BIOL 150A - Class 6: Mendel's Laws & Deviations

Mini-Quiz Assessment Structure and Course Policies

• Weekly Mini-Quiz Schedule and Availability:

  • Mini-Quiz 1 covers course material from Classes 2 through 5.

  • The assessment opens on September 22 and remains available until the end of the semester on December 7.

  • Completing the initial attempt during the first week of availability is strongly recommended to stay aligned with course progression.

• Format and Grading Criteria:

  • Mini-Quiz 1 consists of 6 multiple-choice questions.

  • Questions and correct answers are not displayed upon completion.

  • Unlimited submission attempts are permitted throughout the semester to facilitate learning and exam preparation for midterms and the final exam.

  • To earn the full Weekly Mini-Quizzes grade component, which accounts for $15\%$ of the total course grade, all 10 mini-quizzes must be completed prior to the end-of-semester deadline.

Practice Review Problems in Genetics

• Question 1: Mechanisms of Hereditary Transmission

  • Question: Which of the following options best represents one way in which genetics can address the question of how hereditary information is passed between generations?

  • Option a: Transcription

  • Option b: Translation

  • Option c: RNA

  • Option d: Genetic inheritance

  • Option e: Cross-breeding

  • Correct Answer and Explanation: Genetic inheritance (Option d) directly addresses the mechanism by which hereditary traits and genetic information are transmitted across successive generations.

• Question 2: Monohybrid Cross Genotypic Ratios

  • Question: A homozygous plant dominant for curly seeds ($C$) is crossed with a heterozygous plant expressing curly seeds. What would be the correct proportion of genotypes in the F2 generation from an F1 crossing of two heterozygotes?

  • Option a: $\frac{1}{2}\,Cc$, $\frac{1}{4}\,cc$, $\frac{1}{4}\,CC$

  • Option b: $\frac{1}{2}\,CC$, $\frac{1}{2}\,Cc$

  • Option c: $\frac{1}{4}\,Cc$, $\frac{1}{4}\,cc$, $\frac{1}{2}\,CC$

  • Option d: $\frac{1}{2}\,cc$, $\frac{1}{2}\,CC$

  • Option e: $\frac{1}{2}\,Cc$, $\frac{1}{2}\,cc$

  • Correct Answer and Step-by-Step Derivation:

  • Option a ($\frac{1}{2}\,Cc$, $\frac{1}{4}\,cc$, $\frac{1}{4}\,CC$) is correct.

  • When two heterozygotes ($Cc \times Cc$) are crossed to form the F2 generation, each parent produces $\frac{1}{2}$ $C$ gametes and $\frac{1}{2}$ $c$ gametes.

  • Combining gametes yields:

    • Homozygous dominant ($CC$): $\frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$

    • Heterozygous ($Cc$): $(\frac{1}{2} \times \frac{1}{2}) + (\frac{1}{2} \times \frac{1}{2}) = \frac{2}{4} = \frac{1}{2}$

    • Homozygous recessive ($cc$): $\frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$

  • Thus, the genotypic proportion in the F2 generation is $\frac{1}{2}\,Cc$, $\frac{1}{4}\,cc$, and $\frac{1}{4}\,CC$

• Question 3: Monohybrid Cross Phenotypic Ratios

  • Question: Using the answer from Q2, what would the representative phenotype be?

  • Option a: $\frac{1}{2}$ curly seeds, $\frac{1}{2}$ flat seeds

  • Option b: $\frac{1}{4}$ curly seeds, $\frac{3}{4}$ flat seeds

  • Option c: $\frac{3}{4}$ curly seeds, $\frac{1}{4}$ flat seeds

  • Option d: No curly seeds

  • Option e: All curly seeds

  • Correct Answer and Step-by-Step Derivation:

  • Option c ($\frac{3}{4}$ curly seeds, $\frac{1}{4}$ flat seeds) is correct.

  • Because allele $C$ (curly seeds) is dominant over allele $c$ (flat seeds), both $CC$ ($\frac{1}{4}$) and $Cc$ ($\frac{1}{2}$) express the curly seed phenotype.

