MENDEL AND

HIS PEA PLANTS

The Father of Genetics

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“Mendel & His Pea Plants” 1. What was different about how Mendel approached his research than previous researchers? 2. Why was Mendel concerned with “foreign pollen.” 3. Define the following terms: Purebred Hybrid Dominant Recessive 4. How are dominant traits labeled? 5. How are recessive traits labeled? 6. What is the basic tool for organizing and predicting the results of genetics crosses? 7. Predict the traits of the offspring of P1, if the mom has blue eyes and the dad has green eyes.

The Beginnings of the Science of Heredity

Ever since humans began domesticating plants and animals (at least 10,000 years ago), people have wondered how traits were passed from parent to offspring. One belief was that traits were stored as 'particles' in the parts of each parent's body and 'blended' in the offspring. This theory left many questions unanswered, however.

Johann Gregor Mendel

Johann Gregor Mendel was born in Hyncice (in what is now the Czech Republic) on July 22, 1822 Republic..

In 1843, at the age of 21, Mendel became a friar at the Augustinian monastery in Brno, Czechoslovakia, a center of learning whose members studied theology, philosophy and natural sciences.

Johann Gregor Mendel

Mendel experimented with pea plants in the

monastery's gardens, for twelve years. These

experiments formed the basis for the new branch

of science called GENETICS.

Johann Gregor Mendel

In the 18th and 19th centuries, scientists tried to unravel the problems of heredity, but their experiments returned inconclusive results.

Unlike the others, Mendel studied only one trait at a time. Because of this, he was the first to be able to describe the relations between parents and children

with mathematical symbols.

MENDEL’S EXPERIMENTAL PLAN

Mendel chose a common garden pea for his first experiments because they grew quickly and could reproduce by self-pollination.

The seven traits that Mendel studied in the peas are shown in the table at right.

He studied “pure” traits. That means he could develop plants that always produced seeds with the same traits. Example: Pure tall plants only produced seeds that grew into pure tall plants.

 

Trait

DominantExpression

RecessiveExpression

1. Form of ripe seed Smooth Wrinkled

2. Color of seed albumen Yellow Green

3. Color of seed coat Grey White

4. Form of ripe pods Inflated Constricted

5. Color of unripe pods Green Yellow

6. Position of flowers Axial Terminal

7. Length of stem Tall Dwarf

MENDEL’S MAJOR CONCERNWhen picking a plant to experiment on, Mendel was concerned that they must "during the flowering period, be protected from the influence of all foreign pollen, or be easily capable of such protection [because] accidental impregnation by foreign pollen ... would lead to entirely erroneous conclusions." - J. G. Mendel

Why does pollenmatter?

Why wouldthere be erroneousconclusions?

MENDEL’S EXPERIMENTAL PLAN

In the process of experimenting, he ended up making 287 crosses between 70 different purebred plants. Approximately 28,000 pea plants were used!

This does not take into account the other species of plants he experimented on!

MENDEL’S RESULTS Mendel noticed that when

breeding two peas, of different “pure” traits, a particular variation of a trait in one pea (ie. the yellow color of a pea) would not appear in the next generation.

However, in the following generation, when breeding the offspring together, this variation (yellow color) would appear again.

He concluded that the traits were being "masked,“ or hidden in the second generation, to be exhibited again in the third.

He also concluded that for every trait there must be two hereditary parts, one from each parent.

MENDEL’S RESULTS

Since both the yellow and the green information is being transferred from the parents to the second generation, and the yellow is being masked, we say that the green is dominant, or “stronger” and the yellow is recessive.

LABELING TRAITS The standard way of labeling the variation information of a trait in a

particular organism is by using two letters. Capital letters represent information which is dominant. Lowercase letters represent the recessive. The particular letter that is

chosen usually describes the dominant variation of the trait.

YY

stands for a plant where both pieces of color information are dominant - yellow. The plant is yellow and is called a “purebred”.

Yy

stands for a plant where one piece of color information is dominant - yellow, and the other is recessive - green. The plant is yellow and is called a “hybrid”.

yy

stands for a plant where both pieces of color information are recessive - green. The plant is green and is called a “purebred”.

ORGANIZING and PREDICTING RESULTS

In the table to the left, we can use a box called the “Punnett Square” to figure out what the offspring of parents might be.

If we cross two hybrid parents that are yellow dominant, and green recessive, what might there offspring be?

Offspring will be 25% YY; 50% Yy and 25% yy.

  Y y

Y YY Yy

yY yyy

Inheritance Across Generations:

When creating a diagram showing the relation between generations:

Parents are labeled “P1,” Their children are

labeled “F1” Each generation

thereafter is consecutively numbered “F2,” and so on.

BACK TO MENDEL’S PEAS

Mendel noted the temporary loss of a variation (such as yellow peas) in the first generation of children, "F1" when breeding two purebreds.

WHY?

BACK TO MENDEL’S PEAS

But when breeding the “F1" generation with itself, the next ("Second") generation showed the variation again, in the 3:1 ratio.

