A dominant allele is a variant of a gene that expresses its associated phenotype even when only one copy is present. An individual with at least one dominant allele, whether in a heterozygous (e.g., Bb) or homozygous (e.g., BB) state, will exhibit the dominant trait.

Dominant Alleles

A recessive allele is a variant of a gene that only expresses its associated phenotype when an individual is homozygous for that allele, meaning they have two copies (e.g., bb). The presence of a dominant allele will mask the expression of the recessive trait.

Recessive Alleles

The Punnett square shows different combinations of alleles that can be inherited from parents. Capitalized letters used in Punnet squares denote dominant alleles, while the lowercase letters denote recessive alleles. In this case "B" is a dominant allele and "b" is a recessive allele.

Punnett Square

The relationship between genes, alleles, and traits can be illustrated with hair color. A specific gene provides the code for the general trait of hair color, while the different alleles, or variations of that gene, determine the specific hair color an individual actually has.

Gene-Allele Interaction for Hair Color

An individual is described as homozygous for a particular gene when they possess two identical alleles for that gene. For instance, in the case of the cleft chin trait, genotypes BB (homozygous dominant) and bb (homozygous recessive) are both considered homozygous.

Homozygous

An individual is described as heterozygous for a particular gene when they have a combination of two different alleles for that gene. For example, if 'B' is the allele for a cleft chin and 'b' is for a smooth chin, an individual with the genotype Bb is heterozygous.

Heterozygous

While superficial traits like skin tone, eye color, and hair texture are determined by an individual's genetic makeup, these visible differences do not reflect the full scope of human genetic variation. In fact, scientific analysis of the human genome reveals that there is more genetic diversity within any single racial group than there is between different racial groups.

Genetic Diversity Within and Between Human Races

A single gene is responsible for determining whether a pea plant's flowers are purple or white. What accounts for the difference in flower color between two different pea plants?

A specific gene in a species of plant is responsible for determining the color of its flowers. Within this plant species, some flowers are red, while others are white. What term best describes the different variations of this single gene that result in either red or white flowers?

An allele is a specific version or variation of a gene. While a single gene may provide the code for a general trait, such as hair color, the different alleles of that gene determine the specific variation of the trait that an individual expresses.

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Genes are specific sequences of DNA that provide the code for particular traits. These traits can be both physical and psychological characteristics, and their ultimate expression is determined by the interplay between genetics and environmental factors.

Genes

Different structural units in genetics include:
- Chromosomes
- Deoxyribonucleic acid (DNA)
- Genes
- Alleles

Structural Units in Human Genetics

Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020, April 22). Psychology (2nd ed.). OpenStax. https://openstax.org/books/psychology-2e/pages/1-introduction

OpenStax Psychology (2nd ed.) Textbook

- Coiled lengths of DNA that contain genes
- Found within the cell nucleus 

Chromosome

- Nuclear Receptor Subfamily 3 Group C Member 1 (NR3C1) encodes glucocorticoid receptor.
   - Can function as a transcription factor that binds to glucocorticoid response elements in the promoters of glucocorticoid responsive genes to activate their transcription.
    - Can function as a regulator of other transcription factors.
- It is involved in inflammatory responses, cellular proliferation, and differentiation in target tissues.
- Mutations in this gene are associated with generalized glucocorticoid resistance.

NR3C1 Gene

The manipulation of genetic material, by replacing defective genes or introducing new genes, to treat or prevent disease. Vectors are used to transport DNA into cells.

Gene therapy

Polygenic refers to traits that are controlled by multiple genes (i.e. height, skin color, weight). 

Polygenic

Traits are characteristics of an individual that are at least partially controlled by genes. While commonly associated with observable physical attributes like eye or hair color, the concept of traits also extends to non-physical characteristics, including behavioral patterns, personality, and temperament. The complete set of an individual's observable traits, both physical and behavioral, constitutes their phenotype.

Traits

Two identical twins, who share the exact same genetic makeup, were separated at birth and raised in significantly different environments. As adults, Twin A is outgoing and confident, while Twin B is shy and anxious. Based on the modern understanding of how genes influence traits, what is the most likely explanation for this difference in their psychological characteristics?

Using your understanding of how genetic and environmental factors interact, explain the most likely reason for the different psychological outcomes in the following scenario.

Analysis of Twin Outcomes

Alleles

A mutation is a sudden, permanent change in a sequence of DNA. Mutations serve as a primary source for harmful genes, such as the one causing Phenylketonuria (PKU). While many mutations can be detrimental or lethal, they can occasionally be beneficial, conferring an advantage to an individual that enhances their survival and reproductive success compared to others.

Mutation

Deoxyribonucleic acid (DNA) is a helix-shaped molecule composed of nucleotide base pairs. It serves as the genetic material that constitutes long strings known as chromosomes.

Deoxyribonucleic Acid (DNA)

An individual's genotype is their complete genetic makeup, comprising the genetic material inherited from both parents. This genetic blueprint is established at conception through the combination of DNA from the sperm and the egg.

Genotype

Chromosomes are long, string-like structures composed of the genetic material known as deoxyribonucleic acid (DNA).

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