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mendelonline.notebook
November 01, 2013
VOCABULARY
• TRAITS­ a genetic (inherited) characteristic
• HEREDITY­ The passing of traits from parent to offspring
• GENETICS­ the branch of biology that studies heredity (inherited traits)
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mendelonline.notebook
November 01, 2013
Gregor Mendel
• Who? Monk (priest) • What? Curious gardener trained in math and science
• When? 1851 is when it all began…
•Where? Austria (central Europe)
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mendelonline.notebook
November 01, 2013
Curiosity Leads the Way
• He was curious as to why plants, specifically garden pea plants, had different physical characteristics (short/tall, yellow seeds/green seeds, purple/white flowers, etc.)
• Observed that TRAITS were often similar to those of their parents
• Spent over 10 years experimenting with the thousands of pea plants to understand HEREDITY
• He was the first person to succeed in predicting how traits are transferred from one generation to the next.
• His work formed the foundation of GENETICS
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mendelonline.notebook
November 01, 2013
Why Pea Plants?
• Easy to grow
• Grow quickly
• Produce large number of offspring in one generation (lots of data available at one time) • They have many traits that exist in only two forms (makes it easier to study them)
• Pea plants have both male and female parts on the same flower; so fertilization can be controlled.
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mendelonline.notebook
November 01, 2013
Plant Vocabulary
• Gametes­ male and female sex cells
• Fertilization­ male gamete unites with the female gamete
• Zygote­ the fertilized cell that develops into a seed.
•Pollination­ transfer of pollen grains from male reproductive organ to female reproductive organ.
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mendelonline.notebook
November 01, 2013
Vocabulary Continued
• Self­pollinating­ The pollen of one flower lands on the pistil of the same flower
• Cross­Pollinating­ The pollen of one flower is placed on the pistil of a different flower.
**To prevent self­pollination Mendel removed the stamen from the flowers on the second flower.**
• Pure bred­ always produces offspring with the same form of a trait as the parent….its pure!
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mendelonline.notebook
November 01, 2013
• Pistil­female sex cells
• Stamen­male sex cells (pollen)
• NOTE­ in nature these plants would be self­pollinating
• Mendel developed a way to cross­pollinate pea plants
• Remove pollen from a flower on one plant and brush it on a flower of a second plant
• To prevent self­pollination he carefully removed the stamens from the flowers on the second plants
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mendelonline.notebook
November 01, 2013
Let the Experiment Begin!
• Mendel crossed PUREBRED plants with opposite forms of the same trait. For example he crossed a purebred tall plant with a purebred short plant.
• Why pure breds??? Because those were the plants that he knew what to expect from the offspring. • Mendel carefully controlled his experiments making sure that he studied only one trait at a time to control the variables.
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mendelonline.notebook
November 01, 2013
Experiment #1
• Characteristic: Tall Plant vs Short Plant
• NOTE: The parent plants were called the parental generation or P generation and the offspring from this cross were called the F1 generation. • Results: ALL F1 generation plants were tall! The short trait had “disappeared”.
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mendelonline.notebook
November 01, 2013
• Hybrids­ offspring of parents that have different forms of a trait.
• Monohybrid­ Mendel’s first experiments were monohybrids because they differed only by a single trait. 10
mendelonline.notebook
November 01, 2013
Experiment #2
• He then allowed all of the F1 generation to self­pollinate.
• This generation was called the F2 generation.
• Results: He discovered that about ¾ of them were tall and ¼ were short. Short trait had returned!
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mendelonline.notebook
November 01, 2013
Experiment #3­???
• He tested all of the other main traits
• Flower color, Seed shape, seed color, seed coat color, pod shape, pod color, and flower position.
• Results: All of the traits had the same results as plant height!
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mendelonline.notebook
November 01, 2013
Conclusions
• Individual factors must control the inheritance of traits in peas.
• Factors exist in pairs
• Female contributes one trait and the male contributes the other
•One factor can mask/hide the other factor
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mendelonline.notebook
November 01, 2013
Vocabulary…cont.
