II. Deviations from expected patterns.

Terms

A. Mitochondria are the only cytoplasmic organelles in animals that contain genes. The number of genes is very small compared to the number in the nucleus.

B. Since the cytoplasm of a fertilized egg comes entirely from the mother, the mitochondrial genes all come from the mother. [NB: Sperm have a few mitochondria, but they are not transmitted to offspring.]

C. A trait that is due to mitochondrial DNA is transmitted from a woman to all her children, regardless of sex. Males do not transmit mitochondrial traits to any of their children.

D. There is no recombination among mitochondrial genes as there is in nuclear genes. Therefore, even distantly related persons will have identical mitochondrial DNA if they are related entirely through females.

A. Penetrance is the likelihood that a certain genotype will cause its characteristic phenotype. If a genotype has 100% penetrance, the phenotype is always present. If the penetrance is 50%, the phenotype is present only 50% of the time. Both genetic and environmental factors can cause low penetrance.

B. Expressivity refers to variation in the phenotype of a genotype that is expressed. For example, an inherited defect can be very severe or very mild, often in relatives in which the identical allele is involved.

C. Genetic heterogeneity, often unrecognized, accounts for variations in expression and deviations from Mendelian predictions.

1. Multiple alleles: Alleles with slightly different effects may be lumped together unknowingly.

2. Alleles at different loci can lead to similar phenotypes.

D. Phenocopies are variations in phenotype that mimic inherited traits but that are due to environmental effects rather than genetic.

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