Genetics Study Notes: Alleles, Inheritance, Natural Selection, and Pedigrees Summary & Study Notes

🧬 Dominant vs. Recessive Alleles

Alleles are alternative versions of a gene found at the same locus on homologous chromosomes. An organism's genotype is the combination of alleles (for example, $AA$, $Aa$, or $aa$) and the phenotype is the observable trait.

A dominant allele (usually written as a capital letter, e.g., $A$) expresses its phenotype when present in either the heterozygous ($Aa$) or homozygous ($AA$) state. A recessive allele (lowercase, e.g., $a$) is expressed phenotypically only when homozygous ($aa$). Knowing whether an allele is dominant or recessive helps predict inheritance patterns using Punnett squares.

Key quick rules: affected individuals in every generation often indicate dominant inheritance; traits that skip generations often indicate recessive inheritance.

🎨 Incomplete Dominance & Codominance

Incomplete dominance occurs when heterozygotes show an intermediate phenotype between the two homozygotes. Example: red ($RR$) x white ($WW$) snapdragons produce pink ($RW$). The heterozygote is a blend, not a mixture of distinct parental phenotypes.

Codominance occurs when both alleles in a heterozygote are fully expressed, producing a phenotype that shows both traits simultaneously. Classic example: ABO blood group — alleles $I^A$ and $I^B$ are codominant to each other, while $i$ is recessive. A person with genotype $I^A I^B$ expresses both A and B antigens.

Both patterns deviate from simple dominant/recessive rules and must be interpreted accordingly when predicting offspring phenotypes.

🌱 Natural Selection (Basics for Genetics Tests)

Natural selection is the process by which organisms with traits that increase survival or reproduction become more common in a population over generations. The core components are variation, inheritance, differential survival/reproduction, and time.

Types of selection: directional (favors one extreme), stabilizing (favors the average), and disruptive (favors extremes). Examples: antibiotic resistance in bacteria (directional) and peppered moth coloration during industrial melanism.

Important mechanisms that alter allele frequencies: mutation (introduces new alleles), gene flow (migration), genetic drift (random changes, important in small populations), and selection (non-random change based on fitness). Fitness is the relative reproductive success of a genotype.

Practical points: selection acts on phenotypes but changes genotypes over time; high genetic variation allows faster adaptive responses.

🧾 Constructing a Pedigree

A pedigree is a family tree that shows how a trait is inherited through generations. Common symbols: square = male, circle = female, shaded = affected. A horizontal line between a male and female is a mating; vertical lines lead to offspring. Generations are labeled (I, II, III…) and individuals often numbered.

Steps to analyze a pedigree:

1. Note whether affected individuals appear every generation. If yes, consider dominant inheritance.
2. If the trait skips generations and unaffected parents have affected children, consider autosomal recessive.
3. Check sex distribution: more males affected suggests X-linked recessive; equal males/females suggests autosomal.
4. Look for father-to-son transmission: if present, the trait cannot be X-linked (Y-linked or autosomal). Lack of male-to-male transmission often indicates X-linked inheritance.
5. Identify carriers: for autosomal recessive, parents of affected children are likely carriers. In X-linked recessive, carrier females may be unaffected but can transmit the allele to sons.

Use Punnett-square logic to test possible modes of inheritance against the pedigree data. Mark possible genotypes for each individual as you deduce them.

♂️♀️ Sex-Linked Traits & Inheritance

Sex chromosomes in humans are X and Y. Males are XY (hemizygous for X), females are XX. Sex-linked inheritance commonly refers to X-linked traits.

X-linked recessive: mutant allele on X (e.g., $X^c$) causes disease when males have $X^cY$ (they have no second X). Females must be $X^cX^c$ to be affected; $X^cX$ females are typically carriers. Typical patterns: many more affected males, affected males often have carrier mothers, no male-to-male transmission.

X-linked dominant: a single copy of the mutant allele on X causes disease in females ($X^D X$) and in males ($X^D Y$). Affected fathers pass the trait to all daughters but none of their sons.

Y-linked: only males affected and father-to-son transmission is consistent. These traits are rare.

Practical examples: red-green color blindness and hemophilia are classic X-linked recessive examples. ABO blood type is autosomal and illustrates codominance, not sex linkage.

Final tips:

• Always annotate genotypes when building pedigrees. Use family history and simple logic rules (who transmits to whom) to eliminate impossible modes.
• Draw Punnett squares for suspected crosses to verify whether observed offspring ratios fit a proposed inheritance model.
• Remember carriers and incomplete/codominant patterns can make pedigrees look unusual compared to simple dominant/recessive expectations.

Good luck on your test—focus on recognizing patterns (skip generations, sex bias, father-daughter/son transmission) and practice a few pedigree problems and Punnett squares.