Mendel and the Pea Plants The laws of genetics

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Introduction The cutting edge of science is genetics. From a few drops of blood scientist can test for all sorts of diseases and treat many of them. But the study of genetics is not as new as you would assume. In fact, the study of genetics was started in the 19 th century (1800s) by a monk named Gregor Mendel.

Mendel’s work  Mendel was not a famous scientist. Instead he was a monk and a lowly science teacher who was naturally curious. He became the father of genetics by accident.  Mendel started experimenting with pea plants for the fun of it. He did not set out to found a new field of study.

Mendel’s work  Mendel studied pea plants for two reasons He could control the plant’s pollination Many of the plant’s traits had two possibilities and were very easy to see.  He worked for seven years and performed thousands of experiments before he came up with his laws of segregation.

Mendel’s work  Mendel’s first experiments used two true breeding plants that differed by only one trait. This is called a monohybrid cross.  He observed that one of the traits seen in the true breeding parents disappeared in the first generation but reappeared in the second generation in a predictable ratio.  From these experiments Mendel made 4 hypotheses.

Mendel’s hypotheses  There are multiple forms of genes called alleles.  An organism has two alleles for each gene. They receive 1 allele from each parent. If the two alleles are the same, the organism is Homozygous. If the two alleles are different, the organism is Heterozygous

Mendel’s hypotheses  When only one of the two different alleles in a in a heterozygous individual appears to affect the trait, that allele is called the dominant allele. In such cases, the other allele that does not appear to affect the trait is called the recessive allele.  The two alleles for a character separate during the formation of gametes, so that each gamete carries only one allele for each character.

Pea characteristics  Mendel studied 7 characteristics of the pea plant. For each one he found a dominant trait and a recessive trait as his third hypothesis says.

Pea characteristics  Purple flowers are dominant over white flowers  Axial flower position is dominant over terminal flower position  Yellow seeds are dominant over green seeds  Round seeds are dominant over wrinkled seeds  Green seed pods are dominant over yellow pods  Inflated pods are dominant over constricted pods  Tall plants are dominant over dwarf plants

Probability of a monohybrid cross  When Mendel made a monohybrid cross he found that one trait disappeared in the first generation (the recessive trait) and reappeared in the next generation predictable ratio.  Mendel found that one plant out of four would have the recessive trait and that three plants out of four would have the dominant trait when he crossed the offspring of two true breeding plants that differed by the one characteristic being studied.

Looks versus Genetics  Something was different between the true breeding dominant plant and the first generation dominant plants. They looked the same but the first generation plants could produce a recessive plant and the true breeding plants could not. WHY?  The way a plant looks is called its Phenotype. Two plants can look the same but have different genetics like our Monohybrid example.  A plant’s genetics for a trait is called its Genotype. This is what alleles the plant has for the trait in question.

Looks versus Genetics How can you tell the difference?  Here is the next question. If a plant has the dominant Phenotype how can you tell what the Genotype is? The answer: You do a test cross.  To do a test cross you cross a true breeding recessive plant with the dominant plant in question and look at the offspring.

Test cross offspring.  If you have only dominant offspring the genotype for the plant in question is Homozygous dominant.  If you have dominant and recessive offspring the genotype for the plant in question is Heterozygous dominant.

Lesson Review  Now that you have read the basic principles of genetics let’s see how well you remember them  On the following slides are True and False questions about genetics. Click on the square beside the best answer for the question to see if you are right.

Lesson Review 1. The field of genetics started in the 1900s True False

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Lesson Review 2. Gregor Mendel was a monk, a teacher and the father of genetics True False

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Lesson Review 3. Gregor Mendel studied the pea plant because he could control the pollination and he could easily see the different traits True False

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Lesson Review 4. Monohybrid crosses examine plants that are exactly the same except for one trait. True False

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Lesson Review 5. There are multiple forms of genes called alleles True False

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Lesson Review 6. A homozygous plant has two alleles for a characteristic that are the same. True False

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Lesson Review 7. Mendel studied 3 traits of the pea plant. True False

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Vocabulary Allele - alternative form of a gene Dominant - descriptive of an allele in a heterozygous individual that appears to be the only one affecting a trait Genotype - genetic makeup of an organism; an organism's combination of alleles Heterozygous - having different alleles for a gene Homozygous - having identical alleles for a gene Hybrid - offspring of two different true-breeding varieties Monohybrid cross - mating of two organisms that differ in only one character Phenotype - observable traits of an organism Recessive - descriptive of an allele in a heterozygous individual that does not appear to affect a trait Testcross - mating of an individual of unknown genotype but dominant phenotype with a homozygous recessive individual

Introduction The cutting edge of science is genetics. From a few drops of blood scientist can test for all sorts of diseases and treat many of them. But the study of genetics is not as new as you would assume. In fact, the study of genetics was started in the 19 th century (1800s) by a monk named Gregor Mendel.

Mendel’s work  Mendel was not a famous scientist. Instead he was a monk and a lowly science teacher who was naturally curious. He became the father of genetics by accident.  Mendel started experimenting with pea plants for the fun of it. He did not set out to found a new field of study.

Mendel’s work  Mendel studied pea plants for two reasons He could control the plant’s pollination Many of the plant’s traits had two possibilities and were very easy to see.  He worked for seven years and performed thousands of experiments before he came up with his laws of segregation.

Mendel’s work  Mendel’s first experiments used two true breeding plants that differed by only one trait. This is called a monohybrid cross.  He observed that one of the traits seen in the true breeding parents disappeared in the first generation but reappeared in the second generation in a predictable ratio.  From these experiments Mendel made 4 hypotheses.

Mendel’s hypotheses  There are multiple forms of genes called alleles.  An organism has two alleles for each gene. They receive 1 allele from each parent. If the two alleles are the same, the organism is Homozygous. If the two alleles are different, the organism is Heterozygous

Mendel’s hypotheses  There are multiple forms of genes called alleles.  An organism has two alleles for each gene. They receive 1 allele from each parent. If the two alleles are the same, the organism is Homozygous. If the two alleles are different, the organism is Heterozygous

Pea characteristics  Mendel studied 7 characteristics of the pea plant. For each one he found a dominant trait and a recessive trait as his third hypothesis says.

Lesson Review  Congratulations You have finished the lesson.