Unit 4: Patterns of inheritance, sex-linked genetics and genetic mutations

I. Foundations of Inheritance — From Genes to Phenotypes

Genetic inheritance is the transmission of hereditary information through genes located on chromosomes. Mendel’s laws provide the basic framework, while allele interactions, chromosome behavior, sex, probability, and mutation explain deviations from simple Mendelian ratios.

• Defining properties:
  • Gene: A DNA sequence contributing to a functional product or phenotype.
  • Locus: The physical position of a gene on a chromosome.
  • Alleles: Alternative forms of a gene at the same locus; a diploid individual normally carries two alleles per autosomal locus.
  • Genotype and phenotype: Genotype is the allelic constitution; phenotype is the observable outcome produced by genotype and environment.
  • Segregation: Two alleles separate during meiosis, so each gamete receives one.
  • Independent assortment: Alleles at different loci assort independently when loci are unlinked or sufficiently far apart.
  • Linkage: Genes on the same chromosome may be inherited together; recombination can separate them.
  • Mutation: A heritable alteration in DNA sequence or chromosome structure that generates genetic variation.

II. Multiple Alleles — More Than Two Forms in a Population

A. Multiple alleles

Multiple alleles are three or more alternative forms of one gene found at the same locus within a population.

• Diploid restriction: Although a population may contain many alleles, one diploid individual carries at most two; for example, the human ABO locus has (I^A), (I^B), and (i).
• Origin: Different mutations in the same ancestral gene produce an allelic series.
• Relationships: Alleles may show complete dominance, incomplete dominance, codominance, or an ordered dominance hierarchy.
• Example—rabbit coat color: The classical dominance series is approximately:
  
  TEXT

    C > cch > ch > c

  
  Here (C) permits full color, (c^{ch}) produces chinchilla, (c^h) Himalayan coloration, and (c) albinism.
• Significance: Multiple alleles increase population-level diversity without increasing the number of alleles carried by one individual.

III. Pleiotropism and Pseudoalleles — One Gene, Many Effects and Closely Linked Genes

A. Pleiotropism and pseudoalleles

Pleiotropism concerns one gene affecting several traits, whereas pseudoalleles are separate, tightly linked genes that can mimic alternative alleles of one locus.

1. Pleiotropism:

   • Mechanism: One gene product functions in multiple tissues or biochemical pathways.
   • Example: Mutation of the human HBB gene produces sickle-shaped erythrocytes and also influences anemia, circulation, organ damage, and malaria resistance.
   • Interpretive point: The phenotypes are correlated consequences of one primary molecular defect, not necessarily separate gene actions.

2. Pseudoalleles:

   • Definition: Functionally related mutations occur at distinct but very closely linked loci and may rarely be separated by crossing over.
   • Test: Complementation in trans may restore a normal phenotype, indicating mutations in different functional units.
   • Example: The lozenge region of Drosophila historically revealed closely linked mutations initially treated as alleles.
   • Distinction: True alleles occupy the same locus and cannot be separated by recombination; pseudoalleles can show rare recombination.

IV. Blood Group Genetics — Codominance and Antigen Inheritance

A. Blood group genetics

Blood groups are inherited antigen systems whose alleles determine molecules displayed on red blood cells.

• ABO system: The ABO gene encodes glycosyltransferases; (I^A) adds A antigen, (I^B) adds B antigen, and (i) usually produces no functional enzyme.

• Allelic interaction: (I^A) and (I^B) are codominant, while both dominate (i).

  Genotype	Blood group	Red-cell antigen
  (I^AI^A), (I^Ai)	A	A
  (I^BI^B), (I^Bi)	B	B
  (I^AI^B)	AB	A and B
  (ii)	O	Neither A nor B

• Cross example: (I^Ai \times I^Bi) can produce A, B, AB, or O offspring, each with probability (1/4).

• Rh system: The clinically important D antigen is often simplified as (D) dominant to (d); an Rh-negative mother carrying an Rh-positive fetus may become sensitized.

