Mendelian Genetics

The Laws of Heredity The rules for heredity, how certain traits are passed on was figured out using the pea plant. The scientist named Gregor Mendel is why we call this Mendelian genetics. Pea plants have characteristics that are “either-or”, for example flowers can be either purple or white.

Traits in Pea Plants Characters Dominant Trait Recessive Trait flower colour purple white seed colour yellow green seed shape round wrinkled pod colour stem length tall short

Definitions Character or characteristic – a heritable feature, such as flower colour, varies between individual plants Trait – each variant for a character, such as purple or white flowers If you give an example of a characteristic you have a trait. For the following, are they characters or traits? Wrinkled, seed colour, green, tall, flower colour. Seed shape

Mendel’s experiment Mendel began experiments using only “purebred” plants; meaning that when the plants self-pollinated all the offspring were identical to the parent, i.e. purple flowered plants produce purple flowered offspring Then he bred or crossed two purebred pea varieties – purple flowered plant and a white flowered plant.

What do you expect to happen? What will the first generation produce? They are called the F1 generation Your hypothesis: Result: All the offspring of the F1 generation were purple. Then Mendel crossed two of the F1 generation purple plants, creating a second generation, or F2 generation. What will the F2 generation produce?

The F2 generation produced some white flowers, but mostly purple flowers, in a ratio of about 3 purple to 1 white flowered plant. Mendel observed this same pattern in each of the characters.

Hypothesis: Alternative versions of genes account for variation in inherited characters. The gene for flower colour exists in two versions, one for purple and the other for white. These alternate versions are called alleles. For each character, an organism inherits two alleles, one from each parent. One allele is from the sperm and one allele is from the egg.

If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance. This means the F1 plants had purple flowers because the allele is dominant while the white flower allele is recessive.

Using a common form To write the genes for a certain character, we use common forms: Dominant alleles are symbolized by a capital letter, and recessive are symbolized by a lower-case letter. F is the purple flower allele and f is the white flower allele. So FF and Ff would produce purple flowers and ff would produce white flowers. You can choose which letters to use, but try to pick letters that are distinct lowercase and capitalized: Ex: T & t, H & h, G & g, NOT S & s

Homozygous – An organism that has a pair of identical alleles for a character. Example seed colour, if a pea plant has green seeds it must be homozygous recessive (example gg) Or if a pea plant has yellow seeds and is GG, it is homozygous dominant These organisms can pass ONLY one of the alleles (because they only have one)

Punnett Square If we cross a pea plant that is dominant homozygous (FF) for flower colour with a pea plant that is recessive homozygous (ff) for flower colour, what will the offspring be? The F1 generation will all be purple and Ff… F f Ff

Heterozygous – An organism that has two different alleles for a character. Example seed colour: if a pea plant has yellow seeds but their alleles are Gg. Unlike homozygous, these organisms are not true-breeding because they produce gametes that have one or the other of the different alleles. They pass on either a G or a g.

The F1 generation will be 3:1 purple to white. If we cross a pea plant that is heterozygous (Ff) for flower colour, with another pea plant that is heterozygous (Ff) for flower colour, what will the offspring be? The F1 generation will be 3:1 purple to white. F f FF Ff ff

An organism’s appearance does not always reveal its genetic composition because of dominance and recessiveness. Phenotype – An organism’s appearance (for example a pea plant has purple flowers) Genotype – An organism’s genetic makeup (the pea plant with purple flowers could be homozygous dominant, or heterozygous)

A Punnett square can predict genotype: In the case of flower colour in peas, FF and Ff have the same phenotype – purple flowers, but different genotypes. The genotype of a purple flower has two possibilities, so to find out which one is correct we do a test cross. If we cross the unknown purple flowered plant with a white flowered plant the appearance and ratios of the offspring will reveal the genotype:

Test cross: Case one If FF… P Generation White flower ff Purple flower The F1 Generation will be… 100% purple Ff

Test cross: Case two If Ff… P Generation White flower ff Purple flower The F1 Generation will be… Ff and ff 50% purple : 50% white

Punnett Square – an organized way of showing the possible results of a cross between the gametes of two individuals. Monohybrid Cross – a cross that involves only one allele pair with different traits.