Opener: Get out a clean piece of paper and Title it: #47 Mendel Notes

1) Find your 3:00 Buddy

2) Using your textbook answer the following:

1) What did Gregor Mendel do?

Prime Time Answers…

Gregor Mendel experimented with pea plants to determine how parents pass traits to their offspring.

I. Mendel’s Experiments

A. Crossing Pea Plants

1. Mendel decided to cross purebred pea plant with contrasting traits

a. ex: crossing tall plants with short plants

b. Purebred – an organism which is homozygous for a trait

II. Dominant and Recessive Alleles

A. Mendel determined that each trait exists in pairs (one coming from each parent).

B. Genes and Alleles1. Gene – factors that control a trait

2. Alleles – the different forms of a gene ex: The gene that controls stem height

in pea plants has two different alleles; one for tall plants and one for short plants

3. An organism’s traits are controlled by the alleles it inherits from its parents. Some alleles are dominant and some are recessive.

Dominant Allele – the allele whose trait always shows up in the organism when the allele is present

Recessive Allele – the allele that is hidden whenever the dominant allele is present

Dominant and Recessive Alleles in Pea Plants

Dominant Allele

Recessive Allele

III. Mendel’s Results A. Fertilization1. First Cross

The two purebred “parents” were crossed and all of the offspring were tall.

2. Second Cross The offspring of the first cross were

crossed with each other and Mendel noticed that about ¾ were tall and ¼ were short.

B. Alleles in Mendel’s Crosses

1. Homozygous tall plants had two alleles for tall stems.

2. Homozygous short plants had two alleles for short stems.

3. A hybrid organism is one that is heterozygous for a trait.

C. Symbols for Alleles1. A dominant allele is represented by a capital letter.

Ex: ‘T’ – represents the allele for tall plants

2. A recessive allele is represented by a lowercase letter.

Ex: ‘t’ – represents the allele for short plants3. Possible allele combinations are written as

follows:

TT = Homozygous Tall Plant

Tt = Heterozygous Tall Plant

tt = Homozygous Short Plant

D. Significance of Mendel’s Contribution1. Before Mendel, traits were thought to be a

blend of the parents.

2. Mendel’s experiments showed that traits don’t always blend.

Friday’s Quiz:

Vocab: Allele Gene Genetics Fertilization Trait

A Pea Plant Test Cross Essay question about

why recessive traits cannot be hybrids.

Hybrid Purebred Heredity Recessive Allele Dominant Allele