LECTURE 4: LINKAGE AND RECOMBINATION

Linkage

linear arrangement of non-allelic genes on the same chromosome.

Linkage

Phenomenon wherein genes do not assort independently and can be separated by crossing-over.

Linkage

If the number of genes exceeds the number of chromosomes __________ occurs.

Wild-type

the phenotype most commonly observed in the population

Mutant

the phenotype rarely observed in the population

Wild-type ≈ dominant;

mutant ≈ recessive

Which is the dominant and recessive phenotype?

Wild-type ≈ __________;

mutant ≈ _________

Sex-linked genes

These are genes located in sex chromosomes.

X-linked genes

These are genes located in the X chromosome

Y-linked genes

These are genes located in the Y chromosome

hemizygous

With X-linked recessive genes, males which carry the recessive allele (XᵃY) will express the trait even if there is only one copy, which is called?

carrier

With X-linked recessive genes, females which are heterozygous to the gene will not express the trait, which is called?

• What percentage of female offspring will express the trait?

  • 100%

• What percentage of male offspring will express the trait?

  • 0%

• What percentage of offspring will express the trait?

  • 50%

X-linked Dominant Inheritance

• What percentage of female offspring will express the trait?

  • _____________

• What percentage of male offspring will express the trait?

  • _____________

• What percentage of offspring will express the trait?

  • _____________

• What percentage of female offspring will be normal?

  • 100%

• What percentage of male offspring will be colorblind?

  • 100%

• What percentage of offspring will be carriers?

  • 50%

X-linked Recessive Inheritance

• What percentage of female offspring will be normal?

  • _________________

• What percentage of male offspring will be colorblind?

  • _________________

• What percentage of offspring will be carriers?

  • _________________

Y-linked Inheritance

holandric

Traits which are Y-inherited are called?

Y-linked Inheritance

holandric

Affected fathers will pass the trait to all of their sons.

Linkage

The tendency of genes to be inherited together because of the proximity of their loci in the chromosome

Linkage group

genes located on the same chromosome and are inherited together

Independently assorting genes

These are genes located in different chromosomes

Independently assorting genes

This will produce 50% recombinant types and 50% parental types in the test cross

Independently assorting genes

Independently assorting genes

Complete linkage

Linkage wherein:

genes in the same chromosome and are in close proximity; there is no chance for recombination

Complete linkage

Linkage wherein:

test cross will produce 100% parental types

Complete linkage

Linkage wherein:

Complete linkage

Linkage wherein:

Incomplete linkage

Linkage wherein:

Genes in the same chromosome but far from each other

Incomplete linkage

Linkage wherein:

Test cross → parental types > recombination types

Test cross always results in more parental types than recombination types.

Incomplete linkage

Linkage wherein:

Incomplete linkage

Linkage wherein:

Incomplete linkage

recombinant types are obtained through crossing-over.

• parental types >50%

• recombinant types <50%

Linkage group

physical association of genes on a chromosome

number of linkage group is equal to the n (haploid no.)

number of linkage group is equal to the ___ (______________)

Alfred Sturtevant, Herman Muller and Calvin Bridges

• students of Morgan in 1915

• phenomenon of crossing-over

• used linkage as tool for chromosome mapping

% recombination is used as a measurement of distance.

_______________ is used as a measurement of distance.

Genetic recombination

It is the production of new combination of traits/alleles not found on the parents

Crossing over

It is the process during prophase I in which non-sister chromatids of homologous chromosomes exchange DNA segments

Recombination frequency

It is the percentage of recombinant offspring

Genetic maps

These are ordered lists of genes along particular chromosome

Linkage maps

These are maps constructed from genetic recombination frequency data.

Linkage maps

This is a linear arrangement of non-allelic genes on the chromosome

% recombination

The distance between genes on linkage map is based on _____________

Sturtevant; distance

Linkage Maps

Alfred H. ___________ reasoned that recombination frequency is dependent on the _________ of genes on a chromosome.

farther; higher; higher

Linkage Maps

The _________ the two genes are, the _________ the probability of a crossover, hence the ___________ the recombination frequency.

recombination frequency; cM (centiMorgan)

Linkage Maps

One map unit = 1% ___________ = 1 ____ (__________)

1. Step 1: Identify parentals and double cross-over (DCO) events

   • Parentals (highest): abc= 1498 and ABC = 1506

   • DCO (lowest): Abc = 5 and aBC = 6

2. Step 2: Determine gene order (ABC? BAC? CAB?)

   • Determine two commonly adjacent genes in the parentals and DCOs and then place the third gene in between.