WHY?

 Y y

Y YY Yy

Yy yyy

 Y Y

y Yy Yy

Yy Yyy

F1 F2Purebreds-green

Recessiveyellow

The results look allgreen(dominant)

Hybrids

The results look like a mixture of yellows and greens, but some are purebreds and some are hybrids.

To explain the results, let’s make a Punnet Squarefor each generation.

DNA, GENES & CHROMOSOMESThe complete set of instructions for making an organism is called its genome. It contains the master blueprint for all cellular structures and activities for the lifetime of the cell or organism.

As early as 1900, scientists knew that chromosomes were located in the nucleus of a cell.

They also knew that chromosomes carried hereditary information.

But it wasn’t until 1952 that it became clear that the

genetic material was DNA! Understanding the function of DNA requires some knowledge of

its structure and organization.

DNA DOUBLE HELIX

The molecule of DNA looks like a twisted ladder.

The sides of theladder are made up of alternating molecules of deoxyribose, a sugar (D)and a phosphate (P).

•The rungs of the ladder are made up of nitrogen bases:

(G=guanine, C=cytosine, T=thymine, or A=adenine).

•Each base forms half a rung of the ladder, pairing with only its partner base: G-C and T-A.

•Organisms have different traits because they have different DNA base pair sequences.

DNA DOUBLE HELIX

DNA DOUBLE HELIXIf unwound and tied together, the strands of DNA would stretch more than 5 feet but would be only 50 trillionths of an inch wide. DNA molecules are among the largest molecules now known.

GENES The DNA sequence of base pairs specifies the exact genetic

instructions required to create a particular organism with its own unique traits.

A gene is a specific DNA sequence in the DNA molecule (ranging from fewer than 1 thousand bases to several million) that holds the information about a specific trait;

making it the basic physical and

functional unit of heredity. The human genome is estimated

to comprise more than 100,000

genes. The gene is located in a particular

position on a specific chromosome.

The light and dark bands, on the picture at right, are the genes on a chromosome.

CHROMOSOMES

You have to have a place to keep all the DNA that contains the genes. What better place than a chromosome!

The 3 billion base pairs in the human genome are organized into 23 distinct pairs of microscopic units called chromosomes. Apart from reproductive cells and mature red blood cells, every cell that has a nucleus contains chromosomes made of DNA.

The nucleus of most human cells contains two sets of 23 chromosomes -one set given by each parent. (this includes an XX pair for a female or an XY pair for a male)

CHROMOSOMES Chromosomes can be seen under a light microscope and, when

stained with certain dyes, reveal a pattern of light and dark bands reflecting regional variations in the amounts of A and T vs G and C.

A karyotype is an analysis that distinguishes the chromosomes from each other based on their differences in size and banding pattern.

Upon doing a karyotype, some types of chromosomal abnormalities may be detected, including missing or extra copies of a chromosome or breaks that rejoin upside down (translocations).

Most changes in DNA, however, are too subtle to be detected by this technique and require detailed molecular analysis. These DNA abnormalities (mutations) are responsible for many inherited diseases such as cystic fibrosis and sickle cell anemia or may predispose an individual to cancer, major psychiatric illnesses, and other complex diseases.

Microscopic examination of chromosome size and banding patterns allows medical laboratories to identify and arrange each of the 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes)

into a karyotype, that then serves as a tool in the diagnosis of genetic diseases. The extra copy of chromosome 21 in this karyotype identifies

this individual as having Down's syndrome.

KARYOTYPE

DNA REPLICATION

The bonds between the nucleotide bases on each DNA strand are weak, allowing the bases to come apart when duplication of the strand is needed.

During mitosis, chromosomes make exact copies of themselves. The DNA molecules must also make exact copies of themselves.

The DNA molecule comes apart like a zipper being unzipped. The two halves of the DNA separate between the base pairs forming the rungs. Each half acts as a pattern for a new half to form.

As a result of the DNA molecules making exact copies of themselves, two identical chromosomes are made.

DNA MAKES PROTEINS DNA controls many activities in the cell. One of these activities is to make proteins. A

gene’s sequence can carry information required for constructing proteins (needed for cell and tissues maintenance). Humans can make at least 100,000 different kinds.

Human genes vary widely in length, often extending over thousands of bases, but only about 10% of the genome is known to include the protein-coding sequences (exons) of genes.

Proteins are large, complex molecules made of long chains of subunits called amino acids. Twenty different kinds of amino acids are usually found in proteins.

Within a gene, each specific sequence of three DNA bases, called codons, directs the cell’s protein-making machinery to make specific amino acids. For example, the base sequence “ATG” codes for the amino acid methionine.

Since 3 bases code for 1 amino acid, the protein coded by an average-sized gene (3000 bp) will contain 1000 amino acids. The genetic code is thus a series of codons that specify which amino acids are required to make up specific proteins.

What does protein-making have to do with genetic traits? The arrangement of base pairs to make proteins also codes for genetic traits.