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Extensions to Mendelian Genetics

Image Credits: Biology (Campbell) 9th edition, copyright Pearson 2011, & The Internet Provided under the terms of a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. By David Knuffke. Modified by Eric Friberg.
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Eric Friberg

on 7 January 2015

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Transcript of Extensions to Mendelian Genetics

Extensions to Mendelian Genetics

Dominance
Revisited

Complex
Interactions


Linkage


Non-Mendelian Inheritance


Environmental
Influence

Incomplete Dominance
CoDominance
Multiple Alleles
More Than 2 Choices
Pleiotropy
The interactions among genes and the organism can be highly variable.
Variable Effects
Penetrance
Expressivity
Sex-Linkage
Gene Linkage
Epigenetics
Extra-nuclear genes
Epistasis
Polygenic Inheritance
When the heterozygous condition expresses a third phenotype, different from either homozygous condition.
When the heterozygous condition expresses both of the homozygous conditions simultaneously.
Example: Snapdragon Color
C C
w w
C C
w R
C C
R R
How does this alter ratios?
Example: Human A, B, & AB Bloodtypes
Very few traits are expressed in simple Dominant/Recessive relationships
The body makes anti-blood type antibodies for any carbohydrate that is not on the surface of it's own bloodcells.

These antibodies cause blood cells that have particular carbohydrates to agglutinate (clump)

Transfusion
with the wrong blood type will kill a person.
Many traits are controlled by more than two alleles or one locus
When there are more than two alleles for a gene.
When more than one gene contributes to the expression of a particular trait
Example: Human A,B, & O alleles
Three blood type alleles = six possible genotypes, and four different phenotypes.
Example: Human Skin Color
Interactions at three(?) different loci leads to a wider variation for the trait (distributed along a bell curve).
When one gene has multiple effects on the physiology of an organism
When two or more genes control the expression of a single trait.
Example: Sickle-cell anemia
Example: Coat color in Laborador Retrievers
One locus has wideranging effects on the entire physiology of the organism.
The E/e allele controls wheter pigment will be produced or not.

If at least one E allele is present, the B/b locus determines the color of the pigment produced

__ee - golden lab
bbE_ - chocolate lab
B_E_ - black lab
Some genes travel together
Some traits aren't on chromosomes
Linkage was first studied in the early 1900's by Thomas Hunt Morgan, a biologist who worked at Columbia University.

His lab (and its grad. students) laid the foundation for physically locating genes on chromosomes
Refers to genes that are on a sex chromosome.

Males will show sex-linked traits at a higher frequency than females (why?)

First discovered in fruit flies.
The white eye mutation is on the fruit fly X chromosome
The X chromosome in humans has many genes. The Y chromosome has very few
Map of Y Chromosome...
Different modes of sex determination (fruit flies are essentially like us)
Drosophila melanogaster
The "Fruit Fly": An awesome model organism for genetics (why?)
Refers to any genes that are on the same chromosome

Usually linked genes will travel together during meiosis.

When might they separate?
How linkage affects transmission of genes
Linkage Mapping
Since linked genes are only separated by crossing over events, the higher the frequency of recombinant offspring for any two genes, the greater the likelihood of there being a crossing over event between them.

This correlates to a greater distance between the genes on a chromosome.

This information can be used to generate a "
linkage map
" of genes on a chromosome.

"
Map units
": indicate distance between genes on a linkage map.

1 map unit = 1% chance of crossover (and recombinant offspring)
A linkage map of a Drosophila chromosome
Mitochondria and Chloroplasts have their own chromosomes.

These are inherited "
matrilineally
"

In males, the Y-chromosome is inherited "
patrilineally
"*
* not technically "Extra-nuclear", but still...
Varriegated Leaves: A trait conferred by a chloroplast gene
Mapping of Y-Chromosome mutations informs our understanding of historical human migration.
Refers to any heritable trait that is not determined by DNA sequences
Genomic Imprinting
Methylation
Some traits are only expressed if the gene is inherited from one parent or the other.
Methyl groups (
-CH3
) are added to DNA to inactivate it.

Some evidence to suggest this methlation pattern may be heritable, and affect phenotype.

