Heredity.. An Introduction

Unit 5: Seventh Gradehttp://glencoe.mcgraw-hill.com/sites/007874184x/student_view0/brainpop_movies.html

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Why don’t you look like a rhinoceros?

• The answer seems simple --- neither of your parents is a rhinoceros (I assume).

• But there is more to this answer than meets the eye.

• We look similar to our parents because of heredity.

Heredity• Heredity is the process

of passing characteristics or traits, such as eye color, skin, color, hair, color, height, intelligence, etc from parent to offspring.

• This happens during reproduction

Passing Along Traits• When people, animals,

and plants reproduce, they create new individuals.

• These individuals receive traits from both the female and the male parent.

Passing Along Traits• As you may have guessed, the passing of traits

from parent to offspring is complicated….

• For example:• Why do I have straight hair while both of my

parents have curly hair?• How did my husband and I produce a left-handed

child when we are both righties?• How did my two nephews end up with blue eyes

when both of their parents have brown eyes?

Apparently we aren’t the only ones who

wonder!• About 150 years ago,

an Austrian monk named Gregor Mendel started performing important experiments that helped scientists figure out the answers to such questions.

More on Mendel• Gregor Mendel was born

in 1822 and grew up on a farm in Austria.

• While growing up on the family farm, Mendel observed and learned a lot about flowers and fruit trees.

• But it was not until he was working in the monastery’s garden that he began his research on patterns of inheritance

Unraveling the Mystery

• Mendel knew that patterns of inheritance were not always clear.

• He saw that sometimes a trait that was apparent in one generation (parents) was not present in the next generation (offspring). In the next generation, the trait showed up again.

• Mendel wanted to know why

Self-Pollinating Peas• To start unraveling this mystery, Mendel selected

to study the inheritance pattern of garden peas.

He actually had good reasons for selecting

peas• They grow quickly • They are able to self-pollinate• Plants contain male and

female structures and can pollinate themselves.

• They will be true breeding plant

• All offspring will have the same trait as their parents

• They are also able to cross-pollinate

• The pollen of one plant can pollinate another plant and carry traits from one plant to another

Characteristics• Mendel only studied one characteristic at a time• A characteristic is a feature that has different

forms in a population• For example, hair color is a characteristic• These different forms are called traits.• The different forms of hair color are brown, blond,

red, etc.• Mendel used plants that had different traits for

each of the characteristics he studied.o Characteristic: seed shape Trait: wrinkled, roundo Characteristic: Plant height trait: Tall, shorto Characteristic: Flower color Trait: purple, white

Mix and Match• Mendel was careful to use

plants that were true breeding for each of the traits he was studying.

• By doing this, he would know what to expect if his plants were to cross-pollinate.

• He decided to find out what would happen if he bred, or crossed two plants that had different traits of a single characteristic

• Example: cross a purple flowered plant with a white flowered plant

Mendel’s First Experiment

• In his first experiments, Mendel crossed pea plants to study seven different characteristics.

• He used true-bred plants

• For example, he crossed plants that had purple flowers with plants that had white flowers.

• All of the plants that resulted had purple flowers!

Mendel’s First Experiment

• Mendel got similar results for each cross

• One trait that was always present in the first generation, such as white flowers, seemed to disappear.

• Mendel chose to call the trait that appeared the dominant trait.

Dominant Traits• Dominant traits are the traits that will appear in the

offspring if one or both of the parents contributes it.

• Examples:• Brown eyes• Dark hair• Dimples• Curly hair• Freckles• Extra digits• Normal hearing

Recessive Traits• Because the other trait

seemed to recede into the background, Mendel called it the recessive trait.

• Recessive traits are traits that must be contributed by both parents in order to appear in the offspring.

• Recessive traits can be carried in a person's genes and not appear on that person

• Examples:• Grey, green, hazel,

blue eyes• Blonde hair• Red hair• Straight hair• No freckles• Normal number of

digits• deafness

Mendel’s Second Experiment

• In his second experiment, Mendel allowed the first generation plants to self-pollinate

• He found that even when purple flowered plants crossed with white flowered plants, all of the flowers were purple, showing that purple flowers were the dominant trait.

Mendel’s Second Experiment

• However, in the second generation, for every three purple flowered plants, there was one white flowered plant, showing that the recessive gene was present.

Gone, but not forgotten

• In 1865, Mendel published his findings.

• But they were not understood or accepted until many years after his death.

• Once his research was understood, the door was opened to modern genetics.

• Genetic research is one of the fastest changing fields in science today.

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