Module 4: Mendelian Genetics and Principles of Heredity

Mendelian Genetics Learning Objectives

• Define Key Terms: Students must be able to define terminology related to heredity and genetics, specifically: dominance, recessive, homozygous, heterozygous, allele, loci, phenotype, and genotype.

• Explain Meiotic Actions: Students should explain how specific actions of meiosis, namely segregation and independent assortment, contribute to genetic diversity.

• Apply Punnett Squares: Use Punnett squares to predict outcomes of monohybrid crosses and provide correct genetic or phenotypic ratios.

Overview of Heredity and Gregor Mendel

• Heredity Definition: The transmission of traits from one generation to the next.

• Genetics Definition: The scientific study of heredity.

• Gregor Mendel (The Founding Father of Genetics):

  • He was an Augustinian monk who lived and worked in an abbey in Austria.

  • He was the first person to analyze patterns of inheritance.

  • He deduced the fundamental principles of genetics in 1866 (initial observations dated 1865).

• Experimental Model (Garden Peas): Mendel studied garden peas because they are:

  • Easy to grow.

  • Easily manipulated.

  • Available in many readily distinguishable varieties (e.g., purple or white flowers).

  • Capable of self-fertilization because they possess both male and female reproductive organs.

  • Mendel could strictly control parentage by managing cross-fertilization.

Mendel’s Experimental Methodology: Monohybrid Crosses

• Monohybrid Cross: A cross between parent plants that differ in only ONE character (e.g., flower color).

• P Generation (Parental Plants): Mendel created "true-breeding" varieties, where self-fertilization produced offspring identical to the parent. In his experiments, he crossed two different true-breeding varieties.

• F1 Generation (First Filial): The hybrid offspring of the P generation plants. Mendel observed that in these crosses, only one of the parental traits was expressed.

• F2 Generation (Second Filial): The offspring resulting from the cross or self-fertilization of the F1 plants.

Quantitative Data from Mendel’s Single-Trait Crosses

• Seed Shape:

  • P1: Smooth $\times$ Wrinkled.

  • F1: All Smooth.

  • F2 Total: $7,324$ peas.

  • F2 Counts: $5,474$ smooth; $1,850$ wrinkled.

• Seed Color:

  • F2 Counts: $6,022$ yellow; $2,001$ green.

• Seed Coat Color:

  • F2 Counts: $705$ gray; $224$ white.

• Pod Shape:

  • F2 Counts: $882$ full; $299$ constricted.

• Pod Color:

  • F2 Counts: $428$ green; $152$ yellow.

• Flower Position:

  • F2 Counts: $651$ axial (along stem); $207$ terminal (at tip).

• Stem Length:

  • F2 Counts: $787$ tall; $277$ short.

• General Conclusion: Genetic factors can be hidden or unexpressed. Despite looking identical to P1 plants, F1 plants carry hidden genetic factors. Monohybrid crosses involving heterozygotes (e.g., $Pp \times Pp$) typically lead to a $3:1$ phenotypic ratio.

Essential Genetic Vocabulary

• Dominant Trait:

  • Expressed in the F1 (heterozygous) condition.

  • Masks or "hides" the other trait.

  • Represented by a CAPITAL letter (e.g., $A$).

• Recessive Trait:

  • Repressed in the F1 but re-expressed in some members of the F2 generation.

  • Termed "hidden" by the dominant trait.

  • Represented by a lower case letter (e.g., $a$).

• Phenotype: The observable properties of an organism; what it looks like.

• Genotype: The specific genetic constitution of an organism; its DNA.

• Allele: One possible form of a gene, or a variant. Organisms have two copies (one on each chromosome). These are usually distinguished by their phenotypic effects.

• Locus (Loci, pl.): The specific location of a gene along a chromosome.

• Homozygous: Having identical alleles for one or more genes (e.g., $aa$ or $PP$).

• Heterozygous: Having two different alleles for one or more genes (e.g., $Bb$).

• Wild-Type Alleles: traits seen most often in nature (e.g., budgie plumage). These are not necessarily specified by dominant alleles. For example, the absence of freckles in humans is a frequent trait often specified by recessive alleles.

Mapping Genes on Chromosome 7

• p arm (Short arm):

  • $21.3$: IFNB2 (type of interferon).

  • $15.2$: PSP (phosphoserine phosphatase).

  • $12.2$: EGFR (epidermal growth factor receptor).

• Centromere: Middle of the chromosome.

• q arm (Long arm):

  • $21.11$: EPO (erythropoietin - hormone for red blood cell production).

  • $31.2$: CF (defect causes cystic fibrosis).

  • $35.0$: TCRB (beta chain of antigen receptor on T cells).

