Heredity: Environmental Factors

Environmental Factors

Environmental factors often influence or even override gene expression. The human phenotype is easily changed while the genotype is not. Drugs, pathogens, and other factors of a mother’s life can influence normal gene expression during embryonic development. An example is the thalidomide disaster, in which this sedative was taken by pregnant women to reduce morning sickness, but caused their babies to have ­flipper-like hands and feet. In this case, the drug overrode normal gene expression. The term phenocopies is used to describe environ-mentally produced phenotypes that mimic genetic mutations.

Important environmental factors may influence genetic expression after a baby is born. One example is when poor nutrition affects normal brain growth, height, and general development of the body. A person who has genes that should result in a tall body height can end up shorter because of poor nutrition. Part of a gene’s environment is related to influences of other genes. Hormonal deficits in childhood can result in abnormal skeletal growth and body proportions. An example is cretinism, which is a type of dwarfism that results from hypothyroidism.

Factors that Affect Expression of Single Genes

Because of effects of the environment and various genes, most genotypes are varied between individu-als. Identical twins do not have the exact same symp-toms when they have an inherited illness. Penetrance, expressivity, and pleidotropy are terms that describe various genotype distinctions.

Penetrance

Penetrance is the all-or-none expression of an indi-vidual’s genotype. Some allele combinations that cause disease are completely penetrant. Any individual who inherits a certain genotype has some symptoms. A genotype is called incompletely penetrant when ­certain individuals do not express the associated phenotype­. An example of incomplete penetrance is polydactyly, in which the individual has extra fingers or toes. Some people inheriting the autosomal domi-nant allele will have more than five digits on one hand or foot. Others who are known to have the allele, since they have a parent or child with the condition, have 10 fingers and 10 toes. The majority of traits are incompletely penetrant.

A gene’s penetrance is described numerically. An allele is called 80% penetrant if 80 out of 100 people who have inherited the dominant polydactyly allele have extra digits. Penetrance is also linked to the effects of environment, such as when a person who inherits a genotype that raises the risks for lung cancer do not develop cancer if the lungs are protected from smoke and pollution.

Expressivity

Expressivity is how dramatically a phenotype manifests itself. A phenotype is called variably expressive if symp-toms are of different intensities in different individuals. This is usually how expressivity occurs. A person with polydactyly may have one extra digit on both hands and one foot while another person may have two extra digits on both hands and both feet. Another individual might have only one extra fingertip. Polydactyly is vari-ably expressive as well as being incompletely penetrant.

Pleidotropy

In pleidotropy, a single genetic disorder exhibits sev-eral symptoms. Family members with different symp-toms may seem to have different illnesses, when they really have the identical pleiotropic disorder. Pleido-tropy occurs in genetic disorders affecting just one protein that is present in different parts of the body.

An example is Marfan syndrome, which is an autoso-mal dominant defect in the elastic connective tissue protein called fibrillin. This protein is abundant in the aorta, ribs, fingers, limb bones, and the lenses of the eyes. Therefore, its symptoms include a sunken chest, very thin fingers, lengthening of the limbs, and lens dislocation. The most severe symptom is weakening of the aorta wall, which may cause the aorta to burst. If this weakening is detected early in life, a synthetic graft can patch the affected area and prevent death.

Genetic Heterogeneity

Genetic heterogeneity is defined as when the same phe-notype results from actions of various genes. A good example is the almost 200 types of hereditary ­deafness, which are each due to impaired actions of a differ-ent gene. Different aspects of hearing are affected by ­different genes. Genetic heterogeneity happens when different enzymes that catalyze identical biochemical pathways are encoded by genes or when these genes encode different proteins that make up a pathway. For example, blood clot formation may be caused by 11 different biochemical reactions. Clotting disorders may occur due to mutations in genes that specify any of the enzymes that catalyze these reactions. Several types of bleeding disorders may then develop.

