L2 Basic Genetics

🧠 Concepts Introduced

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genetics: DNA RNA protein, any substances that have nucleic acid

Summary and Key Takeaways

Mendel's laws remain the foundation of classical genetics
Variations exist but principles still guide inheritance studies

Historical Background

Introduce Gregor Mendel as the father of genetics and his landmark work on pea plants.
Gregor Mendel (1822-1884)
Experiments with Pisum sativum (pea plants)
Published in 1866

Why Pea Plants

Easy to cultivate
Distinct traits
Controlled pollination
Short generation time

Key Genetic Terminology

Gene, Allele, Dominant, Recessive, Homozygous, Heterozygous, Genotype, Phenotype

Clarify essential vocabulary with examples (e.g., Tall (T) vs Dwarf (t)).

Mendel's Experimental Design

Cross-pollination of pure lines
Observation of F1 and F2 generations
Statistical analysis

Law of Segregation (1st Law)

Each organism carries two alleles for a trait, which segregate during gamete formation."

Use a simple example (Tt→ T + t gametes) to illustrate allele separation in meiosis.

Punnett Square: Monohybrid Cross

Example: Tall (TT) x Dwarf (tt) → F1 (Tt), F2 ratio 3:1

Demonstrate with a Punnett square how segregation leads to phenotypic ratios

Genotypic and Phenotypic Ratios

F2 generation: Genotype: 1 TT: 2 Tt: 1 tt

Phenotype: 3 Tall: 1 Dwarf

Differentiate between genotype and phenotype outcomes.

Monohybrid Cross Summary

One trait studied

Law of Segregation validated

3:1 phenotypic ratio

Law of Independent Assortment (2nd Law)

Alleles of different genes assort independently during gamete formation

Dihybrid Cross: Example

Seed shape (R/r) and color (Y/y) RrYy x RrYy → 9:3:3:1 ratio

Dihybrid Cross Punnett Square

Gametes: RY, Ry, rY, ry

F2: 9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green

Visualization of Independent Assortment

Diagram showing meiosis I and II separating alleles on different chromosomes

Applications of Mendel's Laws

Prediction of inheritance patterns
Genetic counseling
Plant and animal breeding
Understanding hereditary diseases

Extensions to Mendel's Principles

Incomplete dominance

Red (RR) x White (rr) → Pink (Rr)

Show deviation from Mendelian 3:1 ratio (now 1:2:1)

Codominance

Blood group AB (IAIB)

Codominance is a pattern of inheritance occurring at a single gene locus where both alleles are fully and simultaneously expressed in a heterozygote's phenotype. Neither allele is dominant over the other, and neither masks the other. Key Characteristic: Heterozygotes display both distinct traits concurrently rather than an intermediate blend.

Distinction: Unlike incomplete dominance (where red and white flowers yield pink offspring), codominance causes both parental phenotypes to appear side-by-side.

Example: ABO Blood Type (I^A I^B): An individual inheriting the I^A allele from one parent and I^B from the other produces both A-type and B-type carbohydrate antigens on the surface of their red blood cells, resulting in blood type AB. Roan Coat Color: In cattle and horses, crossing a red-furred parent (R^R R^R) and a white-furred parent (R^W R^W) produces a roan offspring (R^R R^W) with distinct, intermixed red and white hairs.

Multiple alleles

Key Characteristic: Even though a population has a pool of three or more alleles for a given gene, an individual diploid organism can only inherit two of those alleles (one from each parent).

Distinction: Expands the number of possible genotypes and phenotypes beyond traditional two-allele Mendelian genetics.

Example:
Human ABO Blood Group: The single gene controlling ABO blood type has three common alleles: I^A, I^B, and i.
I^A and I^B are codominant to each other.
Both I^A and I^B are completely dominant over the recessive i allele.
Rabbit Coat Color: A single gene locus (C) has four alleles in a dominance hierarchy (C > c^{ch} > c^h > c), producing full color, chinchilla, Himalayan, or albino phenotypes.

Epistasis

Epistasis is a form of gene interaction where the expression of one gene locus masks, modifies, or depends on the alleles present at a completely different gene locus.

Key Characteristic: Involves an interaction between two or more distinct genes operating in a shared biological or biochemical pathway. The gene doing the masking is epistatic; the gene being masked is hypostatic.

Distinction: Codominance and multiple alleles describe interactions between alleles at a single gene locus. Epistasis describes interactions across multiple separate gene loci.

Example:
Labrador Retriever Coat Color: Determined by two separate gene loci (B and E):
Gene B (Pigment Color): Dominant B yields black pigment; recessive b yields brown (chocolate) pigment.
Gene E (Pigment Deposition): Dominant E allows pigment to deposit into the hair shaft; recessive e blocks pigment deposition.
If a dog inherits ee at the second locus, no pigment is deposited regardless of the B locus alleles (B_ee or bbee). This results in a yellow Labrador. The E locus is epistatic to the B locus.

Human Genetics and Mendelism

Cystic fibrosis (autosomal recessive)

Huntington's disease (autosomal dominant)

Blood groups (codominance)

Pedigree Analysis

Symbols and rules for interpreting family trees

Autosomal Dominant Inheritance

Example: Huntington's disease One copy of allele sufficient to cause disease

Autosomal Recessive Inheritance

Example: Cystic fibrosis Both alleles must be defective

Show carrier parents (heterozygous) leading to affected offspring (25%).

Sex-linked Inheritance

Example: Hemophilia, Color blindness

Explain how X-linked traits follow non-Mendelian ratios due to hemizygosity in males.

Limitation of Mendel's Laws

Linkage • Definition: Genes located close together on the same chromosome tend to be inherited together as a unit. • Violation of Mendel: This breaks the Law of Independent Assortment, which assumes genes sort separately into gametes. • Example: Fruit fly studies show that body color and wing size genes on the same chromosome often pass down as a pair unless crossing over (genetic recombination) separates them
Polygenic inheritance • Definition: A single trait is controlled by multiple genes working together with additive effects. • Violation of Mendel: Instead of producing distinct categorical ratios (like 3:1), polygenic traits produce a smooth, continuous spectrum of phenotypes (often forming a bell curve). • Example: Human height is influenced by over 400 different genes, and skin color or eye color involve multiple interacting loci
Environmental effects • Definition: External non-genetic factors—such as diet, temperature, or lifestyle—can alter how a genotype is expressed. • Violation of Mendel: Mendel’s rules assume a fixed genotype always leads to a predictable phenotype. The environment can modify, mask, or mimic genetic traits. • Examples: • Human Height: A person may have the genetic potential to be tall, but poor childhood nutrition or chronic illness can limit actual growth. • Phenylketonuria (PKU): A genetic disorder where the body cannot process phenylalanine; however, severe symptoms can be prevented by following a strict low-phenylalanine diet. • Temperature-sensitive traits: Certain animals or mutants (such as vestigial wings in fruit flies) only display specific physical traits under particular temperature conditions.

Chromosomal Basis of Inheritance

Sutton-Boveri theory linking Mendel's factors to chromosomes:

The Sutton-Boveri Chromosome Theory (also called the chromosome theory of inheritance) states that genes are located at specific positions on chromosomes and that the behavior of chromosomes during meiosis explains Gregor Mendel’s laws of inheritance. Technically, Meiosis phase 1 + Telophase phase 2

Mendelian Ratios Summary Table

Clinical Relevance

Predicting genetic disorders
Prenatal genetic counseling
Carrier screening