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

<b>Dominance</b>: Some alleles are dominant, others recessive. <b>Law of Dominance</b>: In heterozygotes, one trait conceals the other. <b>Segregation</b>: Alleles separate during gamete formation. <b>Independent Assortment</b>: Genes for different traits segregate independently.

Mendel's Principles

<b>P Generation</b>: Parental generation (true-breeding). <b>F1 Generation</b>: First filial (offspring of P). <b>F2 Generation</b>: Second filial (offspring of F1 self-cross).

Generations

<b>Genotype</b>: Genetic makeup (e.g., TT, Tt, tt). <b>Phenotype</b>: Physical traits (e.g., Tall, Short). <b>Homozygous</b>: Identical alleles (TT, tt). <b>Heterozygous</b>: Different alleles (Tt). <b>Hemizygous</b>: Only one allele (e.g., X-linked genes in males).

Genotype vs. Phenotype

<b>Monohybrid</b>: Cross involving one trait. <b>Dihybrid</b>: Cross involving two traits. <b>Test Cross</b>: Cross dominant phenotype with homozygous recessive to determine genotype. <b>Reciprocal Cross</b>: 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

<b>Incomplete Dominance</b>: Blended phenotype (Red x White = Pink). <b>Codominance</b>: Both expressed (Black x White = Speckled; AB Blood). <b>Multiple Alleles</b>: >2 alleles (Blood types A, B, O). <b>Polygenic</b>: Many genes (Skin color). <b>Epistasis</b>: One gene masks another.

Exceptions to Mendel

<b>Continuous Variation</b>: Range of small differences (e.g., height). <b>Discontinuous Variation</b>: Distinct categories (e.g., blood type). <b>Wild Type</b>: Most common phenotype in a population.

Genetic Variation

<b>Linkage</b>: Genes close on same chromosome inherited together. <b>Recombination</b>: 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

<b>Karyotype</b>: Image of chromosomes (46 total, 23 pairs). <b>Sex Chromosomes</b>: XX (Female), XY (Male). <b>Pedigree</b>: 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

<b>Sickle Cell</b>: Codominant, resistance to Malaria. <b>Cystic Fibrosis</b>: Recessive, resistance to Typhoid. <b>Huntington's</b>: 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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