Sex-limited traits:, Traits expressed in only one sex, though genes are present in both (e.g., milk production in females)., Sex-linked traits:, Traits controlled by genes located on sex chromosomes (usually X chromosome), expressed differently in males and females (e.g., color blindness)., Modifying factors & their effect on genotype–phenotype relationship , inter-alelic & intra-allelic interactions. Environmental influences , Terms used to measure environmental influence on a digenic trait (2 marks), Penetrance & Expressivity , Not considered as modifying factors, genetic linkage, independent assortment, crossing over , Oligogenic , An oligogenic trait is a characteristic controlled by a small number of genes (usually 2–5 genes)., polygenic traits, Each gene has a relatively large and noticeable effect on the phenotype., Oligogenic traits, These traits often show limited variation and may appear almost qualitative., Polygenic traits , characteristic controlled by many genes (often more than 5, sometimes dozens or hundreds).Each gene contributes a small additive effect.These traits show continuous variation and are often influenced by the environment, multiple alleles, A gene has more than two possible forms (alleles) in a population., modifying factors , These are other genes that influence how a trait is expressed., Maternal effects , The mother’s genotype directly determines the offspring’s phenotype (especially early in development)., Cytoplasmic Inheritance , Traits are controlled by genes outside the nucleus, usually in organelles like mitochondria (in the cytoplasm).These organelles are inherited almost entirely from the mother via the egg.The trait depends on the cytoplasmic DNA, not nuclear DNA..

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