   • Parentals: abc and ABC

   • DCO: Abc and aBC

   • Commonly adjacent genes: B and C, b and c

   

Steps in Linkage Mapping

Produce a linkage map for the 3 genes A/a, B/b, C/c

Test cross: AaBbCc and aabbcc

coefficient of coincidence (cc)

The ____________ is a measure of strength of interference in linkage.

coefficient of coincidence (cc)

It is a measure of how much the DCO occurs in the population

Whats the formula for coefficient of coincidence (cc)

Interference (I)

It is a measure of how strong a crossover in one region interferes with the crossover in an adjacent region

Formula for Interference (I)

Interpretation of the Interference (I) value

Seeig that the value of I > 0.5, then the genes are likely to be very near to each other.

Given

DCO = 5 + 6

N = 3660

CO₁ = 0.0721

CO₁₁ = 0.1101

Solve for the coefficient of coincidence (cc) and the Interference (I), then interpret.

• Distance of genes to the centromere

  • closer to the centromere, less chances of crossing over

• Distance of gene to one another

  • closer to each other, less chances of crossing over

Interference

What are two factors that affect the probability of a crossing over event.

1. Genetic sex determination

2. Environmental sex determination

   • e.g. in marine worms

     • if free swimming at larval stage

       (female)

     • larval attached to female adult becomes male

     • due to the masculinizing hormones secreted by females
       

   • Another example: Coral reef fish (Labroides dimidiatus)

     • one male in several females

     • when male dies, the most dominant female will take over

     • if successful, there will be sex reversal in two weeks
       

3. Chromosomal sex determination

   • McClung 1900

   • association of sex characteristics with a particular chromosome

Different types of sex determination.

1. Specific genotypes

   • Neurospora

   • Chlamydomonas

     • In these organisms, the genotype mt⁺ produces individuals of the plus mating type and mt^- for the minus mating type and can only reproduce with each other.

2. multiple alleles

   • e.g. in Hymenopterans there are 9 alleles

     If all 9 alleles are heterozygous: female

     If all 9 alleles are homozygous: male

3. multiple genes

   • e.g. in Boniellia, 4 sex gene loci

     female: 7 to 8 female alleles

     male: 7 to 8 male alleles

     hermaphrodite: equal no. of male and female alleles (in this case would be 4 female alleles and 4 male alleles)

Different types of Genetic sex determination.

Specific genotypes

Different types of Genetic sex determination.

These organisms have a mating-type locus (mat in Neurospora and MT in Chlamydomonas), where individuals must have different alleles at this locus to successfully mate

In these organisms, the genotype mt+ produces individuals of the plus mating type and mt- for the minus mating type and can only reproduce with each other.

multiple alleles

Different types of Genetic sex determination.

• e.g. in Hymenopterans there are 9 alleles

  If all 9 alleles are heterozygous: female

  If all 9 alleles are homozygous: male

multiple genes

Different types of Genetic sex determination.

e.g. in Boniellia, 4 sex gene loci

female: 7 to 8 female alleles

male: 7 to 8 male alleles

hermaphrodite: equal no. of male and female alleles (in this case would be 4 female alleles and 4 male alleles)

Environmental sex determination

Different types of sex determination.

• e.g. in marine worms

  • if free swimming at larval stage

    (female)

  • larval attached to female adult becomes male

  • due to the masculinizing hormones secreted by females

Environmental sex determination

Different types of sex determination.

• Another example: Coral reef fish (Labroides dimidiatus)

  • one male in several females

  • when male dies, the most dominant female will take over

  • if successful, there will be sex reversal in two weeks

Chromosomal sex determination

Different types of sex determination.

• McClung 1900

Chromosomal sex determination

Different types of sex determination.

• association of sex characteristics with a particular chromosome

Barr body; Murray Barr; interphase

In the XX-XY system, chromatin body or ___________ (named after its discoverer, _________) is observed in the __________ nucleus of the XX females, such as that observed in humans, cats, and mice.

X-chromosomes

The number of Barr Body is one less than the number of ____________, which explains why Barr Body is not observed in the male cells.

Mary Lyon; Lyonization

__________ (1962) proposed that the Barr Body is an inactive X chromosome, and that any person, regardless of the number of X chromosomes has only one active X chromosome. This is also called ____________.

Mary Lyon

Who proposed that the Barr Body is an inactive X chromosome, and that any person, regardless of the number of X chromosomes has only one active X chromosome.

dosage compensation

The single active X chromosome explains why the genes on the X chromosome have the same expression in males and females (except for those genes that have something to do with sexual function).

Even though the females have twice as many X-linked genes as the males. This adjustment, which happens so that there is the same effective dosage in the two sexes, is called ________________.