prophase I is characterized by crossing over As chromosomes condense, genetic rearrangement occurs Crossing over happens between homologous chromosome pairs Between non-sister chromatids Crossing over involves exchange of corresponding DNA segments Evidence of crossing over appears at X-shaped regions Called chiasma (singular) or chiasmata (plural) Occur where non-sister chromatids have exchanged DNA Like mitosis, Prophase I also includes: Centrosome duplication Centrosome movement Spindle formation Nuclear envelope breakdown 11/26/25, 10:03 PM Homolog pairing, non-sister chromatids & recombinant chromatids Pairing of homologous chromosomes is a unique feature of meiosis During synapsis, all homologous chromosomes pair with each other This forms a bivalent A bivalent = a pair of synapsed chromosomes Forms a four-stranded structure have 4 chromatids = also called tetrad two chromosomes × two chromatids each Non-sister chromatids are attached to different centromeres Sister chromatids Duplications of the same chromosome Genetically identical Non-sister chromatids Belong to different homologous chromosomes Genetically similar, but not identical One homolog is maternal, the other paternal bvalent Crossing over between homologous pairs Causes physical breakage and reunion​between non-sister chromatids Produces chiasmata, visible along paired chromosomes Crossing over is a random process Can occur anywhere along homologous chromosome pairs Produces recombinant chromatids Contain both paternal and maternal DNA segments Contribute to distinct gametes and increased genetic diversity sibling Sexually rproducing species Meiosis I — Metaphase I, Anaphase I, Telophase I As Meiosis I progresses, homologous chromosome bivalents become randomly arranged at the metaphase plate Random orientation is caused by spindle microtubules during Metaphase I Along with crossing over, this increases genetic diversity in gametes 11/26/25, 10:03 PM During Anaphase I: Proteins of the synaptonemal complex (holding homologs together) break down Homologous chromosomes (not sister chromatids) separate Homologs move to opposite poles of the cell key difference from mitotic anaphase, where sister chromatids separate Telophase I marks the end of Meiosis I Each half of the cell has a haploid set of duplicated chromosomes Each chromosome still consists of recombinant sister chromatids Meiosis I is a reductional division () Chromosome number is ​reduced (diploid → haploid) Example: Human parent cell with 46 chromosomes → after Meiosis I → two haploid cells with 23 chromosomes each By the end of Telophase I: Chromosomes slightly uncoil Nuclear envelope reforms Followed by cytokinesis, producing two separate cells Meiosis II — Equational Division No chromosome duplication occurs between Meiosis I and Meiosis II Therefore Telophase I + cytokinesis are followed immediately by Prophase II Meiosis II is an equational division Parent cells at the start of Meiosis II have the same number of chromosomes​as the gametes produced at the end Because ​sister chromatids separate, not homologous chromosomes Example (human gametes): At Prophase II: each cell contains 23 duplicated chromosomes 11/26/25, 10:03 PM By Telophase II: these chromatids separate, producing haploid daughter cells Meiosis II is very similar to mitosis, except for differences in chromosome number Division process: Prophase II: Nuclear envelope breaks down Spindle apparatus forms Chromosomes condense Metaphase II: Chromosomes (each still with two sister chromatids) align at the metaphase plate Due to earlier crossing over, sister chromatids are not genetically identical Anaphase II: Proteins (cohesin) holding sister chromatids at centromeres break down Sister chromatids separate and move to opposite poles as individual chromosomes Telophase II: Nuclear envelope reforms Chromosomes decondense Followed by cytokinesis Final result of Meiosis I + Meiosis II: One parent cell → four haploid daughter cells Each daughter cell is genetically distinct from the others and from the parent cell Nondisjunction — Errors in chromosome separation Ideally, the meiotic spindle distributes the correct number of chromosomes to each daughter cell However, sometimes ​errors occur during meiosis Meiosis I: A pair of homologous