Genetics Unit Study Guide: Key Concepts and Applications

Molecular Genetics & DNA Structure

DNA Components

DNA is the fundamental molecule of heredity, composed of repeating units called nucleotides. Each nucleotide consists of three parts:

• Deoxyribose sugar: A five-carbon sugar unique to DNA.

• Phosphate group: Links nucleotides together via phosphodiester bonds.

• Nitrogenous base: Four types in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G).

Base Pairing Rules

Base pairing is essential for DNA's double helix structure:

• A-T: Adenine pairs with thymine via two hydrogen bonds.

• C-G: Cytosine pairs with guanine via three hydrogen bonds.

• RNA: In RNA, uracil (U) replaces thymine, so A-U and C-G are the pairs.

Hydrogen Bonding

• A-T: Two hydrogen bonds.

• C-G: Three hydrogen bonds (stronger pairing).

DNA vs. Chromosomes

DNA is packaged with proteins (mainly histones) to form chromatin. Chromatin further condenses to form chromosomes during cell division.

• Chromatin: Loosely packed DNA-protein complex.

• Chromosome: Highly condensed structure visible during mitosis/meiosis.

Genomic Vocabulary

• Chromatin: DNA + protein, less condensed.

• Chromatid: One of two identical halves of a duplicated chromosome.

• Purines: Double-ring bases (adenine, guanine).

• Pyrimidines: Single-ring bases (cytosine, thymine, uracil).

Genetic Inheritance Patterns

Mendelian Genetics

Mendel's laws describe how traits are inherited:

• Dominant allele: Expressed in the phenotype if present.

• Recessive allele: Only expressed if both alleles are recessive.

• Homozygous: Two identical alleles (e.g., AA or aa).

• Heterozygous: Two different alleles (e.g., Aa).

Complex Inheritance

• Incomplete Dominance: Heterozygotes show a blended phenotype (e.g., red + white = pink flowers).

• Co-dominance: Both alleles are fully expressed (e.g., AB blood type, spotted animals).

• Multiple Alleles: More than two alleles exist for a gene (e.g., ABO blood group system).

Sex-Linked Inheritance

Traits linked to sex chromosomes (X or Y) show unique inheritance patterns:

• X-linked traits: More common in males (only one X chromosome).

• Y-linked traits: Passed only from father to son.

Test Crosses

A test cross determines an unknown genotype by crossing it with a homozygous recessive individual.

• If offspring show only dominant phenotype, parent is homozygous dominant.

• If offspring show both phenotypes, parent is heterozygous.

Probability & Punnett Squares

Monohybrid Crosses

Predicts offspring ratios for a single trait:

• Typical ratio for heterozygous cross (Aa x Aa): 3:1 phenotype, 1:2:1 genotype.

Dihybrid Crosses

Predicts ratios for two independent traits:

• Typical ratio for heterozygous dihybrid cross (AaBb x AaBb): 9:3:3:1 phenotype.

Gamete Formation

Each parent produces gametes with all possible allele combinations:

• For AaBb: gametes are AB, Ab, aB, ab.

Genotype vs. Phenotype

• Genotype ratio: Proportion of genetic combinations.

• Phenotype ratio: Proportion of observable traits.

Example Punnett Square Equation

Probability of a genotype in a monohybrid cross:

$P = \frac{\text{Number of desired genotype}}{\text{Total number of offspring}}$

Mutations & Genetic Disorders

Point vs. Chromosomal Mutations

• Point mutation: Change in a single nucleotide (e.g., substitution, insertion, deletion).

• Chromosomal mutation: Large-scale changes affecting chromosome structure or number.

Chromosomal Structural Mutations

• Deletion: Loss of a chromosome segment.

• Duplication: Repetition of a segment.

• Inversion: Reversal of a segment's orientation.

• Translocation: Movement of a segment to a non-homologous chromosome.

Non-disjunction

Failure of chromosomes to separate properly during meiosis, leading to disorders like Down syndrome (trisomy 21).

Inheritance of Disorders

• Recessive disorders: Harmful alleles persist via heterozygous carriers who do not show symptoms.

Pedigree Analysis

Reading Pedigrees

• Squares represent males; circles represent females.

• Shaded symbols indicate affected individuals.

• Horizontal lines connect mates; vertical lines connect parents to offspring.

Determining Inheritance

• Autosomal: Trait appears in both sexes equally.

• Sex-linked: Trait appears more in one sex.

• Dominant: Trait appears in every generation.

• Recessive: Trait may skip generations.

Assigning Genotypes

Use logic and family history to deduce individual genotypes based on observed phenotypes.

Biotechnology & Applications

Restriction Enzymes

Restriction enzymes are "molecular scissors" that cut DNA at specific sequences, creating sticky or blunt ends for genetic engineering.

• Sticky ends: Overhanging sequences that facilitate DNA recombination.

Blood Typing Applications

Punnett squares can be used to solve paternity or identity cases based on blood group compatibility.

• ABO blood types: A, B, AB, O.

• Inheritance follows multiple allele and co-dominance patterns.

Blood Type Inheritance Table

Parent Genotypes	Possible Offspring Blood Types
IAIA x ii	A or O
IAIB x ii	A, B, or O
IBIB x IAIA	A or B
ii x ii	O
IAIB x IAIB	A, B, AB, or O

Example: If a child is type O, both parents must carry the i allele.

Study Tip: Practice drawing Punnett squares for each inheritance type and explain the resulting ratios.