STEMscopedia-4
Take a look at the three adult bears shown in these photographs:
Which of these adult bears do you think is most likely to be the parent of the bear cubs shown in the photograph on the right? How did you arrive at your conclusion?
Organisms and Inherited Traits
Heredity is the transmission of traits from one generation to the next. These include physical traits, such as fur color and body shape, that are easy to observe. You probably chose the adult black bear (middle picture) as the parent of the bear cubs above. This is because the bear cubs share more physical features with that adult bear than with the other adult bears.
Other types of traits are also passed from parents to offspring. These include behavioral traits and biochemical traits. An example of an inherited behavioral trait occurs in migrating butterflies. An example of an inherited biochemical trait is the blood type passed from human parents to their children. Biochemical traits can only be observed by studying the chemical composition of biological samples.
Genes: The Units of Inheritance
Inherited traits are determined by information stored in an organism’s genes. Genes are sequences of DNA that are part of the structure of the organism’s chromosomes.
Chromosomes are located in the nucleus of every cell in that organism. An organism has two copies of each chromosome. During sexual reproduction, one copy comes from the organism’s female parent. The other copy comes from the organism’s male parent.
Alleles and Genotypes
Because each chromosome is present in two copies, each gene is present in two copies. However, the DNA sequences of the two copies may not be the same. These variations of a gene are called alleles. As an example, suppose that a scientist inspected the chromosomes present in a flowering plant. The scientist might observe something similar to the situation diagrammed below. Alleles for the same gene have been identified on two chromosomes. One allele codes for purple flower color. The other allele codes for white flower color. One allele came from the plant’s female parent. The other allele came from the plant’s male parent. The pair of alleles an organism inherits for each gene determines the genotype of that individual. In the example shown above, suppose the purple color allele is given the abbreviation, P, and the white color allele is given the abbreviation, p. This plant’s genotype for flower color is Pp.