Comprehensive Study Notes on Principles of Inheritance and Variation

Mendelian Genetics and the Study of Pea Plants

Selection of Garden Pea (Pisum sativum)
- Mendel chose the garden pea for several specific biological advantages:
  - It features clear, contrasting traits (e.g., tall vs. dwarf).
  - It has a short life cycle (annual plant), allowing for multiple generations to be studied quickly.
  - The plant is naturally self-pollinating but can be easily cross-pollinated manually.
  - It produces a large number of offspring in a single generation, providing statistically significant data.
Contrasting Traits Studied by Mendel
- Mendel identified seven pairs of contrasting traits. Beyond plant height (Tall/Dwarf) and seed color (Yellow/Green), these include:
  - Seed Shape: Round (Dominant) vs. Wrinkled (Recessive).
  - Pod Shape: Inflated (Dominant) vs. Constricted (Recessive).
  - Pod Color: Green (Dominant) vs. Yellow (Recessive).
  - Flower Color: Violet (Dominant) vs. White (Recessive).
  - Flower Position: Axial (Dominant) vs. Terminal (Recessive).
Identification of Dominant Traits
- In the case of pod morphology: Inflated and Green are dominant over constricted and yellow.
- In the case of seed morphology: Round and Yellow are dominant over wrinkled and green.

Fundamental Genetic Principles and Terminology

Genes and Alleles
- Gene: A unit of inheritance that controls a specific trait.
- Allele: Slightly different forms of the same gene that occupy the same locus on homologous chromosomes. A gene usually has two or more alternative forms (alleles).
Homozygous vs. Heterozygous
- Homozygous: An organism containing two identical alleles for a particular character (e.g., $TT$ or $tt$ ).
- Heterozygous: An organism containing two different alleles for a particular character (e.g., $Tt$ ).
Purity of Gametes
- Gametes are said to be "pure" because during meiosis, the two alleles of a gene pair segregate from each other so that a gamete carries only one allele for a trait. This ensures that every gamete is haploid and contains no "mixed" characters.
The Punnett Square
- A graphical representation used to calculate the probability of all possible genotypes of offspring in a genetic cross. It is effectively used to predict the phenotypic and genotypic ratios of $F_1$ and $F_2$ generations.

Mendel’s Laws of Inheritance

Law of Dominance
- In a monohybrid cross between two organisms pure for any pair of contrasting characters, the character that appears in the $F_1$ generation is called the dominant character, and the one that is suppressed is the recessive character. This law explains why only one parental character is expressed in the $F_1$ generation.
Law of Segregation (Law of Purity of Gametes)
- This law is considered universally acceptable and without exception. It states that during the formation of gametes, the two alleles for a trait segregate (separate) so that each gamete receives only one allele.
Law of Independent Assortment
- This law is based on observations of dihybrid crosses. It states that when two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair. For example, in seeds, the inheritance of shape (round/wrinkled) does not influence the inheritance of color (yellow/green).

Analyzing Genetic Crosses

Monohybrid Cross Ratios (F2 Generation)
- Phenotypic Ratio: $3:1$ (Ex: 3 Tall : 1 Dwarf).
- Genotypic Ratio: $1:2:1$ ( $1$ Homozygous Tall ( $TT$ ) : $2$ Heterozygous Tall ( $Tt$ ) : $1$ Homozygous Dwarf ( $tt$ )).
- Heterozygous population percentage: $50\%$ .
- Homozygous population percentage: $50\%$ ( $25\%$ dominant + $25\%$ recessive).
The Test Cross
- Definition: A cross between an individual of unknown genotype (showing the dominant phenotype) and a homozygous recessive parent ( $tt$ , $vv$ , etc.).
- Purpose: To determine the zygosity (homozygous or heterozygous) of the dominant parent.
- Mechanism:
  - If the offspring are $100\%$ dominant, the parent was homozygous dominant.
  - If the offspring show a $1:1$ ratio of dominant to recessive, the parent was heterozygous.
Dihybrid Crosses
- Standard dihybrid phenotypic ratio: $9:3:3:1$ .
- Standard dihybrid test cross ratio: $1:1:1:1$ .
- If a dihybrid cross ( $AaBb \times aabb$ ) results in a deviation where parental forms are far higher than $25\%$ , it indicates that the two genes are linked on the same chromosome and do not assort independently.

