September 28, 2004

More about sex linked genes

We went over a couple more problems about sex-linked traits:

(See Fig. 11.13b Pedigree of Queen Victoria (III-2) and her descendants, showing the X-linked recessive inheritance of hemophilia)

Question:

In 1995, a sixty-three year old man named Eugene Romanov, a resident of the former Soviet Union, turned up. He shared both the disease and last name; claimed to be the grandson of Anastasia, daughter of Czar Nikolas II.

(Said Anastasia (whose body was never found) was raised by a farmer, and later she married a nephew of her adopted parents and had a daughter, Eugene's mother.)

According to Eugene's argument, determine the hemophilia status of his mother, father, grandmother and grandfather.

Draw the pedigree corresponding to his story, and figure out how to determine who might be a carrier of the hemophilia gene.

Is his story plausible?

Another Question

White eyes (w) is a recessive gene on the X chromosome of Drosophila melanogaster and ebony body (eb) is a recessive gene on an autosome.

A true breeding white-eyed female with normal body is crossed to a normal-eyed, ebony male (to make an F1).

How would you notate this cross?

What is/are the phenotypes of the F1?

Now the F1 are intercrossed to make an F2 generation.

What are the expected phenotypes?

We did this one in class. The secret is to "divide and conquer". Solve for the phenotypes of each locus separately and the multiply the probabilities for the two loci. The autosomal locus is the same as before. For the sex-linked locus you have to keep track of males and females separately.

Incomplete dominance

Heterozygous snapdragons are pink

RR = red, Rr = pink, rr = white

Cross Red x white

F1 = ??

F2 = ??

Nothing fancy here. It simply means that heterozygotes are intermediate and three phenotypic classes are possible.

Make sure you understand why the F2 will have a 1:2:1 ratio

Codominance means both alleles are expressed

Example: blood groups

Degrees of Dominance

A single gene can show different kinds of dominance, depending on the scale at which you look

Example: Smooth/Wrinkled peas is a result of starch production in the peas.

(Starch grains absorb water which makes the pea swell. When too little starch is present the peas don't swell and look wrinkled).

Whole plant level: resessive
- both homozygous dominants and heterozygotes look smooth.
Physiological level: Incomplete dominance
Heterozygous plants have less starch than homozogous smooth.
- It is still enough to make smooth seeds, but close analysis would show that heterozygotes are in fact intermediate.
Molecular level: co-dominant
- If you look at gene sequences, you can see that both alleles are present in heterozygotes; i.e. they are codominant at the molecular level.

This is true of almost all genes. At the molecular level any allele is codominant. Thus dominance is not an intrinsic property of genes. Instead it is just a shorthand way of saying that we can't distinguish the different gene effects when we look at the organism.

Multiple alleles

In a large population, lots of genetic variation is present.

Most of it is not expressed; occasionally you find a mutant with obvious phenotype.

Still, well studied genes show lots of variation (e.g. sickle cell: you book gives a table of about 15 different alleles in Chapter 4).

How does that affect inheritance?

Example with multiple alleles: Rabbit coat color

Variation at a single locus

C>c^ch>c^h>c (normal, chinchilla, Himalayan, albino)

Different defects in the pigment gene

What do you see in these crosses:

normal x albino?
Chinchilla x Himalayan?

Composite cross:

Normal x Himalayan

and

Chinchilla x albino

Now cross the two F1s

What are the phenotypic ratios in the F2?

We solved this cross in class. The trick is to realize that each individual rabbit still only has two alleles of the gene. Figure out what gametes mom and dad can make, then combine them to figure out the genotypes of the F2. Then use the dominance relationships (above) to get the phenotypes.

See http://home.pacbell.net/bettychu/genetics.html for more about rabbit color genes

Epistasis

Gene interactions may modify the phenotypic ratios

Example:

Agouti, albino and black mice: two genes give only three F2 phenotypes

(A-/C-, --/cc, aa/C-)

This is because cc blocks all pigment, so it doesn't matter what the genotype at A is.

Your book says there is also a third interacting gene. Black (B) is dominant to brown (b). What would you expect from the trihybrid cross

AaBbCc x AaBbCc?

Another example

Black Lab x Yellow Lab

Two genes:

B determines types of pigment (B=black, b=brown or chocolate)

E determines deposition of pigment on hair (E = pigment on hairs, e = none)

Cross BBEE x bbee

F1 = ??

F2 = 9:4:3 (black:yellow:brown)

e is epistatic to B and b

Notice that both of these examples are cases where knocking out a gene early in a metabolic pathway blocks the expression of later genes (i.e. if they can't make any pigment in the first place, genes that affect the patterning of color don't matter).

Sets of genes in a common metabolic pathway often show epistasis.

Redundant genes

Fruit shape in Shepherd's Purse

Most plants have triangular fruits; occasionally you find a plant with round fruits.

Cross Round x Triangular

F1: all Triangular

F2: 1/16 round, all the rest triangular

Propose a mechanism to explain those results

Epistasis: things to remember:

The genes are inherited just as before, and the genotypic ratios in the F1 and F2 are just the same.

The interaction of gene products can affect the phenotypes, but the genes are still genes, following the same rules.

Don't try to memorize all of the different ratios (12:3:1, 9:6, etc). Instead, relate them back to combinations of the familiar 9:3:3:1

Here are two final examples:

White squash x green squash

F2 gives 12:3:1 white, yellow, green

Why?

White flowers in morning glories

Anthocyanin pathway has several different steps that can be blocked

Determines white/red/purple flowers

Red vs Blue determined by P or p. Blue is dominant.
White vs colored: determined by A or a, (chalcone synthase gene) which is the very first step in the pathway. If no chalcone is produced, no pigment can be produced at all.

White flowers can arise from defects in in several different genes (e.g. DFR and ANS)

Cross two purple morning glories and see 9:7 purple:white offspring-

How can that happen? Again, relate that back to the 9:3:3:1 ratio to figure out which genotypes must be in each phenotypic class.

Chapter 11 problems

2, 3, 4, 6, 12, 16, 17, 18, 21, 25