In this chapter, you will learn
- —Understand Mendel's laws of inheritance
- —Learn about dominant and recessive traits
- —Apply monohybrid and dihybrid crosses
- —Understand DNA structure and function
- —Learn about genes and alleles
- —Understand genetic variation and mutations
- —Study human genetics and inheritance patterns
- —Apply Punnett squares to predict inheritance
Mendel's Laws of Inheritance
Gregor Mendel discovered laws of inheritance by studying pea plants. His work laid foundation for genetics. Key findings: traits pass from parents to offspring in predictable patterns.
Mendel's Laws:
(1) Law of Segregation: Alleles separate during gamete formation. (2) Law of Independent Assortment: Different traits assort independently.
Law of Segregation:
- Each trait controlled by pair of factors (now called alleles)
- During meiosis, alleles segregate - each gamete gets one allele
- During fertilization, alleles recombine in offspring
- Offspring genotype determined by inherited alleles
- Example: P (tall × short) → F1 (all tall) → F2 (3 tall: 1 short)
Law of Independent Assortment:
- Different traits segregate independently during meiosis
- Inheritance of one trait doesn't affect inheritance of another
- Applies when genes on different chromosomes or far apart on same chromosome
- Dihybrid cross (two traits) shows 9:3:3:1 ratio
- Creates genetic variation through different combinations
Exam Tip
Memorize Mendel's two laws! Law of Segregation explains 3:1 ratio in monohybrid cross. Law of Independent Assortment explains 9:3:3:1 in dihybrid.
Common Mistake
Students confuse dominant/recessive with stronger/weaker. Dominant allele just masks recessive phenotypically, both are equally valid alleles.
Dominant and Recessive Traits
Alleles are different forms of same gene. Dominant alleles are expressed in heterozygous individuals. Recessive alleles are masked by dominant in heterozygotes.
Key Concepts:
- Homozygous dominant (TT): Two dominant alleles. Expresses dominant trait
- Heterozygous (Tt): One dominant, one recessive. Expresses dominant trait
- Homozygous recessive (tt): Two recessive alleles. Expresses recessive trait
- Genotype: Genetic composition (TT, Tt, or tt)
- Phenotype: Observable trait (Tall or Short)
- Dominant always masks recessive in heterozygotes
Monohybrid Cross Example (Tt × Tt):
- Gametes: T and t from each parent
- Punnett square: TT (1), Tt (2), tt (1)
- Genotypic ratio: 1:2:1 (TT:Tt:tt)
- Phenotypic ratio: 3:1 (Tall:Short)
- 3 Tall (1 TT + 2 Tt), 1 Short (tt)
Exam Tip
3:1 ratio in F2 from heterozygous cross (Tt × Tt). Know genotype vs phenotype. Recessive only shows in homozygous recessive (tt).
Common Mistake
Students think heterozygous shows both traits. WRONG! Heterozygous shows only dominant trait. Recessive hidden.
Monohybrid and Dihybrid Crosses
Monohybrid cross follows inheritance of one trait. Dihybrid cross follows inheritance of two traits simultaneously.
Monohybrid Cross (One Trait):
P: TT (Tall) × tt (Short)
F1: All Tt (Tall)
F2 (Tt × Tt): 3 Tall : 1 Short (9:3:3:1 becomes 3:1)
Dihybrid Cross (Two Traits):
- P: TTGG (Tall, Green) × ttgg (Short, Yellow)
- F1: All TtGg (Tall, Green) - dominant traits
- F2 (TtGg × TtGg): 9:3:3:1 ratio
- 9 Tall Green : 3 Short Green : 3 Tall Yellow : 1 Short Yellow
- 16 total combinations from 4×4 Punnett square
Using Punnett Squares:
- Monohybrid: 2×2 grid (16 cells when expanded), 4 combinations
- Dihybrid: 4×4 grid, 16 combinations
- Lists alleles on sides, fills squares with combinations
- Count phenotypes to get ratios
- Shows probability of each genotype/phenotype
Exam Tip
Monohybrid 3:1 ratio, Dihybrid 9:3:3:1. Know how to construct and analyze Punnett squares. Practice with different dominant/recessive combinations.
Common Mistake
Students incorrectly draw Punnett squares or forget to count all combinations. Always check that grid is complete and count matches 3:1 or 9:3:3:1.
DNA Structure and Function
DNA (Deoxyribonucleic Acid) is molecule containing genetic instructions for all organisms. Structure determines how it stores and transmits information.
