Genetics Exam 1 Review: Mendelian Inheritance, Chromosome Behavior, and Genetic Mapping

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Mendelian Genetics

Law of Independent Assortment

The Law of Independent Assortment states that alleles of different genes are distributed independently of one another during gamete formation. This is observed in dihybrid crosses, where the phenotypic ratio of offspring can be explained by the independent segregation of two gene pairs.

Dihybrid Cross: Crossing two heterozygotes (AaBb x aabb) produces a 1:1:1:1 ratio of gametes.
Gamete Number: The number of possible gametes is calculated as , where N is the number of heterozygous gene pairs.
Test Cross: Involves crossing an individual with a homozygous recessive to determine genotype.
Phenotypic Ratios: Monohybrid cross yields 3:1 ratio; dihybrid cross yields 9:3:3:1 ratio.
Example: In a dihybrid cross AaBb x aabb, the gametes produced are AB, Ab, aB, ab.

Law of Segregation

The Law of Segregation states that each individual has two alleles for each gene, which segregate during meiosis so that each gamete receives only one allele.

Observed in Monohybrid Crosses: Crossing two heterozygotes (Aa x Aa) produces a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio.
Chromosome Behavior: Homologous chromosomes separate during Anaphase I of meiosis.

Allele Types and Inheritance Patterns

Understanding the terminology and patterns of inheritance is crucial for interpreting genetic crosses and pedigrees.

Dominant Allele: Expressed in both homozygous and heterozygous states.
Recessive Allele: Expressed only in homozygous state.
Codominance: Both alleles are fully expressed (e.g., blood type AB).
Incomplete Dominance: Heterozygote shows intermediate phenotype (e.g., pink flowers from red and white parents).
Pleiotropy: One gene affects multiple traits.
Recessive Lethality: Homozygous recessive genotype is lethal, producing a 2:1 ratio in offspring.
Vertical Inheritance: Dominant trait appears in every generation.
Horizontal Inheritance: Rare recessive trait appears among siblings but not in parents.
Example: Cystic Fibrosis (recessive), Huntington Disease (dominant detrimental allele).

Extensions to Mendelian Inheritance

Epistasis

Epistasis occurs when one gene masks or modifies the expression of another gene. It can be recessive or dominant.

Recessive Epistasis: 9:3:4 phenotypic ratio.
Reciprocal Recessive Epistasis: 9:7 phenotypic ratio.
Epistatic vs Hypostatic: The epistatic gene masks the effect of the hypostatic gene.
Example: Dog coat color (recessive epistasis), purple flower color (reciprocal epistasis).

Penetrance and Expressivity

Penetrance refers to the proportion of individuals with a particular genotype that actually display the phenotype. Expressivity describes the degree to which a phenotype is expressed.

Complete Penetrance: All individuals with genotype show phenotype.
Variable Expressivity: Phenotype varies among individuals with same genotype.

Chromosome Structure and Cell Division

Mitosis and Meiosis

Mitosis and Meiosis are processes of cell division, each with distinct stages and outcomes.

Mitosis: Produces two identical diploid cells. Stages: Interphase (G1, S, G2), Prophase, Prometaphase, Metaphase, Anaphase, Telophase, Cytokinesis.
Meiosis: Produces four haploid gametes. Stages: Meiosis I (Prophase I, Metaphase I, Anaphase I, Telophase I), Meiosis II (Prophase II, Metaphase II, Anaphase II, Telophase II).
Key Differences: Homologous chromosomes separate in Meiosis I; sister chromatids separate in Meiosis II.
Crossing Over: Occurs in Prophase I, increases genetic diversity.
Interphase: G1 (growth), S (DNA synthesis), G2 (preparation for division), G0 (non-dividing state).
Prometaphase: Spindle fibers attach to chromosomes via kinetochores.
Types of Microtubules: Kinetochore, polar, and astral microtubules.
Diploid vs Haploid: Diploid (2N) cells have two sets of chromosomes; haploid (N) cells have one set, produced during meiosis.

Chromosome Behavior and Structures

Centrosome: Organelle organizing microtubules during cell division.
Kinetochore: Protein structure on chromosome where spindle fibers attach.
Synaptonemal Complex: Protein structure facilitating pairing of homologous chromosomes.
Chiasmata: Sites of crossing over between homologous chromosomes.
Cohesin Proteins: Hold sister chromatids together.
Tetrad: Group of four chromatids formed during meiosis.
Recombination Nodules: Sites of crossing over.

