Theoretical Genetics (3.4)
SECTION LINKS: |
Notes:HL Genetics NotesSL Genetics NotesHL Extension:
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Gregor Mendel and the Laws of Inheritance
Gregor Mendel is known as the Father of Modern Genetics
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Mendel's Laws of Inheritance
1. Law of SegregationNo two alleles of the same gene from one individual can be inherited together. The two alleles in a person a "segreated" or seperated from each other.
First Law (Segregation) |
2. Law of Independent AssortmentAlleles from two different genes are not inherited together, they are inherited independently of each other. Both genes alleles are segregated having nothing to do with the other, therefore they are inherited independently.
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3. Law of DominanceSome alleles overpower the expression of other alleles within an organsim. So the presence of a dominant (A) alele and a recessive (a) allele in the same organsims (Aa) results in the dominant phenotype always being expressed.
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Terminology
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Characteristic/ Trait: Some physical aspect that is coded for in DNA (ex. flower color)
Gene: a sequence of DNA that codes for a specific protein within the body. Leads to traits. Allele: a variation of a gene (ex. purple or white) The number of variations of a specific gene vary from two to dozens.
Recessive: an allele that is overshadowed by a different allele (identified with a lower case letter) Homozygous: Having two the same allele of one gene within one individual (AA or aa) Heterozygous: Having two different alleles of one gene within one individual (Aa) Phenotype: The physical characteristics of an organsim (ex. purple flowers Genotype: The genetic make up of the organsims, represented through letters (ex. AA, Aa, aa) |
Punnett Square basics
ONE letter represents ONE gene.
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Steps to a punnett square:
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Monohybrid Crosses
Monohybrid problems deal with a single gene with two alleles, one that is dominant other is recessive.
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Some specific disease to know:
Cystic FibrosisRecessive allele mutation of the gene CFTR on chromosome 7
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Huntington's DiseaseDominant mutation of the HTT gene on chromosome 4
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Co-dominant Crosses and Multiple Alleles with blood types
Mendel’s laws we now know are rather simplistic. We have since learned that inheritance is a lot more complicated than originally thought. Two of those ways are co-dominant inheritance and multiple alleles.
Practice Problems- and their Key Ms. Noller's Co-dominant Problems and answer key |
ABO blood type and an example problem.
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Sex-Linked Crosses
Between 1911-1914, Thomas Hunt Morgan noticed a peculiar pattern with fruit flies. This pattern did not follow Mendel's laws of his ratios. Around the same time, the chromosomal theory of XY determining sex plus Mendel's laws developed a mode of inheritance called Sex-linked genes.
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Some diseases to know (+ practice problems)
Red-Green Color BlindnessX-linked recessive disorder that affects the cone cells in the eyes limiting their ability to differentiate these two colors.
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HemophiliaAn X-linked recessive disorder that affects the ability of the body to blood clot
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Pedigrees (Family Trees)
A pedigree (or family tree) is a way of tracking human inheritance of traits. Because human generations occur relatively slow (every ~25-40 years), the only way to determine inheritance (dominant/ recessive/ x-linked etc) is by looking at how traits are carried through a family lineage.
Rules:
By analyzing how a specific trait is passed down through the generations, and using Punnett squares with predictive ratios, we can determine the method of inheritance.
Explanation 2 Explanation 3 Practice building and analyzing pedigrees Practice Problems + the answer key |
Analyzing a Pedigree
Dominant Traits
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Recessive Traits:
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Mutation and Nuclear Radiation
Most genetic disease are overall very rare and recessive. Those that have been identified generally run in specific families or specific people groups (example Tay-Sachs Disease)
- What is a genetic mutation?
- A change in the DNA or RNA code which causes problems with the folding of proteins
- Mutations can be caused by many factors including: sunlight, food, toxins, chemicals, radiation, etc
- The effects of radiation on life have been extensively study through the after affects of the Chernobyl nuclear disaster in Ukraine in April of 1986 and the after affects of the atomic bomb dropped on Hiroshima and Nagasaki in 1945.
- Effects include: increased birth defects, stillborn births, damage to thyroid glands, bioaccumulation of radiation in aquatic ecosystems.
- The scientific link between cancer and radiation exposure is limited but many still consider it a side affect of the exposure.
- Effects include: increased birth defects, stillborn births, damage to thyroid glands, bioaccumulation of radiation in aquatic ecosystems.
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Classical Genetics Simulator
The Classical Genetics Simulator is an easy to use, relatively inexpensive way of introducing students to classical genetics problems using "real" data. Most genetics experiments either require lots of equipment and money, or extensive amounts of time. In the IB program this is not something we often have time for. However, this simulator is straight forward and easy to use. It replicates real data and makes it easy to analyze.
Classical Genetics Simulator
CGS student instructions
Classical Genetics Simulator
CGS student instructions