- Maternal Effects are influences of the mother on the phenotype of her offspring that occur through mechanisms beyond the offspring’s own nuclear genotype. They are an important component of quantitative genetics because offspring can resemble their mothers not only because they inherit genes from them, but also because the maternal environment, maternal physiology, maternal behavior, cytoplasmic inheritance, and conditions during development can influence offspring traits. Maternal effects can therefore contribute to phenotypic variation, covariance among relatives, estimates of heritability, and the observed response to selection.
- A maternal effect occurs when characteristics of the mother influence the phenotype of her offspring. These effects can begin before birth through the uterine environment or during egg formation and can continue after birth through nutrition, nursing, behavior, protection, and other forms of parental care. The offspring’s phenotype is therefore influenced by its own genotype, its direct environment, and factors originating from the mother.
- In quantitative genetic models, a simplified representation of an offspring phenotype can include its direct genetic effect, maternal genetic effect, maternal environmental effect, and residual environmental variation. Conceptually, the phenotype can be represented as a combination of direct genetic effects, maternal effects, and environmental effects. Maternal effects are particularly important when individuals raised by different mothers experience substantially different developmental environments.
- Maternal effects can have several biological sources. Prenatal effects occur when the mother’s physiological condition, nutrition, hormones, immune status, or exposure to environmental conditions influences embryonic or fetal development. In mammals, the uterine environment can affect offspring growth and development before birth. In birds, reptiles, fish, and other egg-laying organisms, maternal investment can influence the contents and developmental environment of eggs. Egg size, nutrient composition, hormones, antibodies, and other maternal contributions can affect offspring phenotypes.
- After birth or hatching, maternal environmental effects can continue through nutrition, milk production, nursing, protection, temperature regulation, social interactions, and other forms of maternal care. These effects can influence traits such as growth rate, survival, behavior, development, disease resistance, and reproductive performance. Such effects may persist for substantial periods even though the maternal environment is not part of the offspring’s inherited nuclear DNA sequence.
- Maternal effects can also have a genetic component. A mother’s genotype may influence the environment she provides to her offspring. For example, genes affecting maternal body size, milk production, hormone levels, immune function, behavior, or reproductive physiology can indirectly influence offspring traits. This creates an important distinction between a mother’s direct genetic effects on her own phenotype and her maternal genetic effects on offspring phenotypes.
- Maternal effects can also arise from cytoplasmic inheritance. The cytoplasm of an egg contains cellular components that are contributed by the mother to the offspring. Mitochondrial DNA is a major example because mitochondria are generally inherited maternally in many animals. Maternal transmission can therefore affect offspring phenotypes through genetic information outside the nuclear genome. However, cytoplasmic inheritance should not be treated as identical to maternal environmental effects because they have different biological mechanisms.
- Another important mechanism involves maternal epigenetic effects. Maternal conditions can influence patterns of gene expression in developing offspring through molecular mechanisms such as DNA methylation, histone modification, chromatin regulation, and regulatory RNAs. These processes can alter developmental trajectories without necessarily changing the underlying DNA sequence. Maternal epigenetic effects are therefore one possible mechanism connecting maternal conditions with offspring phenotypes.
- Maternal effects are closely related to phenotypic variation because differences among mothers can contribute to differences among their offspring. If offspring from some mothers consistently grow faster or survive better than offspring from other mothers, the pattern may reflect differences in maternal environments or maternal characteristics rather than differences in the offspring’s direct genetic effects alone. Ignoring maternal effects can therefore lead to incorrect interpretations of the sources of phenotypic variation.
- Maternal effects are particularly important when estimating genetic variance and heritability. Standard quantitative genetic models often assume that resemblance between relatives is primarily caused by shared genetic effects. However, offspring sharing the same mother also share aspects of their maternal environment. If maternal effects are not modeled, environmental resemblance among maternal siblings can be incorrectly attributed to genetic similarity, potentially inflating estimates of genetic variance or heritability.
- This issue is especially important for covariance among relatives. Maternal half-siblings, for example, share the same mother but have different fathers. Their phenotypic resemblance can therefore contain information about maternal effects as well as genetic relationships. Similarly, full siblings share both parents and may also share maternal prenatal and early-life environments. Quantitative genetic analyses can use these family structures to separate direct genetic effects from maternal effects when sufficient information is available.
- Maternal effects can be incorporated into linear mixed models and other statistical models used in quantitative genetics. A model may include a direct additive genetic effect for the offspring and a separate maternal effect associated with the mother. Depending on the biological system and experimental design, maternal effects may be modeled as maternal genetic effects, maternal permanent environmental effects, or both. REML and related variance-component methods can be used to estimate the corresponding sources of variation.
- A useful conceptual distinction is between direct genetic effects and maternal genetic effects. A direct genetic effect occurs when the offspring’s own genotype influences its phenotype. A maternal genetic effect occurs when the mother’s genotype influences the offspring phenotype through the maternal environment or maternal contribution. The two effects can be genetically correlated because genes influencing maternal characteristics may also be related to genes influencing the offspring’s own traits.
- Maternal effects can therefore create complex patterns of resemblance among relatives. A mother’s genes may affect her own phenotype, her ability to provide resources, and consequently the phenotype of her offspring. At the same time, offspring inherit genes from both parents that influence their own traits. This creates a potentially important relationship between maternal genotype, offspring genotype, maternal environment, and offspring phenotype.
- Maternal effects can also influence estimates of the breeding value of individuals. If offspring performance is partly determined by the maternal environment, observed offspring phenotypes may not accurately represent the offspring’s own genetic potential. In animal and plant breeding, separating maternal effects from direct genetic effects can therefore improve the accuracy of breeding decisions, particularly for traits expressed early in life.
