Genetics – Bio 111 Chapter 10 Study Guide

Key Vocabulary Refresher

• Karyotype: Photographic inventory of an individual's chromosomes; used to detect chromosomal abnormalities (e.g., trisomy 21).

• Genes: DNA segments coding for functional RNA/proteins; fundamental units of heredity.

• Alleles: Alternative versions of a gene found at the same locus; may be dominant, recessive, codominant, etc.

• Chromosomes: Discrete packages of DNA and associated proteins.

• Diploid Cells (2n): Cells with two sets of homologous chromosomes (humans 2n = 46).

• Haploid Cells (n): Gametes possessing one set of chromosomes (n = 23 in humans).

• Autosomes: Non-sex chromosomes (human pairs 1–22).

• Sex Chromosomes: Chromosomes that determine biological sex (X, Y in most mammals).

• Homologous Chromosomes: Chromosome pairs—one paternal, one maternal—carrying the same genes in the same order.

• Meiosis: Two-division process yielding four non-identical haploid gametes; introduces genetic variation via crossing-over and independent assortment.

• Sister Chromatids: Identical DNA copies joined at a centromere after replication.

• Gametes: Reproductive cells (sperm, egg) that fuse during fertilization.

• Zygote: Diploid cell formed when two gametes merge; first stage of a new organism.

• Fertilization: Fusion of nuclei of two gametes restoring diploidy.

Mendelian (Complete) Dominance

Mendel’s Choice of Garden Peas

1. Short generation time → multiple generations within one growing season.

2. True-breeding varieties readily available → ensured genetic consistency.

3. Controlled pollination (self- vs cross-fertilization possible with simple emasculation) → precise mating design.

From Blending to Particulate Inheritance

• **Mendel’s “factors” = modern *alleles*.

• Blending theory predicted intermediate offspring indefinitely; Mendel’s F1 hybrids showed dominant phenotype but F2 re-expressed recessive trait, demonstrating discrete, non-blending units.

Essential Terminology

• True breeding: Population that, when self-crossed, produces offspring identical for the trait across many generations.

• Carrier: Heterozygote for a recessive allele; phenotypically normal but can transmit recessive condition.

• Genotype: Specific allele combination (e.g., Aa).

• Phenotype: Observable trait (e.g., purple flowers).

• Dominant allele: Masks expression of recessive allele in heterozygote.

• Recessive allele: Expressed only when homozygous.

Generational Labels

• Parental (P): Original true-breeding lines.

• F1: First filial generation (offspring of P).

• F2: Second filial (offspring of F1 × F1).

• Backcross: F1 crossed back to one parental genotype to reveal hidden recessive alleles.

Allele Abbreviation Rules

• Dominant: Capital letter, often first letter of dominant phenotype (e.g., A for purple flowers).

• Recessive: Same letter but lowercase (e.g., a for white flowers).

Zygosity Conditions

• Homozygous Dominant: AA

• Heterozygous: Aa

• Homozygous Recessive: aa

• These are genotypic, though they often correlate with phenotype.

Test Cross

• Cross an individual of unknown genotype (showing dominant phenotype) with a homozygous recessive.

• Reveals whether unknown is homozygous dominant or heterozygous.
  • 2:2 phenotypic ratio → unknown is heterozygous.
  • 4:0 phenotypic ratio → homozygous dominant.

Punnett Squares & Expected Ratios

Monohybrid Cross (P: true-breeding AA × aa)

• F1 Genotype: 100\%\ Aa

• F1 Phenotype: 100\% dominant.

• F2 Genotype: 1\ AA : 2\ Aa : 1\ aa

• F2 Phenotype: 3 dominant : 1 recessive.

Dihybrid Cross (P: AABB × aabb)

• F1 Genotype: 100\%\ AaBb (dihybrid).

• F1 Phenotype: shows both dominant traits.

• F2 Phenotype (if genes assort independently): 9:3:3:1.

Mendel’s Laws

• Law of Segregation: Allele pairs separate during gamete formation; each gamete carries only one allele for each gene.

• Law of Independent Assortment: Alleles of genes on different (or widely separated) chromosomes assort independently into gametes.

Probability Tools

• Product Rule: P(A\;and\;B) = P(A) \times P(B) for independent events. • Example: Heterozygote × heterozygote ( Aa \times Aa ).

  • Probability of homozygous dominant female = P(AA) \times P(female) = \frac14 \times \frac12 = \frac18.

  • Two females in a row = \left(\frac12\right)^2 = \frac14.

  • Two homozygous recessive males = \left(\frac14\times\frac12\right)^2 = \frac1{64}.

• 66% Carrier Logic: If a dominant-phenotype child from heterozygous parents is known not to be aa (ruled out), only AA or Aa remain. Probability of Aa among those is \frac{2}{3} = 66\%.

Beyond Mendel: Modified Patterns

Incomplete Dominance / Codominance

• Incomplete Dominance
  • Heterozygote shows blended intermediate phenotype (e.g., red \times white snapdragons → pink).

• Codominance
  • Both alleles fully expressed side-by-side (e.g., human ABO blood type I^A I^B = AB; roan cattle).

• Notation Issue: Upper/lower-case implies dominance hierarchy; instead use superscripts (C^R, C^W) or different letters.

Linked Traits / Sex-Linked Traits

• Linked Traits
  • Genes located close together on same chromosome; inherited together unless crossing-over separates them.
  • Example: Drosophila body color and wing size on chromosome 2.

• Sex-Linked Traits
  • Genes on sex chromosomes (usually X).
  • Examples: Red-green color blindness, hemophilia A, Duchenne muscular dystrophy.