• Clinical importance: ABO incompatibility can cause acute transfusion reactions; anti-D immunoglobulin reduces Rh hemolytic disease of the fetus and newborn.

V. Sex Determination and Sex Linkage — Chromosomal Control of Sex-Associated Inheritance

A. Sex determination and sex linkage

Sex determination establishes sexual development, while sex linkage describes inheritance of genes located on sex chromosomes.

• XX–XY system: Human females are typically XX and males XY; the Y-linked SRY gene initiates testis development.
• Other systems:
  • XX–XO: Females XX and males XO, as in some insects.
  • ZZ–ZW: Males ZZ and females ZW, as in birds.
  • Haplodiploidy: Fertilized diploid eggs produce females and unfertilized haploid eggs produce males in many Hymenoptera.
  • Environmental determination: Temperature affects sex in several reptiles.
• X-linked inheritance: Males are hemizygous for most X-linked genes, so one recessive allele can be expressed; red–green color blindness is an example.
• Transmission pattern: An affected father passes his X chromosome to all daughters but no sons; sons receive their X chromosome from their mother.
• Y-linked inheritance: A Y-linked trait passes from father to son only.
• Dosage compensation: In female mammals, one X chromosome is largely inactivated, forming a Barr body and producing cellular mosaicism.

VI. Sex-Limited and Sex-Influenced Traits — Sex-Dependent Gene Expression

A. Sex-limited and sex-influenced traits

These are usually autosomal traits whose expression differs between sexes because of anatomy, hormones, or physiological environment.

1. Sex-limited traits:

   • Definition: Genes occur in both sexes but are phenotypically expressed in only one.
   • Examples: Milk production is expressed in female mammals; cock-feathering is normally expressed in male chickens.
   • Inheritance: An unexpressed individual can still transmit the relevant alleles.

2. Sex-influenced traits:

   • Definition: A genotype is expressed in both sexes but with different dominance, penetrance, or intensity.
   • Example: Pattern baldness is traditionally modeled as an autosomal allele that behaves more dominantly in males than females.
   • Distinction from sex linkage: The genes need not lie on X or Y chromosomes; sex modifies autosomal expression.

VII. Probability and Chi-Square — Predicting and Testing Genetic Ratios

A. Probability and chi-square

Probability predicts outcomes of genetic crosses, while the chi-square test evaluates whether observed data fit an expected genetic ratio.

• Product rule: For independent events, multiply probabilities:
  
  TEXT

    P(A and B) = P(A) × P(B)

  
  (P) denotes probability; (A) and (B) are independent events.
• Sum rule: For mutually exclusive outcomes, add their probabilities:
  
  TEXT

    P(A or B) = P(A) + P(B)

• Binomial model:
  
  TEXT

    P(r) = [n! / (r!(n-r)!)] p^r q^(n-r)

  
  (n) is the number of trials, (r) the specified outcome count, (p) its probability, and (q=1-p).
• Chi-square statistic:
  
  TEXT

    χ² = Σ[(O − E)² / E]

  
  (O) is an observed count, (E) an expected count, and (\Sigma) means sum over categories.
• Degrees of freedom: Usually (df=k-1), where (k) is the number of phenotypic categories.
• Interpretation: A small (p)-value, conventionally below 0.05, suggests that chance alone poorly explains the deviation.
• Conditions: Observations should be independent, categories mutually exclusive, and expected counts generally at least about five.

VIII. Mutation — The Source of New Genetic Variation

A. Mutation

A mutation is a permanent change in genetic material that may be inherited by daughter cells or offspring.

• Somatic mutation: Occurs in body cells and affects a cell lineage; it is not normally transmitted to offspring.
• Germ-line mutation: Occurs in gametes or their precursors and can enter future generations.
• Spontaneous origin: Replication errors, tautomeric base changes, depurination, deamination, and transposable elements can alter DNA.
• Phenotypic effects: Mutations may be neutral, harmful, lethal, beneficial, conditional, or silent.
• Evolutionary role: Mutation creates new alleles; selection, drift, migration, and recombination determine their population frequencies.
• Repair: Proofreading, mismatch repair, base-excision repair, nucleotide-excision repair, and double-strand-break repair limit mutation rates.

