BIOL 310 | Chapter 7

Gene linkage

2 genes on the same chromosome (but close together) so they are inherited together during meiosis. Don’t independently sort.

Linkage groups

the chromosomes that contain genes that are linked together and inherited as a unit.

Genes that are far apart

likely will independently assort from one another

Crossing over occurs during 

Prophase 1

WITHOUT crossing over

linked alleles segregate togetherW

WITH crossing over

linked alleles can be reassorted

Non-recombinant cells are called

parental

Non-parental/recombinant cells

Cells that contain a combination of alleles NOT found on original chromosomes 

Most common genes

those with parental traits, but still had recombinants

It was proposed that crossing over was a physical exchanged, is this true?

Yes, Morgan confirmed that with his workp

Likelihood of crossing over between genes is:

proportional to the distance between them

Crossover between eye color and wing length genes: 

fairly common 

Crossover between body color and eye color genes:

uncommon

Double-crossover between genes:

Very uncommon

You can spot a double-crossover:

when you see the “middle” of the 3 genes as being different from the parental phenotype.
* also they’re normally the lowest number of recombinant events

Using Chi Square Analysis to determine if the outcome of a cross:

is consistent with linkage or independent assortment 

Independent assortment hypothesis predicts:

each phenotype has an equal probability of occurring

Law of Independent Assortment states:

two different genes randomly assort their alleles during gamete formation

How many classes can we expect based off the law of independent assortment?

2; nonrecombinant, and recombinant

What were the two relevant genes for corn in the crossing over experiment? 

• kernel color near knobbed end

• kernel endosperm texture near translocated end

Recombination of alleles resulted in what?

a physical exchange of DNA from one chromosome to the other

(colorless, waxy; colored, starchy)put

Genetic mapping / chromosome mapping:

finding linear order of linked genes in chromosome

Uses of genetic maps

• diagnosing human disease

• cloning of disease genes

• offspring prediction

How can genetic maps be made? 

• recombination analysis 

• molecular approaches, like sequencing

Recombination analysis estimates:

relative distance between linked genes, based on number of times crossover event occurs 

If genes are far apart:

many recombinant offspring

If genes are close:

very few recombinants

How are genetic mapping experiments accomplished?

Using a testcross

Map distance is found by dividing ________ by _________ and multiplying by 100

recombinant offspring, Total offspring 

1 map unit (mu) is equivalent to

1% recombination

The relationship between genes is determined by

only two genes at a time

Number of events should correlate with:

the distance between the genes

The closer the genes, the estimation of distance is 

more accurate W

We can order genes based on

their distance

A testcross is expected to yield a maximum of only ______ recombinant offspring

50%

Parental phenotypes occur

most frequently

Double crossover phenotypes occur

the least frequently

Single crossover phenotypes are the most

common

Product rule allows us to predict the likelihood of:

a double crossover

Mitotic recombination

when crossing over occurs in mitosis: pair of recombinant chromosomes that have a new combination of alleles

Twin spot

2 adjacent regions being different from the rest of the body and each other

What causes twin sopts?

single mitotic recombination within 1 cell during embryonic development