Principles of Inheritance and Meiosis

• Mendel’s Principle of Segregation:

  • Gene pairs separate from each other during gamete formation (meiosis).

  • P1 ($SS \times ss$) results in gametes ($S$) and ($s$).

  • Fertilization leads to F1 ($Ss$).

• Mendel’s Principle of Independent Assortment:

  • The random distribution of alleles into gametes during meiosis.

  • Leads to all possible combinations of gametes with equal probability in a cross between two individuals.

• Gamete Prediction:

  • If parent genotype is $Jj$, gametes are $J$ and $j$.

  • If parent genotype is $AA$, gametes are only $A$.

  • If parent genotype is $Cc$, gametes are $C$ and $c$.

How to Solve Genetics Problems (Punnett Squares)

1. Determine Parent Genotypes: Identify the alleles from the problem description.

2. Set up the Square: Place the parental gametes on the top and left side of the square.

3. Fill the Square: Combine gametes to show possible fertilization outcomes.

4. Calculate Genotypic Ratio: List the frequency of specific allele combinations (e.g., $1:2:1$).

5. Calculate Phenotypic Ratio: List the frequency of expressed traits (e.g., $3:1$).

Practical Case Studies

• Earlobes (Human): Unattached lobes ($A$) are dominant to attached lobes ($a$).

  • Female with attached lobes: $aa$.

  • Male with unattached lobes whose mother has attached lobes: $Aa$.

  • Results: Genotypic ratio of $2\,Aa : 2\,aa$ ($50\%$ each); Phenotypic ratio of $2\,\text{unattached} : 2\,\text{attached}$ ($50\%$ each).

• Beetle Exterior Color: Black ($B$) is dominant over brown ($b$).

  • $BB \times bb$: All offspring are $Bb$ (Black).

  • $Bb \times bb$: $50\%$ Black ($Bb$), $50\%$ brown ($bb$).

  • $bb \times bb$: $100\%$ brown ($bb$); black offspring are impossible.

• Achondroplasia (Dwarfism): Dominant ($D$) over typical height ($d$).

  • $DD$: Lethal (child dies before age one).

  • $Dd$: Individual has dwarfism (carrier).

  • $dd$: Individual has typical height.

  • Cross between two carriers ($Dd \times Dd$):

  • Outcomes: $1\,DD$ (lethal), $2\,Dd$ (dwarfism), $1\,dd$ (typical height).

  • Probability of $Dd = 2/4 = 50\%$.

Course Logistics and Schedule

• Week 14: Lecture Exam 3; Mendel and Genetics; Evolution and Statistics Lab.

• Week 15: Dihybrid crosses; Human Heredity; Genetics Lab.

• Week 16: Non-Mendelian genetics; Evidence of Evolution; Animal Diversity Lab.

• Week 17: Origin of Life (Online); Evolution mechanisms; Lab Exams.

• Final Exam (Wednesday 5/27, 10:00 AM – 11:50 AM):

  • Exam 4 Content (100 pts): Covers Module 4; review via SLOs (Student Learning Objectives).

  • Cumulative Review (50 pts): "Big ideas" of the course; Multiple Choice, T/F, Matching, Labeling (no open-ended).

  • Materials: Bring pencil and a $3 \times 5$ inch "cheat sheet" card.

• Extra Credit: Create a video response summarizing understanding of AlphaFold or CRISPR for Module 3.

Questions & Discussion

• TPS Discussion (Purple vs White Plants):

  • Question: If you cross a purple and white plant, what color do you expect? Why? Would a "true-breeding" purple plant produce a white flower?

  • Context: Results showed F1 flowers were all purple, while F2 showed a $3:1$ ratio of purple to white. A true-breeding purple plant ($PP$) cannot produce a white flower because it only carries the $P$ allele.

• TPS Discussion (P and F1 Purple Similarity):

  • Question: How are purple flowers in P and F1 similar or different?

  • Answer: Phenotypically they look identical. Genotypically, P plants are homozygous dominant ($PP$) and F1 plants are heterozygous ($Pp$).

• TPS Discussion (Chromosomal Analysis):

  • Questions regarding diagrams: How many chromosomes? How many genes? How many alleles? How many loci?

  • Answer: Gene loci designate physical positions on chromosomes; alleles are the specific versions ($A$ vs $a$) at those locations.

• TPS Discussion (Earlobes):

  • Question: Can you determine the genotype of someone with attached lobes? With unattached lobes?

  • Answer: Attached lobes must be $aa$ (recessive). Unattached lobes can be either $AA$ or $Aa$, and may require pedigree analysis (like checking the male's mother) to confirm.