Misconceptions: (What students may think) Genes and chromosomes are the same. The homophones “Gene” and “Jean” are the same. All hereditary traits are passed through the blood. All mutations are harmful. Dominant traits are those that will take over in a population. All traits are determined by a single gene. Since humans are more complex, they have more chromosomes.

Heredity Notes Chapter 5 Please fill in the blanks on your notes handout.

Why don’t you look like a rhinoceros? You don’t look like a rhinoceros because of heredity, or the passing of traits from parents to offspring.

Who figured all this out? Gregor Mendel performed experiments on pea plants. During these experiments he noticed that sometimes patterns of inheritance seemed simple whereas others did not. Mendal picked plants with a wide variety of characteristics, such as height, flower color, and seed shape to cross-pollinate, or inter-breed.

Mendel’s First Experiment In his first experiment, Mendel crossed plants with two different traits of the same characteristic. For example, he cross-pollinated one pea plant with a pure white flower with another that had a bright purple flower.

What he discovered… Mendel noticed that in each of the cross pollinations, one trait would disappear and one would stay for the next generation. He chose to call the trait that stayed the dominant trait. He called the other trait that disappeared the recessive trait since it seemed to recede into the background.

Mendel’s Second Experiment Mendel allowed the new generation of plants to self-pollinate, or fertilize themselves. He was surprised to see the recessive traits show up in the third generation. Mendel realized that the recessive traits show up again because each plant has two sets of instructions or genes that cause them to have different characteristics. The two forms of the genes are known as alleles.

So how do we figure out which plant will have what allele? We use a device known as a Punnett Square to visualize all of the possible combinations of alleles. Dominant alleles are symbolized with capital letters written as PP. Recessive alleles are symbolized with lowercase letters written as pp.

The inherited combination of alleles is known as the offspring’s genotype. Ex: Pp An organism’s appearance is known as its phenotype. Ex: Homozygous organisms have two alleles for one trait that are the same (written AA or aa). Heterozygous organsims have two alleles for one trait that are different (written Aa).

How do we make one of those things? 1.Draw a square with four sections, so it should look kinda like a window. 2.Write the letters that represent alleles from one parent across the top 3.Write the letters that represent the alleles from the other parent along the side. 4.Fill in the boxes using one letter from the top and one letter from the side.

Dad: XY Mom: XX Kids: XX XX XY XY

Let's try some samples and see how these squares can help us. A is a dominant characteristic. a is a recessive characteristic. This bird has two genes for red feathers. Its genotype is AA. This bird has two genes for blue feathers. Its genotype is aa.

You try: Mom: AA Dad: aa

Answer… All the offspring have the genes Aa. They will all have red feathers, but will carry a recessive gene for blue feathers.

Next generation… Mom: Aa Dad:Aa

Next generation… AaAa AaAa AA aa Mom: Aa Dad: Aa Kids…

Let’s Review: Why do I look the way I do? How can I predict what traits will be passed from one generation to another?