Genetics Midterm Study Guide: Mendelian Genetics, Cell Division, Pedigrees, Prokaryotic Genetics, Sex Determination, Chromosomal Variation

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Mendelian Genetics & Extensions

Core Concepts

Mendelian genetics forms the foundation of classical inheritance, describing how traits are passed from parents to offspring via discrete units called genes. Extensions to Mendelian principles account for more complex patterns of inheritance.

Unit factors: Mendel's term for genes; variant forms are alleles; locus is the gene's position on a chromosome.
Genotype vs phenotype: Genotype is the genetic makeup; phenotype is the observable trait.
Dominant vs recessive: Dominant alleles mask recessive alleles in heterozygotes.
Monohybrid cross: Typical phenotypic ratio is 3:1 (dominant:recessive).
Autosome: Non-sex chromosome.
Sex-linked: Gene located on a sex chromosome, often X.

Extensions to Mendelian Inheritance

Incomplete dominance: Heterozygote shows intermediate phenotype (e.g., pink flowers from red and white parents).
Codominance: Both alleles are fully expressed (e.g., ABO blood types, 1:2:1 ratio).
Penetrance: Proportion of individuals with a genotype who express the phenotype.
Expressivity: Degree to which phenotype is expressed among individuals with the same genotype.
Epistasis: One gene masks the effect of another; important in biosynthetic pathways.
Double mutant phenotype: Determined by the earliest block in a pathway.
Sex-linked recessive: More common in males (hemizygous for X); carrier females (e.g., XB Xb) are phenotypically normal but can pass the allele to sons.

Example: ABO Blood Types

ABO blood group exhibits codominance: IA and IB alleles are both expressed in AB individuals.

Cell Division

Mitosis

Mitosis is the process by which somatic cells divide, producing genetically identical diploid daughter cells.

S phase: DNA synthesis (replication).
Karyokinesis: Nuclear division.
Cytokinesis: Cytoplasmic division.
Result: Daughter cells are diploid and genetically identical.

Meiosis

Meiosis produces haploid gametes through two consecutive divisions, increasing genetic diversity.

Meiosis I: Homologous chromosomes pair (synapsis) and separate (Anaphase I).
Meiosis II: Sister chromatids separate (Anaphase II).
Crossing over: Occurs during Prophase I, increasing genetic diversity.
Independent assortment: Random distribution of homologs to daughter cells.

Example: Chromosome Combinations

With two chromosome pairs, meiosis produces four possible haploid combinations.

Pedigree Analysis

Modes of Inheritance

Pedigree analysis helps determine the mode of inheritance for genetic traits by examining family history.

Autosomal dominant: Trait appears in every generation; affected × unaffected → ~50% affected.
Autosomal recessive: Can skip generations; unaffected carriers.
X-linked recessive: More males affected; carrier mothers; no male-to-male transmission.
X-linked dominant: Affected father passes trait to all daughters, no sons.
Mitochondrial (maternal): Only affected mothers pass trait; affects both sexes equally.

Rules for Exclusion

Unaffected parents with affected child → rules out autosomal dominant.
Affected daughter of unaffected father → rules out X-linked recessive.
Affected father passes trait to son → rules out X-linked (unless dominant).
Paternal transmission → rules out mitochondrial.
Affected mothers pass trait to all offspring → think mitochondrial.

Example Table: Modes of Inheritance

Mode	Transmission Pattern	Key Features
Autosomal Dominant	Every generation	Both sexes, 50% affected if one parent is heterozygous
Autosomal Recessive	Can skip generations	Carriers, both sexes equally affected
X-linked Recessive	More males affected	No male-to-male transmission
X-linked Dominant	All daughters of affected father	No sons affected by father
Mitochondrial	Maternal only	All children of affected mother

Prokaryotic Genetics & Bacteriophages

Plasmids

Plasmids are small, circular, double-stranded DNA molecules found in the cytosol of bacteria, separate from the chromosome.

Replicate independently of the bacterial chromosome.
Not found in nucleus (bacteria lack nuclei).

