Genetic variation II

One more extension of Mendelian genetics

Morgan and Sturtevant – gene linkage


Morgan wanted to distinguish among the evolutionary theories of the Darwinists, neo-Lamarckists, and Mendelists such as de Vries

Methods – Drosophila genetics

Morgan began working with fruit flies (Drosophila melanogaster) in 1908 because he wanted a system for genetic analysis in which he could grow large numbers of individuals in small quarters quickly


Crossed white-eyed mutant male with red-eyed female

F1progeny all red-eyed
What hypotheses can you make about inheritance of eye-color?

How many genes?  Probably 1

How many alleles?  Can’t tell

What allelic effects?  Red allele dominant to white

Crossed the F1 sisters and brothers

F2 progeny
3 red-eyed:1white-eyed
Seems normal –
What hypotheses can we make about inheritance of eye color?

How many genes?  Probably 1

How many alleles?  2 in this cross

What allelic effects?  Red dominant to white

But, noticed something weird
All white-eyed flies were male!
What were ratios of sex and eye color?

Red-eyed flies were 50:50 male but white flies were 100% male

What hypotheses can we make about inheritance of eye color?

What would product law predict if sex and eye color were independently inherited?

Make a Punnett Square


½ red female:0 white female: ¼ red male:¼ white male

Sex and eye color do not follow product law

Thus, hypothesize that they are not inherited independently

Not independently segregating

Crossed F2 progeny and obtained white-eyed females
Bred them with white-eyed males to produce pure-breeding white-eyed line

Reciprocal cross:  crossed purebreeding white-eyed female with purebreeding red-eyed male

F1 progeny observed
½ red-eyed females : ½ white-eyed males  Weird!!!
Recessive trait expressed in males
Crossed F1 siblings with each other

¼ red-eyed female: ¼ white-eyed female: ¼ red-eyed male: ¼ white-eyed male


Sex inherited by modified Mendelian mechanism

Females are XX (homogametic)
Males are XY (heterogametic)

Eye color

Eye color inherited by single gene, two allele system, red dominant to white

But, sex and eye color not inherited independently

Eye color is linked to sex
Eye color gene on X chromosome

Males have only one eye color allele


Females have two eye color alleles


Ellen:  What about dosage???

Called sex linkage
Gene or genes for sex determination are on same chromosome (X) as gene for eye color

Check out sex-linked inheritance problem set on web:


More linkage

As Morgan and colleagues accumulated more mutants in their fly colony

They quickly discovered that linkage is not unique to X-chromosome

Found many linked traits on autosomes

Autosomes are all chromosomes that are not sex chromosomes


Brown/Red eyes, Thin/Heavy veins

Two traits with genes both linked on same autosome

Predict:  assort together

Phenotype frequencies:  1:2:1
Tightly linked

Tightly linked genes act like a single gene

In this case, like a single gene with codominant alleles

But, also found odd ratios

Example, yellow body and white eyes

Crossed yellow-bodied, white-eyed males x brown-bodied, red-eyed females

F1 progeny as expected for sex-linked trait
What phenotypes do you expect?

Crossed F1s to get F2s

What ratios might be expected?
9:3:3:1 if unlinked
But, they knew they were both sex-linked and therefore on same chromosome
Linked to one another
So, expected what outcome?
1:1:1:1 ratio

¼ female yellow, white : ¼ female brown,red : ¼ male yellow, white : ¼ male brown,red

Most progeny fit that description

But a few did not!

Non-parental types

Further, when Morgan’s group did other crosses, looking at other genes, they got different rates of non-parental type progeny

Raises questions

What mechanism allowed the linked genes to separate in meiosis?
Why did the rate of separation vary, depending on the traits being studied?

Answer to question 1:  what mechanism?

Morgan knew of cytological work by the Belgian cytologist, F. A. Janssens

In 1909 Janssens published La théorie de la chiasmatypie
Janssens called his discovery: chiasmatypie.
Nowadays we use the term crossing-over
Contradicted the opinions held at that time about the individuality of chromosomes.
The progeny’s chromosomes differ from their parental chromosomes.

Morgan postulated that the crossing over that Janssens observed created new chromosomal types in a few gametes

Leading to the non-parental progeny phenotypes

Crossing over between non-sister chromatids in meiosis can break up linked allelic combinations

Recombination: re-arrangement of DNA

Chromosomal cross-overs are associated with the genetic recombination of alleles

This results from breakage & reunion of homologous DNA molecules

Postulated mechanism

Holliday-Whitehouse Model

Nicking of complementary strands of paired DNA (figure 1)

Strand displacement

RecA protein causes "track jumping" (figure 2)
During branch migration, H-bonds reform between complementary strands (figure 3)

Heteroduplex formation & rotation

Two double-stranded DNA molecules are covalently joined
Molecular "chiasmata" can be visualized by EM (photo)

Resolution & reunion

Repair (ligation) of nicks forms two new recombinant DNA molecules (figure 4)

FISH of Recombinant chromosomes

Creates new combinations of traits

Important source of genetic variation

Describe recominant and parental progeny types

Undetected recombination

Calculating recombinant frequencies

What about the varying frequencies of non-parental types?

Morgan postulated that the distance between genes determined how frequently they would be separated by crossing-over

Genes close together would separate more frequently

Morgan, T.H. (1911). Random segregation versus coupling in Mendelian inheritance. Science 34:384

This idea was later applied by Alfred Sturtevant, then an undergraduate in Morgan's laboratory

Used the frequency of crossovers to generate the first genetic map, a map of six X-linked genes in Drosophila (Journal of Experimental Zoology 14: 43-59 (1913).

Different kinds of crossovers

Double crossover

Maximum frequency of recombination is 50%

Linkage and recombination

Tightly linked genes assort together

Like a single gene

Crossing over allows alleles to separte

Creating new allelic combinations
Recombinant progeny

Alters phenotype frequencies from expected

Intemediate between frequencies expected for independent assortment and linked genes
However, max rate of recombinants is 50%

Distance between loci dictates frequency of crossing over between them

Thus, dictates frequency of recombinant progeny

Can use altered phenotype frequencies to map loci

Assume equal rate of crossing over at each point along chromosome
But, this rate is not equal
So, map is not a measure of physical distance along chromosome
Not a physical map, but a linkage map

Can use larval salivary gland polytene chromosomes in Drosophila to map linkage map onto physical map

Polytene chromosomes
One chromosome, stretched out by manipulating photograph
Fine scale mapping of polytene chromosomes
Now can use genome sequencing to produce physical map for other organisms

Extra information:  Three-point test cross for mapping