Table of Contents Mendel’s Work Probability and Heredity The Cell and Inheritance The DNA Connection

Heredity, Traits and Genetics *Heredity is the passing of traits from parent to offspring *A trait is a characteristic of an organism, such as hair color, eye color, etc. *Genetics is the scientific study of heredity. Trait Quiz

The Father of Modern Genetics *Gregor Mendel outlined the basic laws of heredity. *Mendel was an Austrian monk born in 1822. *Experimented with garden pea plants *Presented the first paper on genetic research in 1866 titled, “Experiments with Plant Hybrids”

Mendel’s Experimental Garden Mendel chose garden pea plants for his experiments. Why?

*Pea plants usually self-pollinate, producing purebred plants. *Mendel found a way to cross-pollinate them so he could mix traits. *Easy to grow large numbers of plants. *Many different traits such as height, seed color, seed shape, pod color, pod shape, and flower color can be easily studied.

wrinkled seeds Side or end flowers

How did he do it? - Mendel’s Work Gregor Mendel crossed pea plants that had different traits. The illustrations show how he did this.

Details of Mendel’s First Experiment Mendel’s first step was to cross pure tall plants with pure short plants. What were his results? X

The first generation of offspring were all tall. X P generation How can this be? F1 generation

Mendel’s Explaination In pea plants, tall traits overpower short traits. Each plant, although it looked tall, was hiding a factor for short height. Will the short trait reappear in the next generation?

To find out, Mendel now crossed tall plants of the first generation with each other. x What were his results?

In the second generation, the offspring were both tall and short In the second generation, the offspring were both tall and short. Tall outnumbered short 3 : 1 X F1 generation F2 generation

- Mendel’s Work In all of Mendel’s crosses, only one form of the trait appeared in the F1 generation. However, in the F2 generation, the “lost” form of the trait always reappeared in about one fourth of the plants.

Mendel’s Conclusion Tall is a dominant factor for height since it overpowers or hides the short factor. Short is a recessive factor for height since it appears to disappear if the tall factor is present. We now call these “factors” alleles.

Dominant and Recessive Alleles - Mendel’s Work Dominant and Recessive Alleles Mendel studied several traits in pea plants.

Traits, Genes and Alleles Sometimes the words "gene", "allele" and "trait" seem to be used to mean the exact same thing.  Although they all are closely related, there is a difference between genes, alleles, and traits. 

A trait is a characteristic of an organism, such as tall height. A gene is a small section of DNA on a chromosome that controls a trait. (Mendel didn’t know about chromosomes). The different forms a gene may come in are its alleles (example—short or tall are the alleles for the gene that controls the height of a plant). Gene Allele Trait Homologous Chromosomes Tall

Mendel showed that organisms have two alleles for every trait Mendel showed that organisms have two alleles for every trait. (One that we get from mom and one that we get from dad). He also showed that these alleles can be either dominant or recessive. Capital letters are used to represent dominant alleles and lower case letters are used to represent recessive alleles.

Dominant traits will be seen even if there is only one dominant allele in the pair. TT = tall plants Tt = tall plants Recessive traits will only be seen when both recessive alleles are present in the pair. tt = short plants This explains the 3:1 ration of tall to short plants observed by Mendel.

Heterozygous and Homozygous An individual with two alleles for a trait that are exactly the same (TT or tt) is said to be homozygous or “pure” for that trait. An individual with two different alleles for a trait (Tt) is said to be heterozygous or “hybrid” for that trait.

Genotypes and Phenotypes Genotype is an organism’s genetic make-up: it includes hidden traits and is represented by letters: Examples: TT Tt or tt Phenotype is the physical trait that you see in an organism: it is a description Examples: tall plants or short plants

Phenotypes and Genotypes - Probability and Heredity Phenotypes and Genotypes

Remember, Mendel did not know how factors were passed from parent to offspring. In 1903, Walter Sutton discovered that Mendel’s “factors” were actually genes carried on chromosomes. These genes are passed to the offspring in the sperm and egg cells produced during meiosis.

