BME 80H Lecture Notes - Lecture 7: Kinsey Scale, Haplotype, Single-Nucleotide Polymorphism
4 Jun 2018
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Lecture #7- Inheritance of Complex Traits (chapter 5 all)
I. Polygenic (quantitative) Inheritance: when more than one gene contributes
to a phenotype.
A. Mendelian vs Polygenic Traits (figure 5.1)
A cross of two true-breeding lines:
Mendelian traits (involve a single gene)
F1 show one of the parental traits
F2 phenotypes are distinct = discontinuous variation
• Qualitative = description of phenotype
Polygenic traits (involve >1 gene)
F1 show a phenotype intermediate to that of the parents.
F2 show a range of continuously varying phenotypes
Characteristics of Polygenic Inheritance
• Quantitative, measured on a scale of numbers
• Need large sample sizes so it is best to use population data.
• Population= group of individuals of same species in the same place at
the same time
• Not okay to mix different populations in the data
B. The Additive Model for Polygenic Inheritance (Handout, figure 5.5)
• Each dominant allele contributes an equal and additive amount to
the phenotype
• Recessive alleles contribute nothing
• Uses incomplete dominance
• Each gene has 2 alleles
• Assumes the genes are autosomal
• Does NOT include he environmental influence
• Makes the assumption that there is independent assortment (absense
of linkage)
C. Example cross involving a trait controlled by three genes: A, B and C.
Let’s say the trait is height and, for simplicity sake, let’s assume no
environmental impact.
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cross of two persons with medium height:
woman AabbCC x man AABbcc
AbC ABc
abC Abc
II. Multifactorial Traits: Interaction between a single gene or multiple genes
with the environment leads to the phenotype.
A. Mendelian and Polygenic Multifactorial traits:
Abc
Abc
AbC
AABbCc
4 Dominant alleles
AAbbCc
3 Dominant alleles
abC
AaBbCc
3 Dominant alleles
AabbCc
2 Dominant alleles
0
0.5
1
1.5
2
2
3
4
# Dominant alleles
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B. Epigenetics (“on top of genetics”)
• Addition of chemical groups (e.g. methyl groups) to the chromatin.
• Impacts gene expression
• Can have an effect on phenotype
C. Multifactorial Threshold Traits (figure 5.9, table 5.1)
• Polygenic
• Only people with genotypes above a certain threshold liability have
the condition
• Shows a discontinuous distribution in a population
• Relative risk decreases substantially with each degree of separation.
• Since there are fewer alleles in common
• Example conditions: Heart disease, cancer, Parkinsons, bipolar
condition, Schizophrenia, Alzheimers, Diabetes, other
D. Heritability: the contribution of genetic differences to phenotypic
variety in a population
genetic variance:
• Influence of genes on variation in a population
environmental variance:
• Influence of environment on variation in population
Measuring heritability involves comparing the expected
correlation coefficients with those observed
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Lecture #7- inheritance of complex traits (chapter 5 all: polygenic (quantitative) inheritance: when more than one gene contributes to a phenotype, mendelian vs polygenic traits ( gure 5. 1) F2 phenotypes are distinct = discontinuous variation: qualitative = description of phenotype. F1 show a phenotype intermediate to that of the parents. F2 show a range of continuously varying phenotypes. Let"s say the trait is height and, for simplicity sake, let"s assume no environmental impact. cross of two persons with medium height: woman aabbcc x man aabbcc. Measuring heritability involves comparing the expected correlation coef cients with those observed. Correlation coef cients (cc): fraction of genes shared by related individuals. Siblings, fraternal twins = approximately 50%, cc of 0. 5. Grandparent > grandchild = approximately 25%, cc of 0. 25. The members of a family share many environmental factors. How to separate the environmental in uences and genetic ones? solution: use twin studies! Comparing phenotypes in identical twins reared apart.
