Height in Humans Height in pea plants is controlled by one of two alleles; the allele for a tall plant is the dominant allele, while the allele for a short plant is the recessive one. What about people? Are the factors that determine height more complicated in humans?

Bell Work 1. Make a list of 10 adults whom you know. Next to the name of each adult, write his or her approximate height in feet and inches. 2. What can you observe about the heights of the ten people? 3. Do you think height in humans is controlled by 2 alleles, as it is in pea plants? Explain your answer.

Exploring Mendelian Genetics

Key Concept What inheritance patterns exist aside from simple dominance Incomplete dominance and Codominance The influence of the environment on genetic expression

A Summary of Mendel’s Principles: Gregor Mendel experimented with concluded that Pea plants “Factors” determine traits Some alleles are dominant, and some alleles are recessive Alleles are separated during gamete formation which is called the which is called the Law of Dominance Segregation

A Summary of Mendel’s Principles: 1) The inheritance of biological characteristics is determined by individual units know as genes → genes are passed from parent to offspring Example: Offspring tend to resemble their parents

A Summary of Mendel’s Principles: 2) In cases in which two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive → this is known as The Principle of Dominance Example: Tongue Rolling → R = can roll tongue (dominant), r = can’t roll tongue (recessive)

A Summary of Mendel’s Principles: 3) In most sexually reproducing organisms, each adult has two copies of each gene (one from each parent) → These genes are segregated from each other when gametes are formed → known as The Principle of Segregation Example: Rr → body cell / \ gametes→ R r

The Principle of Independent Assortment The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes This helps account for the many genetic variations observed in plants, animals, and other organisms

A Two Factor Cross

Beyond Dominant and Recessive Despite the importance of Mendel’s work, there are important exceptions to most of his principles Some alleles are neither dominant nor recessive Many traits are controlled by multiple alleles or multiple genes

Incomplete Dominance One allele is not completely dominant over another one The heterozygous phenotype is somewhere in between the two homozygous phenotypes (both alleles contribute to the phenotype)

Example: The Dosage Effect Flower color in 4 o’clock plants R = red flowers W = white flowers P-generation: RR x WW

Codominance Another situation in which both alleles contribute to phenotype A heterozygous phenotype shows distinct characteristics from each allele

Example: Chickens In chickens there is an allele for white feathers and black feathers A heterozygous individual will have both color feathers

Multiple Alleles Genes that have more than two alleles (individuals will still only have two copies) Example: Fig. 11-12 Coat color in rabbits

Polygenic Traits Traits that are produced by the interaction of two or more genes Example: skin color in humans is influences by at least 4 different traits

Applying Mendel’s Principles Mendel’s principles don’t just apply to plants Example: Drosophila (the common fruit fly) is a popular organism for genetic research. Why? Fruit flies are small, develop quickly, and produce many offspring

Genetics and the Environment The characteristics of any organism are not solely determined by the genes it inherits → characteristics are determined by the interaction between genes and the environment Example: Many genes affect a Sunflower’s height and color; however these traits are also influenced by climate, soil conditions, and the availability of water

Conclusion Genes provide a plan for development, but how that plan unfolds also depends on the environment