introduction to mendelian genetics monohybrid crosses
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Introduction to Mendelian Genetics Monohybrid Crosses. Lesson 4. Learning goals. Understand the basic terminology of mendelian genetics, not limited to (phenotype, genotype, homozygous, heterozygous, P,F1,F2 generation(s), dominant, recessive, alleles, gene - PowerPoint PPT PresentationTRANSCRIPT
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Introduction to Mendelian GeneticsMonohybrid Crosses
Lesson 4.
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Learning goals
• Understand the basic terminology of mendelian genetics, not limited to (phenotype, genotype, homozygous, heterozygous, P,F1,F2 generation(s), dominant, recessive, alleles, gene
• Solve monohybrid problems using Punnett square method
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Gregor Mendel (1822-1884)• Used pea plants to study
genetics, heredity and variation.
• Looked at 7 different traits that only had two variations.
• From his meticulous work he came up with many “key terms” and, more importantly, two generalizations that later became known as Mendel’s Laws.
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1677 Anton van Leewenhoek
• First discovered living sperm (also designed first microscope)
• Believed he saw a complete miniature person in the head of a sperm; homunculi
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1919thth Century British Century British
• From breeding ornamental plants, scientists believed in a blending theory of inheritance.
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• Charles Darwin – theorized that offspring have variations of their parents' characteristics, but he was unable to explain why (1870s).
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Gregor Mendel (1822-1884)Gregor Mendel (1822-1884)• Austrian monk
• Considered the father of genetics (now aka Mendelian Genetics)
• Why the pea plant?– grows easily and quickly – has seven traits that are easily
identified – it is easy to manipulate
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7 traits Mendel looked at7 traits Mendel looked at
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Mendel’s true breeding experiments
• Pea plants reproduce sexually, but usually self fertilize
• True breeding: organisms that exhibit the same traits generation after generation
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Mendel's Pea Plant ExperimentMendel's Pea Plant Experiment• Mendel selected true bred plants for different traits and
cross-fertilized them to see what would happen.
• True breeding: Individuals that only contain one variation of a trait and therefore can only pass this one variation on to future generations. We now call these individuals homozygous, or having two alleles that are the same.
• For example:– Tall x dwarf
• What do you think the offspring looked like?X
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• 100% of the offspring plants were tall! (Instead of a blending – where all plants would be medium sized...)
X
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x
• Mendel then let the F1 generation of offspring self-pollinate...
• F1 generation – (filial (son or daughter) generation) offspring from the cross (reproduction) of the Parent or P generation.
• this time most were tall, but some were short and ALWAYS 3:1 ratio
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Why?Why?• Each plant in the F1 generation carried an allele from the P
generation; a tall allele and a dwarf allele. They are what are called hybrids.
• Hybrids: Individuals that contain more than one variation of a trait and therefore can pass on more than one variation to future generations. We now call these individuals heterozygous, or having two alleles that are different from each other.
• Traits are dominant or recessive. If a dominant trait is inherited, it will be expressed.
• What is the dominant trait for height in pea plants?
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Mendel’s “Key Terms”• Phenotype: the appearance of traits in an organism (eg.
short or tall)
• Genotype: the specific genes that an organism has (which alleles, dominant or recessive)
• Every individual has a phenotype and genotype for every trait.
• The phenotype is easy to figure out, but occasionally the genotype is hard to determine, especially for dominant traits.
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Alleles: Alternate forms of a gene
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Genotype vs. Phenotype
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Which Allele is expressed?
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Can you tell the genotype of an individual by their phenotype?
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Mendel's Laws and PrinciplesMendel's Laws and Principles
• Law of Segregation: when any individual produces gametes, the copies of a gene separate so that each gamete receives only one copy of a gene and therefore only one allele for all possible traits.
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Principle of Segregation Demystified
The principle of segregation is explained by the behavior of homologous chromosomes at meiosis.
Segregation
Law of Segregation
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• Law of Independent Assortment: alleles of different genes assort independently of one another during gamete formation, so different traits are inherited independently of each other
• In plain English, this means that if you are able to be dominant for one trait, it will have no influence on whether you will be dominant for another trait.
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Punnett Square
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Punnett Square
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Dominant traits
This phenomenon can be explained by thefact that some ________ are dominant. Adominant allele is an allele that __________its phenotypic effect when present in theindividual.
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Recessive traits
9. A recessive allele is expressed only when _____ alleles are of the recessive form. The genotype of an individual showing a recessive phenotype must be ____________ for the recessive allele.
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Punnett Square
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Punnett Square
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Genotypic proportions
11. The genotypic proportions of the offspring from the cross of two heterozygous parents (Bb) are:
• BB __/4• Bb __/4• bb __/4
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Phenotypic proportions
12. The phenotypic proportions of the offspring are:
• Purple __/4 or __%• White __/4 or __%
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Monohybrid cross problem
Curly hair is dominant over straight hair. If amother is homozygous dominant for curlyhair and the father has straight hair, whattype of hair will the children have?
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Test Crosses
• When geneticists want to know if an individual is heterozygous or homozygous– Why? For breeding purposes
• Cross between unknown phenotype and homozygous recessive individual– (Pp or PP) x pp
• Pp x pp = 50% purple, 50% white• PP x pp = 100% purple