Human Genetics chapter 5 Flashcards
0
Q
Continuous variation
A
- A distribution of phenotypic characters, often controlled by two or more genes, that is distributed from one extreme to another in an overlapping, or continuous fashion.
- EX. Height illustrates a phenotype that falls into a series of overlapping phenotypic classes (5ft 3in, 5ft 3.5in, 7ft)
1
Q
Discontinuous variation
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- Phenotypes that fall into two or more distinct, nonoverlapping classes
- EX. Tall and short pea plants
2
Q
Complex traits.
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- Traits controlled by multiple genes, the interaction of genes with each other, and with environmental factors where the contributions of genes and environment are undefined
- Can be characterized as polygenic and multifactorial
- EX. Heart disease, obesity, and depression
3
Q
Polygenic traits
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- Traits controlled by two or more genes in which each contributes a small and equal amount to the phenotype
- Traits are usually quantified by measurement rather than by counting
- As the number of genes controlling a trait increases so does the phenotypic classes (less difference between them)
- As the number of phenotypic classes increases, the distribution of phenotypes becomes a normal curve
- Eye color is a polygenic trait
- Best analyzed in populations, not individuals
4
Q
Additive model for polygenic inheritance
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- EX. 3 genes with 2 alleles (Aa, Bb, Cc)
- Each dominant alleles make equal contribution to the phenotype, and recessive alleles make no contribution
- The effect of each active dominant allele on the phenotype is small and additive
- The genes controlling the trait are not linked, they assort independently
- The environment acts equally on all genotypes
5
Q
Multifactorial traits
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- Polygenic inheritance (controlled by several genes) + environment
- Genes involved are inherited in Mendelian fashion but the influence of the environment yields phenotypes that don’t show Mendelian ratios
- Each gene contributes a small amount
- Some multifactorial traits do not show a continuous phenotypic distribution or bell shaped phenotypic curve
- Multifactorial traits use indirect heritability estimates to be analyzed
- EX. Height, Results in wide distribution of phenotypes
- Threshold measurement, those above develop a disease (evidence comes from family histories)
6
Q
Regression to the mean
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- In a polygenic system, offspring of parents with extreme differences in phenotype tend to exhibit a phenotype that is the average of the two parental phenotypes
- Phenotype only, not genotype
7
Q
Epigenetics
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Reversible chemical modifications of chromosomal DNA (such as methylation of bases) and/or associated histone proteins that change the pattern of gene expression without affecting the nucleotide sequence of the DNA
8
Q
Genetic variance
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The phenotypic variance of a trait in a population that is attributed to genotypic differences.
9
Q
Environmental variance
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The phenotypic variance of a trait in a population that is attributed to differences in the environment.
10
Q
Heritability
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- An expression of how much of the observed variation in phenotype is due to differences in genotype.
- If heritability is high, (it is 100% when H= 1) the observed variation in phenotypes is genetic, with little or no environmental contribution.
- If heritability is low (H= 0.0) there is little or no genetic contribution to the observed phenotypic variation and environmental contribution is high.
- Finger prints used to measure heritability (first three months of development, polygenic and influenced by the environment for only a short period of time, twins have same genotype but different fingerprints)
11
Q
Correlation coefficient
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- Measures the degree of interdependence of two or more variables, the fraction of genes shared by two relatives.
- EX. A child receives half of his genes from each parent, this half set of genes corresponds to a correlation coefficient of 0.5.
- Identical twin genetic relatedness is 100% and the C C is 1.0 (all phenotypic differences in identical twins may be due to environmental factors)
- Parents, if unrelated, should also be genetically unrelated and have a C C of 0.0.
- EX. If the heritability value for a trait is 0.72 then 72% of the phenotypic variability seen in the population is caused by genetic differences in the population.
- Brother- sister CC is .50
- Grandparent - grandchild and aunt/uncle- niece/ nephew CC is .25
- Great grand parent CC is .12
12
Q
Monozygotic (MZ)
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Twins derived from a single fertilization involving one egg and one sperm; such twins are genetically identical.
13
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Dizygotic (DZ)
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Twins derived from two separate and nearly simultaneous fertilizations, each involving one egg and one sperm. Such twins share, on average, 50% of their genes
14
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Concordance
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- Agreement between traits exhibited by both twins (concordance if both twins have a trait, discordant if only one twin has that trait)
- If a trait is completely controlled by genes, concordance should be 1.0 for monozygotic twins and close to 0.5 in dizygotic twins
- Greater the Difference of concordance between MZ and DZ twins, greater the heritability
- Cleft lip for MZ and DZ twins is 42% to 5%, suggesting a genetic component but because it is so far from 100% environmental factors are also important in cleft lip cases
