john d. nagy

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


Outline

1 Introduction: A New Type of Pattern

2 Simple Non-Mendelian Traits

3 Epistasis

4 Linkage

5 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?



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.



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 → ???



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.



Example 2: ABO blood groups



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 → ???



Example 2: ABO blood groups



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.



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.



Example 3: Duchenne Muscular Dystrophy

Inherited muscle weaknessand wasting disease.

Most common childhooddystrophy.

Progressive; usually leads toearly death.



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.



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?



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?



Example 4: Coat color in mice

Agouti×Agouti

↓Approximately 9/16 were Agouti

Approximately 3/16 were Black

Approximately 4/16 were Albino



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.



Example 4: Coat color in mice—Epistasis

Definition: Epistasis

Suppression of action ofone gene by anothergene (not an allele ofthe first gene).



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?



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?



Contradicts: the traits are linked



Rare phenotypes caused by recombination



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 d. nagy

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