- The importance of maternal effects varies among traits and species. They are often particularly strong for early-life traits such as birth weight, hatch weight, growth, survival, immune development, and juvenile behavior. As offspring mature and become increasingly independent, some maternal effects may decline. However, maternal influences can sometimes persist into adulthood, especially when early developmental conditions have long-lasting effects on physiology, behavior, or reproductive success.
- Maternal effects can also interact with the environment through genotype–environment interaction (G×E). The influence of a particular maternal genotype or maternal environment may differ under different environmental conditions. For example, maternal nutritional effects may be stronger under resource-limited conditions than under abundant conditions. Maternal effects can therefore contribute to context-dependent phenotypic variation and may influence genetic correlations and estimates of genetic architecture.
- Maternal effects are closely related to phenotypic plasticity because mothers can alter the developmental environment experienced by their offspring in response to environmental conditions. A mother exposed to temperature stress, limited food, disease, or social stress may produce offspring with different phenotypes. Such changes can represent environmentally induced maternal effects rather than changes in the offspring’s DNA sequence.
- Maternal effects are important in animal breeding because traits measured early in life may be strongly influenced by the dam. In livestock, for example, maternal milk production, body size, uterine environment, and maternal behavior can influence offspring growth. Genetic evaluation systems may therefore include both direct breeding values and maternal breeding values when maternal effects are biologically important.
- Maternal effects are also important in plant breeding and seed biology. Seed size, nutrient reserves, maternal plant condition, and the environment experienced during seed development can influence germination, early growth, and subsequent plant performance. Researchers must therefore distinguish genetic differences among offspring from maternal effects transmitted through seeds.
- In evolutionary biology, maternal effects can influence natural selection and adaptation because mothers can modify the phenotype and fitness of their offspring without directly changing offspring DNA. Maternal effects may provide a mechanism through which environmental conditions experienced by one generation influence phenotypes in the next generation. If these effects affect survival or reproduction, they can influence evolutionary dynamics.
- Maternal effects can also generate transgenerational effects, although the terminology should be used carefully. An effect observed in offspring after maternal exposure does not necessarily demonstrate stable inheritance across generations. In mammals, for example, an environmental exposure during pregnancy can directly affect the developing offspring, meaning that the offspring has been directly exposed rather than inheriting an effect exclusively through the germline. Demonstrating true transgenerational inheritance requires careful experimental designs that distinguish direct exposure from effects transmitted across generations.
- Maternal effects can influence genetic covariance and genetic correlation between traits. If a maternal characteristic affects several offspring traits simultaneously, it may create correlations between those traits that are not caused entirely by direct genetic effects in the offspring. Multivariate quantitative genetic models can therefore include maternal covariance structures when studying several traits simultaneously.
- Maternal effects also have implications for genomic selection and genomic estimated breeding values (GEBVs). Genomic information can help estimate direct genetic relationships, but genomic data alone do not automatically eliminate maternal environmental effects. Accurate prediction may require models that separately account for direct genetic effects, maternal genetic effects, and maternal permanent environmental effects.
- Experimental designs can help distinguish maternal effects from direct genetic effects. Cross-fostering, in which offspring are raised by mothers other than their biological mothers, can separate genetic inheritance from postnatal maternal environment. Reciprocal cross-fostering designs can be particularly informative. Common-garden experiments, controlled breeding designs, embryo transfer, and comparisons among maternal relatives can also help identify maternal contributions.
- Maternal effects should not be confused with maternal inheritance. Maternal inheritance usually refers to transmission of genetic or cytoplasmic material through the mother, such as mitochondrial inheritance. Maternal effects are broader and include environmental, physiological, behavioral, nutritional, and developmental influences. A maternal effect can therefore occur even when the mechanism does not involve inheritance of a specific genetic sequence.
- Maternal effects should also be distinguished from parent-of-origin effects. Parent-of-origin effects occur when the phenotypic effect of an allele depends on whether it was inherited from the mother or father. Genomic imprinting is one mechanism that can produce parent-of-origin effects. Although maternal effects and parent-of-origin effects can both produce differences associated with parental origin, they are conceptually different biological phenomena.
- Maternal effects are also relevant to the interpretation of heritability. A high resemblance between mothers and offspring does not necessarily mean that the trait has high additive heritability. Similarity can arise from inherited genes, maternal genetic effects, shared environments, prenatal conditions, or combinations of these factors. Correctly estimating heritability therefore requires models that account for important sources of maternal covariance when they are present.
- In evolutionary quantitative genetics, maternal effects can be incorporated into models of multigenerational inheritance. The phenotype of an individual may depend partly on the phenotype or genotype of its mother, creating an indirect pathway between generations. This can alter the expected response to selection because selection on offspring phenotypes may change maternal effects as well as direct genetic effects.
- Maternal effects are therefore an important part of the broader framework of quantitative genetics. They help explain why offspring can resemble their mothers for reasons extending beyond direct inheritance of nuclear genes. By separating direct genetic effects, maternal genetic effects, maternal environmental effects, and other sources of variation, researchers can better understand phenotypic variance, genetic variance, heritability, breeding value, and response to selection.
- Understanding maternal effects is particularly important whenever early development, parental investment, family structure, or reproductive biology strongly influences a trait. In breeding, ecology, evolution, developmental biology, and human genetics, accounting for maternal effects can prevent environmental and indirect parental influences from being incorrectly interpreted as direct genetic effects. Maternal effects therefore provide an essential link between genetic inheritance, development, environmental variation, and the expression of complex traits.
