3.3 Mendelian and Non-Mendelian Inheritance

Why Inheritance Questions Are Evidence Questions

A Regents inheritance item may look like a vocabulary question, but it usually tests evidence. A family chart, offspring ratio, blood-type table, or short scenario gives clues about how a trait is passed on. Your job is to decide what the evidence supports. Begin by separating genotype, the allele combination, from phenotype, the observed trait or function.

An allele is a version of a gene. In a simple dominant-recessive pattern, a dominant allele is expressed when at least one copy is present, while a recessive phenotype appears only when two recessive alleles are present. A person with two identical alleles is homozygous. A person with two different alleles is heterozygous. A heterozygous individual who has one recessive disease allele but does not show the recessive condition may be called a carrier.

Use letters carefully. If T represents a dominant allele for tall stems and t represents a recessive allele for short stems, TT and Tt can both produce tall plants, but tt produces short plants. The phenotype tall does not tell you whether the genotype is TT or Tt unless more evidence is provided.

Punnett Squares as Probability Models

A Punnett square organizes possible gametes from each parent. It does not predict the exact children in order, and it does not guarantee that four offspring will match a 3:1 ratio. It gives expected probabilities for each fertilization event.

A common Regents trap is saying a 25% probability means exactly one of four children must have the trait. Probability applies to each independent fertilization event. Four children could all have the same phenotype by chance, especially in a small sample.

Pedigrees and Family Evidence

A pedigree shows family relationships and which individuals show a trait. For a recessive trait, unaffected parents can have an affected child if both parents are carriers. For a dominant trait, an affected child usually has at least one affected parent, unless a new mutation occurs. For sex-linked patterns, males may be affected more often because they have only one X chromosome. Regents questions usually provide enough context; do not assume sex-linkage unless the pedigree or prompt supports it.

Beyond Simple Dominance

Real traits often do not fit a single dominant allele hiding a recessive allele. In incomplete dominance, the heterozygous phenotype is intermediate, such as red and white flower alleles producing pink flowers in a simplified model. In codominance, both alleles are expressed, such as a blood-type example where A and B alleles together produce type AB. In multiple-allele traits, more than two allele versions exist in the population, even though each individual still has only two alleles for that gene. Human ABO blood type is a common classroom example because IA, IB, and i are three allele versions.

Many traits are polygenic, meaning they are influenced by several genes. Height, skin pigmentation, and many disease risks involve multiple genes plus environmental factors. A graph of a polygenic trait often shows a range of values rather than two neat categories. Regents questions may ask which explanation best fits a wide distribution. The answer is usually multiple genes and environmental influence, not one dominant allele.

Science-Data Example

A class grows two groups of genetically similar plants. Group 1 receives high light and enough minerals. Group 2 receives low light and fewer minerals. Both groups carry alleles associated with tall growth, but Group 2 plants are shorter on average.

The data do not prove the alleles changed. They show that environment affected phenotype. A strong explanation says the plants had similar genetic potential, but light and mineral availability affected photosynthesis, growth, and final height. This kind of reasoning helps avoid the trap that every observed difference is inherited.

Choosing the Best Inheritance Claim

Use this checklist when a Regents item asks for a claim:

• Identify the phenotype pattern: two categories, blended categories, both traits expressed, or continuous range.
• Check parent and offspring data before naming the inheritance pattern.
• Decide whether the evidence supports a probability claim or only a possible explanation.
• Include environment if genetically similar organisms show different traits under different conditions.
• Avoid claiming a trait is dominant just because it is common; dominance and frequency are different ideas.

A dominant allele can be rare in a population, and a recessive allele can be common. Dominance describes expression in a heterozygote, not how often the allele appears. Likewise, a Punnett square predicts genetic probability, not survival, reproduction, or natural selection. Those topics connect to evolution, but they are not the same as inheritance.

Regents Traps to Avoid

• Phenotype does not always reveal genotype.
• Probability is not a guarantee for a small number of offspring.
• Dominant does not mean stronger, better, or more common.
• Carriers can pass on recessive alleles without showing the recessive condition.
• Environment can influence phenotype without changing DNA sequence.

On constructed response items, name the evidence and the reasoning. For example: The affected child has unaffected parents, so each parent could be a carrier; the recessive phenotype appears only when the child inherits two recessive alleles. That answer uses pedigree evidence and explains the mechanism, which is stronger than only writing recessive.