Regulation of Gene Expression

The human genome has three basic levels of controls. Protein-coding genes make up only the first control level. They account for less than 2% of human cell DNA. This part is considered the “blueprint” for pro-tein structure in humans. The other two control levels are referred to as small RNAs and epigenetic marks.

Small RNAs

The second level of controls appears to be related to many RNA-only genes in the body. It is now believed that 80% of the genome may form a parallel regulatory system. It may generate different small RNAs, includ-ing microRNAs and small interfering RNAs. These molecules are mobile controllers with direct actions upon DNA, other RNAs, or proteins. They can reduce the effects of or inactive aggressive jumping genes known as retrotransposons, which usually copy themselves and insert these copies into distant DNA sites. When this occurs, the ­retrotransposons disable or hyperactivate their target­ genes.

Small RNAs control the timing of programmed cell death during development and may prevent translation of other genes. When mutations occur, conditions such as schizophrenia or cancers of the prostate gland and lungs may develop. Human genetic complexity is linked to small RNAs and their control of gene expression, mostly during growth and differentiation. Nucleotide sequences of RNA-­ specifying DNA areas are being utilized for gene therapy research. Drugs that interfere with RNA are being developed to slow or stop the effects of genes related to cancer, Parkinson’s disease, age-related macular degeneration, and many other ­conditions.

Epigenetic Marks

The third level of controls involves epigenetic marks, stored in proteins and chemical groups that bind to DNA. Epigenetic marks are always changing. They are also found in the packaging of chromatin inside cells. Chemical tags such as acetyl and methyl groups are bound to DNA segments inside cells as well as to histones. They determine if DNA is available for transcription, also called acetylation, or silencing, also called methylation. Via methylation, epigenetic marks also cause inactivation of one of the female’s X chromosomes in the early ­embryonic phase.

The presence or absence of epigenetic marks may predispose cells for transformation from normal states to cancer. Slight deviations in these marks on certain chromosomes can result in severe diseases. Most mater-nal and paternal genes turn on or off at the same time. However, this balance is altered during gametogenesis, when certain genes are modified by addition of a methyl group. This process, known as genomic imprinting, tags genes as either “paternal” or “maternal.” The embryo then utilizes these tags and expresses either the mother’s gene, while the father’s version remains idle or the reverse. With every generation, old imprints are erased when new gametes are produced. All chro-mosomes are newly imprinted. Epigenetic marks are easily “wiped away.” Sometimes they are inherited by the next generation, causing changes to occur.

When imprinted genes mutate, pathological con-ditions can occur. Examples of such conditions include:

■■ Angelman’s syndrome: An autosomal recessive syndrome characterized by severe mental retar-dation, incoherent speech, uncontrolled laughter, and movements that are sudden and jerky; it is caused by a deletion on chromosome 15 inherited from the mother.

■■ Prader–Willi syndrome: A congenital metabolic condition characterized by mild-to-moderate retardation, shortness, extreme obesity, hypo-gonadism, hypotonia, and hyperphagia; it is caused by a deletion on chromosome 15 inherited from the father. The genetic cause of these two syndromes is identical: the deletion of a certain region of chromosome 15. The same allele can have different effects based on the ­parent it came from.

Mitochondrial Inheritance

There are also 37 genes found in the mitochondria of cells. These genes are referred to as mtDNA. They are transmitted almost always from the mother to the off-spring. This is because the ovum donates almost all the cytoplasm in the fertilized egg. Also, sperm mtDNA is selectively destroyed by elimination factors found in both the sperm and the egg. Many rare disorders are linked to mitochondrial inheritance, which is also called extranuclear inheritance. While the majority of these disorders involve mitochondrial oxidative phos-phorylation abnormalities, some cause unusual neu-rological problems or degenerative muscle disorders. It is suspected that Alzheimer’s disease and Parkinson’s disease may be linked to mitochondrial inheritance.

1. Identify factors that may alter gene expression.

2. Explain what small RNAs control.

3. What does genomic imprinting do in relation to genes?