HeredityThe passing of traits from parent to offspring.

DNA and Heredity

• Pieces of DNA called genes tell your cells how to make your traits.

• Traits are simply a specific characteristic. For instance the characteristic, “hair color” can have many different

traits – blond, black, brown, red, etc.

Both your Parents are to Blame!• You get your genes from your parents. • 23 Chromosomes from Mom.• 23 Chromosomes from Dad.• Making 23 homologous pairs.• Each pair makes the same characteristic, but

they may make different traits.For Instance: Mom’s chromosome might make

attached earlobes while Dad’s makes unattached.

Different traits are the result of differences in the genes (sections of DNA).

• For instance: a small change in one base pair (one rung on the DNA ladder) can completely change a trait.

(Remember the replacement mutation?)

Mendel is the father of genetics• Gregor Mendel studied heredity in __. They

could “self-pollinate” (they could fertilize themselves). In the plant world, there’s nothing wrong with being both, mom and dad.

• This made it easy for him to gather up a bunch of true breeding or pure bred plants.

Mendel’s first Experiment• One characteristic Mendel looked at was

flower color in pea plants. He took pea plants that always made purple flowers (true breeding purple, pure bred purple, or homozygous purple) and crossed them with plants that always made white flowers.

• What other termscould you use for the plants that always made white flowers?

DOMINANT vs recessive

• A DOMINANT trait is one that always shows up or dominates when paired with a recessive trait.

• A recessive trait is one that is hidden by a DOMINANT trait. A recessive trait will only show up if paired with another recessive trait.

• Different forms of a gene (genes for the same characteristic that make different traits) are called, ALLELES.

• Having two of the same alleles is called “HOMOZYGOUS” and would be a true breeding or pure bred.

• Having two different alleles is called “HETEROZYGOUS” and would be a hybrid.

Alleles• A dominant allele is represented by a capital

letter. • Usually, the letter comes from the 1st letter of the

dominant trait. • Purple flowers are a dominant trait and would be

represented by a “P”• Recessive alleles are represented by a lower case

letter. • White flowers are recessive and would be

represented by a “p”.

Remember most characteristics have 2 possible genes that result in 2 possible traits.

• A pea plant that is homozygous for purple flowers would have alleles PP.

• A pea plant that is homozygous for white flowers would have alleles pp.

• A pea plant that is heterozygous for purple flowers would have alleles Pp.

• A pea plant that is heterozygous for white flowers is impossible…why? ____________________________________________________________________________

Genotype and Phenotype• A genotype is an organism’s genes (get it

GENO type). For instance a genotype for the characteristic, “flower color” could be any of the following: PP, pp, or Pp.

• A phenotype is an organism’s physical appearance of the trait (get it PHENO type). For instance a phenotype for, “flower color” could be any of the following: purple flowers or white flowers.

Do you remember?1. These are different forms of a gene.

_____________2. The prefix homo means? ____________3. The prefix hetero means? ____________4. “Aa” is an example of a _____________.5. “Widows Peak” is an example of a

_____________.6. A hybrid would have the dominant or

recessive phenotype? __________ Why? ___________________________________

Were you paying attention?In pea plants round seeds is dominant(represented by the allele R) and wrinkled seeds are recessive.1. What would be the genotype for plants that are

homozygous for round seeds? ______2. What would be the phenotype for a plant with the

genotype Rr? ____________3. Is a plant with wrinkled seeds a hybrid or a

purebred? _________ How do you know? _____________________________________

Punnett Squares• Punnett Squares are used to figure out the

possible genotypes (which would then tell us the phenotypes) for the offspring of parents with known genotypes.

• Mendel’s first cross would look like this for flower color.

• As you can see, allof the offspring would be hybrids with the genotype Pp.

P P

P

P

P

P

p p

p p

p

p

What wouldthe phenotypebe for all of theoffspring? ______________

Punnett Squares Cont.• Mendel then had all of the hybrids (or

heterozygotes) from this first generation self pollinate. Pp crossed with Pp.

• Complete the Punnettsquare of this cross below.

Co-dominance and Incomplete Dominance• Although Mendel’s work was extremely important

to understanding heredity, a number of exceptions have been found.

Co-dominance in pig hair color.Wavy hair is the result of incomplete dominance.

Many genes can result in one trait. • Eye color in people is the result of many genes

acting on one trait.

• Some characteristics have more than one allele possible. Blood type has 3 (A, B, and O). Some traits in strawberries have 8!

Why is heredity important?• To know why we look the way we do.• To track genetic diseases.• To know if we might be at risk because of our

genes.• Many of the foods we eat are possible because of the manipulation (controlling) of heredity.

One more quiz1. What are Punnett squares used for?A: To find the possible offspring of 2 parents.2. What would the probabilities be for each

offspring of a cross between a pea plant with hybrid purple flowers and white flowers?

A: 50% hybrid purple and 50% white3. What are some of the exceptions to Mendel’s

theories?A: Co-dominance, Incomplete dominance,

Multiple genes affecting one trait, More than 2 alleles for a trait.