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Mendel’s Genetics. Monohybrid Cross. Gregor Mendel. As discussed last class, he was an Austrian monk He worked at St. Thomas Monastery He studied both mathematics and botany. Why did he actually Succeed?. He chose the appropriate organism to study
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Mendel’s Genetics Monohybrid Cross
Gregor Mendel • As discussed last class, he was an Austrian monk • He worked at St. Thomas Monastery • He studied both mathematics and botany
Why did he actually Succeed? • He chose the appropriate organism to study • He designed and performed his experiments correctly • Analysis of the data was done properly
The Patient • The common pea plant also known as Pisum sativium • It was chosen for 4 reasons • It was commercially available • Easy to grow and mature • The sex organs are entirely enclosed in the flower • Different varieties had different traits
Purebred • What is a purebred organism? • It is an organism that has descended from ancestors of a distinct type or breed • Why was this important to Mendel and his pea plants? • This is because he used purebred plants to conduct all of his experiments
How did he make sure? • How did Mendel make sure that his organisms (pea plant) were pure bred? • He bred them as everyone else bred plants, and he looked at one trait at a time • He selected from the progeny, only the plants that had the trait that he was looking for • He then bred them together (F1 x F1) and then he continued this for several generations until no more of the other trait showed up
What happened Next? • He repeated this for all the other traits until he had pure bred plants with each trait.
The first of the Crosses • So what did he end up doing next? • He started to cross breed • He designated the parents as the “p” generations and crossed a true-breeding tall pea plant, with a true-breeding short pea plant • The offspring were of course the F1 generation and he also denoted them as Hybrids
That F1 Generation • What do you think he saw? • What was the phenotype of the plants? • All of the plants were tall • What is this type of crossing is called?
Cross? • Well what is the Greek word for a single unit? • Well, think back to the first unit, what did we call a single sugar • MONOSACCARIDE • So now what is the Greek word for a single unit? • MONO (Greek word for single or alone)
Creatively • Creatively the name given to this type of cross is Monohybrid • It is a single cross between two purebred plants giving us a hybrid species
That’s the Phenotype • We have now seen the phenotype • All of the F1 generations plants were tall • What do you think that the genotype must be of the offspring? • First we have to learn some more terms
Terms • Homozygous Dominant (TT) – Two alleles for a trait that are the same as a result of pure breeding • Heterozygous (Tt) – Having two alleles for a trait that are different • Homozygous Recessive (tt) - Two alleles for a trait that are the same as a result of pure breeding
The First Cross PF1 Tall Short
Lets back track • Yesterday we looked at the different combinations of alleles that an individual can have • We also talked about dominance and recessiveness • What did we say the dominant allele was? • What about the recessive allele
What were the parents? • The parents that was tall had a double dominant allele • The parent that was short had a double recessive allele • Both of these parents were homozygous
When he Crossed • When the parents were crossed, the F1 generation was completely heterozygous • They all expressed the dominant trait, but why? • This was because the dominant allele trumps the recessive allele
Mendelian Ratio • As Mendel completed the experiment, he found that 75% of the offspring of the F1 cross were the dominant (tall) and 25% were the recessive (short) • The Ratio of 3:1 is known as the Mendelian ratio
Based on observations, Why did this happen? • Each parent in his F1 Generation starts with two hereditary “factors.” One factor is dominant and the other recessive • The factors separate out in the and only one of the two factors contribute to the phenotype of the offspring
Why did this happen? • The offspring inherit one factor from mom and one from dad. If the dominant factor is present, it will be expressed, even if the recessive one is present • The recessive factors will be express only if the recessive factors are present
The first Law of heredity • His results gave rise to his first law The law of Segregation
The law of segregation • His law of segregation states that inherited traits are determined by pairs of “factors” • These factors segregate in the gametes (after meiosis) • Mendel did not know that his factors were actually genes, we know this today
What do they do for us? • Well, they are used by geneticists so that they can calculate the expected ratio (probability) and to suggest possible combinations of alleles in the offspring • They also tell us something about the phenotype (the appearance of a trait in an organism)
So we saw TT x tt • And for that cross we ended up having a ratio of? • All were Heterozygous (100% - 0%) • What about when we cross the F1 x F1 generations?
F1 X F1 • What’s the ratio when we are talking about phenotype? • 3:1 as well, three of the offspring will present the dominant gene while the fourth will present the recessive gene • What about the genotype ratio? • 1:2:1 (one homozygous dominant, two heterozygous, and one homozygous recessive)
Class Work • Read Pages 123 – 129 • Complete Questions P135, 1-6, 8-16