  • Total proportion of curly seeds = $\frac{1}{4} + \frac{1}{2} = \frac{3}{4}$.

  • Only the homozygous recessive genotype $cc$ ($\frac{1}{4}$) expresses the flat seed phenotype.

  • This yields a phenotypic ratio of $\frac{3}{4}$ curly seeds to $\frac{1}{4}$ flat seeds ($3:1$).

Principle of Allelic Segregation (Mendel's First Law)

• Definition and Mechanism of Segregation:

  • The Principle of Allelic Segregation states that two alleles of a gene pair separate (segregate) from each other during gamete formation in meiosis.

  • As a result of this physical separation, half ($\frac{1}{2}$) of the gametes carry one allele, and half ($\frac{1}{2}$) carry the alternative allele.

  • Upon fertilization, gametes combine randomly to restore the diploid state in the next generation.

• Definition of a Gene in Classical Mendelian Genetics:

  • In classical genetics, a gene is functionally defined by a $3:1$ phenotypic ratio observed in the F2 generation following a monohybrid cross between two true-breeding parental lines.

• Monohybrid Cross Model System:

  • Parent (P) Generation: A cross between a true-breeding homozygous dominant parent with purple flowers ($PP$) and a true-breeding homozygous recessive parent with white flowers ($pp$).

  • P Gametes: The purple parent contributes exclusively $P$ gametes, while the white parent contributes exclusively $p$ gametes.

  • F1 Generation: All offspring are heterozygous hybrids ($Pp$) displaying the dominant purple flower phenotype ($100\%$ purple flowers).

  • F1 Gamete Production: Heterozygotes produce two gamete types in equal frequencies: $\frac{1}{2}$ $P$ and $\frac{1}{2}$ $p$

  • F2 Generation Cross ($F_1 \times F_1$): Self-fertilization or intercrossing of F1 plants ($Pp \times Pp$) yields a Punnett square matrix of 4 combinations.

  • Genotypic Outcome: $1\,PP : 2\,Pp : 1\,pp$ (Genotypic ratio = $1:2:1$).

  • Phenotypic Outcome: $\frac{3}{4}$ purple flowers to $\frac{1}{4}$ white flowers (Phenotypic ratio = $3:1$).

• Real-World Biological Example: Plant Pathogen Resistance

  • Inheritance of disease resistance against specific fungal pathogens in plant crop species follows simple Mendelian segregation.

  • Parental Cross: A true-breeding susceptible plant ($RR$) crossed with a true-breeding resistant plant ($rr$).

  • Disease resistance phenotypes segregate predictably according to Mendel's First Law in subsequent generations.

Principle of Independent Assortment (Mendel's Second Law)

• Definition and Chromosomal Basis:

  • The Principle of Independent Assortment states that alleles of two or more different gene pairs segregate independently of one another during gamete formation in meiosis.

  • This principle applies specifically to unlinked genes located either on different chromosomes or far apart on the same chromosome.

• Dihybrid Cross Demonstration:

  • Parental (P) Generation: Cross between true-breeding pea plants with yellow round seeds ($RRYY$) and plants with green wrinkled seeds ($rryy$).

  • P Gametes: $RRYY$ produces $RY$ gametes; $rryy$ produces $ry$ gametes.

  • F1 Generation: All offspring are dihybrids with genotype $RrYy$, manifesting the dominant yellow round seed phenotype.

  • Test Hypotheses for F1 Cross ($RrYy \times RrYy$):

  • Hypothesis 1: Dependent Assortment (Refuted):

    • Assumes allele pairs ($R$ with $Y$ and $r$ with $y$) remain linked and travel together into gametes.

    • Gametes produced: Only $\frac{1}{2}$ $RY$ and $\frac{1}{2}$ $ry$

    • Predicted F2 phenotypic ratio: $3:1$ (Yellow round : Green wrinkled).

    • Experimental outcome: Refuted by empirical data.

  • Hypothesis 2: Independent Assortment (Supported):

    • Assumes seed shape ($R/r$) and seed color ($Y/y$) segregate completely independently.

