How does the phenotypic ratio of an F2 generation in incomplete dominance differ from Mendelian dominance?

In Mendelian dominance, the F2 phenotypic ratio is 3:1 because the heterozygote looks like the dominant homozygote. In incomplete dominance, the ratio is 1:2:1 because the heterozygote has a unique, intermediate phenotype.

What is the difference between codominance and incomplete dominance?

Incomplete dominance results in a blend or intermediate phenotype (e.g., pink from red and white). Codominance results in both parental phenotypes being expressed distinctly and simultaneously (e.g., red and white patches).

How does gene linkage affect the Law of Independent Assortment?

Gene linkage violates the Law of Independent Assortment because genes located close together on the same chromosome tend to be inherited as a unit. They do not segregate independently unless a crossover event occurs between them.

What is a common mistake when calculating probabilities for X-linked recessive traits?

A common error is failing to account for the sex of the offspring. Males only need one recessive allele to express the trait, while females need two; therefore, the probability is often different for sons versus daughters.

Why is it incorrect to assume a 9:3:3:1 ratio for all dihybrid crosses?

The 9:3:3:1 ratio only applies if the two genes are on different chromosomes and show complete dominance. If the genes are linked or show epistasis, the observed ratios will deviate significantly from this prediction.

What error occurs if you confuse pleiotropy with polygenic inheritance?

Pleiotropy is one gene affecting multiple traits, whereas polygenic inheritance is multiple genes affecting one single trait. Confusing them leads to a misunderstanding of how mutations impact the organism's overall phenotype.

Define 'Hemizygous' in the context of sex-linked inheritance.

Hemizygous refers to having only one copy of a gene or chromosome instead of the usual two. In humans, males are hemizygous for genes on the X and Y chromosomes.

What are 'Map Units' (centimorgans)?

Map units are a measurement of the relative distance between genes on a chromosome, where 1 map unit equals a 1% chance of recombination occurring between those genes during meiosis.

Epistasis is a type of gene interaction where the expression of one gene masks or interferes with the expression of another, separate gene, often altering expected Mendelian ratios.

Why are mitochondrial traits inherited only from the mother?

During fertilization, the egg cell contributes the cytoplasm and organelles (including mitochondria) to the zygote, while the sperm contributes only nuclear DNA. Thus, all mitochondrial DNA comes from the maternal lineage.

Non-Mendelian Genetics | College Board AP Biology

Non-Mendelian Genetics

Summary

Non-Mendelian genetics encompasses inheritance patterns that deviate from the simple dominant-recessive relationships and independent assortment described by Gregor Mendel. These patterns arise from complex interactions between alleles, the physical proximity of genes on chromosomes, and the influence of non-nuclear DNA, leading to phenotypic ratios that differ significantly from the classic 3:1 or 9:3:3:1 predictions.

1. Variations in Dominance: Incomplete and Codominance

Incomplete Dominance occurs when the phenotype of a heterozygote is an intermediate blend between the two homozygous phenotypes. In this pattern, neither allele is completely dominant, resulting in a third phenotype; for example, crossing a red-flowered plant with a white-flowered plant might produce pink-flowered offspring.
Codominance is a relationship where both alleles in a heterozygote are fully and simultaneously expressed in the phenotype. Unlike blending, both parental traits appear distinctly in the offspring, such as a cross between a black-feathered bird and a white-feathered bird resulting in offspring with both black and white feathers.
Notation and Ratios: These patterns are often represented using a base letter with superscripts (e.g., $C^R$ and $C^W$ ). In a monohybrid cross of two heterozygotes, both incomplete dominance and codominance typically yield a 1:2:1 phenotypic ratio, matching the genotypic ratio.

Diagram comparing Complete Dominance (Red), Incomplete Dominance (Pink blend), and Codominance (Red spots on White).

2. Gene Linkage and Recombination

Autosomal Linkage occurs when genes are located close together on the same chromosome (autosome). Because they are physically part of the same DNA molecule, they tend to be inherited together as a single unit rather than assorting independently during meiosis.
Breaking Linkage: The process of crossing over during prophase I of meiosis can swap segments of homologous chromosomes, creating new combinations of alleles known as recombinant phenotypes. The frequency of this recombination depends on the physical distance between the linked genes.
Mapping Distance: Recombination frequency is used to map the relative positions of genes on a chromosome. One percent recombination frequency is defined as one map unit (or centimorgan); the further apart two genes are, the more likely they are to be separated by a crossover event.

3. Sex-Linked Inheritance Patterns

4. Non-Nuclear and Multi-Gene Inheritance

5. Key Distinctions in Inheritance

6. Exam Strategy & Tips