“Factors” that control traits are called GENES, they are located on chromosomes.
Different forms of a gene are called ALLELES
ex: two alleles for height: short and tall, purple vs. white
An organism's two alleles are located on different copies of a chromosome one from mom and one from dad.
Individual alleles control the inheritance of traits. Some alleles are dominant while others are recessive.
*Dominant alleles will always show up if they are present.
*Recessive alleles are covered up when dominants are present.
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mendelonline.notebook
November 01, 2013
Take out your puppy­dogs!
• On the back of your pet, write what alleles your pet has for each trait.
• Use XX for female, XY for male
• Dominant alleles are: F (Floppy ears ), R (round eyes), T (triangular nose), and P (pointed teeth)
• Example: My dog has pointed ears. Since pointed ears is recessive my two alleles must be ff.
• Note: If your pet has a trait controlled by a dominant allele, you can choose which of the possible combinations of alleles your pet has.
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mendelonline.notebook
November 01, 2013
Understanding the Crosses
• P­generation: Tall Purebred (TT) and Short Purebred (tt)
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November 01, 2013
All four possibilities posses a tall gene so in every case the recessive, short gene, will be hidden.
All F1 plants are considered to be hybrids­ they have two different alleles for the same trait, rather than being purebreds.
Always use the same letter for different alleles of the same gene.
Capital Letters stand for dominant alleles.
Lower case letters stand for recessive alleles
Dominant allele always comes first in the boxes.
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mendelonline.notebook
November 01, 2013
The Law of Segregation
• Recall the results of Mendel’s cross between F1 plants
• Rules: Each tall plant in the F1 generation carried one dominant allele for tallness and one unexpressed recessive allele for shortness. (F1 plants received 1 allele from each P­
generation parent)
• The Law: Every individual has two alleles of each gene and when gametes are produced, each gamete receives one of these alleles. During Fertilization, these gametes randomly pair to produce four combinations of alleles.
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mendelonline.notebook
November 01, 2013
One more Rule from Mendel:
• Rule of Dominance­ One of the two factors (dominant allele) governs a trait and the alternate form of the trait (recessive) is only present if the dominant trait is not present.
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mendelonline.notebook
November 01, 2013
History Rap­up
• Mendel tried to present his results to other scientists; however, some felt that he had over­simplified inheritance while others would not even read his findings.
• It took 34 years before people began to realize how important his work really was.
• Today Mendel is often referred to as the FATHER OF GENETICS
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mendelonline.notebook
November 01, 2013
Mendel’s Dihybrid Crosses
• He used Peas that differed from each other by 2 traits instead of just one.
• The question he was asking: Will the two traits stay together in the next generation or will they be inherited independently of each other?
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mendelonline.notebook
November 01, 2013
Dihybrid Cross Experiment
• Characteristics: True­bred round, yellow seeds and a true­bred wrinkled, green seeds
• He already knew: Round and Yellow were dominant from his monohybrid crosses.
• Results: All Seeds were round and yellow.
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mendelonline.notebook
November 01, 2013
He allowed the F1 generation to self­pollinate
Results: F2 varied greatly, round/yellow, wrinkled/yellow, round/green and wrinkled/green
It was a definite ratio 9:3:3:1 (9 round/yellow, 3 wrinkled/
yellow, 3 round/green and 1 wrinkled green)
Were the results expected: Yes, because the dominant traits appeared more often.
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mendelonline.notebook
November 01, 2013
Conclusions to Dihybrid Cross Experiments
• Mendel was able to express his second Law from the results and he called it: The Law of Independent Assortment
• Genes of different traits are inherited independently
• Why was he able to make this conclusion? Because if the alleles for seed shape and color were inherited together only 2 kinds of pea seeds would have been produced instead of variations of all combinations.
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mendelonline.notebook
November 01, 2013
Vocabulary Once more…
• Phenotype­ its physical appearance (visible traits)
• Genotype­ genetic makeup or allele combinations.