    • Gametes produced by F1: Four distinct gamete types in equal frequencies ($\frac{1}{4}$ $RY$, $\frac{1}{4}$ $rY$, $\frac{1}{4}$ $Ry$, $\frac{1}{4}$ $ry$).

    • Punnett Square Matrix ($4 \times 4$ grid, 16 equal-probability outcomes):

    • $\frac{9}{16}$ Yellow round ($R\_Y\_$)

    • $\frac{3}{16}$ Green round ($rrY\_$)

    • $\frac{3}{16}$ Yellow wrinkled ($R\_yy$)

    • $\frac{1}{16}$ Green wrinkled ($rryy$)

    • Predicted F2 phenotypic ratio: $9:3:3:1$

    • Experimental outcome: Supported by empirical cross data.

Extensions and Deviations from Mendelian Inheritance

• Incomplete Dominance:

  • Definition: A form of intermediate inheritance where neither allele is completely dominant over the other, resulting in a heterozygous phenotype that is an intermediate blend of both homozygous phenotypes.

  • Classical Example: Flower color in Snapdragon (Antirrhinum majus).

  • Parental Generation: Red flowers ($RR$) crossed with White flowers ($rr$).

  • F1 Generation: All offspring have Pink flowers ($Rr$).

  • F2 Generation Cross ($Rr \times Rr$):

    • Genotypic Ratio: $1\,RR : 2\,Rr : 1\,rr$

    • Phenotypic Ratio: $1\text{ Red} : 2\text{ Pink} : 1\text{ White}$ ($1:2:1$).

    • Key Distinguishing Feature: The phenotypic ratio directly mirrors the genotypic ratio ($1:2:1$), unlike complete dominance which exhibits a $3:1$ phenotypic ratio.

• Codominance and Multiple Alleles:

  • Multiple Alleles Concept: While an individual diploid organism can possess a maximum of two alleles for any given gene, more than two allelic variants can exist within a broader population.

  • Codominance Definition: A phenotypic condition in which two different alleles for a gene are fully and simultaneously expressed in the heterozygote without blending.

  • Classical Example: The Human ABO Blood Group System.

  • Governed by three alleles: $I^A$, $I^B$, and $i$

  • Alleles $I^A$ and $I^B$ are codominant with respect to each other, and both are completely dominant over allele $i$

  • Phenotype and Genotype Relationships:

    • Blood Type A: Genotypes $I^A I^A$ or $I^A i$. Displays Antigen A on red blood cell (RBC) surface; contains Anti-B antibodies in blood plasma. Can receive A or O blood; cannot receive B or AB blood.

    • Blood Type B: Genotypes $I^B I^B$ or $I^B i$. Displays Antigen B on RBC surface; contains Anti-A antibodies in blood plasma. Can receive B or O blood; cannot receive A or AB blood.

    • Blood Type AB: Genotype $I^A I^B$. Displays both Antigen A and Antigen B on RBC surface; contains neither Anti-A nor Anti-B antibodies in plasma. Known as the Universal Recipient (can receive blood from A, B, AB, or O donors).

    • Blood Type O: Genotype $ii$. Displays neither Antigen A nor Antigen B on RBC surface; contains both Anti-A and Anti-B antibodies in plasma. Known as the Universal Donor (can donate blood to all types; can receive only Type O blood).

• Pleiotropy:

  • Definition: A genetic phenomenon wherein a single gene mutation or allele influences multiple, distinct, and seemingly unrelated physical and physiological phenotypic characteristics.

  • Classical Example: Human Sickle-Cell Anemia (Allele $Hb^S$).

  • Homozygous Genotype: $Hb^S Hb^S$

  • Primary Cellular Effect: Production of abnormal sickle-cell hemoglobin. Under low oxygen conditions, abnormal hemoglobin crystallizes, deforming spherical red blood cells into rigid, sickle-shaped structures.

  • Pleiotropic Downstream Consequences:

    • Organ Damage:

    • Kidney failure

    • Heart failure

    • Spleen damage

    • Brain damage (resulting in impaired mental function and paralysis)

    • Systemic Clinical Manifestations:

    • Severe pain and recurrent fevers

    • Joint problems and swelling

    • General physical weakness

    • Severe chronic anemia

    • Pneumonia and heightened susceptibility to bacterial infections

• Polygenic Inheritance:

  • Definition: The additive physiological effect of two or more independent genes on a single phenotypic trait, producing continuous phenotypic variation across a population.