DNA Structure:
Double helix with sugar-phosphate backbone and nitrogenous bases.
DNA Components:
- Sugar (Deoxyribose): 5-carbon sugar in backbone
- Phosphate: Links sugars forming backbone
- Nitrogenous bases: Purines (Adenine, Guanine) & Pyrimidines (Cytosine, Thymine)
- Base pairing: A pairs with T, G pairs with C (Complementary)
- Double helix: Two strands coiled around each other (Watson and Crick model)
DNA Functions:
- Information storage: Sequence of bases codes for protein synthesis
- Replication: Makes exact copies before cell division
- Mutation: Changes in sequence create genetic variation
- Gene expression: Instructions for making proteins
- Inheritance: Passed to offspring determining traits
DNA Replication:
- Semi-conservative: Each new DNA has one original strand + one new strand
- Enzyme helicase unwinds double helix
- DNA polymerase adds complementary bases following base-pairing rules
- Results in two identical DNA molecules
- Essential for passing genetic info to daughter cells
Exam Tip
DNA = double helix with sugar-phosphate backbone and bases. A-T and G-C pairing rules. Semi-conservative replication. Base sequence codes for proteins.
Common Mistake
Students confuse DNA with RNA. DNA has deoxyribose and thymine; RNA has ribose and uracil. Both have genes but DNA is main genetic material.
Genes, Alleles, and Genetic Variation
Gene is stretch of DNA coding for specific protein/trait. Allele is different form of same gene. Genetic variation is differences in DNA between individuals.
Genes and Alleles:
- Gene: Specific location on chromosome, controls trait
- Allele: Different versions of same gene (dominant T, recessive t)
- Multiple alleles: Some genes have more than 2 versions (blood types: A, B, AB, O)
- Homozygous: Same alleles (TT or tt)
- Heterozygous: Different alleles (Tt)
Sources of Genetic Variation:
- Mutation: Change in DNA sequence (spontaneous or induced). Creates new alleles
- Sexual reproduction: Gamete combinations create genetic diversity
- Crossing over: Exchange of genetic material during meiosis
- Independent assortment: Random distribution of chromosomes in meiosis
- Beneficial mutations: Basis for natural selection and evolution
Types of Mutations:
- Point mutation: Single base change (sickle cell anemia - A→T)
- Insertion: Addition of bases causing frameshift
- Deletion: Loss of bases causing frameshift
- Duplication: Repetition of DNA segment
- Inversion: Reversal of DNA segment direction
- Beneficial/Harmful/Neutral: Effects depend on coding impact
Exam Tip
Gene = location on DNA. Allele = version of gene. Variation from mutations and sexual reproduction. Know mutation types and their effects.
Common Mistake
Students think all mutations are harmful. Some neutral (no effect), some beneficial (advantageous), some harmful. Evolution selects for beneficial.
Human Genetics and Inheritance Patterns
Human genetics studies inheritance in humans. Pedigree analysis traces traits through families. Many human traits follow Mendelian inheritance.
Human Genetic Traits:
- Autosomal dominant: Brown eyes, widow's peak, attached earlobes dominant
- Autosomal recessive: Blue eyes, straight hairline, cystic fibrosis (disease)
- Sex-linked (X-linked): Traits on X chromosome. Males only need one allele (hemophilia, colorblindness)
- Polygenic: Multiple genes control trait (height, skin color - shows continuous variation)
X-linked Inheritance (Sex-linked):
- Females: XX (two X chromosomes). Can be homozygous or heterozygous
- Males: XY (one X chromosome). Express all X-linked traits
- Carrier females: Heterozygous for recessive allele, don't show disease but can pass to offspring
- Colorblindness/Hemophilia: Mostly affect males. Females rarely show (need two copies)
- Example: Color-blind mother (XbXb) × Normal father (XBY) → All daughters carriers, all sons colorblind
Pedigree Analysis:
- Square = Male, Circle = Female
- Filled/shaded = Affected (shows trait/disease)
- Empty = Unaffected
- Horizontal line = Marriage/mating
- Vertical line = Offspring
- Traces inheritance patterns through generations
- Identifies carriers, homozygous/heterozygous individuals
Exam Tip
Know autosomal vs sex-linked inheritance. X-linked: Males need 1 copy to show, females need 2. Pedigree analysis crucial for identifying inheritance patterns.
Common Mistake
Students think sex-linked traits equally common in males and females. WRONG! Recessive X-linked rare in females (need 2 copies) but common in males (1 copy).