Might explain phenomena like genomic imprinting
Big Questions
Make Sure You Can
"Dominance Hierarchy":
I = I > i
A
B

How does this alter ratios?
How does this alter ratios?
How does this alter ratios?
The likelihood of a genotype expressing a phenotype in an organism (or population)
The extent to which a trait is expressed in an organism.
Down's Syndrome is a genetic condition with total penetrance, but highly variable expressivity.
Several possible sex-linked transmission situations
The interaction of an organism's genome and its environment leads to complex patterns of gene expression.
The flower color of some species of hydrangea depends upon the pH of the soil (more acidic = bluer)
The coat color of Arctic Hare's changes on a seasonal basis.
How much of a human is a product of genes?
Summing It Up:
How are traits inherited?

How are traits expressed?
Note: Gigantificated!
Describe how each of the situations that is discussed in this presentation extends Mendelian understanding of genetics.

Use your understanding of these situations to analyze/predict data from experiments and genetics problems.

Provide examples of each of the situations described in this presentation.

Use data from crosses involving linked genes to determine the distance between linked genes on a chromosome (in map units).

Explain why most traits result from the interplay between an organism and its environment.

Illustrate how genetics illuminates the relationship between science and society.
Actual Size
Meiosis...makes gametes
Human Genetic Conditions
Autosomal

Pedigree
Analysis

Sex-Linked
Big Questions
Make Sure You Can
How are traits inherited?

How are traits expressed?

What is the relationship between genetics and human health?
Interpret pedigrees and use them to identify the mode of inheritance for particular traits.

Identify each of the examples in this presentation as belonging to particular modes of inheritance.

Explain how each of the examples discussed in this presenation affects human physiology at the molecular, cellular and organismal levels of organization.
A way of tracking genetic relationships
circles: females
squares: males
shaded: expresses phenotype
unshaded: no phenotype
An example of an genetically unhealthy pedigree (why?)
Modern genetic testing can detect genotype directly
A few points to keep in mind
Very few traits are inherited in strictly Mendelian patterns.

From an evolutionary perspective, traits that confer fitness in some circumstances can be detrimental in others.
Dominant
Recessive
Widow's Peak shows a dominant inheritance pattern
Examples of other Dominant Conditions
Achondroplastic Dwarfism
Acromegaly
Huntington's Chorea
One of several forms of Dwarfism in humans.

Affects cartillage formation and bone growth.

Lethal in homozygous condition
Peter Dinklage
One of several forms of Gigantism in humans.

Develops from an overactive pituitary gland

Can lead to serious health complications
Andre The Giant
Results from an abnormally large huntingtin protein.

Leads to degeneration of the nervous system.

Symptomatic and lethal in middle age.
Woody Guthrie
Attached earlobes show a a recessive inheritance pattern
Examples of other Recessive Conditions
Albinism
A mutation in the melanin production pathway.

Can be associated with vision problems, increased photosensitivity.

Seen in many different animals
Phenylketonuria (PKU)
An inability to digest the amino acid phenylalanine (common in many foods and artificial sweeteners).

Untreated can lead to mental retardation.

Can be managed with diet regulation.
Cystic Fibrosis
A defect in chloride ion membrane channel proteins, leads to a buildup of stick mucus

Has many symptoms, particularly in the respiratory and digestive system.

Must be intensively managed

Death usually by age 50.
X-Linked Recessive
X-Linked
Dominant

Y-Linked
Most common sex-linked inheritance pattern in humans.

Tend to show up in males due to hemizygosity.
Hemophilia
A defect in the proteins involved in blood clotting.

2 different forms are X-linked recessive conditions.

Can lead to death from hemorrhage (not typical in modern times).
Red-Green Color Blindness
A defect in the proteins that detect particular wavelengths of light.

Harmless...unless you want to be a pilot.
Not particularly common...

...but make sure you can spot it on a pedigree
Even less common than X-linked Dominant (the Y chromosome has almost no genes on it).
Duchenne Muscular Dystrophy
Due to an abnormality in the dystrophin protein.

Early onset muscle degeneration disease.

Symptoms appear by age 6, terminal by mid-20's
Inheritance of Hemophilia in a European Royal Lineage.
The Dystrophin protein is encoded in the longest gene in the human genome (2.4 MB)
An example of a vision test given to children to determine color-blindness.
Full transcript