Genetic Mapping and Linkage

Gene Loci and Linkage

Genes located close together on the same chromosome are said to be syntenic and may be inherited together. The frequency of recombination between loci is used to estimate their distance apart.

Recombinant Frequency: 50% indicates genes are either on different chromosomes or far apart on the same chromosome.
Gene Distance: Greater physical distance increases probability of crossing over.
Parental vs Recombinant Genotypes: Parental genotypes are most frequent; recombinants are less frequent.
Double Crossover (DCO): Lowest frequency among recombinants.
Gene Mapping: Map units (centimorgans) are calculated based on recombinant progeny percentage.
Coefficient of Coincidence: Measures the degree of interference in crossing over.
Example: Establishing gene map from progeny data.

Chi Square Analysis

The Chi Square Test is used to determine if observed genetic ratios deviate significantly from expected ratios.

Null Hypothesis: No difference between observed and expected ratios.
Degrees of Freedom: Number of classes minus one.
Formula:

p-value: Probability that deviation is due to chance.
Example: Calculating chi square for a dihybrid cross.

Chromosomal Variation and Sex Determination

Nondisjunction and Chromosomal Disorders

Nondisjunction is the failure of chromosomes to separate properly during meiosis, leading to abnormal chromosome numbers.

First Division Nondisjunction: Homologous chromosomes fail to separate.
Second Division Nondisjunction: Sister chromatids fail to separate.
Resulting Genotypes: XXY, XYY, XXX, Turner Syndrome (XO), Klinefelter Syndrome (XXY), Trisomy 13, Trisomy 21.
Barr Bodies: Inactivated X chromosomes; number depends on genotype (e.g., XX, XXY).

Sex Determination and Gametogenesis

SRY Gene: Located on Y chromosome, determines male sex.
Oogenesis: Oogonia undergo mitosis, primary oocyte undergoes meiosis I at ovulation, producing a polar body; meiosis II occurs after fertilization.
Spermatogenesis: Spermatogonia undergo mitosis, produce spermatocytes, which undergo meiosis to form spermatids and mature sperm.

Pedigree Analysis

Types of Inheritance

Pedigrees are used to track inheritance patterns in families.

X-linked Recessive: More common in males; females must be homozygous to express trait.
X-linked Dominant: Affected males pass trait to all daughters, not sons.
Autosomal Recessive: Trait appears in siblings, not parents.
Autosomal Dominant: Trait appears in every generation.
Y-linked: Only males affected; passed from father to son.

Genetic Crosses and Problem Solving

Performing Crosses

Understanding how to set up and analyze genetic crosses is fundamental for genetics.

Monohybrid, Dihybrid, Trihybrid Crosses: Use multiplication rules to determine probabilities.
Self Cross: Crossing an individual with itself.
Recognizing Recombinants: Identify offspring with new allele combinations.
Establishing Map Units: Use recombinant frequencies to calculate genetic distances.

Summary Table: Types of Genetic Crosses and Ratios

Cross Type	Genotypic Ratio	Phenotypic Ratio	Example
Monohybrid (Aa x Aa)	1:2:1	3:1	Pea color
Dihybrid (AaBb x AaBb)	1:2:1:2:4:2:1:2:1	9:3:3:1	Pea color & shape
Recessive Epistasis	-	9:3:4	Dog coat color
Reciprocal Recessive Epistasis	-	9:7	Purple flower color
Recessive Lethality	2:1	-	Yellow mice

Summary Table: Chromosomal Disorders

Disorder	Genotype	Phenotype
Turner Syndrome	XO	Female, short stature, infertility
Klinefelter Syndrome	XXY	Male, tall, infertility
Trisomy 13	13,13,13	Patau syndrome
Trisomy 21	21,21,21	Down syndrome

Summary Table: Pedigree Patterns

Pattern	Key Features
X-linked Recessive	More males affected, skips generations
X-linked Dominant	Affected males pass to daughters
Autosomal Recessive	Appears in siblings, not parents
Autosomal Dominant	Appears in every generation
Y-linked	Only males affected

Key Equations

Number of Gametes:
Chi Square:
Recombinant Frequency:

Additional info: Academic context and explanations have been expanded for clarity and completeness. Tables have been inferred and reconstructed for study purposes.