Post on 31-Jan-2022
Intro Simple Epistasis Linkage Summary
Lecture 2.5: Exceptions to Mendelian Patterns
John D. Nagy
Scottsdale Community College
BIO 181, General Biology for Majors
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Outline
1 Introduction: A New Type of Pattern
2 Simple Non-Mendelian Traits
3 Epistasis
4 Linkage
5 Summary
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Example: Flower color in snapdragons
Red snapdragons have genotype AA, white snapdragons havegenotype aa, and pink snapdragons have genotype Aa.
Which allele is dominant, A or a?
Which trait is recessive?
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Example 1: Thalassemia
Inherited disorder of red blood cells; caused by incorrectsynthesis of hemoglobin.
Thalassemia minor: usually asymptomatic; sometimesslightly less hemoglobin with elevated RBC count.
Thalassemia major: much more severe; onset of anemia ininfancy or childhood leads to early death.
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Possible outcomes—Thalassemia
Knowing what you know, predict what we would see from thefollowing crosses:
1 Healthy × Healthy → ???.
2 Healthy × Th. minor → ???.
3 Th. minor × Th. minor → ???
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Possible outcomes—Thalassemia
Actual observations:
Healthy × Healthy → Always Healthy.
Healthy × Th. minor → 1/2 Healthy, 1/2 Th. minor.
Th. minor × Th. minor → 1/4 Healthy, 1/2 Th. minor,1/4 Th. major.
Genotypes:
TT × TT → TT .
TT × Tt→ 12TT + 1
2Tt.
Tt× Tt→ 14TT + 1
2Tt + 14 tt.
TT × tt→ Tt.
Incomplete dominance
The heterozygote looks like a blending of the two homozygotes.Resembles generative fluid.
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Example 2: ABO blood groups
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Example 2: ABO blood groups
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Possible outcomes—ABO blood types
Predict the outcomes of the following crosses:
A × A → ???
A × B → ???
B × B → ???
A × AB → ???
B × AB → ???
AB × AB → ???
A × O → ???
B × O → ???
AB × O → ???
O × O → ???
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Example 2: ABO blood groups
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Possible outcomes—ABO blood types
Observed outcomes:
A × A → A, O
A × B → A, AB, O
B × B → B, O
A × AB → A, B, AB
B × AB → A, B, AB
AB × AB → A, B, AB
A × O → A, O
B × O → B, O
AB × O → A, B
O × O → O
Explanation
Two things are going on here:
More than 2 alleles;
Co-Dominance: Genotype IAIB is both type A and typeB; since both are expressed in the heterozygote, both aredominant by the definition of dominance.
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Examples: ABO blood groups
Here are a series of examples:
IAi× IAi→ 14I
AIA (A) + 12I
Ai (AB) + 14 ii (O).
IAi× IBi→ 14I
AIB (AB) + 14I
Ai (A) + 14I
Bi (B) + 14 ii (O).
IAIA × IBi→ 12I
AIB (AB) + 12I
Ai (A).
Thought questions:
Can an AB individual have an O child? Explain.
Lupita has blood type A, her brother Luca has blood typeO and so does her mother. What could her father’sgenotype be? List all possibilities.
Show, using the proper cross, why parents who are typeAB and O should not expect to have a child with either’sblood type.
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Example 3: Duchenne Muscular Dystrophy
Inherited muscle weaknessand wasting disease.
Most common childhooddystrophy.
Progressive; usually leads toearly death.
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Inheritance pattern of DMD
Suppose 2 healthy parents have a child with DMD. Then wecan deduce the following:
The child is a boy.
The mother probably has a family history of DMD; thefather’s history is irrelevant.
Sex-linked traits
A trait is sex-linked if it is determined mainly by one gene onone sex chromosome but not the other. There are 2 types:
X-linked: Gene on the X chromosome, not Y.
Y-linked: Gene on the Y chromosome, not X.
NOTE: Sex-linked does not mean it can occur only in one sex.
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Details of an X-linked trait
We represent the 2 possible alleles as XD and Xd; the gene is Don chromosome X.
Phenotypes Genotypes
Healthy female XDXD or XDXd
DMD female XdXd
Healthy male XDYDMD male XdY
Carrier of a trait
An individual is called a carrier of a trait if they (i) do notexpress the trait, but (ii) can pass it on to their offspring.
How did we know that a DMD child of 2 healthy parents mustbe male?
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
X-linked trait examples
Solution: Here’s what we know:
The father is healthy ⇒ XDY .
Therefore, if the child were a girl, she must have inheritedthe XD allele, which means she’s healthy.
Since the child is sick, the pathological allele must havecome from mom.
Since mom’s healthy, her genotype must be XDXd.
Thought questions:
Suppose mom gets pregnant again. What is the probabilitythat they will have another child with DMD?
Suppose we know she’s pregnant with a girl. What’s theprobability now?
Oops! We made a mistake. She actually is pregnant withanother boy. What’s the probability now?
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Example 4: Coat color in mice
Agouti×Agouti
↓Approximately 9/16 were Agouti
Approximately 3/16 were Black
Approximately 4/16 were Albino
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Example 4: Coat color in mice
Five genes contribute to coat color in mammals (including mice): A,B, C, D, and S. In the following we consider only A and C.
Against a particular genetic background, A determines if yellow bandsare made–yellow banding is dominant. (Ignoring certain other alleles.)
Gene C determines albinism (no pigment is made); genotype cc isalbino, which is recessive. Black in this case is dominant.
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Example 4: Coat color in mice—Epistasis
Definition: Epistasis
Suppression of action ofone gene by anothergene (not an allele ofthe first gene).
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Example 5: Fruit fly body color and wing length
Wild type Ebony Vestigial wing
Body color: Gene B; Wild type dominant to ebony.
Wing length: Gene V g: Long wing dominant to vestigial.
What are the phenotypes of BbV gvg and bb vgvg?
What is the expected phenotypic ratio from the cross,BbV gvg × bb vgvg?
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Example 5: Fruit fly body color and wing length
T. H. Morgan
Thomas Hunt Morgan performed gotthe following results in an experiment:
He crossed BbV gvg × bb vgvg.
Of the 2300 offspring,
965 were Wild/Long wing.185 were Ebony/Long.206 were Wild/Vestigial.944 were Ebony/Vestigial.
Hypothesis: Body color andwing length areindependent.
Does this result support, contradictor say nothing about the hypothesis?
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Contradicts: the traits are linked
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Rare phenotypes caused by recombination
John Nagy Lec 2.5: Exceptions to Mendel
Intro Simple Epistasis Linkage Summary
Summary
Mendel’s principle is only the beginning of modern genetics!
Most gene systems do not obey Mendel’s simple principle,a fact Mendel knew.
Recessiveness is very rare; most alleles are expressed in theheterozygote, at least at the molecular level.
By convention, a given gene is represented by a symbol,and alternate alleles are denoted with variant forms of thatsymbol.Example: If the gene for seed shape in peas is denoted S,then its alleles are written S and s (not S and W ).
If an allele is recessive, it is often denoted with a lower caseletter or a ‘−’ (minus) sign.
Mendel’s principal contribution to modern genetics is therecognition that genes are particles, not fluids.
John Nagy Lec 2.5: Exceptions to Mendel