Inheritance and Punnett Squares

Inheritance

How are traits passed from parents to offspring?

• Arm folding

• Tongue rolling

• Ear lobes; attached or detached

• "hitchhiker's thumb"

• "widow's peak"

• hand clasping

• Toe length

Gregor Mendel

• "Father of genetics"

• Studied pea plants; seed and flower color

• Called different forms of a characteristic traits

Mendel's Investigation

• Crossed two true-breeding plants; one with purple flowers and one with white flowers.

• True-breeding means that it will always produce the same offspring. "Purebred" is another term for this.

Results

• When he crossed purple flowers with white flowers ALL offspring had purple flowers.

• He called the purple characteristic DOMINANT

• He called the white characterstic RECESSIVE

​*REMEMBER, purple showed up over white.

Why do you think Mendel used the terms "dominant" and "recessive" to describe alleles?

Multiple Choice

A true-breeding red rose will produce mostly red roses, but some pink roses occasionally.

• True

• False

Results

• When he crossed offspring from the first generation (purple to purple flower) he got mostly purple, but SOME had white flowers.

• He wondered why the white trait "skipped" a generation.

Mendel hypothesized that the plants inherited two "factors" for the trait, one from each parent.

• Some traits only occurred if a plant inherited two factors for the same trait.

• Other traits only required one of the factors.

Open Ended

Mendel crossed two true-breeding plants; one with yellow peas and one with green peas. The first generation of offspring had green peas. When offspring from that generation were crossed, most had green peas but some offspring had yellow peas. Based on these results, explain which seed color is dominant and which is recessive.

To understand traits we need to understand DNA.

• A molecule that contains instructions that determine an organism's traits.

Multiple Choice

In humans, where is DNA found?

• In the cell wall

• In chloroplasts

• In the cell membrane

• In the nucleus

DNA is organized into structures called CHROMOSOMES.

• An individual has paired sets of chromosomes. (Humans have 23 pairs for a total of 46)

• Genes are pieces of information carried on chromosomes.

Genes have different forms called ALLELES

• Alleles come in pairs (like a pair of shoes)

• Each parent contributes one allele for each pair (think of a left and right shoe)

• The combined pair produces the trait

Multiple Choice

One of the traits that Mendel experimented with in peas was flower color; some plants had purple flower and some had white. How many alleles controlled the flower color of his plants?

• 1 allele

• 2 alleles

• 4 alleles

• there's no way to know

Multiple Choice

Where do the alleles come from that control an organism's traits?

• from their siblings

• from their parents

• nobody really knows

• from their grandparents

Alleles carry codes for PROTEINS

• Proteins are large molecules that doe much of the work in the cell and also make up much of its structure.

• They're responsible for how our bodies look and function.

• Proteins give our hair and eyes their color, and our muscles strength.

Multiple Choice

The different forms of a gene for a particular trait are called

• alleles

• genes

• chromosomes

• proteins

Multiple Choice

DNA is organized into structures called

• genetic material

• alleles

• traits

• chromosomes

Multiple Choice

Which order is most accurate in describing how genetic material is organized?

• DNA contains many genes and is organized into chromosomes.

• Chromosomes contain many DNA and is organized into genes.

• Genes contain many chromosomes and are organized into DNA.

First we need to separate the two alleles of each parent (since they can only pass ONE to their offspring at a time).

Let's practice!

Write the combination (genotype) into each box to determine all possible combinations.

​

A Punnett Square is a tool used to make predictions based on PROBABILITY, or the chance that offspring will have a certain trait.

​

The results are NOT 100%!!!!!!!!

How can we predict the results of a cross?

The Structure of DNA

The shape of DNA is a "double helix"

• "Twisted Ladder"

• ​The entire structure is a large molecule made of many atoms.

• ​If you stretched the DNA in one cell all the way out, it would be about 2m long and all the DNA in all your cells put together would be about twice the diameter of the Solar System.

The "rungs" of the ladder are called base pairs.

• The order they're in is what makes up a gene.

• Like writing a sentence.

• It's a HUGE molecule

• ​There are about 3 BILLION base pairs in each DNA molecule!

Open Ended

How is DNA like a recipe? What happens if a recipe is changed slightly?

How can genes change?

• Mistakes during DNA replication while the cells is dividing.

• UV light damage

• Carcinogenic chemicals

• Some viruses and bacteria

• Radiation

If genes get changed, sometimes a mutation can occur.

• Some mutations cause dramatic changes

Some mutations are very rare

Some mutations have made people famous.

Siamese cats have a "temperature sensitive" mutation

Most mutations are actually pretty normal...

• Red hair

• Blue eyes

• The ability to drink milk

• The ability to taste certain things

• Freckles, dimples, cleft chin...