• Homozygous (pure bred)­ organism with two identical alleles for a trait (purebred)
• Homozygous dominant (TT)
• Homozygous recessive (tt)
•Heterozygous (hybrid)­ two different alleles for the same trait (hybrid)
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mendelonline.notebook
November 01, 2013
What is probability?
• The likelihood that a particular event will occur.
• For example we would say that any coin that is tossed will have a 1 in 2 chances of landing heads up…BUT if you were to toss a coin 20 times you might expect it to be 10 heads and 10 tails but that might not be the case.
• Laws of probability PREDICT what is likely to occur not necessarily what WILL occur. However, the more tosses you make, the more accurate you’ll be.
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mendelonline.notebook
November 01, 2013
Probability in Genetics
• Mendel, mathematician and scientist, was the first to recognize that the principles of probability can be used to predict the results of genetic crosses.
• Mendel carefully counted the offspring from every cross. Every time he crossed two hybrid plants (Tt) ¾ of the F1 generation would be tall and ¼ would be short.
• Mendel could say that the probability of such a cross producing a tall plant was 3 in 4 chances.
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mendelonline.notebook
November 01, 2013
PUNNETT SQUARES
• Used to relate probability to genetics.
• Shows all of the possible outcomes of a genetic cross • Allows you to determine the probability of a particular outcome (possible genotypes in the offspring)
• You can predict probabilities of the offspring IF you know the genotypes of the parents.
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mendelonline.notebook
November 01, 2013
Monohybrid Crosses
• You need a 2x2 box because each parent can produce two kinds of gametes or alleles for this trait.
• Once the boxes are filled in you have all of the possible genotypes for a particular trait and from that information you can determine the phenotypes.
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mendelonline.notebook
November 01, 2013
Dihybrid Punnett Squares
• 4x4 box because each combination must be considered.
• 2 traits per box
• See figure on page 186 in your book.
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mendelonline.notebook
November 01, 2013
Dihybrid Cross Punnett Square
• Example: What happens if a man who is heterozygous brown eyes AND heterozygous tongue roller has children with a woman who is homozygous blue eyes AND heterozygous tongue roller?? What is the chance that their child will be a blue eyed non­tongue roller?
• Due to Mendel’s Law of Independent Assortment, we must look at all possible allele combinations that can go into sperm and egg. We will use a method of distribution you may have seen in math class called FOIL. FOIL stands for First, Outside, Inside, Last.
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mendelonline.notebook
November 01, 2013
Dihybrid Punnett Squares
• 4x4 box because each combination must be considered.
• 2 traits per box
• See figure on page 186 in your book.
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mendelonline.notebook
November 01, 2013
Dihybrid Cross Punnett Square
• Example: What happens if a man who is heterozygous brown eyes AND heterozygous tongue roller has children with a woman who is homozygous blue eyes AND heterozygous tongue roller?? What is the chance that their child will be a blue eyed non­tongue roller?
• Due to Mendel’s Law of Independent Assortment, we must look at all possible allele combinations that can go into sperm and egg. We will use a method of distribution you may have seen in math class called FOIL. FOIL stands for First, Outside, Inside, Last.
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mendelonline.notebook
November 01, 2013
• Foil the following genotype: BbWw (F) _____, (O) ______, (I) _____, (L) ____
• Now we’re ready to tackle the example problem below.
• What happens if a man who is heterozygous brown eyes AND heterozygous tongue roller has children with a woman who is homozygous blue eyes AND heterozygous tongue roller??
• Dad’s Genotypes: ____________
_____________
• Possible Sperm Alleles: ______ or _______ or ______ or _______
• Mom’s Genotypes: _____________ _____________
• Possible Egg Alleles: ______ or _______ or _______ or _________
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mendelonline.notebook
November 01, 2013
What is the chance their child will be a blue­eyed non­tongue roller?
______ out of _____ or ______%
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November 01, 2013
Example 2
Dad: Homozygous blue eyes and heterozygous widow's peak. __________________
Mom: Heterozygous brown eyes and heterozygous widow's peak. _______________________
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