Before genetics, a quick review… What cell organelle houses our genetic information? Nucleus What molecule is our genetic information found on? Deoxyribonucleic Acid (DNA) What process allows for organisms to be unique from their parents? Meiosis

Do you look like your parents? Most of us share some traits with our parents Traits are distinguishing characteristics that are inherited, such as eye color or skin color. Give some other examples of traits… Most of us also differ from our parents in some ways. This is because during meiosis, there are different actions taken to ensure a unique individual! Do you remember these actions?

So, what’s responsible for all this? Genetics is the study of biological inheritance patterns and variation in organisms. Heredity is the passing of traits from one generation to the next

Who discovered all this stuff? Gregor Mendel Considered the “Father of Genetics” Austrian Monk and Horticulturist Studied the garden pea plant 1st to publish findings on heredity

Mendel’s Experiment Mendel used pollen to fertilize selected pea plants. Purebred first generation (P) crossed to produce second generation (F1) interrupted the self-pollination process by removing male flower parts Mendel controlled the fertilization of his pea plants by removing the male parts, or stamens. He then fertilized the female part, or pistil, with pollen from a different pea plant.

Mendel allowed the resulting plants to self-pollinate. F1 generation, all plants had purple flowers F1 plants are all heterozygous (different alleles) Among the third (F2) generation, some plants had purple flowers and some had white

Mendel’s Results

Mendel’s Conclusions Traits are inherited as separate discrete units. This is the Principle of Independent Assortment. Today these are called genes. Organisms inherit two copies of each gene, one from each parent Organisms donate only one copy of each gene in their gametes Then the two copies of each gene segregate, or separate, during gamete formation One factor in a pair may mask the effect of the other. This is the Principle of Dominance and Recessiveness. Together, these two make up the Principle of Segregation

Mendel called them discrete units, we call them genes. A gene is a piece of DNA that provides a set of instructions to a cell to make a certain protein Every gene has a locus (a specific position on a pair of homologous chromosomes) Think about how your house has a specific address. The locus is the gene’s address

Most genes exist in many forms An allele is any of the alternative forms of a gene that may occur at a specific locus Your cells have TWO alleles for each gene, one on each of the homologous chromosomes Each parent gives ONE allele

Genes influence traits. A genome is all of an organism’s genetic material This genome determines ALL of your traits A genotype refers to the genetic makeup of a specific set of genes For example, eye color What is it? A phenotype refers to the physical characteristic or trait For example, eye color of blue How does it appear?

Remember the Principle of Dominance and Recessiveness? Alleles will be dominant or recessive A dominant allele is the allele that is expressed when two different allele are present Represented by capital letters A recessive allele is the allele that is masked, or only expressed when there are two recessive alleles present. Represented by lowercase letters Example: blue eyes are recessive and brown eyes are dominant

So, how do we figure all this stuff out? Glad you asked!!!  Shortly after Mendel’s experiments became widely known, a poultry geneticist named R.C. Punnett developed the Punnett square.

Punnett squares illustrate genetic crosses. A cross is the mating of two organisms. The Punnett square is a grid system for predicting all possible genotypes resulting from a cross. The axes represent the possible gametes of each parent. The boxes show the possible genotypes of the offspring. The Punnett square yields the ratio of POSSIBLE genotypes and phenotypes.

A monohybrid cross involves one trait. Monohybrid crosses examine the inheritance of only one specific trait. For example: a cross between a homozygous dominant and a homozygous recessive yields all heterozygous, all dominant

And another example… Cross between a heterozygous and a heterozygous yields a ratio of 1:2:1 1 homozygous dominant: 2 heterozygous: 1 homozygous recessive OR a ratio of 3:1 3 dominant to 1 recessive

And, another… A cross between a heterozygous and a homozygous recessive yield a ratio of 1:1 1 heterozygous 1 homozygous recessive OR a ratio of 1:1 1 dominant to 1 recessive

A dihybrid cross involves two traits. Mendel’s dihybrid crosses with heterozygous plants yielded a 9:3:3:1 phenotypic ratio. Two Traits are represented by two different letters: Yy and Rr

Sometimes we use Punnett squares to find unknown genotypes A testcross is a cross between an organism with an unknown genotype and an organism with the recessive phenotype. The offspring will show whether the organism with the unknown genotype is heterozygous or homozygous dominant.

Let’s Practice! If you are crossing a heterozygous purple parent with a homozygous dominant purple parent, what percent of offspring will have purple flowers? Represent purple with P and white with p.

PP Pp PP Pp Step 1: Decide the genotype for each parent. Heterozygous purple = Pp Homozygous dominant purple = PP Step 2: Draw your Punnett square and label your axes. Step 3: “Cross” your parents P p PP Pp P PP Pp P

So, What is the percentage of flowers with purple flowers? 100% What is the ratio of purple flowers to white flowers? 4:0

Now, you try one. You are crossing a heterozygous black dog and a homozygous recessive brown dog. What percentage of offspring will be black? What percentage will be brown? Represent black with B and brown with b