Sutton discovered this while studying the development of sperm and egg cells in grasshoppers. This is now known as the chromosome theory of inheritance.

Click the Video button to watch a movie about chromosomes. - The Cell and Inheritance Chromosomes Click the Video button to watch a movie about chromosomes.

- The Cell and Inheritance A Lineup of Genes Chromosomes are made up of many genes joined together like beads on a string. The chromosomes in a pair may have different alleles for some genes and the same allele for others.

When a pair of chromosomes separate during meiosis, the different alleles for a trait move into separate sex cells. Ee = parent genotype ee = parent genotype EE = parent genotype

The genotype of the offspring will be determined by which sperm cell fertilizes which egg cell. Ee = genotype of offspring Ee

Punnett Squares Punnett squares can be used to show the possible ways that genes can combine at fertilization

- The Cell and Inheritance A Punnett square is actually a way to show the events that occur at meiosis.

Monohybrid Cross A Punnett Square that shows only one trait is known as a Monohybrid Cross.

T T t T t T t t T t T t Example: TT x tt The only possible combination for the offspring of these two parents is one tall allele (T) and one short allele (t). All offspring will be hybrid (heterozygous) tall. Genotype = Tt Phenotype = tall T T t T t T t t T t T t

Let’s look at another example: T t x T t There are 3 combinations for the offspring of these two parents. They occur in the following ratios: Pure (homozygous) tall TT = 25% Hybrid (heterozygous) tall Tt = 50% Pure (homozygous) short tt = 25% T t T T T T t t T t t t

What do you actually see? 75% tall plants and 25 % short plants.

Mendel’s Law of Independent Assortment Mendel discovered that it was possible to study more than one trait at a time. Example: There are 4 possible outcomes for the gametes. One letter does not affect the selection of the other.

RY Ry rY ry RY Ry rY ry Dihybrid Crosses show two traits: Example: Rr Yy x Rr Yy R = round seeds Y = yellow seeds r = wrinkled seeds y = green seeds RY Ry rY ry RY RRYY RRYy RrYY RrYy 9 = round/ yellow 3 = round/ green 3 =wrinkled/ yellow 1 = wrinkled/ green Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy

Pure Red Flowers X Pure White Flowers Result: All Pink Flowers Codominance Codominance occurs when neither allele in the pair is dominant over the other. Example: Pure Red Flowers X Pure White Flowers Result: All Pink Flowers

X RR WW RW RW RW RW

Punnett Squares for codominance do not use lower case letters, since neither trait is dominant. Example: RR X WW R R W RW RW All offspring are PINK W RW RW

- Probability and Heredity Codominance Another example of codominance shows each trait appearing together rather than a blending.

Sometimes mistakes happen in the genetic code. Mutations Without correctly coded proteins, an organism can’t grow, repair, or maintain itself. Sometimes mistakes happen in the genetic code. Any permanent change in a gene or chromosome of a cell is called a mutation.

Mutations can: *be harmless, beneficial, or harmful *happen randomly or be caused by an outside factor (x-rays, radiation, chemicals etc.) *add variety to a species over time

Three Types of Mutations *Substitution—one base pair is substituted for another. *Deletion—one base pair is omitted *Addition—one base pair is added

Mutations - The DNA Connection Mutations can cause a cell to produce an incorrect protein during protein synthesis. As a result, the organism’s trait, or phenotype, may be different from what it normally would have been.

Data Sharing Lab - Mendel’s Work Click the PHSchool.com button for an activity about sharing data for the Skills Lab Take a Class Survey.

Links on Probability and Genetics - Probability and Heredity Links on Probability and Genetics Click the SciLinks button for links on probability and genetics.

Click the SciLinks button for links on meiosis. - The Cell and Inheritance Links on Meiosis Click the SciLinks button for links on meiosis.

Protein Synthesis Activity - The DNA Connection Protein Synthesis Activity Click the Active Art button to open a browser window and access Active Art about protein synthesis.

Click the Video button to watch a movie about protein synthesis. - The DNA Connection Protein Synthesis Click the Video button to watch a movie about protein synthesis.