UNIT 2: Non-allelic Interactions and Chromosome Morphology

Exam Focus: Distinguish between allelic and non-allelic interactions. Remember the altered dihybrid phenotypic ratios (e.g., 9:7, 12:3:1) for epistasis. For chromosome morphology, know the key terms (cistron, exon, intron) and the salient features of giant chromosomes.

Non-allelic Interactions
Chromosome Morphology

1. Non-allelic Interactions

Non-allelic interactions, also known as **Gene Interactions**, involve the modification of the normal dihybrid 9:3:3:1 ratio due to the interaction between **two or more independent genes** (i.e., genes located at different loci) that affect the same trait.

Complementary Genes (9:7 Ratio)

Two separate genes interact to produce a single trait. The dominant allele of both genes must be present together to produce the characteristic trait; the presence of one dominant allele alone is not enough.

Example: Flower color in Sweet Peas (Lathyrus odoratus).

C_P_ → Purple (needs both dominant C and P)
C_pp, ccP_, ccpp → White (any homozygous recessive condition leads to white)

Ratio: 9:7 (e.g., 9 Purple : 7 White).

Epistasis (Dominant & Recessive)

Epistasis occurs when one gene (the **epistatic** gene) masks or suppresses the expression of another gene (the **hypostatic** gene) at a different locus.

(A) Dominant Epistasis (12:3:1 Ratio):

A dominant allele (e.g., 'W') at one locus masks the expression of alleles at a second locus (e.g., 'G').
Example: Fruit color in Summer Squash. W (White, epistatic to Y/y) and Y (Yellow, hypostatic to W). W_Y_ and W_yy are White (12 parts); wwY_ is Yellow (3 parts); wwyy is Green (1 part).

(B) Recessive Epistasis (9:3:4 Ratio):

The recessive alleles (e.g., 'ee') at one locus mask the expression of alleles at a second locus (e.g., 'B/b').
Example: Coat color in Labrador Retrievers. B (Black), b (Brown), E (allows color), e (inhibits color). B_E_ (Black), bbE_ (Brown), B_ee and bbee are Yellow.

Duplicate Genes (15:1 Ratio)

The dominant allele of **either** of the two genes is sufficient to produce the same phenotype. The recessive phenotype is only expressed when both genes are homozygous recessive.

Example: Shape of seed capsule in Shepherd's Purse (Capsella bursa-pastoris).

A_B_, A_bb, aaB_ → Triangular (15 parts)
aabb → Ovoid (1 part)

Inhibitory Genes (13:3 Ratio)

A dominant allele at one locus (the Inhibitor gene, e.g., 'I') prevents the expression of a dominant allele at a second locus (the color gene, e.g., 'C'). The color is only expressed when the inhibitor gene is homozygous recessive ('ii') and the color gene is dominant ('C_').

Example: Feather color in White Leghorn chickens.

I_C_, I_cc, iicc → White (13 parts: Dominant I prevents color OR both recessive for color)
iiC_ → Colored (3 parts: No inhibitor AND dominant color)

2. Chromosome Morphology

Packaging of DNA Molecule into Chromosomes

The human genome contains approximately 3 × 10⁹ base pairs of DNA. This long thread must be condensed to fit inside the cell nucleus through a hierarchical process of packaging:

Naked DNA: The basic double helix structure.
Nucleosome (Beads-on-a-string): The DNA helix wraps around an octamer core of basic proteins called **histones** (H2A, H2B, H3, H4) twice, forming a nucleosome, the fundamental unit of chromatin.
30 nm Fiber (Chromatin Fiber): Nucleosomes are compacted into a coil structure, stabilized by the H1 histone protein.
Loop-Scaffolds: The 30 nm fiber forms large loops anchored to a non-histone protein scaffold.
Metaphase Chromosome: The looped domains are further condensed to form the highly compact and visible metaphase chromosome structure.

Polytene Chromosomes

These are giant chromosomes found in the salivary gland cells of fly larvae (e.g., Drosophila) and some other insect tissues.

Formation: Result from multiple rounds of DNA replication (**endomitosis**) without cell division or separation of the daughter chromosomes. This creates a bundle of thousands of chromatids.
Structure: Characterized by distinct **banding patterns** (chromomeres) and non-staining areas (interbands). These bands can be used as genetic markers.
Puffs: Localized swellings called **chromosomal puffs** or Balbiani rings represent sites of active gene transcription (RNA synthesis).

Lampbrush Chromosomes

These are giant chromosomes found in the growing oocytes (egg cells) of vertebrates (e.g., amphibians), insects, and some plants.

Structure: They are typically found during the prophase of meiosis. The chromosome axis forms a dense line from which many pairs of characteristic **lateral loops** project, giving them a brush-like appearance (like a lamp cleaning brush).
Function: The loops are sites of intense **RNA synthesis** (transcription) necessary for the massive protein and nutrient accumulation required for egg development.

Concept of Cistron, Exons and Introns

These terms define the functional units within a gene.

Cistron: A segment of DNA encoding a single polypeptide chain. It is considered the **functional unit of a gene**. (The term 'gene' is often used interchangeably with 'cistron').
Exons: The segments of a gene that encode a part of the protein and are **retained in the final mature mRNA**. They are expressed regions.
Introns: The intervening segments of a gene that **do not code for protein** and are removed from the pre-mRNA during RNA processing (splicing). They are non-expressed regions.

Mnemonic: Exons are Expressed; Introns are Intervening/Intentionally discarded.

Genetic Code

The set of rules by which information encoded in genetic material (DNA or RNA sequences) is translated into proteins (amino acid sequences).

Codons: The genetic code is a **triplet code**, meaning three sequential nucleotide bases (a codon) specify one amino acid.
Features: The code is **universal** (mostly), **degenerate** (multiple codons code for one amino acid), **non-overlapping**, and **commaless**.
Start and Stop Codons: **AUG** typically serves as the start codon (Methionine), while **UAA, UAG,** and **UGA** are stop codons that signal the termination of translation.