Deviations from Mendelian Inheritance

Incomplete Dominance
- Observed in Snapdragon (Antirrhinum majus).
- A cross between red ( $RR$ ) and white ( $rr$ ) flowers produces pink ( $Rr$ ) offspring.
- The $F_1$ phenotype does not resemble either parent but is an intermediate. This is not "blending" because the original parental traits (red and white) reappear in the $F_2$ generation ( $1:2:1$ ratio).
Codominance and Multiple Alleles
- ABO Blood Grouping: Controlled by gene $I$ , which has three alleles: $I^A$ , $I^B$ , and $i$ .
- Multiple Allelism: More than two alleles governing the same character (investigated only in populations).
- Codominance: In blood type AB ( $I^AI^B$ ), both alleles express their own sugars on red blood cells simultaneously.
- Genotypes and Phenotypes:
  - Total possible genotypes: $6$ ( $I^AI^A, I^Ai, I^BI^B, I^Bi, I^AI^B, ii$ ).
  - Total possible phenotypes: $4$ (A, B, AB, O).
Pleiotropy
- Occurrence where a single gene can exhibit multiple phenotypic expressions.
- Example: In garden peas, the gene for starch grain size also affects seed shape.
  - Large grains ( $BB$ ) lead to round seeds.
  - Small grains ( $bb$ ) lead to wrinkled seeds.
  - Intermediate grains ( $Bb$ ) also lead to round seeds, showing that dominance is not an autonomous feature of a gene if we look at grain size (incomplete dominance) vs. shape (full dominance).
Polygenic Inheritance
- Traits controlled by three or more genes where the environment also plays a role.
- Example: Human skin color. The phenotype reflects the additive effect of each allele.

Chromosomal Theory and Linkage

Chromosomal Theory of Inheritance
- Proposed by Sutton and Boveri. They noticed that the behavior of chromosomes was parallel to the behavior of genes.
T.H. Morgan and Drosophila Genetics
- Morgan used Fruit flies (Drosophila melanogaster) because:
  - They can be grown on simple synthetic medium.
  - They complete their life cycle in two weeks.
  - A single mating produces many progeny.
  - There is clear sexual dimorphism (males and females are easily distinguishable).
  - It has only 4 pairs ( $2n=8$ ) of chromosomes (linkage groups).
- Linkage: The physical association of genes on a chromosome.
  - Strong Linkage: Genes are very close; low recombination (e.g., $1.3\%$ in Morgan's yellow-bodied/white-eyed cross).
  - Weak Linkage: Genes are far apart; higher recombination (e.g., $37.2\%$ ).
- Alfred Sturtevant: Morgan's student who first used the frequency of recombination between gene pairs as a measure of the distance between genes to "map" their position on the chromosome.

Practical Exercises & Quantitative Data

Gamete Calculation
- For a diploid organism heterozygous at $n$ loci, the number of types of gametes produced is $2^n$ .
- Example: Heterozygous for 4 loci ( $n=4$ ) produces $2^4 = 16$ types of gametes.
Specific Cross Results
- Cross between $TtYy$ and $Ttyy$ (Tall Yellow x Tall Green):
  - Expected phenotypes include Tall-Green, Tall-Yellow, Dwarf-Green, and Dwarf-Yellow.
- Cross between $AaBb$ and $aabb$ :
  - This is a dihybrid test cross with a phenotypic ratio of $1:1:1:1$ .

Questions & Discussion

Q: Why did Mendel self-pollinate F1 to get F2?
- A: To observe the segregation of alleles and the reappearance of the recessive trait that was hidden in the $F_1$ generation.
Q: Why did Mendel cross a tall plant with a dwarf plant in a test cross?
- A: To ensure the recessive parent only contributes recessive alleles, making the phenotype of the offspring directly representative of the unknown parent's alleles.
Q: Why was Mendel's work unrecognized for so long?
- A: (1) Poor communication in that era; (2) His use of mathematics and statistics in biology was new and unacceptable to many; (3) He could not provide physical proof of "factors" (genes) as discrete units.