Quantitative Inheritance: Continuous Traits

• Quantitative inheritance involves traits with continuous variation, categorized into:
• Continuous traits involving gradation from one phenotype to the next, such as growth rate, crop yield, and weight.
• Meristic traits, where phenotypes fall into discrete, integral categories, such as the ability to roll the tongue or litter size in cats.
• Threshold traits, which have only 2 or a few phenotypic classes, but are determined by multiple genes and environment, requiring individuals to surpass a liability threshold for expression. Diabetes, schizophrenia, and certain cancers are examples.

Continuous traits:

• Do not fall into discrete categories.
• Values follow a normal distribution, also known as a Gaussian distribution or bell curve.
• Characterized by the mean (mid-point) and variance (width) of the distribution.
• Standard deviation is the square root of variance and measures the curve's width.

Meristic Traits:

• Mendelian traits are discontinuous traits.

Threshold Traits:

• It has an underlying quantitative distribution.
• The trait appears only if a threshold is crossed.
• Only those individuals exceeding the threshold on the liability scale will express the trait.
• All factors influencing a polygenic disorder, whether genetic or environmental, can be considered as a single entity known as liability.
• Liabilities of all individuals in a population form a continuous variation.
• Curves for relatives are shifted to the right relative to the closeness of their relationship to the affected index case.
• Liability is not directly observable.

Consequences of Liability Model:

• The incidence of the condition is greatest among relatives of the most severely affected patients.
• The risk is greatest among close relatives and decreases among distant relatives.
• The risks for other relatives are increased if there is more than one affected close relative.

Multiple Genes:

• Polygenic trait: Is one whose phenotype is influenced by more than one gene.
• Polygene is any group of non-allelic genes with small quantitative effects that produces a wide range of phenotypic variation.
• It is also called multiple factor or quantitative gene.
• A polygenic trait involves several, but not an unlimited number of genes.
• The loci act in concert in an additive fashion.
• The phenotype results from the interaction of the genotype and the environment.
• Human traits that exhibit a normal distribution are not entirely polygenic.

Regression to the Mean:

• Most offspring of extreme parents are more average than their parents.

Statistical Analysis of Quantitative Traits:

• Segregating populations exhibit a continuous distribution for quantitative traits and cannot be analyzed using traditional Mendelian genetic techniques.
• They are described by statistical parameters.
• Variance = s² = Σ (xi - x)² / (n-1)
• Standard Deviation = s = √s²

Genetic sources of variation:

• They can be divided into several subcategories, including additive variance (VA), dominance variance (VD), and epistatic variance (VI).
• These subcategories yield the total amount of genetic variation (VG) responsible for a particular phenotypic trait: VG = VA + VD + VI
• Additive variation (VA) represents the cumulative effect of individual loci.
• Dominance variation (VD) represents interaction between alleles.
• Epistatic variance (VI) represents interaction between alleles at another loci.
• The overall mean is equal to the summed contribution of these loci.
• If a trait is controlled by a dominant allele, both homozygous and heterozygous individuals will display the same phenotypic value.

Components of Phenotypic Variance:

• All instances of phenotypic variance (VP) within a population are the result of genetic sources (VG) and/or environmental sources (VE).
  • VP = VG + VE
• It's the sum of the variation in additive (A), dominance (D), gene-interaction (I), environmental (E) and gene-environment interaction (GE) effects:
• VP = VA + VD + VI + VE + VGE
• Meaning:
• VA = Expression of a trait IS NOT controlled by the other allele at the locus.
• VD = Expression of a trait IS affected by the other allele at the locus.
• VI = Expression of a trait is affected by alleles at another loci.
• VGE = A given genotype is superior to another in one environment (differential local adaptation).

Heritability

• Estimating the amount of phenotypic difference in a trait is due to genetic variance but not absolute.
• Estimating the amount is due to non-genetic environmental factors.
• Broad-sense heritability: h² = VG/VP
• Narrow-sense heritability: h² = VA/VP

Heritability Problems:

• Two inbred lines of beans are intercrossed and variance must be V, The F, is selfed. F. variance must be V or VP = VG + VE.
• Calculate V = 6.1 - 1.5 = 4.6 and broad heritability is h² = VG/VP = 4.6/6.1 = 0.75 or 75%
• Experiment with Tribolium Body length shows a with a mean of 6 mm. Calculate the heritability in the narrow sense:
• Calculate the selection differential or VP: 9- 6 = 3 mm
• Calculate the section response or VA: 7.2 - 6 = 1.2.
• h² = VA/VP = 1.2/3 = 0.4 or 40%

Heritability:

• h² indicates the proportion of variability passable from parent to offspring.
• h² = 0 means no phenotypic variance comes from genetic differences.
• Offspring do not easily resemble parents in this trait.
• h² = 1 indicates variation is due to genetic differences in offspring.
• Offspring will easily resemble parent in this trait.

Heritability Facts:

• Its value can range from 0 (no genetic contribution) to 1 (all differences on a trait reflect genetic variation).
• It does NOT apply to individuals but to populations.
• All living things differ in many ways, both between and among each other.
• The gene must express in an environment and the environment has to act on the genotype
• A heritable trait isn’t always adaptive.
• No demonstration that genes are irrelevant; rather in the population, no genetic variation exist or that population developed were such that different genotypes had the same phenotype. A high heritability does effect a trait within its environment. Its heritability varies within sets and populations.
• It cannot be taken from one population and to another. Non-heritable traits: Language or where u worship the church. Twin studies:
• Differences between MZ twins are likely environmental while DZ twins may be from environmental and genetic differences.
• High heritability: MZ act very similar and DZ twins may be less similar
• Low heritability: MZ aren’t very similar to fraternal twins.
• Complete heritable: MZ twins correlation near 1 while DZ are near 0.5