chromosomes may fail to separate 11/26/25, 10:03 PM Meiosis II: Sister chromatids may fail to separate Result of these failures: Some gametes receive extra chromosomes (n+1) Other gametes have missing chromosomes (n-1) This error is called nondisjunction Nondisjunction can lead to detrimental or lethal effects in offspring Trisomy (2n+1) — Example: Down syndrome (Trisomy 21) Monosomy (2n−1) — Example: Turner syndrome (XO) It can also occur during mitosis, and is often seen in cancer cells Unit 2: Mendelian Inheritance Mendel, heredity & early genetics Many ​heritable variations exist naturally in a population Examples: eye colour, hair colour In the early 1800s, heredity was explained by the blending hypothesis Example: Red + White = Pink Tall mom + Short dad = Medium child Belief that parental genetic material mixed to produce traits in offspring This concept changed with Gregor Mendel Mendel identified two laws of inheritance His work laid the foundation for modern transmission genetics Mendel used the scientific method long before chromosomes were ever seen under a microscope Mendel chose pea plants because many traits vary between individuals But Mendel only studied characteristics with two distinct, alternate forms No intermediate forms Studied 7 traits: Seed colour Seed shape Pod colour Pod shape Flower colour Flower position Plant height Pea plants normally self-fertilize, but Mendel controlled fertilization He began with ​true-breeding plants (plants that produce the same trait over many self-fertilized generations) These plants are homozygous​(two identical alleles) Example: a purple-flower strain produces only purple-flowered plants Mendel’s experimental approach: Cross true-breeding parents Then cross the offspring Look for mathematical patterns that predict outcomes of future crosses Mendel identified dominant and recessive​traits Dominant traits appear in the offspring of a cross between two true-breeding (homozygous) parents Recessive: F1 Mendel’s P generation → F1 generation Mendel always began with two contrasting, true-breeding pea varieties He cross-fertilized these parents to study inheritance Example cross: One true-breeding yellow-seed parent One true-breeding green-seed parent These parents = P generation (parental generation) The offspring of the P generation = F1 generation (“filial 1”) In this cross, all F1 offspring​had yellow seeds Mendel followed and recorded traits across generations again From this result, Mendel deduced: The yellow seed trait is dominant The green seed trait is recessive If Mendel had stopped at only observing the F1 generation He would have missed the deeper patterns of inheritance Evidence against the blending model If the blending model were correct, F1 offspring would have a mixed seed color Example: an “average” color between yellow and green (lime green) But Mendel’s F1 generation showed only yellow seeds This disproved the blending hypothesis Question: What happened to the green parent’s genetic contribution? If it were truly lost, F1 plants could only produce yellow seeds in the F2 generation Mendel allowed F1 plants to self-pollinate He discovered F1 plants did not breed true In the F2 generation, the traits reappeared in a ​3:1 ratio 3 dominant (yellow) 1 recessive (green) This showed that recessive traits are not lost, only masked in the F1 generation Mendel’s 3:1 Ratio & Law of Segregation Mendel examined F2 generation results across all 7 pea plant traits With large sample sizes and careful record-keeping, he consistently observed a 3:1 ratio 3/4 dominant phenotype 1/4 recessive phenotype Interpretation of the ratio: For every 4 offspring, 3 show the dominant trait, 1 shows the recessive trait Mendel concluded: The recessive “heritable factor” does not disappear It is masked by the dominant allele in F1 To explain this, Mendel proposed the Law of Segregation: Two alleles of a gene segregate during gamete formation Each parent passes only one allele to each gamete Offspring inherit two alleles total (one from each parent) Assumptions of the Law of Segregation: Alternative versions of a gene (alleles) produce variation in traits 11/26/25, 10:03 PM If alleles differ, the ​dominant allele determines the visible trait The recessive allele has no noticeable