mendelian genetics. how genetics began a monk named gregor mendel first studied how certain traits...
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Mendelian Genetics
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How Genetics Began
• A monk named Gregor Mendel first studied how certain traits could be passed on by studying his pea plants.
Heredity (or inheritance): the passing of traits to the next generation
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What Mendel did in his experiments:
• He found two plants that were “pure-breds” or true-breeding of certain traits.
• Those two plants were called the parent or “P” generation
• Once he pollinated those two plants, he waited until seeds were produced, planted them and then observed what characteristics they had
• The seeds from the parent plants make up the “F1” generation
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Traits in pea plants that Mendel studied
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• Allele: two forms of one trait (example #1 : seed color could be yellow or green example #2: plants could be tall or short)
• Dominant traits- the trait that shows through in the organism
• Recessive Trait- the trait that is “masked” (doesn’t show)
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Dominance
• Homozygous- an organism with two of the same alleles for a particular trait
• Heterozygous- an organism with two different alleles for a particular trait
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*The outward appearance of an organism does not always indicate which pair of alleles is present
Genotype: the organism’s allele pairsPhenotype: the observable characterisitics
(what the organism actually looks like)
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Mendel’s Genetic Laws
1. Law of segregation: the 2 alleles for each trait separate during meiosis
2. Law of Independent Assortment: a random distribution of alleles occurs during gamete formation.
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Practice: Monohybrid Crosses
• Tongue rolling is a dominant trait. Suppose both parents can roll their tongues and are heterozygous (Tt) for the trait. What possible phenotypes and genotypes could their children have?
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Practice: Monohybrid Crosses• A person with a widow's peak (Pp) is married to a
person with a straight hairline (pp), what percentage of their children will have a straight hairline?
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Practice: Monohybrid Crosses
Two people who are both heterozygous for the widow's peak trait are married. What percentage of their children will have a straight hairline?
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Review: Monohybrid Crosses
• Gregor Mendel is out working in his pea garden one day and decides to cross a plant that is homozygous for round seeds (RR). He crosses this plant with one that is heterozygous for round seeds (Rr).
• Create a Punnett Square that shows the possible genotypes that could be seen in the F1 generation.
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What if you wanted to look at TWO traits at one time???
You do a dihybrid cross:
• Example: Mendel crossed two pea plants, both were heterozygous for yellow color and round seeds (YyRr) and wanted to see the chances that the offspring would have green, wrinkled seeds?
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Complex Patterns of Inheritance
• Incomplete Dominance• Codominance• Mulitple Alleles• Epistasis• Sex Determination
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Incomplete Dominance• Heterozygous phenotypes is an intermediate phenotype
between two homozygous phenotypes
• Example: A red-flowered plant (RR) is crossed with a white-flowered plant (rr) produces pink flowered plants that are heterozygous (Rr).
• How do you know for sure you’re seeing Incomplete Dominance? Look for a “new color phenotype” that’s in the middle of the parents.
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Codominance
• Both alleles are expressed in the heterozygous condition
• Example: A white cow mates with a red bull. The offspring has a mixture of red and white fur
• How do you know? Both alleles are expressed
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Codomanice: Roan coloration
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Multiple Alleles
• More than two alleles can influence the trait.
• Example: Human blood groups (three forms of blood alleles: IA IB i). A, B, AB, or O
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Sex Determination
• There are two types of sex chromosomes: X and Y
• Individuals with two X chromosomes are female
• Individuals with an X and a Y chromosome are male
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Karyotype: Chromosomes are arranged by size and structure
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• Pairs 1-22 are called autosomes • The 23rd pair are called sex chromosomes: They control
what sex the organism will be
• What is the sex of the karyotype on this slide?