Bacteriophage Life Cycles

Lytic cycle: Phage replicates, lyses host cell, releases progeny.
Lysogenic cycle: Phage DNA integrates into host chromosome as prophage.

Bacterial Recombination

Conjugation: Direct cell-to-cell DNA transfer via pilus.
Transduction: DNA transferred via bacteriophage.
Transformation: Uptake of free DNA from environment.
Auxotroph: Requires external nutrient.
Prototroph: Can synthesize its own nutrients.

Example Table: Bacterial Recombination Mechanisms

Mechanism	Process	Key Features
Conjugation	Cell-to-cell transfer	Requires pilus
Transduction	Bacteriophage-mediated	Phage transfers DNA
Transformation	Uptake from environment	Free DNA incorporated

Sex Determination

Genetic Mechanisms

Sex determination varies among species and is controlled by specific genetic systems.

ZW system: Female is heterogametic (ZW) in birds and some insects.
XY system: Male is heterogametic (XY) in humans and Drosophila.
SRY gene: Located on Y chromosome; triggers male development.
Xist gene: Controls X-inactivation; inactive X forms a Barr body.
Barr body rule: Number of Barr bodies = number of X chromosomes minus 1.

Example Table: Sex Determination Systems

System	Heterogametic Sex	Species
XY	Male (XY)	Humans, Drosophila
ZW	Female (ZW)	Birds, some insects

Chromosomes

Chromosome Number Disorders

Chromosomal variation can result in genetic disorders due to abnormal numbers or structures of chromosomes.

Aneuploidy: Gain or loss of individual chromosomes (not a full haploid set).
Turner syndrome: 45,X; phenotypically female, 0 Barr bodies.
Klinefelter syndrome: 47,XXY; phenotypically male, 1 Barr body.
Polyploidy: More than two complete sets of chromosomes.
Euploidy: Exact multiples of the haploid set.

Nondisjunction

Meiosis I: Homologs fail to separate; all four products may be aneuploid.
Meiosis II: Sister chromatids fail to separate; two of four products aneuploid.

Haploid/Diploid Numbers

Diploid (2n): Two sets of chromosomes.
Haploid (n): One set; gametes.
Relationship: Diploid number is twice the haploid number.

Example Table: Chromosomal Disorders

Disorder	Karyotype	Barr Bodies	Phenotype
Turner Syndrome	45,X	0	Female
Klinefelter Syndrome	47,XXY	1	Male

Practice Questions & Applications

Sample Calculations and Concepts

Punnett Square for Autosomal Dominant: Hh x hh → 50% chance of affected child.
Codominance Cross: FF' x FF' → 1:2:1 ratio; with 80 plants: 20 frilly, 40 frilly and lobed, 20 lobed.
Chi-squared Test: Used to compare observed vs expected ratios; fits expected ratio in example.
X-linked Recessive: Carrier daughters have 50% chance of passing affected allele to sons.
Chromosome Combinations: After mitosis: diploid, identical cells; after meiosis: four possible haploid combinations.

Key Formulas

Chi-squared formula:

Number of possible gamete combinations:

(where n = number of chromosome pairs)

Definitions & Vocabulary

Gene: Unit of heredity; segment of DNA encoding a trait.
Allele: Variant form of a gene.
Phenotype: Observable trait.
Genotype: Genetic makeup.
Penetrance: Proportion of individuals with genotype expressing phenotype.
Expressivity: Degree of phenotype expression.
Recessive: Allele not expressed in heterozygote.
Plasmid: Circular, double-stranded DNA in cytosol.
Barr body: Inactive X chromosome in female cells.

Summary Table: Key Terms

Term	Definition
Unit factor	Gene
Allele	Variant form of gene
Penetrance	Proportion expressing phenotype
Expressivity	Degree of phenotype expression
Epistasis	One gene masks another
Auxotroph	Requires external nutrient
Prototroph	Can synthesize own nutrients

Additional info: Academic context was added to clarify definitions, provide examples, and expand brief points into full explanations. Tables were reconstructed for clarity and completeness. Equations are provided in LaTeX format as required.