1.2 Basic Principles of Inheritance

Basic Principles of Inheritance

Inheritance explains how traits pass from parents to offspring through genes and alleles. Understanding these principles gives you the tools to predict genetic outcomes using crosses and probability. Mendel's laws of segregation and independent assortment form the core framework, and non-Mendelian patterns build on that foundation.

Key Terms in Inheritance

A gene is a segment of DNA that codes for a specific trait or characteristic. Each gene sits at a specific position on a chromosome called its locus (plural: loci). Think of the locus as the gene's "address" on a particular chromosome.

An allele is one of the alternative forms a gene can take. For example, a gene for flower color might have a purple allele and a white allele. Alleles can be dominant (expressed when one or two copies are present) or recessive (expressed only when two copies are present, with no dominant allele to mask it).

Genotype refers to the actual allele combination an organism carries for a trait (e.g., Aa or aa). Phenotype is the observable result of that genotype, like purple vs. white flowers. Phenotype is shaped by genotype but can also be influenced by environmental factors such as temperature or nutrition.

A few more terms you'll use constantly:

• Homozygous means both alleles are the same (AA or aa)
• Heterozygous means the two alleles differ (Aa)
• The heterozygous individual is often called a carrier of the recessive allele when discussing recessive traits

Mendel's Laws of Inheritance

Law of Segregation

Each organism carries two alleles for every gene (one from each parent). During gamete formation (meiosis), those two alleles separate so that each gamete carries only one allele. When two gametes fuse at fertilization, the offspring again has two alleles.

This law directly explains the 3:1 phenotypic ratio you see in a monohybrid cross of two heterozygotes. Cross Aa × Aa, and you get 3 offspring showing the dominant phenotype for every 1 showing the recessive phenotype.

Law of Independent Assortment

Alleles for different genes sort into gametes independently of one another, as long as the genes are on different chromosomes (or far apart on the same chromosome). This means inheriting one allele for gene A doesn't affect which allele you inherit for gene B.

This law explains the 9:3:3:1 phenotypic ratio in a dihybrid cross of two double heterozygotes (AaBb × AaBb).

Why Mendel's laws matter:

• They let you predict offspring genotypes and phenotypes using simple probability
• They provide the foundation for understanding more complex patterns like epistasis, pleiotropy, and linkage
• Deviations from expected Mendelian ratios are often your first clue that something more complex is going on

Genetic Problem-Solving

Monohybrid Cross (one gene, two alleles)

A Punnett square is the standard tool here. For a cross between two heterozygous individuals (Aa × Aa):

1. Write one parent's possible gametes across the top (A and a)
2. Write the other parent's possible gametes down the side (A and a)
3. Fill in each box by combining the row and column alleles
4. Count up the resulting genotypes and phenotypes

Results:

• Genotypic ratio → 1 AA : 2 Aa : 1 aa
• Phenotypic ratio → 3 dominant : 1 recessive

Each box in the Punnett square represents an equally likely outcome, so each has a probability of 1/4. That means there's a 3/4 chance of the dominant phenotype and a 1/4 chance of the recessive phenotype for any single offspring.

Dihybrid Cross (two genes, each with two alleles)

For AaBb × AaBb, each parent produces four gamete types: AB, Ab, aB, and ab. You'll need a 4×4 Punnett square (16 boxes).

The expected phenotypic ratio is 9:3:3:1:

1. 9/16 dominant for both traits ($A\_B\_$)
2. 3/16 dominant for trait A, recessive for trait B ($A\_bb$)
3. 3/16 recessive for trait A, dominant for trait B ($aaB\_$)
4. 1/16 recessive for both traits ($aabb$)

The underscore notation ($A\_$) means "at least one dominant allele," so it covers both AA and Aa.

Non-Mendelian Inheritance Patterns

Not all traits follow simple dominant/recessive rules. Here are three common exceptions you need to know.

Incomplete Dominance

Neither allele is fully dominant. The heterozygote shows a blended, intermediate phenotype between the two homozygotes. Cross a red-flowered plant (RR) with a white-flowered plant (rr), and all $F_1$ offspring (Rr) are pink. Cross two pink $F_1$ plants, and you get a 1:2:1 phenotypic ratio (1 red : 2 pink : 1 white) instead of the typical 3:1.

Codominance

Both alleles are fully expressed at the same time in the heterozygote. This is different from incomplete dominance because you see both phenotypes, not a blend. In roan cattle, a cross between a red-haired individual (RR) and a white-haired individual (WW) produces offspring (RW) with distinct red and white hairs visible side by side.

Multiple Alleles

Some genes have more than two alleles in the population, even though any single individual still carries only two. The ABO blood group system is the classic example, with three alleles: $I^A$, $I^B$, and $i$.

• $I^A$ and $I^B$ are codominant with each other (genotype $I^A I^B$ → type AB blood)
• Both $I^A$ and $I^B$ are dominant over $i$
• Genotype $ii$ → type O blood

This means ABO inheritance involves both codominance and simple dominance within the same system, which is why it shows up on exams so often.