BASIC GENETIC CONCEPTS
M.Tevfik Dorak, MD, PhD
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Dominant vs Recessive
Mendelian inheritance patterns usually apply to traits governed by a single gene. Most characters, however, are determined by multiple genes or by an interaction of genes and the environment. The inheritance of quantitative characteristics that depend on several genes is called polygenic inheritance. A combination of genetic and environmental influences is known as multifactorial inheritance. Diseases with multifactorial inheritance are called complex genetic diseases.
A gene may act as dominant or recessive, although dominance or recessiveness concerns the phenotype. But these are not the only modes of action (incomplete dominance and co-dominance are possible). Examples of dominant characters include dark hair (to blonde hair); brown eyes to blue eyes (as originally thought by Davenport & Davenport in their 1907 Science paper 'available through JSTOR, but also see more recent views here); and lobed ears to unlobed ears. Possible examples of recessive characters are, therefore, blonde hair and blue eyes. Ability to taste a chemical (PTC) is dominant to inability to taste it. The grey body colour of Drosophila is dominant to black. Yellow is dominant to green in seed cotyledons of Pisum sativum used in Mendel's experiments. A dominant allele usually codes for a functional product and continues to do so in heterozygous form (however, beware of dominant negative and haploinsufficiency; see glossary and Clinical Genetics). A recessive allele usually directs the synthesis of a non-functional product causing the lack of the product in homozygous form. For example, a green seed is always homozygous but a yellow one may be heterozygous (yellow is dominant to green).
Recessive sex-linked genes do not require homozygosity for expression (coat color in cats, various sex-linked diseases such as hemophilia and red-green color-blindness in humans). A rare X-linked dominant trait is the blood type Xga (males expressing this trait would transmit it to all daughters but not to a son).
Incomplete dominance (also called blending, semi-dominance): The four-o'clock plant (the snapdragon, Antirrhinum), for example, may have flowers that are red, white, or pink. Plants with red flowers have two copies of the allele R for red flower color (pure line for red) and hence are homozygous RR. Plants with white flowers have two copies of the allele r for white flower color (pure line for white) and are homozygous rr. Plants with one copy of each allele, heterozygous Rr, are pink—a blend of the colors produced by the two alleles. Fur color of Andalusian Fowl is another example: black and white are partial-dominant, therefore heterozygotes are grey.
Co-dominance: ABO blood groups (A and B are co-dominant; O is recessive) and HLA transplantation antigens (HLA-A,B,C,DR,DQ,DP) are co-dominantly expressed. The gene responsible for sickle cell anemia has two co-dominant alleles: HbA (normal gene) and HbS (sickle cell gene). HbAA is normal, HbSS is diseased and HbAS is mildly anemic and protective for malaria (hence its selection). White or Dutch clover (Trifolium repens) leaf patterns are co-dominantly expressed. A heterozygous plant shows both patterns superimposed, while homozygotes for the other two patterns show a single pattern.
Sex-influenced dominance: A dominant expression that depends on the sex of the individual. An example is the horns in sheep (dominant in males, recessive in females). Another one is the plumage in domestic fowl: autosomal alleles whose expression is modified by sex hormones; the same genotype (hh) results in long, more curved and pointed plumage in cocks; shorter and more rounded in hens. Certain coat patterns in cattle, baldness patterns, breast development and facial hair (as well as all secondary sex characters) in humans are also sex-influenced traits. An autosomal dominant trait that controls precocious puberty is expressed in heterozygous males but not in heterozygous females. Affected males undergo puberty at 4 years of age or earlier. Heterozygous females are unaffected but pass this trait on to half of their sons (confusing the pattern with a sex-linked trait).
A different phenomenon is temperature-influenced expression (in Himalayan rabbit and Siamese cat fur color). The temperature-sensitive allele causes say, darker patches in extremities, ears and nose.
Further points: As in rabbit fur and mouse coat color determination, dominance relationships between alleles may be more complicated. The reasons for dominance may be related to the activity of an enzyme coded by the relevant gene. Mendel's yellow pea allele is dominant because the gene involved codes for the breakdown of chlorophyll (which is green). In the homozygous recessive case, no functional enzyme will be present, chlorophyll breakdown cannot occur and the seeds remain green. In a situation called epistasis, one gene affects the expression of another gene that is not linked (multigenic determination of a phenotype). The masked gene is said to be hypostatic to the epistatic gene. An epistatic-hypostatic relationship between two loci is similar to a dominant-recessive relationship between alleles at a particular locus.
Genomic imprinting, methylation and penetrance are other factors that may influence the expression of characters. If one of the possible genotypes is an embryonic lethal, the observed proportions would be different from expected ones (also remember the cytoplasmic male sterility in plants).
Having a disease-causing gene does not always mean having the disease. In phenylketonuria (PKU), avoiding phenylalanine-containing food prevents ill effects of the mutant gene; and in xeroderma pigmentosum, avoiding ultraviolet radiation prevents the development of melanoma. On the other hand, having a protective gene may prevent the ill effects of an environmental agent. The gene for the xenobiotic enzyme CYP1A1 on chromosome 15 may activate the polycyclic aromatic hydrocarbons (PAH) depending on the allele. PAHs are present in cigarette smoke and those who have the non-activating allele may not be affected from the carcinogenic effects of PAH.
M.Tevfik Dorak, MD, PhD
Last updated on 14 July 2013