Genetics and Heredity — Practice Quiz

A Biology cheat sheet for Genetics and Heredity — every key formula with its symbols defined — plus a medium-level practice quiz to test recall.

Formulas & key concepts

Dominance: Some alleles are dominant, others recessive. Law of Dominance: In heterozygotes, one trait conceals the other. Segregation: Alleles separate during gamete formation. Independent Assortment: Genes for different traits segregate independently.
Mendel's Principles
P Generation: Parental generation (true-breeding). F1 Generation: First filial (offspring of P). F2 Generation: Second filial (offspring of F1 self-cross).
Generations
Genotype: Genetic makeup (e.g., TT, Tt, tt). Phenotype: Physical traits (e.g., Tall, Short). Homozygous: Identical alleles (TT, tt). Heterozygous: Different alleles (Tt). Hemizygous: Only one allele (e.g., X-linked genes in males).
Genotype vs. Phenotype
Monohybrid: Cross involving one trait. Dihybrid: Cross involving two traits. Test Cross: Cross dominant phenotype with homozygous recessive to determine genotype. Reciprocal Cross: Swapping traits of male/female parents to test for sex-linkage.
Types of Crosses

Diagram to predict genetic cross outcomes. Uses probability. <b>Monohybrid</b>: 4 squares. <b>Dihybrid</b>: 16 squares (9:3:3:1 ratio for heterozygotes).

Punnett Squares
Incomplete Dominance: Blended phenotype (Red x White = Pink). Codominance: Both expressed (Black x White = Speckled; AB Blood). Multiple Alleles: >2 alleles (Blood types A, B, O). Polygenic: Many genes (Skin color). Epistasis: One gene masks another.
Exceptions to Mendel
Continuous Variation: Range of small differences (e.g., height). Discontinuous Variation: Distinct categories (e.g., blood type). Wild Type: Most common phenotype in a population.
Genetic Variation
Linkage: Genes close on same chromosome inherited together. Recombination: Exchange of DNA between homologous chromosomes (Crossing Over) disrupts linkage.
Linkage & Mapping

Production of <b>Gametes</b> (Haploid, N). Two rounds of division. <b>Crossing Over</b> (Prophase I): Homologous chromosomes exchange DNA, increasing variation. Result: 4 genetically different cells.

Meiosis
Karyotype: Image of chromosomes (46 total, 23 pairs). Sex Chromosomes: XX (Female), XY (Male). Pedigree: Chart tracking traits through generations.
Human Heredity

Genes on sex chromosomes (usually X). Males (XY) more likely to express recessive X-linked traits (Colorblindness, Hemophilia) as they have only one X. Females (XX) can be carriers.

Sex-Linked Traits
Sickle Cell: Codominant, resistance to Malaria. Cystic Fibrosis: Recessive, resistance to Typhoid. Huntington's: Dominant, late onset.
Genetic Disorders

Caused by <b>Nondisjunction</b> (failure to separate). <b>Down Syndrome</b> (Trisomy 21). <b>Turner's</b> (X). <b>Klinefelter's</b> (XXY).

Chromosomal Disorders

Practice quiz

  1. A plant with a dominant allele for tallness ($T$) and a recessive allele for shortness ($t$) will exhibit which phenotype?

    • Tall
    • Short
    • Medium
    • Varies

    Answer: Tall

  2. If a homozygous dominant parent ($AA$) is crossed with a homozygous recessive parent ($aa$), what will be the genotype of the $F1$ generation?

    • $AA$
    • $Aa$
    • $aa$
    • $AA$ and $aa$

    Answer: $Aa$

  3. A monohybrid cross between two heterozygous parents ($Rr \times Rr$) is expected to produce what phenotypic ratio in the offspring?

    • $1:1$
    • $3:1$
    • $9:3:3:1$
    • $1:2:1$

    Answer: $3:1$

  4. In a species of flower, red color ($R$) is incompletely dominant over white color ($r$). A cross between a red flower and a white flower produces pink offspring. What would be the phenotypic ratio if two pink flowers were crossed?

    • $1 \text{ Red}: 2 \text{ Pink}: 1 \text{ White}$
    • $3 \text{ Red}: 1 \text{ White}$
    • $1 \text{ Red}: 1 \text{ Pink}$
    • All Pink

    Answer: $1 \text{ Red}: 2 \text{ Pink}: 1 \text{ White}$

  5. A person with $AB$ blood type is an example of which genetic phenomenon?

    • Incomplete Dominance
    • Codominance
    • Polygenic Inheritance
    • Epistasis

    Answer: Codominance

  6. What process during meiosis leads to the exchange of DNA between homologous chromosomes, thereby disrupting gene linkage?

    • Segregation
    • Independent Assortment
    • Crossing Over
    • Fertilization

    Answer: Crossing Over

  7. A normal human karyotype typically contains how many total chromosomes?

    • $23$
    • $46$
    • $48$
    • $22$

    Answer: $46$

  8. Why are males more likely to express recessive X-linked traits like colorblindness than females?

    • Males have two X chromosomes
    • Females have only one X chromosome
    • Males are hemizygous for X-linked genes
    • Females cannot be carriers

    Answer: Males are hemizygous for X-linked genes

  9. Which of the following genetic disorders is caused by a recessive allele and provides resistance to Typhoid in heterozygotes?

    • Huntington's Disease
    • Sickle Cell Anemia
    • Cystic Fibrosis
    • Down Syndrome

    Answer: Cystic Fibrosis

  10. Down Syndrome is a chromosomal disorder resulting from an extra copy of chromosome $21$. This condition is primarily caused by what error during cell division?

    • Crossing Over
    • Independent Assortment
    • Nondisjunction
    • Gene Mutation

    Answer: Nondisjunction

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