Chapter 5 - Linkage, Recombination, and Gene Mapping

Unlinked Genes Assortment

Assort independently in Meiosis 1, and genes are on separate chromosomes.

Unlinked Gene Ratio in F₁ Generation (Homozygous Dominant x Homozygous Recessive Parents)

Just 1 Phenotype (AaBb)

Linked Genes Assortment

Stick together, usually assort into the same gamete cell, genes are on the same chromosome.

Linked or Unlinked?

Unlinked

Linked or Unlinked?

Linked

Linked or Unlinked?

Unlinked

In a dihybrid cross, offspring phenotypes deviate significantly from the expected 9:3:3:1 ratio.

Linked Genes

A dihybrid individual produces four types of gametes in which the two parental types significantly outnumber the two recombinant types.

Linked Genes

A cross between a heterozygote and a homozygous recessive individual yields offspring in four phenotypic classes that deviate significantly from a 1:1:1:1 ratio.

Linked Genes

The two genes are on a single chromosome.

Linked Genes

In a dihybrid cross, offspring phenotypes occur in a 9:3:3:1 ratio.

Unlinked Genes

The two genes are on different chromosomes, or very far apart on the same chromosome.

Unlinked Genes

A cross between a heterozygote and a homozygous recessive individual yields offspring in four phenotypic classes exhibiting a 1:1:1:1 ratio.

Unlinked Genes

A dihybrid individual produces four types of gametes in a 1:1:1:1 ratio.

Unlinked Genes

Dihybrid Cross

Both crossed individuals are Heterozygotes

What ratio is expected in a Testcross with unlinked genes?

1:1:1:1

What ratio is expected in a Testcross with linked genes?

1:1, only two groups are seen.

Make up the largest groups of phenotypes/genotypes for linked genes.

Parental genotypes/phenotypes

Most offspring have this pictured genotype. What are the parental genotypes?

Parental genotypes would be b⁺ vg⁺/b vg and b vg/b vg.

How can you determine what the P generation looked like?

Parental genotypes/phenotypes (most common offspring genotypes/phenotypes)

What does the P generation look like?

The P generation would be 2 homozygotes → b⁺vg⁺/b⁺vg⁺ and b vg/b vg

Parental Groups

Offspring match the parental phenotype

Recombinant Groups

Offspring don’t match the parental phenotypes

Recombinants NEVER account for more than what percentage of the progeny?

50%

What process leads to recombinant groups?

Crossing over/Recombination! → Occasionally breaks the physical connections between genes on the same chromosome.

Recombination frequency of 50% resembles…

Unlinked genes → closer to a 1:1:1:1 ratio

Homologous chromosomes exchange…

Genetic Material

When does crossing over occur?

Prophase 1

How many sister chromatids usually recombine?

Only around 1/2

Compared probabilities of NCO, SCO, and DCO?

NCO>SCO>DCO → Crossing over does not always happen

More distance between genes leads to what?

Higher probability of crossing over between them and a higher frequency of recombination

Recombination frequencies show what?

How often two alleles on a chromosome separate and form new combinations during crossing over and all recombinants between two genes.

Genes close together: chance of recombination?

Lower

Genes far apart: chance of recombination?

Higher

Recombination Frequency Formula

RF = (# of recombinants/total # of progeny) x 100

Steps to Gene Mapping

1. Identify Parental and Recombinant Groups

2. Calculate RF

3. Draw linkage map

DCOs can only occur in crosses with how many genes?

3 or more

What type of crossover is the only kind that results in recombinants when mapping 2 genes?

SCO → DCOs require at least 3 genes to occur

Distance between 2 genes on a chromosome is indicated by what?

Recombination Frequency = the distance in map units (m.u.) between two genes

How to draw a Linkage Map?

Find the DCO groups (2 smallest)

Figure out which gene is the middle gene (which one flips?)

Draw the linkage map with flipped gene in the middle

Calculate the RF of the first two genes and the RF of the second two genes. Use the recombinant groups of the two genes (whichever gene combos don’t match the parental groups)/total # of progeny x 100.

RF/2 gives what?

The RF for each group of a pair.

DCO

Middle Gene exchanged, outer genes are paired like parentals.

Creating one recombinant type (in Diploid organisms) creates…

The opposite recombinant type, because Diploid organisms have pairs of chromosomes.

Interference

Happens when one crossover reduces the likelihood of another crossover on the same chromosome. Why there are fewer observed DCOs than expected

Interference Formula

1 - coefficient of coincidence

Coefficient of Coincidence

Observed DCO/Expected DCO

If observed DCO = expected DCO

Interference = 0

If no DCO occurred…

Interference = 1

Expected DCOs

Probability of one cross AND the other cross (RF x RF)

Observed DCOs

(#/Total) x 100

True or False: Genetic Linkage doesn’t always correlate to physical distance

True, because the frequency of crossing over is not uniform across a chromosome.

Chi-Square Test is used for…

Determining if genes are linked or unlinked

In Chi-Square analysis, if genes are not linked, the observed ratios…

Do no significantly deviate from the expected ratios

Chi-Square Formula

X² = ∑(O-E)²/E

Degrees of Freedom # is equal to…

1 minus the number of groups

Typical two point crosses have how many groups? And what Degrees of Freedom?

4 groups so DF = 3