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Discrete or discontinuous traits: traits occur in distinct Categories: Trait is there or it is not (examples: albinism, cystic fibrosis, Huntington’s disease) Mendelian inheritance, single genes, dominance, recessiveness Continuous traits: Distribution of phenotypes in the population varies along a continuum. Individuals differ by small degrees. (examples include height, blood pressure, reaction time, learning ability) Polygenic quantitative or multifactorial inheritance. Genes act additively.
TWO TYPES OF TRAITS
A
A
1
B
B
1 An allele with a prime 1 , adds one unit of measure to the phenotype
INHERITANCE OF CONTINUOUS TRAITS
A simple case with two loci, two alleles each
AABB AA^1 BB
AABB
1
AA
1 BB 1 A 1 A 1 BB AAB 1 B 1
A
1 A 1 B 1 B A 1 AB 1 B 1
A
1 A 1 B 1 B 1 short semi- short average semi- tall tall 0 1 2 3 4
Two loci, Two alleles Three loci, Two alleles
Why are the traits continuous? Because of the environment. In height, for example, nutritional differences can play a major role in variation For skin color, exposure to sun can modify the phenotype How do we know how much of the variability we see among people is due to genetic differences between them as opposed to environmental differences?
Heritability: the proportion of the variability among Individuals in a population that is due to genotypic differences among them or Vg/Vt
Vt = Ve + Vg
Vt = total variability Ve = environmental variability Vg = genotypic variability
What are the components of variability
in a group of people?
TWIN STUDIES TO MEASURE HERITABILITY Dizygotic (DZ) twins: two egg twins, same degree of genetic relatedness as normal siblings (25% genes in common) Monozygotic (MZ) twins: one egg twins genetically identical (100% genes in common)
Concordant: have the same trait Discordant: do not have the same trait
Twin 1 Twin 1 MONOZYGOTIC TWINS
CORRELATION?
Trait Heritability
- Body Mass Index (BMI) 0.50-0.
- Plasma cholesterol 0.37-0.
- Triglyceride level 0.68-0.
- Systolic Bood Pressure 0.17-0.
- Weight 0.42-0.
- Height 0.60-0.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
M F
Who has the greater probability of having a child with cleft lip and palate? A person with unilteral or with bilateral expression of the trait?
