THINK ABOUT IT
What is an inheritance?
It is something we each receive from our parents—a contribution that determines our blood type, the color of our hair, and so much more.
Where does an organism get its unique characteristics?
An individual’s characteristics are determined by “factors” that are passed from one parental generation to the next.
The delivery of characteristics from parent to offspring is called heredity.
The scientific study of heredity, known as genetics, is the key to understanding what makes each organism unique.
A trait is a variation of a particular characteristic of an individual, such as AB blood, red hair and purple flowers.
Lesson Overview
The Work of Gregor Mendel
Gregor Mendel
Lesson Overview
The Work of Gregor Mendel
Mendel Worked with True-Breeding Pea Plants
Lesson Overview
The Work of Gregor Mendel
Mendel Cross-Fertilized Plants by Hand
Cross-pollination allowed Mendel to breed plants with traits different from those of their parents and then study the results.
Lesson Overview
The Work of Gregor Mendel
Monohybrid Cross: Parents Differ in Only One Character
Lesson Overview
The Work of Gregor Mendel
Mendel Examined Inheritance Patterns of 7 Pea-Plant Characters
Lesson Overview
The Work of Gregor Mendel
. . . reappeared in ~1/4 of the F2 generation ……why?
Mendel’s 2nd experiment told the story mathematically!
Lesson Overview
The Work of Gregor Mendel
Mendel’s principles (Laws)
1. Alleles are alternative forms of “factors” (genes) which are passed from parent to offspring.
An organism has a pair of alleles that controls each character trait.
Lesson Overview
The Work of Gregor Mendel
Alleles are Alternative Versions of a Gene
Lesson Overview
The Work of Gregor Mendel
Mendel’s principles (Laws)
1. Alleles are alternative forms of “factors” (genes) which are passed from parent to offspring.
An organism has a pair of alleles that controls each character trait.
2. Dominant allele masks the recessive allele in a heterozygous individual
Lesson Overview
The Work of Gregor Mendel
Dominant and Recessive Traits
In Mendel’s experiments, the allele for tall plants was dominant and the allele for short plants was recessive.
Lesson Overview
The Work of Gregor Mendel
Dominant and Recessive Traits
In Mendel’s experiments, the allele for tall plants was dominant and the allele for short plants was recessive. Likewise, the allele for yellow seeds was dominant over the recessive allele for green seeds
Lesson Overview
The Work of Gregor Mendel
Mendel’s principles (Laws)
1. Alleles are alternative forms of “factors” (genes) which are passed from parent to offspring.
An organism has a pair of alleles that controls each character trait.
2. Dominant allele masks the recessive allele in a heterozygous individual
3. Principle of segregation: The 2 alleles for a character segregate (separate) during the formation of gametes (sex cells).
Lesson Overview
The Work of Gregor Mendel
Segregation = Meiosis
How are different forms of a gene distributed to offspring?
During gamete (sex cell) formation, the alleles for each gene segregate from each other, so that each gamete carries only one allele for each gene.
Mendel assumed that a dominant allele had masked the corresponding recessive allele in the F1 generation.
The reappearance of the recessive trait in the F2 generation indicated that, at some point, the allele for shortness had separated from the allele for tallness.
Lesson Overview
The Work of Gregor Mendel
Let’s assume that each F1 plant—all of which were tall—inherited an allele for tallness from its tall parent and an allele for shortness from its short parent.
The Formation of Gametes (Meiosis)
When each parent, or F1 adult, produces gametes (meiosis), the alleles for each gene segregate from one another, so that each gamete carries only one allele for each gene.
One gamete from each parent will randomly combine during fertilization.
The F2 generation has new combinations of alleles.
Lesson Overview
The Work of Gregor Mendel
Probability and Punnett Squares
How can we use probability to predict traits?
Probability is the likelihood that a particular event will occur.
Punnett squares use mathematical probability to help predict the genotype and phenotype combinations in genetic crosses.
The probability of a particular combination is the product of the separate probabilities for each coin.
