Patterns of Inheritance (Mendelian Genetics)

Gregor Mendel 1890’s Central European Monk Conducted research on pea plants Used garden peas Easy to grow Can produce many generations quickly Cross-pollination

Mendel’s experimental design Statistical analyses: Worked with large numbers of plants counted all offspring made predictions and tested them Excellent experimentalist controlled growth conditions focused on traits that were easy to score chose to track only those characters that varied in an “either-or” manner

GENE: the unit of heredity a section of DNA sequence encoding a single protein

Genetic Vocabulary Character: a heritable feature, such as flower color Trait: a variant of a character, such as purple or white flowers Each trait carries two copies of a unit of inheritance, one inherited from the mother and the other from the father Alternative forms of traits are called alleles

ALLELE: Forms of genes Ex: Purple flowers vs. white flowers

Mendel’s experimental design Mendel also made sure that he started his experiments with varieties that were “true-breeding” X X X X X X

Genetic Vocabulary Generations: P = parental generation F1 = 1st filial generation, progeny of the P generation F2 = 2nd filial generation, progeny of the F1 generation (F3 and so on) Crosses: Monohybrid cross = cross of two different true-breeding strains (homozygotes) that differ in a single trait. Dihybrid cross = cross of two different true-breeding strains (homozygotes) that differ in two traits.

GENOTYPE the specific allelic combination for a certain gene or set of genes (PP, Pp, pp)

PHENOTYPE the physical appearance of a particular trait (Purple, White)

Phenotype vs Genotype Figure Phenotype Purple White Genotype PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Ratio 3:1 Ratio 1:2:1

Mendel’s Experimental Design Mendel mated 2 contrasting, true-breeding varieties, a process called hybridization True-breeding parents are called the P generation The hybrid offspring of the P generation are called the F1 generation When F1 individuals self-pollinate the F2 generation is produced

Mendel’s Observations When Mendel crossed true-breeding white and purple flowered pea plants all of the offspring were purple When Mendel crossed the F1 plants, many of the plants had purple flowers, but some had white flowers A ratio of about three to one, purple to white flowers, in the F2 generation P Generation (true-breeding parents) Purple flowers White flowers  F 1 Generation (hybrids) All plants had purple flowers F 2 Generation EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by  ). The resulting F 1 hybrids were allowed to self- pollinate or were cross- pollinated with other F 1 hybrids. Flower color was then observed in the F 2 generation. RESULTS Both purple-flowered plants and white- flowered plants appeared in the F 2 generation. In Mendel’s experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white.

Law of Segregation States that 2 alleles coding for the same trait separate during gamete formation

HOMOZYGOUS: Having the same alleles for a gene HOMOZYGOUS (PP, pp) HETEROZYGOUS: having different alleles for a gene (Pp)

DOMINANCE An organism with a dominant allele for a particular form of a trait will ALWAYS have that form (P = purple)

RECESSIVE An organism with a recessive allele for a particular trait will have that form only when the dominant allele is not present

Practice use 2 letters to represent genotype A capital letter represents the dominant form of a gene (allele) and a lowercase letter represents the recessive form of the gene (allele). Example below: P=dominant purple and p= recessive white The phenotype for this flower is violet while its genotype (if homozygous) is PP. The phenotype for this flower is white while its genotype is pp (must have 2 of the recessive copies of the allele).

Punnett Squares standard way of working out what the possible offspring of 2 parents will be helpful tool to show allelic combinations and predict offspring ratios

Before we go further lets review how to set up a Punnett Square… We begin by constructing a grid of two perpendicular lines.

Next, put the genotype of one parent across the top and the other along the left side. For this example lets consider a genotype of BB crossed with bb. B B bbbb Notice only one letter goes above each box It does not matter which parent’s genotype goes on either side.

Fill in the boxes by copying the column & row letters down and across into the empty spaces B B b B B B B b b b b b

Lets say: W- dominant white w- recessive violet W w W Parents in this cross are heterozygous (Ww). Note: Make sure I can tell your capital letters from lowercase letters. What percentage of the offspring will have violet flowers? ANSWER: 25% (homozygous recessive) Usually write the capital letter first w W W w w

Red hair (R) is dominant over blond hair (r). Make a cross between a heterozygous red head and a blond. Rrrr Rrrr R r rrrr What percentage of the offspring will have red hair? 50%

Let’s try some more… In pea plants, tall pea plants (T) are dominant over short pea plants (t). Construct a Punnett Square for a heterozygous tall pea plant and a short pea plant. Tt tt Tt tt T t tttt What are the percentage of phenotypes? 50% tall 50% short

Black eyes (R) is dominant over red eyes (r) in rats. Make a cross between a homozygous rat with black eyes and a rat with red eyes. Rr R rrrr What is the possibility of a red eye off springs? 0% 