Performance Indicator 7.L.4A.3 Develop and use models (Punnett squares) to describe and predict patterns of the inheritance of single genetic traits from parent to offspring

Punnett Square Punnett square is used to predict the ratio or percentage of the possible genes that an offspring will have based on the genotype of the parents. In a Punnett square, the top of the table shows the alleles provided by one parent. The each parent is placed in the individual squares, forming a new gene pair. The individual squares show the possibilities of allele pairs in the offspring.

Punnett square For example, the following table shows the cross Tt x tt:

Punnett square In this example, tallness (T) is the dominant trait and shortness (t) is the recessive trait. As the Punnett square shows, TT, Tt, and tt are all possible genotypes for the height of the offspring. The offspring with the genotypes TT and Tt will have a phenotype of tall: the offspring with the genotype of tt will have a phenotype of short.

Punnett square If the two alleles are the same (TT or tt), the genotype is considered homozygous (also referred to as purebred). If the two alleles are different (Tt), the genotype is considered heterozygous ( also referred to as hybrid)

Punnett square The example above shows the inheritance of a single characteristic (height in bean plants). A cross that shows the inheritance of a single characteristic is known as a monohybrid cross. It is sometimes difficult to predict certain traits in humans (for example hair color or eye color) because there are multiple alleles that control these traits

Incomplete Dominance Incomplete dominance is a condition when the dominant allele does not completely mask the recessive. As a result some of the recessive trait will be observed in the phenotype. An example of an incomplete dominance in plants may be color of flowers.

Incomplete Dominance The color red is dominant over the recessive color white. Pink flowers are a result of a blending of red and white.

Co-dominance Co-dominance is a condition when there is more than one dominant allele. As a result, both alleles will be expressed in the phenotype. An example of co-dominance in plants may be the color of flowers. If both red and white alleles are dominant, a mottled or variegated phenotype will be observed. Other examples may be the human AB blood type or spotted fur/feathers color in some animals