Mendel worked with pea plants

Pea plants:• grow and reproduce

quickly• have a variety of different

traits (characteristics):

Mendel crossed2 short (dwarf) plants

What size pea plants

do you think he expected to see grow?

Only short plants grew

Mendel crossed2 tall plants

What size pea plants

do you think he expected to see grow?

Seeds from some of the plants produced only tall

plants….and…..

AND…

Result A Result B

Seeds from some of the plants produced mostly tall and some

short plantsWait! That doesn’t

make sense!

Why is there a short pea plant? This means tall plants can pass on short traits …hmmm…

Result B

Crossed pure tall with a pure short plant…..

So is tall the stronger trait?

Mendel’s Theory:

The plants are only

showing one trait! Tall must be

stronger.

• Mendel self pollinated offspring from the previous experiment…

Mendel’s Theory:

Mendel’s Discovery (p.14)

• In plants there were traits for many different characteristics…. – Only one trait showed up either short or

tall, not medium height

Mendel’s Discovery (p.14)

If true breeding plants were crossed (tall and short)…

The plant only showed one trait: tallness

Dominant trait (stronger)

Upper case letter TThe trait that did not show up was

called…

Recessive trait (weaker) Lower case letter t

Tt

Please take out “Mendel’s Experiment Review” and

complete the back side of the paper.

When you are done take out your minitext and review with

the following slides.

Cells (p. 21)…make sure you have the following red information is highlighted in your minitext…

• All organisms are made up of cells. Cells are the basic building blocks of all life, including plants and animals. The simplest organisms are made of a single cell. More complex organisms (like sixth graders) are composed of trillions of cells. The control-center of a cell is the nucleus. In the nucleus are thin fibers called chromosomes. The chromosomes are like “blueprints” for the cell.

• Tiny segments of chromosomes are genes. This term is the basis of

the word genetics. Genes can be thought of as a unit of inheritance. Gregor Mendel used the term “traits”. Now scientists use the term “specific genes”. Humans have about 25,000 different genes in each our cells. Genes and chromosomes are composed of Deoxyribonucleic Acid or DNA.

Inheriting Traits (p. 22)…make sure you have the following red information is highlighted in your minitext…

• In humans, during sexual reproduction male and female sex cells combine to form one stem cell.

• Non-sex cells in humans contain 46 chromosomes. Sex cells only contain 23 chromosomes. During sexual reproduction, both parents contribute one sex cell, each donating 23 chromosomes, which forms the 46 chromosome stem cell.

• Humans begin life as a single stem cell. This single cell makes copies of itself and becomes two cells. This copying process happens many times (two become four; four become eight, etc.). At some point in the copying process cells begin to change into specific types of cells (i.e. bone cells, blood cells, heart cells). The beginning stem cell is a very general type of cell, called “undifferentiated”. Stem cells can become any specific cell if the right signals are given to it.

Predicting Traits (p. 24-25)…make sure you have the following red information is highlighted in your minitext…

• Traits you see in an organism are the result of many different genes working together (half from the female and half from the male). In the simplest case a trait can result from only two genes- one from the female and one from the male.

PURE AND HYBRID TRAITS• Scientists use letters to represent genes. Upper case letters

represent dominant traits; lower case letters signify recessive traits. The specific letter is not important, however often a logical letter is used such as T for “tallness”. An uppercase letter, T, represents the dominant gene (tall adult pea plant), and a lowercase letter, t, represents the recessive gene (short adult pea plant).  

• Traits (genes) come in pairs, one from each parent. There are three different combinations of genes a pea plant can have for height: TT, Tt, tt.

• An organism has pure traits if both its genes for a certain characteristic are the same (two dominants or two recessives). TT and tt are pure traits.

• If an organism has one dominant and one recessive gene, Tt, it is said to have hybrid traits.

Predicting Traits (p. 24-25)…make sure you have the following red information is highlighted in your minitext…

Word Definitions

Pure Traits Traits that have the same gene for a characteristic

Example: TT or tt

Word Definitions

Hybrid Traits Traits that have one dominant and one recessive gene

Example: Tt

• One cannot look at an organism and see its genes. The type of genes an organism has (TT, Tt, or tt) is referred to as its genotype (think “type of genes”). Although one cannot see the type of genes an organism has, one can see how the genes make the organism look (tall adult pea plant or short adult pea plant). How the genotype makes an organism appear is called the phenotype.

Genotype and Phenotype(p. 26)…make sure you have the following red information is highlighted in your minitext…

Word Definitions

Genotype Genes an organism has

Example: TT, Tt, or tt

Word Definitions

Phenotype The physical appearance of an organism

Example: Short or tall