Non-Mendelian Genetics
Mendel's model is clean: one gene, two alleles, one fully dominant over the other, so the heterozygote looks exactly like the dominant homozygote. Most real inheritance does not stay that tidy. Non-Mendelian genetics is the set of patterns that break the clean picture — where the heterozygote shows something new, where a gene has more than two possible alleles, or where a trait's inheritance depends on which sex chromosome carries it. The underlying rules of segregation still hold; what changes is how the two alleles are expressed, how many alleles exist to draw from, and where on the chromosomes the gene sits.
Two of these patterns are constantly confused. In incomplete dominance the heterozygote is a blended intermediate — a red and a white parent giving pink, a phenotype neither parent shows. In codominance both alleles are fully and separately expressed at once — think of AB blood, where A and B antigens both appear, unmixed. Multiple alleles adds a population-level twist: a gene like ABO has more than two variants circulating, even though any single individual still carries only two of them. And in sex linkage, a gene on the X or Y inherits differently in males and females — an X-linked recessive trait surfaces far more often in males, who are hemizygous and need only one copy to show it.
Interactive · Non-Mendelian
Set up crosses across the four patterns and read the offspring ratios out directly. Flip a locus between incomplete dominance and codominance to see how the same cross gives a blended intermediate in one and both alleles at once in the other, expand a gene to multiple alleles, and put a gene on the X to watch how the phenotype ratios split by sex.
Non-Mendelian · Open the full sandbox →The mistakes here all come from forcing non-Mendelian crosses back into Mendel's single mold. The first is collapsing incomplete dominance into codominance — treating a blended pink heterozygote and a both-antigens-at-once AB as the same thing, when one mixes and the other shows both alleles fully and separately (U5-BIO10, U5-BIO11). The second is assuming a dominant allele must be stronger or more common — when "dominant" only says which allele's phenotype shows in a heterozygote, and with multiple alleles a population may carry many variants even though any one organism still holds exactly two (U5-BIO7). The third is flattening sex linkage — expecting the usual autosomal ratios and forgetting that a hemizygous male shows an X-linked recessive from a single copy, so the phenotype ratios differ by sex (U5-BIO14). Every scenario in this topic asks you to read the cross for which pattern is in play before you predict the offspring.
The work
3 ways in · any order
Lesson
Non-Mendelian Genetics
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Non-Mendelian patterns break the clean one-gene, two-allele, fully-dominant model: incomplete dominance blends the heterozygote, codominance expresses both alleles at once, multiple alleles give a gene more than two population variants, and sex linkage ties inheritance to the X or Y. The lesson walks the ways students misread these — fusing incomplete dominance with codominance, over-loading an individual with too many alleles, and flattening sex linkage back to autosomal ratios. It closes with a ten-scenario applet that asks you to name the pattern before you predict the cross.
Diagnostic
10-item topic check
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Ten items across the non-Mendelian patterns — telling a codominant cross (both alleles fully expressed) from an incompletely dominant one (a blended intermediate) (U5-BIO10, U5-BIO11); recognizing that a dominant allele is not automatically stronger or more common, and that a multiple-allele system has many variants in the population but two in any individual (U5-BIO7); and tracking a sex-linked trait whose phenotype ratios split between males and females (U5-BIO14). Take it cold to surface which patterns are still tangled, or after the lesson to confirm they hold.
Targeted Practice
Drill a single misconception
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Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.