Genetic Recombination, Linked Genes, and Crossing Over

Summary

This script delves into Mendel's Law of Independent Assortment and its deviations, focusing on the concept of genetic linkage and recombination. It uses fruit flies as an example to illustrate how genes for eye color, body color, and wing type can recombine during meiosis, explaining non-recombinant and recombinant types. The script further explores recombination frequency and genetic distance measured in centimorgans, highlighting how close genes are on a chromosome and their likelihood to recombine. It concludes with the significance of linkage in gene mapping, which helps in understanding gene locations and distances on chromosomes.

Takeaways

• 🧬 Mendel's Law of Independent Assortment states that genes are assorted independently during meiosis, but this can be deviated by the concept of linkage.
• 🔍 Linkage is a newer development that helps understand deviations from the Mendelian rule and is crucial for gene mapping.
• 📚 The example of fruit flies is used to illustrate the concepts of recombination and linkage, focusing on traits like eye color, body color, and wing type.
• 🌟 Recombination is the process where genes on homologous chromosomes switch places during meiosis, leading to new combinations of alleles.
• 🔄 Nonrecombinant types are combinations of alleles that are passed on unchanged through meiosis, while recombinant types result from crossing over.
• 📏 Recombination frequency is the percentage of meiotic events where alleles recombine, indicating the likelihood of genes switching places.
• 📊 Genetic distance, measured in centimorgans (cM), represents the likelihood of genes recombining, with lower values indicating closer proximity on a chromosome.
• 🔗 Linkage is the likelihood that two alleles will be inherited together and is directly related to their physical distance on a chromosome.
• 🧭 Gene mapping uses linkage and recombination frequencies to determine the relative positions of genes on chromosomes.
• 🧬 The closer two genes are on a chromosome, the more likely they are to be inherited together and the less likely they are to recombine.
• 🔬 Test crosses are used to experimentally determine whether recombination has occurred by observing the phenotypes of offspring from a cross with a homozygous recessive organism.

Q & A

What is Mendel's Law of Independent Assortment?

Mendel's Law of Independent Assortment states that each pair of alleles segregates independently of any other pair of alleles during the process of meiosis, meaning that the inheritance of one gene does not affect the inheritance of another.

What is the concept of linkage in genetics?

Linkage refers to the tendency of certain genes to be inherited together because they are located close to each other on the same chromosome. It is a deviation from the Law of Independent Assortment when genes are not assorting independently.

What is recombination in the context of genetics?

Recombination is the process during meiosis where genetic material is exchanged between homologous chromosomes, resulting in new combinations of alleles in the offspring.

How are fruit flies used to explain the concept of recombination and linkage?

Fruit flies (Drosophila melanogaster) are used as a model organism in genetics because they have a short generation time and easily observable traits like eye color, body color, and wing type, which are genetically determined and can be used to demonstrate recombination and linkage.

What are the dominant and recessive phenotypes for eye color, body color, and wing type in fruit flies as mentioned in the script?

In the script, red eye is the dominant phenotype while brown eye is recessive. For body color, yellow-brown or tan is dominant and black is recessive. Regarding wing type, functional wings are dominant while vestigial wings are recessive.

What does it mean for an individual to be heterozygous for a trait?

An individual is heterozygous for a trait if it has two different alleles for that trait, one from each parent, which may result in a blend of the two phenotypes or the expression of one over the other.

What are nonrecombinant and recombinant types in the context of genetic recombination?

Nonrecombinant types are the combinations of alleles that are passed on without any recombination, maintaining the original configuration from the parent chromosomes. Recombinant types are the new combinations of alleles that result from recombination events during meiosis.

How is recombination frequency calculated and what does it indicate?

Recombination frequency is calculated as the percentage of meiotic events that result in recombination, rather than the nonrecombinant configuration. It indicates the likelihood of alleles being inherited together versus being separated during meiosis.

What is the significance of centimorgans in genetics?

Centimorgans (cM) is a unit of genetic distance that represents the likelihood of alleles being inherited together. It is used to measure the distance between genes on a chromosome and is inversely proportional to the recombination frequency.

How is the concept of linkage related to gene mapping?

Linkage is related to gene mapping as it helps determine the relative positions of genes on chromosomes. By studying the frequency of recombination between genes, researchers can create a genetic map that shows the arrangement of genes and their distances from one another.

What is a test cross and how is it used to determine recombination?

A test cross is a breeding experiment where an individual with an unknown genotype is crossed with a homozygous recessive individual. The phenotypes of the offspring can reveal whether recombination has occurred by showing new allele combinations.

