Genetics: The Science of Heredity

7th Grade Life Science

SECTION 1: MENDEL’S WORK

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A. Gregor Mendel

1. The first recorded scientific study of how traits pass from one generation to the next was done by Gregor Mendel, a monk!

2. In 1856, he began experimenting with garden peas.

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3. He believed he could predict the kinds of flowers and fruits a plant would produce. a. To do this he believed that something had to be

known about the parents of the plants.

4. Each different form of a characteristic, such as a stem height, is called a TRAIT.

5. Today, Mendel’s discoveries form the foundation of Genetics. a. Father of Genetics

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B. Mendel’s Experiments1. Fertilization occurs when egg and sperm

join. a. Before this can happen in pea plants, pollen

must reach the pistil of the flower. This process is called pollination.

2. Mendel developed a method by which he cross pollinated, or “crossed” pea plants.

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C. Crossing Pea Plants1. An organism that always produces the same

traits in its offspring is called a PUREBRED. a. Example: PUREBRED short pea plants

always come from short parent plants.

b. Tall plants that always produce tall plants are considered PUREBRED tall plants.

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C. Crossing Pea Plants2. In one experiment, Mendel crossed

purebred tall plants with purebred short plants.

a. Scientists call the parent plants the parental generation or the “P generation”

b. The offspring from this cross are the first fillial generation or the “F1 generation”.

i. Fillial comes from the word fillia: the Latin word for daughter

c. All of the offspring from this cross were TALL.

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C. Crossing Pea Plants3. When the plants in the F1 generation were

full grown, Mendel allowed them to self pollinate.

a. Some of the plants in the F2 generation were a mix of tall and short plants.

i. ¾ were tall

ii. ¼ were short.

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D. Dominant and Recessive Traits1. Genes are factors that control

traits.

a. Genes are made up of DNA

b. Genes are found on chromosomes.

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D. Dominant and Recessive Traits2. The different forms a gene may have for a trait are its alleles.

3. Each new organism inherits two alleles from its parents.

a. One from the egg (mother) and the other from the sperm

(father).

b. A pea plant may inherit two alleles for tall stems, two alleles

for short stems, or one of each.

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D. Dominant and Recessive Traits4. An organisms traits are controlled by the alleles it inherits from

its parents.

a. Dominant Allele is one whose trait always shows up in the

organism when the allele is present.

b. Recessive Allele is hidden whenever the dominant allele is

present.

i. A trait controlled by a recessive allele will only show up

if the organism does not have the dominant allele.

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Trait Dominant RecessiveDominant and Recessive Traits in Pea Plants are shown in the table to the right.

If the dominant allele is present it will mask the recessive allele.

Example: If a pea plant has both purple and white flower alleles the purple allele will be the one that is visible to you.

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A. Probability

1. Probability is the number that describes how likely it is that

an event will occur.

2. Probability is different then the odds.

a. Probability = what you want / total chances

b. Odds= what you want: what you don’t want

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B. Punnett Squares

1. A handy tool to use to predict

results in Mendelian Genetics is

the Punnett Square.

2. A Punnett Square is a chart that

shows all the possible

combinations of alleles that can

result from a genetic cross.

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Y= Yellow

y= Green

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B. Punnett Squares

3. Dominant alleles are

represented by capital letters.

4. Recessive alleles are

represented by lower case

letters.

5. An organism that has two

identical alleles is called

HOMOZYGOUS.

6. An organism that has two

different alleles is called

HETEROZYGOUS.

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Y= Yellowy= GreenYy= Heterozygousyy= Homozygous

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C. Using a Punnett Square

1. Monohybrid Cross deals with

only one trait.

a. Yellow pea plants being crossed

with green pea plants.

2. Di-hybrid cross deals with two

traits.

a. Tall Yellow Pea Plants crossed

with Tall Green Pea Plants

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TtYy X TtyyTall Yellow Pea Plants X Tall Green Pea Plants

Plant one is

Heterozygous

Tall;

Heterozygous

Yellow.

Plant Two is

Heterozygous

tall green pea

plants.

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D. Phenotype

1. An organisms phenotype is its physical appearance, or its visible

traits.

2. Phenotypic Ratio

a. The ratio of phenotypes found in the Punnett Square.

b. Pea plants can have one of two different phenotypes for stem height-

Short or Tall.

E. Genotype

1. An organisms genotype is its genetic makeup, or allele

combinations.

2. Although all of the tall plants have the same phenotype, they can

have two different genotypes-TT or Tt

a. A Genotypic Ratio is the ratio of genotypes found in a Punnett Square.

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F. Comparing Phenotype and Genotype

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Genotype Phenotype

Genetic Make-Up Physical Appearance

Cannot see with eye Visible to the eye

Example: Tall = TT or Tt Example: Tall=Tall

Two Different Genotypes Only one Phenotype-Tall

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G. CODOMINANCE

1. For some alleles, an inheritance pattern called co-dominance exists.

a. In co-dominance, the alleles are neither dominant nor recessive.

b. As a result, both alleles are expressed in the offspring.

c. We will discuss an example in class.

1. When two traits are not completely dominant over each other.

a. This results in a blending of the two traits.

b. Example:

i. RR = Red Flowers

ii. WW= White Flowers

iii. RW = Pink flowers

H. INCOMPLETE DOMINANCE

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