Ex: Probability of 2 heads showing is 1/2 x 1/2 = 1/4
Lesson Overview
The Work of Gregor Mendel
Punnett Square: Step 1) show the parent genotypes & cross [Pp X Pp]
Step 2) draw square & separate parent alleles into possible gametes [ P or p]
Lesson Overview
The Work of Gregor Mendel
Step 3) show all possible combinations of gametes� (fertilization events)
Step 4) record Genotype and Phenotype ratios
(pairs of alleles/genes) G: ¼ PP, ½ Pp, ¼ pp
(physical appearance) P: ¾ Purple, ¼ White
Lesson Overview
The Work of Gregor Mendel
1) Write the genotypes of the two organisms that will serve as parents in a cross.
In this example we will cross a male and female osprey that are heterozygous for large beaks. They each have genotypes of Bb.
Bb and Bb
2) Draw a table and determine what alleles would be found in all of the possible gametes that each parent could produce.
Lesson Overview
The Work of Gregor Mendel
3) Fill in the table by combining the gametes’ genotypes.
4) Determine the genotypes and phenotypes of each offspring.
Calculate the percentage of each. In this example:
G: ¼ BB, ½ Bb, ¼ bb
P: ¾ large beaks, ¼ small beaks
Lesson Overview
The Work of Gregor Mendel
4) Mendel’s principle of Independent Assortment
How do alleles segregate when more than one gene is involved?
The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes.
Mendel performed an experiment that followed two different genes as they passed from one generation to the next.
Because it involves two different genes, Mendel’s experiment is known as a two-factor, or dihybrid cross. Single-gene crosses are monohybrid crosses.
Lesson Overview
The Work of Gregor Mendel
The Two-Factor Cross: F1
Mendel crossed true-breeding plants that produced only round yellow peas with plants that produced wrinkled green peas.
The round yellow peas had the genotype RRYY, which is homozygous dominant.
The wrinkled green peas had the genotype rryy, which is homozygous recessive.
Step 1) RRYY X rryy
Step 2) FOIL method: RRYY = RY, RY, RY, RY
rryy = ry, ry, ry, ry
Lesson Overview
The Work of Gregor Mendel
The Two-Factor Cross: F1
Step 4:
100% of the F1 offspring had the phenotype round yellow peas. These results showed that the alleles for yellow and round peas are dominant over the alleles for green and wrinkled peas.
The Punnett square shows that the genotype of each F1 offspring was 100% RrYy, heterozygous for both seed shape and seed color.
Step 3) combine all possible gametes
- keep like letters together
- capital first
Lesson Overview
The Work of Gregor Mendel
The Two-Factor Cross: F2
Mendel then crossed the F1 plants to produce F2 offspring.
Step 1) RrYy X RrYy
Lesson Overview
The Work of Gregor Mendel
Principle of Independent Assortment
RrYy = RY, Ry, rY, ry
RrYy = RY
Ry
rY
ry
Lesson Overview
The Work of Gregor Mendel
Step 3) combine all possible gametes
- keep like letters together
- capital first
Step 4) Four Possible Phenotypes
Lesson Overview
The Work of Gregor Mendel
A Summary of Mendel’s Principles
What did Mendel contribute to our understanding of genetics?
Mendel’s principles of heredity, observed through patterns of inheritance, form the basis of modern genetics.
1) The inheritance of biological characteristics is determined by individual units called genes, which are passed from parents to offspring.
2) Where two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.
3) In most sexually reproducing organisms, each adult has two copies of each gene—one from each parent. These genes segregate from each other when gametes are formed.
4) Alleles for different genes usually segregate independently of each other.
Lesson Overview
The Work of Gregor Mendel
A Summary of Mendel’s Principles
At the beginning of the 1900s, American geneticist Thomas Hunt Morgan decided to use the common fruit fly as a model organism in his genetics experiments.
The fruit fly was an ideal organism for genetics because it could produce plenty of offspring, and it did so quickly in the laboratory.
Before long, Morgan and other biologists had tested every one of Mendel’s principles and learned that they applied not just to pea plants but to other organisms as well.
The basic principles of Mendelian genetics can be used to study the inheritance of human traits and to calculate the probability of certain traits appearing in the next generation.
Lesson Overview
The Work of Gregor Mendel