Patterns of Inheritance

The Vocabulary of Genetics Heredity: the passing of traits from parents to offspring Self-Pollination: the process by which plant pollen is transferred from the male reproductive organs to the female reproductive organs to form seeds. In flowering plants, pollen is transferred from the anther to the stigma, often by the wind or by insects. Cross pollination: sperm (pollen) from one plant fertilizes the eggs of a different plant. True-breeding plant: this plants offspring will have the same trait as the parent Characteristic: a feature that has different forms in a population (hair color)

Trait: a genetically determined characteristic or condition (red, brown or blonde hair color) Dominant trait: the trait observed in the first generation when parents that have different traits are bred Recessive trait: a trait that reappears in the second generation after disappearing in the first generation when parents with different traits are bred The Vocabulary of Genetics

Blending Inheritance Classical assumptions (prior to Gregor Mendel--1866) First, heredity occurs within species.  All species have been maintained without significant change since time of creation Second, traits are transmitted directly and independently. Paradox - all members of same species should eventually have the same appearance. Thus, if a brown rabbit mates with a white rabbit, the offspring would be tan. Yet, in reality when these two rabbits mate the offspring often have brown fur, or one might have white fur.  hybrids differ in appearance

Gregor Mendel Born in 1822 in Heinzendorf, Austria. He grew up on a farm and learned a lot about flowers and fruit trees. He entered a monastery in 1843 to study religion. As a monk, Mendel put most of his energy into plant research. He studied garden peas in depth.

Pea plants have both male and female reproductive structures. And many plants reproduce through cross-pollination Cross pollination: sperm (pollen) from one plant fertilizes the eggs of a different plant. Pollen may be carried by insects, bats, birds, or the wind.

Plant Structures

Pollination and Cross Pollination During self-pollination, pollen from the anthers (male) is transferred to the stigma (female). Fertilization occurs when a sperm from the pollen travels through the stigma and enters the egg in an ovule. Self-pollination: A plant, that when self- fertilized, only produces offspring with the same traits. Thus, a plant with purple flowers will have offspring with purple flowers. For cross pollination to occur most plants need another plant to reproduce. Pea plants can reproduce through cross- pollination or self-pollination

Characteristics Characteristics: a feature that has different forms in a population. E.g. different hair colors in humans. The different colors are called traits. Traits: a genetically determined characteristic or condition. Traits may be physical, such as hair color or leaf shape, or they may be behavioral, such as nesting in birds and burrowing in rodents. Traits typically result from the combined action of several genes, though some traits are expressed by a single gene.

Mendel’s Experiments Mendel used plants that had different traits for each of the characteristics he studied. For example, he chose plants that had purple flowers and plants that had white flowers. Mendel also studied seed shape, pod color, and plant height.

Mendel also studied seed shape, pod color, and plant height.

Mendel’s Experiments Mendel used plants that were true-breeding. By choosing these plants he knew what to expect when they self-pollinated. He also cross-bred two plants of different characteristics. He removed the anthers of one plant and then fertilized it with the pollen of another plant.

Mendel’s First Experiments In the first experiments seven different characteristics were examined. Mendel used only true-breeding plants that were true- breeding for different traits for each characteristic. The offspring from such a cross are called first- generation plants. In Mendel’s experiment he crossed purple plants with white plants and all of the first generation offspring were purple. Mendel got the similar results for each cross. One trait seemed to disappear, while one trait was always present in the first generation. Mendel called this the dominant trait. The trait that seemed to disappear was called the recessive trait. Recede means “to go away or back off.”

Mendel’s second experiments sometimes bred plants with recessive traits. In this example, for every three plants that have purple flowers, there is one plant that has white flowers. Mendel’s Second Experiments

Mendel then counted the number of plants that had each trait and that turned up in the second generation. Clearly, the recessive trait did not show up as often as the dominant trait. Mendel then formulated a ratio of dominant traits to recessive traits