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5.4 Non-Mendelian Genetics

Syllabus
2025
Topic
5.4
Level
—

Learning objectives

5.4A—Explain deviations from Mendel’s model of the inheritance of traitsExplain deviations from Mendel’s model of the inheritance of traits.• Patterns of inheritance of many traits do not follow the ratios predicted by Mendel’s laws and can be identified by quantitative analysis, when the observed phenotypic ratios statistically differ from the predicted ratios.- i. Genes located on the same chromosome are referred to as being genetically linked. The probability that these linked genes segregate together during meiosis can be used to calculate the map distance (or map units) between them on a chromosome. This calculation is called gene or genetic mapping.- ii. Codominance occurs when the phenotype from both alleles is expressed such that the heterozygote would have a different phenotype than either homozygote.- iii. Incomplete dominance occurs when neither allele of a gene can mask the other, so the phenotype of the heterozygote is a blended version of the dominant and recessive phenotypes.• Some traits, known as sex-linked traits (X- or Y-linked), are determined by genes on sex chromosomes. The pattern of inheritance of sex-linked traits can often be predicted from data, including pedigrees, indicating the genotypes and phenotypes of both parents and offspring.• Pleiotropy is a phenomenon in which the expression of a single gene results in multiple traits or effects; these traits therefore do not segregate independently.• Some traits result from non-nuclear inheritance.- i. Chloroplasts and mitochondria are randomly assorted to gametes and daughter cells; thus, traits determined by chloroplast and mitochondrial DNA do not follow simple Mendelian rules.- ii. In animals, mitochondria are usually transmitted by the egg and not by sperm; thus, traits determined by the mitochondrial DNA are typically maternally inherited.- iii. In plants, mitochondria and chloroplasts are transmitted in the ovule and not in the pollen; as such, mitochondria-determined and chloroplast-determined traits are typically maternally inherited.

Why Inheritance Can Deviate from Mendelian Ratios

A non-Mendelian pattern occurs when observed inheritance does not fit the ratios expected from a simple model of complete dominance and independent assortment. The deviation can come from chromosome location, allele expression, gene effects, or inheritance outside the nucleus.

Pattern Biological basis Evidence or consequence
Genetic linkage Genes on the same chromosome tend to travel together; crossing over can produce recombinant combinations Parental combinations exceed recombinant combinations; recombination frequency estimates map distance
Codominance Both alleles are expressed in a heterozygote The heterozygote displays both allele products and differs from either homozygote
Incomplete dominance Neither allele fully masks the other The heterozygote has an intermediate or blended phenotype
Sex-linked inheritance The gene is on a sex chromosome Transmission depends on the offspring's sex-chromosome genotype and can be inferred from pedigrees
Pleiotropy One gene influences multiple traits or effects Several phenotypic effects follow the same inherited gene
Non-nuclear inheritance The trait is encoded by mitochondrial or chloroplast DNA In animals, mitochondrial traits are typically maternal; in plants, mitochondrial and chloroplast traits are typically transmitted through the ovule

$\text{map distance (map units)}=\dfrac{\text{recombinant offspring}}{\text{total offspring}}\times100$

Analyze a deviation in order: state the expected Mendelian ratio → compare it with observed counts → determine whether the difference is statistically meaningful → use the pattern of offspring or pedigree transmission to identify which model assumption may not hold. A significant difference shows that the simple prediction is inadequate; the detailed pattern is needed to identify the mechanism.

Non-Mendelian does not mean alleles ignore meiosis. It means that linkage, allele expression, chromosome type, multiple gene effects, or organelle transmission changes the ratios or patterns predicted by the simplest Mendelian model.