Heredity, Genetics, and Evolution

If an affected father passes a trait to all his daughters, it suggests the trait is X-linked dominant, as he passes his only X chromosome to all his daughters.

Autosomal traits are located on non-sex chromosomes, meaning they are inherited equally by males and females. X-linked traits show different inheritance patterns between sexes due to the difference in X chromosome count.

Genes located on the X chromosome are referred to as X-linked. These traits often show distinct inheritance patterns because males have only one X chromosome.

The Hardy-Weinberg principle, which describes allele and genotype frequencies in a population, is named after G.H. Hardy and Wilhelm Weinberg.

Directional selection occurs when conditions favor individuals exhibiting one extreme of a phenotypic range, thereby shifting the population's frequency curve in one direction.

Artificial selection is the process by which humans breed plants and animals for particular traits, rather than allowing natural environmental pressures to dictate survival.

Autosomal refers to any chromosome that is not a sex chromosome (X or Y).

Catastrophism (misspelled as Castratrophism) proposes that rapid, catastrophic events drive major biological or geological changes, as opposed to gradual change over long periods.

Polygenic inheritance occurs when one characteristic is controlled by two or more genes, often resulting in a range of phenotypes.

Sickle cell anemia is an autosomal recessive disorder. An individual must have two copies of the recessive allele (ss) to express the disease.

Pleiotropy describes how a single gene can affect multiple, seemingly unrelated phenotypic traits. This is common with genes involved in fundamental biological processes.

A testcross is a cross between an individual with a dominant phenotype (but unknown genotype) and a homozygous recessive individual to determine the unknown genotype.

Homozygous recessive means having two identical copies of the recessive allele, represented by lowercase letters (e.g., gg).

A heterozygous genotype has two different alleles, one dominant and one recessive (Gg or gG). This means the individual carries both versions of the gene.

Homozygous dominant means having two identical copies of the dominant allele, represented by two uppercase letters (e.g., GG).

Natural selection acts on the phenotype, which is the physical expression of the genotype, because the phenotype determines an organism's interaction with the environment.

The phenotype is the set of observable characteristics or traits of an organism resulting from the interaction of its genotype with the environment.

The genotype is the genetic makeup of an organism, representing the specific set of alleles it carries.

Natural selection occurs because individuals with traits better suited to their environment are more likely to survive and reproduce, passing those traits to the next generation.

Thomas Malthus wrote about population growth and the struggle for existence due to limited resources. Darwin applied this concept of competition for finite resources to his theory of natural selection.

Natural selection is the process where individuals with traits better suited to their environment are more likely to survive and reproduce. This is the core mechanism of evolutionary change.

In complete dominance, the dominant allele completely masks the presence of the recessive allele in a heterozygote. Thus, the phenotype of the heterozygote is identical to the dominant homozygote.

Incomplete dominance occurs when the phenotype of the heterozygote is an intermediate blend of the two homozygous phenotypes. Red and white alleles blending to create pink is the classic example.

Codominance occurs when both alleles are expressed simultaneously in the heterozygote. Unlike incomplete dominance, the traits do not blend; both are visible.