Laws of Inheritance (10.2)
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Notes:HL Genetics NotesFOR CORE See:
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Independent Assortment
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Dihybrid Crosses
(Punnett Squares with two genes)
Rules of a Punnett Square apply as they do in Monohybrid crosses. (See 3.4 for Review)
A few differences:
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Sample Problem:
In pea plants, yellow is dominant to green seeds and round is dominant to wrinkled seeds. If two plants both heterozygous for both traits are crossed, what are the genotypic and phenotypic ratios of the offspring?
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Polygenic Inheritance
Polygenic traits are traits controlled by more than one gene. They tend to occur on a continuum (such as hair and eye color). The more genes controlling them the more likely that the phenotype distribution will fit on a normal statistical distribution.
- Normal Statistical Distribution is a bell curve. The most middle variation being the highest frequency with the most dominant or most recessive being outliers. Try this site for more specifics
Linked Genes
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Thomas Morgan’s further investigations demonstrated that 2 traits in fruit flies (yellow body and white eyes) were inherited in the same way (X-linked) AND that they were NOT independent of each other.
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Linked genes are those that occur on the same chromosome in close proximity to each other.
Independent Assortment vs Linkage (toward end) Linked Genes- University of Utah |
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Recombinants and Non-Recombinants
Recombination is the term given to those genes that are "mixed up" during crossing over from the original parent genotypes.
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Chi-Squared and Linked Genes
Chi-squared is a statistical test that tests for the independence of two things by comparison of expected and observed ratios. In data where the expected numbers are unknown these numbers must be calculated (See Ecology 4.1). In genetics, these expected numbers come from Punnett Squares that predict what ratios should look like assuming Independent Assortment.
Two hypotheses always govern a chi-squared test:
Question: In the garden pea, yellow cotyledon color is dominant to green, and inflated pod shape is dominant to the constricted form. Considering both of these traits jointly in self-fertilized dihybrids, the progeny appeared in the following numbers: 263 green, inflated; 1774 yellow constricted; 279 yellow, inflated ; 420 green, constricted.
Two hypotheses always govern a chi-squared test:
- H0- the two events (genes) are independent of each other (Follow the Law of Independent Assortment)
- H1- the two events (genes) are NOT independent of each other (Do NOT follow the Law of Indepdent Assortment) and are therefore probably linked.
- Punnett Square of the cross, assuming independent assortment, with phenotypic ratios
- Calculator
- Observed Numbers of offspring
- Chi-Squared critical value table
Question: In the garden pea, yellow cotyledon color is dominant to green, and inflated pod shape is dominant to the constricted form. Considering both of these traits jointly in self-fertilized dihybrids, the progeny appeared in the following numbers: 263 green, inflated; 1774 yellow constricted; 279 yellow, inflated ; 420 green, constricted.