  • Classical Example: Adult Human Height.

  • Human height is a continuously varying quantitative trait influenced by multiple genetic loci operating additively, alongside environmental influences such as childhood nutrition.

  • Simplified Three-Gene Model ($A, B, C$):

    • Parental Cross: Very short ($aabbcc$, 0 dominant alleles) crossed with Very tall ($AABBCC$, 6 dominant alleles).

    • F1 Generation: Medium height trihybrids ($AaBbCc$, 3 dominant alleles).

    • F2 Generation Distribution ($AaBbCc \times AaBbCc$, 64 total gametic combinations):

    • 0 Dominant Alleles ($aabbcc$): $\frac{1}{64}$ (Very short)

    • 1 Dominant Alleles: $\frac{6}{64}$

    • 2 Dominant Alleles: $\frac{15}{64}$

    • 3 Dominant Alleles: $\frac{20}{64}$ (Medium height)

    • 4 Dominant Alleles: $\frac{15}{64}$

    • 5 Dominant Alleles: $\frac{6}{64}$

    • 6 Dominant Alleles ($AABBCC$): $\frac{1}{64}$ (Very tall)

    • Population Distribution Curve: Produces a symmetric, bell-shaped normal distribution curve centered around the intermediate phenotype.

Linked Genes and Violations of Independent Assortment

• Chromosomal Mechanism of Linkage vs. Unlinked Genes:

  • Unlinked Genes: Genes located on separate non-homologous chromosomes assort independently during Meiosis I because maternal and paternal chromosome pairs align randomly at Metaphase I and segregate independently at Anaphase I.

  • Linked Genes: Genes located near one another on the same chromosome do not assort independently. Because they physically reside on the same DNA molecule, alleles close together tend to remain linked during meiotic division and migrate into the same gamete and zygote.

• Crossing Over (Meiotic Recombination):

  • Definition: Homologous non-sister chromatids break and exchange reciprocal segments of DNA during Prophase I / Metaphase I of meiosis.

  • Impact on Linked Genes: Crossing over breaks physical genetic linkage between syntenic genes, producing novel combination alleles.

  • Chromatid Classification:

  • Non-Recombinant Chromatids: Chromatids that retain the original parental allele configuration.

  • Recombinant Chromatids: Chromatids that carry new combinations of alleles resulting from crossing over.

• Recombination Frequency and Classical Mapping:

  • Concept: The frequency with which two linked genes are separated by recombination during Meiosis I serves as a direct quantitative measure of the physical distance separating them on a chromosome.

  • Linear Distance Principle: Greater physical distance between two gene loci increases the probability of a crossover occurring between them, resulting in a higher recombination frequency.

  • Representative Recombination Frequency Map:

  • Consider linked genes $g$, $c$, and $l$ arranged linearly on a chromosome section:

    • Recombination frequency between gene $g$ and gene $c$ = $9\%$ ($9$ map units / cM).

    • Recombination frequency between gene $c$ and gene $l$ = $9.5\%$ ($9.5$ map units / cM).

    • Recombination frequency between gene $g$ and gene $l$ = $17\%$ ($17$ map units / cM).

• Genetic Chromosome Map Example in Fruit Flies (Drosophila melanogaster):

  • Gene loci map positions on a single fruit fly chromosome:

  • Position $0$: Short aristae (mutant phenotype: bristled appendages on head short) vs. Long aristae (wild-type phenotype).

  • Position $48.5$: Black body (mutant phenotype) vs. Gray body (wild-type phenotype).

  • Position $57.5$: Cinnabar eyes (mutant phenotype) vs. Red eyes (wild-type phenotype).

  • Position $65.5$: Vestigial wings (mutant phenotype) vs. Normal wings (wild-type phenotype).

  • Position $104.5$: Brown eyes (mutant phenotype) vs. Red eyes (wild-type phenotype).