IX. Classification of Mutations — Molecular, Chromosomal, and Functional Categories

A. Classification of mutations

Mutations are classified by the scale of change, molecular effect, origin, cell type, or phenotypic consequence.

• Base substitutions:
  • Transition: Purine replaces purine or pyrimidine replaces pyrimidine.
  • Transversion: Purine and pyrimidine replace one another.
  • Coding outcomes: Silent, missense, or nonsense mutations.
• Insertions and deletions: Addition or loss of nucleotides causes a frameshift when the number is not divisible by three.
• Repeat expansion: Repeated sequences increase in copy number, as with CAG expansion in Huntington disease.
• Chromosome-structure mutations: Deletion, duplication, inversion, and translocation rearrange chromosome segments.
• Genome-number mutations: Aneuploidy changes individual chromosome numbers, while polyploidy changes complete chromosome sets.
• Functional classes: Loss-of-function, gain-of-function, dominant-negative, lethal, suppressor, and conditional mutations describe biological effects.

X. Methods of Inducing Mutations — Experimental Mutagenesis Techniques

A. Methods of inducing mutations

Experimental mutagenesis deliberately increases mutation frequency to study gene function or generate useful variation.

• Physical treatment: Seeds, cells, or organisms are exposed to calibrated X-rays, gamma rays, ultraviolet light, or particle radiation.
• Chemical treatment: Material is treated with a selected mutagen at controlled concentration, temperature, pH, and duration.
• Insertional method: Transposons or transferred DNA disrupt genes and provide a molecular tag for locating the affected locus.
• Targeted method: CRISPR–Cas systems create a chosen DNA break; repair by non-homologous end joining or homology-directed repair produces defined changes.
• Screening method: Forward genetics begins with a phenotype and identifies its gene; reverse genetics alters a known gene and observes the phenotype.
• Safety principle: Dose must balance mutation yield against sterility, lethality, and extensive chromosome damage.

XI. Mutagenic Agents — Physical, Chemical, and Biological Causes

A. Mutagenic agents

Mutagenic agents increase DNA damage or replication errors above the spontaneous background rate.

• Physical mutagens:
  • Ultraviolet radiation: Produces pyrimidine dimers that distort DNA.
  • Ionizing radiation: X-rays and gamma rays generate free radicals, strand breaks, and chromosome rearrangements.
• Chemical mutagens:
  • Base analogues: 5-bromouracil can mispair during replication.
  • Alkylating agents: Ethyl methanesulfonate modifies bases and commonly causes substitutions.
  • Deaminating agents: Nitrous acid alters base-pairing properties.
  • Intercalating agents: Acridine compounds insert between base pairs and promote frameshift mutations.
• Biological mutagens: Transposons, some viruses, and mobile genetic elements can insert into genes or disturb regulation.
• Specificity: Each agent produces a characteristic mutation spectrum rather than all mutation types equally.

XII. Induction of Mutation — Treatment, Recovery, and Detection

A. Induction of mutation

Mutation induction is a controlled process of treatment, biological recovery, screening, and confirmation.

• Baseline determination: Untreated controls establish spontaneous mutation frequency and normal survival.
• Dose selection: Pilot experiments identify an exposure producing useful mutation rates without excessive mortality; median-lethal or growth-reduction measurements may guide dosage.
• Treatment: Uniform biological material is exposed under standardized conditions, with chemical mutagens subsequently removed or neutralized.
• Recovery and fixation: Cells must replicate after treatment; recessive mutations usually become detectable only after segregation or inbreeding produces homozygosity.
• Screening: Morphological, biochemical, resistance-based, reporter, or sequencing assays identify candidate mutants.
• Confirmation: Repetition, complementation tests, linkage analysis, or DNA sequencing distinguishes heritable mutations from temporary physiological injury.
• Application: Induced mutation supports gene discovery, microbial strain improvement, functional genomics, and mutation breeding of crops.