Multiple Alleles / Polygenic Inheritance

• Multiple Alleles
  • More than two allele forms at a locus (population level).
  • Example: ABO blood system (I^A, I^B, i).
  • Results in several discrete phenotypes, not a continuum.

• Polygenic Inheritance
  • Many genes contribute additive effect to a single trait.
  • Examples: Human height, skin pigmentation.
  • Produces a continuous range (bell curve) of phenotypes.

Environmental Effects

• Phenotype influenced by non-genetic factors.

• Examples: Hydrangea flower color (soil pH), Himalayan rabbit fur darkening in cooler body regions, identical twins diverging due to nutrition/lifestyle.

X-Inactivation

• In female mammals one X condenses into a Barr body → dosage compensation.

• Example: Calico cats’ patchy fur arises from differential inactivation of X-linked coat-color alleles.

Pleiotropy / Epistasis

• Pleiotropy
  • Single gene affects multiple traits.
  • Examples: Sickle-cell gene (anemia, stroke risk, malaria resistance); Marfan syndrome (long limbs, heart defects).

• Epistasis
  • One gene masks/modifies expression of another.
  • Example: Labrador coat color—gene for pigment deposition (E/e) overrides B/b pigment color gene.

Pedigree Interpretation (Dominant/Recessive Focus for Exam)

• Male: square.

• Female: circle.

• Breeding pair: horizontal line connecting symbols.

• Relative ages/siblings: horizontal sibship line; oldest on left.

• Unaffected: open symbol.

• Carrier: half-shaded or dot inside.

• Affected: fully shaded symbol.

Foundational & Real-World Connections

• Mendel’s work laid groundwork for modern genetics, influencing agriculture (selective breeding) and medicine (gene mapping).

• Understanding non-Mendelian patterns essential for accurate genetic counseling and ethical decision-making in gene-editing technologies (CRISPR, PGD).

Key Vocabulary Refresher

• Genetics Fundamentals: Karyotype (chromosome inventory), Genes (DNA for functional RNA/proteins), Alleles (gene variants), Chromosomes (DNA packages).

• Cell Types: Diploid Cells (2n) (two sets of homologous chromosomes, e.g., human 2n = 46), Haploid Cells (n) (one set, e.g., human gametes n = 23).

• Chromosome Types: Autosomes (non-sex), Sex Chromosomes (determine sex).

• Reproduction: Meiosis (two divisions, four non-identical haploid gametes, introduces variation), Sister Chromatids (identical DNA copies), Gametes (reproductive cells), Zygote (diploid, fused gametes), Fertilization (gamete fusion).

Mendelian (Complete) Dominance

• Mendel's Experiments: Used garden peas due to short generation time, true-breeding varieties, and controlled pollination.

• Particulate Inheritance: Disproved blending; traits are inherited as discrete, non-blending units (alleles).

• Essential Terminology: True breeding (produces identical offspring when self-crossed), Carrier (heterozygote for recessive allele), Genotype (allele combination, e.g., Aa), Phenotype (observable trait), Dominant allele (masks recessive), Recessive allele (expressed only when homozygous).

• Generational Labels: P (parental), F1 (first filial), F2 (second filial).

• Allele Notation: Dominant as capital (e.g., A), recessive as lowercase (e.g., a).

• Zygosity: Homozygous Dominant (AA), Heterozygous (Aa), Homozygous Recessive (aa).

• Test Cross: Crosses an unknown dominant phenotype with homozygous recessive to determine genotype (2:2 ratio for heterozygous unknown, 4:0 for homozygous dominant unknown).

• Punnett Squares & Ratios:

  • Monohybrid Cross: F2 genotype 1 ext{ }AA : 2 ext{ }Aa : 1 ext{ }aa; F2 phenotype 3 dominant : 1 recessive.

  • Dihybrid Cross: F2 phenotype 9:3:3:1 for independently assorting genes.

• Mendel’s Laws:

  • Law of Segregation: Allele pairs separate during gamete formation, each gamete gets one allele.

  • Law of Independent Assortment: Alleles of genes on different chromosomes assort independently.

Probability Tools

• Product Rule: P(A\;and\;B) = P(A) \times P(B) for independent events.

• 66% Carrier Logic: A dominant-phenotype child from heterozygous parents has a 2/3 probability of being a carrier (Aa) if homozygous recessive (aa) is ruled out.

Beyond Mendel: Modified Patterns

• Incomplete Dominance: Heterozygote shows intermediate/blended phenotype (e.g., pink snapdragons).

• Codominance: Both alleles fully expressed (e.g., AB blood type).

• Linked Traits: Genes physically close on the same chromosome; inherited together unless separated by crossing-over.

• Sex-Linked Traits: Genes on sex chromosomes (usually X), e.g., color blindness.

• Multiple Alleles: More than two allele forms at a single locus (e.g., ABO blood system), resulting in several discrete phenotypes.

• Polygenic Inheritance: Multiple genes contribute to a single trait, creating a continuous range of phenotypes (e.g., height).

• Environmental Effects: Non-genetic factors influencing phenotype (e.g., hydrangea color by soil pH).

• X-Inactivation: In female mammals, one X chromosome is inactivated (Barr body), leading to dosage compensation (e.g., calico cats).

• Pleiotropy: Single gene affects multiple traits (e.g., sickle-cell).

• Epistasis: One gene masks or modifies another's expression (e.g., Labrador coat color).

Pedigree Interpretation

• Symbols: Male (square), Female (circle), Affected (shaded), Carrier (half-shaded/dot).

Foundational & Real-World Connections

• Mendel’s work established modern genetics, impacting agriculture and medicine. Understanding these patterns is crucial for genetic counseling and gene-editing technologies.