Complex Inheritance
Complex Inheritance
Learning Objectives
By the end of this class you should understand:
The difference between continuous and discontinuous traits
The different factors that produce a complex trait
The expected proportions of inheritance from polygenic traits
The threshold model of polygenic genetic disorders
The factors increasing risk of multifactorial disorders
The purpose of studying twins
The genetics of obesity, height, and intelligence
The Convenience of Mendel Pea plant color and missing
enzyme disorders are discontinuous traits Obvious difference between
phenotypes Most important traits are
continuous A range of possible outcomes What are some continuous
traits in humans?
Continuous Traits
Major examples today: Height Weight Skin Color Intelligence
Most traits in humans are continuous Multiple loci Multiple alleles per locus Environmental effects
Polygenic Traits
Some traits are controlled solely by genetics but at multiple loci Referred to as polygenic
traits Best examples are eye
color and hair color Wide range of traits but
not influenced by environment
(Not counting cosmetics...)
Why So Much Variation?
The various hair proteins have many different genes
Recall what monohybrid and dihybrid cross ratios looks like 1:2:1 1:4:6:4:1
With three and four alleles the possibilities stack up quickly
Thought Experiment
The additive model for polygenic inheritance suggests certain codominant alleles increase a certain trait AaBbCc, where A B C
increase height +3 inches A dihybrid cross between
AaBbCc x AaBbCc will produce a normal (bell) curve
Multiallele variation
Polygenic Phenotypes
Bell Curve in Offspring
Due to heterozygosity, offspring rarely match parents in unusual traits This phenomenon of a bell
curve of offspring is called regression to the mean
Even a dominant allele will only be expressed in 50% of offspring A result of the rare-allele
phenomenon
What About Environment?
Eye and hair color are independent of environment
Height can be influenced by diet, endocrine function, and disease Nature vs. Nurture
Even monozygotic twins are different heights if they are raised differently
Multifactorial Trait
Height is a multifactorial trait which means it is influenced by genetic and nongenetic factors
The best way to study whether a trait is multifactorial is to examine monozygotic twins “Identical twins”
Why Monozygotic Twins?
Monozygotic twins have essentially identical DNA except for stray mutations
All genetic factors are the same, so any difference is solely due to environment
Adopted monozygotic twins tell us a lot about how much influence the environment has
Dizygotic Twins
Dizygotic twins come from two different sperm-egg fertilizations
Identical nurture in the womb but are otherwise only 50% identical (siblings)
Still a secondary valuable source of information on heredity
Percent DNA-related
When a trait is shared by twins it is referred to as concordance When the trait is different between the two it is
called discordance
Heritability
The more genetic in origin a condition is, the more heritable it is said to be
A multifactorial disorder such as diabetes or cleft lip has a genetic component but may also be partially triggered by the environment Diabetes trigger: eating lots of
refined sugars
Threshold Model
Some disorders (such as cleft palate) appear discontinuous but are polygenic and multifactorial
A combination of genes puts the child at risk
Risk Factors for DiscontinuousMultifactorial Disease
A threshold model disease has the following risk factors: Consanguinity Previous affected child Severity of defect Higher frequency in one sex
Risk Factors
Consanguinity: related parents (first cousins) Previous Affected Child: parents most likely
carry many markers Severity of Defect: the more severe the defect,
the more extreme the genetic markers are Higher Frequency in One Sex: most likely there
are markers on the X or Y chromosome
Heritability of Multifactorial Traits
Obesity can be studied as a disorder Has many genetic factors but also heavily
dependent on environment/behavior
Heritability of Other Traits
Skin color is also a complex trait
Intelligence (indirectly measured through IQ) is also highly heritable “IQ” is a very poor estimate for
complexity of cognitive function
Heritability of IQ
The Return of Bioethics
Many people believed that race and IQ are strongly correlated Some people still believe this This is not true
IQ has also been rising Score of 100 (“average”) is a higher performance
than it was 50 years ago This is still a sticky topic for many people
See you Thursday!