effect on phenotype when paired with a dominant allele In true-breeding organisms, ​all gametes contain the same allele Punnett Squares Punnett square is a simple visual tool used to predict the possible allele combinations of offspring from parents with known traits Punnett squares use the outcomes of meiosis Each parent contributes one allele for a given gene to their gametes For any given gene, there are two possible alleles, and each parent can produce gametes containing either allele To construct a Punnett square: List the possible gametes produced by Parent 1 List the possible gametes produced by Parent 2 Fill in the boxes by combining one allele from each parent Each box in the Punnett square represents a possible genotype of the offspring after fertilization Using these genotype combinations, one can determine: Genotypic ratios of the offspring Example: 1 YY : 2 Yy : 1 yy Phenotypic ratios based on dominant and recessive allele interactions Example: 3 yellow : 1 green Gametes & Mendel’s Equal Segregation True-breeding dominant (yellow seeds) → AA True-breeding recessive (green seeds) → aa Heterozygous individual → Aa F1 During meiosis: 11/26/25, 10:03 PM Homologous chromosomes separate Each gamete gets only one allele​(either A or a) If an organism is homozygous: AA → all gametes = A aa → all gametes = a If an organism is heterozygous (Aa): Gametes can be A or a 50% A, 50% a This follows ​Mendel’s Law of Equal Segregation Key point: Homozygous parents → one type of gamete Heterozygous parents → two types of gametes (equal probability) F2 Generation Genotype Probabilities Gametes combine randomly → zygote genotype follows probability rules Probability of AA (dominant homozygous) Parent gametes: A (50%) × A (50%) 0.5 × 0.5 = 0.25 → 25% Probability of aa (recessive homozygous) Parent gametes: a (50%) × a (50%) 0.5 × 0.5 = 0.25 → 25% Probability of Aa (heterozygous) Two ways: A from one × a from other, OR a from one × A from other 0.25 + 0.25 = 0.5 → 50% Final genotype ratio (F2): AA : Aa : aa = 1 : 2 : 1 F2 Phenotype Ratio & Hidden Genotypes F1 gametes: 50% → A, 50% → a Expected F2 phenotype ratio: Dominant : Recessive = 3 : 1 Punnett square explains this 3:1 phenotypic ratio in F2 Phenotype alone ≠ genotype Cannot tell genotype just from dominant/recessive appearance Mendel’s hypothesis: Underlying genotypic ratio = 1 : 2 : 1 (AA : Aa : aa) This genotypic ratio can be tested experimentally Test Cross & Evidence for Segregation Test cross: Cross heterozygous F1 (Aa) × true-breeding recessive (aa) Used to determine if an individual with a dominant phenotype is AA or Aa Why recessive parent? Recessive parent makes only a gametes So offspring genotype depends entirely on the mystery parent's gametes If mystery parent = Aa (heterozygous): Offspring: 50% Aa (yellow), 50% aa (green) Phenotype ratio = 1:1 11/26/25, 10:03 PM If mystery parent = AA (homozygous dominant): Offspring: 100% Aa All yellow Mendel’s results: Observed 1:1 outcome (heterozygous test cross pattern) twice as often Cross A = Aa × aa → 1:1 (yellow : green) Cross B = AA × aa → 100% yellow Mendel’s Law of Segregation & Meiosis Mendel’s principle of segregation Explained by how chromosomes separate during meiosis Diploid precursor → 4 gametes Each parent produces four possible gametes from one precursor cell Meiosis I: Homologous chromosomes (maternal + paternal) align at the metaphase plate Anaphase I: homologous chromosomes separate Meiosis II: Sister chromatids separate As a result, each allele copy is separated into different gametes Outcome: Each gamete carries only one allele for each gene This physical separation of chromosomes = basis of Mendel’s Law of Segregation 11/26/25, 10:03 PM Unit 3: Independent Assortment Law of Independent Assortment (Mendel’s 2nd Law) Law of Segregation Derived from following one trait e.g., seed color F1 from true-breeding parents = monohybrids (heterozygous for one trait) Cross between monohybrids → monohybrid cross Studying two traits at once → discovery of 2nd law Mendel examined seed shape + seed color together Yellow (dominant) vs green Smooth (dominant) vs wrinkled Parental cross (true-breeding for two traits): Yellow + wrinkled × Smooth + green Because yellow & smooth are dominant → F1 = all smooth + yellow Goal of the experiment: Determine how one gene pair’s alleles segregate compared to another gene pair’s alleles Are they linked? Or do they assort independently? Key question: When F1 (double heterozygotes) self-fertilize, what genotypes + phenotypes appear in the F2 generation? Discovery: Different allele pairs behave ​independently during gamete formation → Law of Independent Assortment 11/26/25, 10:03 PM TTRR x ttrr = dihybrids dihybrid corse consist of heterozy Example: TtRr x TtRr or TTRR x ttrr According to the Law of Independen Assortment, hybrid will four different gametes Dihybrid Cross & 9:3:3:1 Ratio Mendel observed an F₂ phenotypic ratio of: 9 yellow-round Dominant-dominant 3 green-round Recessive-dominant 3 yellow-wrinkled Dominant-recesive 1 green-wrinkled Recessive-recessive Using a Punnett square, these phenotypes match corresponding F₂ genotypes For seed color: alleles A (yellow) and a (green) For seed shape: alleles B (round) and b (wrinkled) 11/26/25, 10:03 PM Alleles segregate equally into gametes: Half of gametes carry A, half carry a Half carry B, half carry b This produces 4 possible gamete combinations These 4 gamete types result from independent assortment Observed in F₁ parents Sperm from pollen & eggs from ovules each contribute one gamete type to the F₂ offspring Punnett square analysis of a dihybrid cross predicts 4 phenotype categories in the F₂ generation with a ​9:3:3:1 ratio Important: The law of independent assortment only applies to genes (allele pairs) located on different chromosomes Calculating Probabilities in a Dihybrid Cross To find genotype probabilities in the F2 generation, we calculate the probability of ​two independent events​happening together Example 1 — Genotype: aa bb Only one gamete combination produces aa bb: Ovule: ab Pollen: ab Probability = ¼ × ¼ = 1/16 Example 2 — Genotype: Aa Bb To be Aa, one gamete must contribute A and the other a To be Bb, one gamete must contribute B and the other b There are four possible gamete combinations that produce Aa Bb Example shown: Ovule: ab (probability ¼) Pollen: AB (probability ¼) 11/26/25, 10:03 PM Combined probability = ¼ × ¼ = 1/16 Since four different combinations produce the Aa Bb genotype: Total probability = 1/16 + 1/16 + 1/16 + 1/16 = 4/16 = ¼ (25%) Ovule Pollen ab AB AaBb aB Ab AaBb Ab aB AaBb AB ab AaBb Independent Assortment & Chromosome Alignment The law of independent assortment applies to genes located on different chromosomes This law depends on how nonhomologous chromosomes align​at the metaphase plate during meiosis I Scenario 1: A chromosome carrying the dominant allele of one gene can align so that it segregates to the same pole as the chromosome carrying the dominant allele of another gene Scenario 2: The dominant allele of one gene may instead segregate with the recessive allele of another gene on a different chromosome Because both orientations are equally likely, all possible gametes (four combinations) are produced in a 1:1:1:1 ratio​by the end of meiosis II However, this equal ratio occurs only when genes are on different chromosomes If two genes are on the same chromosome, they will not assort independently If they are physically close, they tend to be inherited together These genes are called linked genes 11/26/25, 10:03 PM Unit 4: From gene to protein Connecting Mendelian Traits to Molecular Function Understanding inheritance principles helps explain how single genes encode proteins that are essential for cellular function One of Mendel’s studied genes was the B gene: Dominant allele (B) → smooth seeds Recessive allele (b) → wrinkled seeds In 1990, the B gene was identified as SBE1 (Starch-Branching Enzyme 1) SBE1 gene encodes the starch-branching enzyme converts sugar → starch during pea seed development In wrinkled peas (bb): SBE1 gene is interrupted, so sugar cannot be efficiently converted to starch Seeds accumulate ​nearly double the sucrose compared to BB plants As a result: Young bb seeds take in more water and become ​larger due to high sucrose content As seeds mature and lose water, they shrink unevenly, producing the wrinkled phenotype Mendel’s inheritance principles therefore reveal not only phenotypic patterns, but also the molecular basis behind gene function and variation