Chapter 3 Section 1

Mendel’s Work

Gregor Mendel

Young priest that worked in the garden at a monastery in Vienna.

Considered the “Father of Genetics”

Mendel’s Work - Vocabulary

Traits A physical characteristic that an

organism can pass on to its offspring through its genes.

Heredity Passing of traits from parent to offspring

Genetics Scientific study of heredity

Mendel’s Experiments

What type of plant did Gregor Mendel use for most of his studies? The Pea Plant

Mendel’s Peas

Peas are good plants to use for genetic studies because: Many traits exist only in two

forms (i.e. tall and short) And….

Peas produce many offspring in one generation making it is easy to collect and analyze large amounts of data.

How did Mendel make his crosses? – Basics First..

Pea plants reproduce by the same method as most plants. The pollen (male reproductive part of a plant) is transferred to the pistil (female reproductive part of a plant) and the ovary is fertilized producing a fruit (baby plant). Pollen can be transferred by many methods such as wind, water and bees.

Pollen

How did Mendel make his crosses?

Tall Plant Short Plant

1. Mendel transferred the pollen from one plant to the pistil of another plant. For example, pollen from a tall plant was used to fertilize the pistil of a short plant essentially making the father plant tall, and the mother plant short.

2. In order to prevent self pollination, Mendel removed the pollen producing parts of the short plant

Mendel’s Experiments

Purebred Plant One that always produces offspring

with the same form of a trait as the parent

For example….

Purebred pink plants will always produce pink plants

X

X

If a white flower is produced by two pink flowers, the parents CANNOT be purebred

Mendel’s Experiments

How did Mendel make sure he had purebred plants to make his crosses? By allowing pea plants of one

particular trait self pollinate for several generations.

Mendel’s Experiments In Mendel’s first experiment he crossed

PUREBRED TALL plants with PUREBRED SHORT plants. These plants were called the parental

generation or the P generation The offspring of this cross was called the

first filial generation or the F1 generation. After the F1 plants self pollinated they

produced the second filial generation or the F2 generation

Mendel’s Experiments What types of plants (Short/Tall) did

Mendel observe in the F1 generation? All Tall plants

What types of plants (Short/Tall) did Mendel observe in the F2 generation? A mix of short and tall plants. The ratio of tall to short was 3 to 1 or

75% tall and 25% short.

Mendel’s Experiments What conclusions did Mendel arrive at

from his experiments with peas? Some sort of “factor” controls the

inheritance of traits in pea plants. These “factors” exist in pairs with the

female contributing one factor and the male contributing the other.

One factor can “mask” the presence of the other.

Mendel’s Experiments What modern words do we now use for

Mendel’s discoveries. Mendel said “factors.” Today scientist call

these factors that control traits genes. Mendel concluded that these genetic

factors exist in pairs. Today we call these two forms of a gene alleles.

Mendel realized that one factor in the pair can mask the other. Today we know these as dominant alleles and recessive alleles.

Mendel’s Work - Vocabulary Dominant Allele

One whose trait always shows up in the organism when the allele is present.

Recessive Allele One that is masked or covered up when the dominant

allele is present. Recessive traits only show up when two recessive alleles are present.

Hybrid An organism that has two different alleles for a trait.

Mendel’s Experiments Other “opposite” traits that Mendel studied

(Figure 3 on page 83). Seed Shape – round (dominant) wrinkled (recessive) Seed Color – yellow (dominant) green (recessive) Seed Coat Color – gray (dominant) white (recessive) Pod Shape – smooth (dominant) pinched (recessive) Pod Color– green (dominant) yellow (recessive) Flower Position– side (dominant) end (recessive)

Dominant and Recessive Alleles

In modern genetics scientist represent the dominant allele with a capitol letter and the recessive allele with the lower case version of the same letter.

The same letter is used for the dominant and the recessive allele. For example….

Tall plants would be represented by T Short plants would be represented by t

Dominant and Recessive Alleles

Assigning the letter to the allele is random. However scientist often use the dominant trait. Examples… Seed color: Yellow Y Green y Seed coat color: Gray G White g Seed shape: Round R Wrinkled r

Dominant and Recessive Alleles

Genes exist in pairs and are made up of two alleles. One allele comes from mom and the other allele comes from dad. Assign the following allele combinations. A plant that inherits two alleles for Yellow seed color YY A plant that inherits one allele for Yellow seed color

and one allele green seed color Yy A plant that inherits two alleles for wrinkled seeds rr A plant that inherits one allele for round seed shape

and one allele wrinkled seed shape Rr A plant that inherits two alleles gray seed coat color

GG

Phenotypes and Genotypes

Phenotype Physical appearance or visible trait Example is flower color, stem height etc.

Genotype The genetic makeup or allele

combination Examples are TT or Tt

Genotypes determine phenotypes

Phenotypes and Genotypes

Figure 9 on the top of page 92 Phenotypes are Tall and Short Genotypes for tall plants are TT and Tt Only genotype that can produce a short

plant is tt

Homozygous and Heterozygous

Homozygous An organism that has two identical alleles for a

trait TT – homozygous dominant tt – homozygous recessive

Heterozygous An organism that has two different alleles for a

trait Tt – heterozygous – will show the dominant trait

Homozygous and Heterozygous

What term did Mendel use that means the same thing as heterozygous?

Hybrid

And The Secret Message is…