DNA, Genes, and Chromosomes
Location of Gene within DNA Strand
A heterozygous gene will express a phenotype related to the dominant allele. However, an organism with a heterozygous gene can pass the recessive allele to offspring. For example, the genotype of a brown-eyed individual may include two genes for the color brown. Alternatively, a brown-eyed individual may have one dominant gene for the color brown and one recessive gene for the color blue. If a brown-eyed person with the recessive gene for blue eyes has children with someone who also has a gene for blue eyes, their child can have blue eyes.Heritability is the proportion of phenotypic variance attributable to genetic variance. For example, if individuals in a population differ in weight, these differences may be due to differences in their genetic makeup or their diet. The heritability of weight can be calculated as
Natural selection is a mechanism of evolution in which individuals that are better adapted to their environment survive and reproduce more successfully than less well adapted individuals do. Natural selection was proposed by English naturalist Charles Darwin in his book On the Origin of Species, published in 1859. Variation exists in the traits of members of most species, some of which is heritable. If these heritable traits confer survival advantages, individuals with these traits will be healthier or survive longer. This allows them to produce more offspring than members of their species with less successful traits. Individual reproductive success is referred to as fitness, defined as the number of copies of one's genes passed to subsequent generations. This is the type of fitness referred to in the common phrase survival of the fittest. The phrase does not mean that the physically strongest survive. Instead it means that those best suited to their environment survive. The outcome of natural selection is adaptation to specific environments. An adaptation is a process or trait that improves an organism's ability to survive or reproduce in a given environment.
Structural adaptations are the heritable physical features of an organism. An example is sickle cell anemia, an inherited blood disorder in which the body makes an abnormal form of hemoglobin (a protein that transports oxygen in blood). The disease is caused by inheriting two defective copies of a particular gene. The gene has spread because inheriting only one defective copy offers protection against the mosquito-borne disease malaria. The highest proportion of the gene variant is found among people from regions where malaria is prevalent, such as West Africa, Southeast Asia, the Middle East, and China.
Behavioral adaptations are the things organisms do to survive in their physical and social environments. These adaptations can be inherited, learned, or a combination of both. For example, birds migrating south in the winter to find food is an inherited behavioral adaptation. The tendency of humans and other animals to cooperate with others is an adaptation that is inherited but also learned.
Psychologists use twin and adoption studies to understand genetic and environmental influences. In these studies, identical and fraternal twins raised in the same family are compared to twins who were separated at a young age and raised in separate families. These studies show that genes contribute to many physical and psychological traits, including intelligence, personality, careers, hobbies, weight, political beliefs, religious beliefs, and mental health. Twin studies also reveal the importance of the environment. Identical twins have very similar gene expression early in life. As they age, twins raised in different environments develop more epigenetic differences. Twins raised separately differ more in psychological traits and health histories than those raised together.
Genetic factors influence risk for mental illness. For instance, depression occurs when levels of a neurotransmitter called serotonin drop too low. Variation exists in a serotonin transporter gene (5-HTTLPR) that influences risk of depression following stressful life events. People with a short variant of the gene are more vulnerable to depression than those with a long variant. Individuals with the short variant experience increased activity in the amygdala (part of the brain involved with emotions such as fear and aggression) following a stressful event and are more likely to dwell on negative experiences. Researchers estimate that genes account for 50% of depression risk, environmental events account for 10%, and intentional activities (e.g., rumination) account for 40%.
Environmental factors can also lead to changes in the expression of the genetic code. Epigenetics is the study of these heritable changes in the expression of genes that are caused by the environment. Many genes require specific environmental circumstances to be expressed and may therefore never be expressed.
Gene expression is regulated through the process of methylation. Each gene has a "tail" called a histone. The addition of a methyl (CH3) molecule to this tail deactivates the gene. Removing the tag allows the gene to become active. Environmental factors such as diet and stress levels can influence gene methylation. Epigenetic change can be passed down across generations because children can inherit methyl tags from parents.For example, all mammals have a gene called agouti. Mice with methylated agouti genes are slim, healthy, and covered in brown fur. Mice with unmethylated agouti genes are obese, prone to diabetes and cancer, and covered in yellow fur. Pregnant yellow mice who are fed a methyl-rich diet give birth to pups with brown fur who remain healthy for life.