Outlines

00:00

🧬 Mendel's Law of Independent Assortment and Linkage

This paragraph introduces Mendel's Law of Independent Assortment, which states that genes are assorted independently during meiosis. It explains the concept of linkage and recombination, which can cause deviations from this law. The use of fruit flies as an example illustrates how traits like eye color, body color, and wing type are inherited and how their genetic linkage can be studied. The paragraph also explains the process of recombination during meiosis, where genes can switch places on homologous chromosomes, leading to new combinations of traits in offspring. It discusses non-recombinant and recombinant types, which are crucial for understanding genetic linkage and deviations from Mendel's law.

05:03

🔍 Understanding Recombination Frequency and Genetic Distance

This paragraph delves into the concept of recombination frequency, which is the measure of how often two alleles recombine during meiosis. It uses the example of alleles for eye color (E) and body color (B) to explain how the frequency of recombination can indicate the likelihood of alleles staying together or separating. The paragraph introduces the term 'centimorgans' as a unit of genetic distance, which quantifies the probability of alleles recombining. It explains that the closer two genes are on a chromosome, the less likely they are to recombine, and vice versa. The paragraph also discusses the concept of linkage, which is the tendency of genes to be inherited together based on their proximity on a chromosome, and how it differs from genes on separate chromosomes.

10:04

🗺️ The Significance of Linkage in Gene Mapping

This paragraph discusses the importance of linkage in gene mapping, explaining how the understanding of recombination and linkage can help determine the location of genes on chromosomes. It highlights that genes that are closely linked are less likely to recombine and are therefore located closer together on the chromosome. The paragraph describes how linkage analysis has been instrumental in identifying the positions of various genes on human chromosomes over the past few decades. It also touches on the experimental method of test crosses to determine whether recombination has occurred, which is vital for understanding the genotype of an organism with a known phenotype but an unclear genetic makeup.

Keywords

💡Mendel's Law of Independent Assortment

Mendel's Law of Independent Assortment is a fundamental principle in genetics that states that the alleles of different genes assort independently of one another during gamete formation. In the video, this law is contrasted with the concept of linkage, which explains deviations from this law. The script uses the example of fruit flies to illustrate how alleles for eye color, body color, and wing type might be inherited together or separately, depending on their genetic linkage.

💡Meiosis

Meiosis is the process of cell division that results in four daughter cells, each with half the number of chromosomes of the parent cell. It is crucial for sexual reproduction. The script discusses how genes are assorted differently during meiosis, leading to the concept of independent assortment and, in cases of linkage, deviations from this pattern.

💡Gene mapping

Gene mapping refers to the process of determining the location of genes on chromosomes. The video explains that understanding recombination and linkage is essential for gene mapping, as it helps scientists identify the relative positions of genes and their distance from each other, which is measured in centimorgans.

💡Recombination

Recombination is the process by which genetic material is exchanged between different chromosomes during meiosis, leading to new combinations of alleles. The script uses the concept of recombination to explain how alleles that are not on the same chromosome can be inherited together, which is a deviation from Mendel's law.

💡Linkage

Linkage is the tendency of genes that are located close together on the same chromosome to be inherited together. The video script discusses how linkage is a newer development in understanding gene behavior and how it affects the likelihood of alleles being passed on together, which is measured by recombination frequency.

💡Centimorgans

Centimorgans (cM) is a unit of measurement used in genetics to express the distance between genes on a chromosome. The script explains that a lower centimorgan value indicates a higher likelihood that two genes will be inherited together, while a higher value suggests a greater chance of recombination.

💡Dominant and Recessive Phenotypes

In genetics, dominant and recessive refer to the expression of traits controlled by alleles. A dominant phenotype will be expressed if at least one dominant allele is present, while a recessive phenotype requires two recessive alleles. The script uses the example of fruit flies to illustrate this concept, with red eyes being dominant over brown eyes, and tan body color being dominant over black.

💡Heterozygous

Heterozygous describes an organism that has two different alleles for a particular gene. In the script, the example fruit fly is heterozygous for eye color, body color, and wing type, meaning it carries both dominant and recessive alleles for these traits.

💡Nonrecombinant and Recombinant Types

Nonrecombinant types are the combinations of alleles that are passed on without any exchange between homologous chromosomes, maintaining the original parental configuration. Recombinant types result from recombination events, where alleles are exchanged between chromosomes, leading to new combinations. The script explains these concepts using the example of fruit fly traits and how they can be inherited together or separately.

💡Test Cross

A test cross is a genetic cross between an individual with an unknown genotype and a homozygous recessive individual to determine the genotype of the unknown individual. The script mentions test crosses as a method to experimentally determine whether recombination has occurred by observing the phenotypes of the offspring.

💡Synapsis

Synapsis is the pairing of homologous chromosomes during the early stages of meiosis. The script refers to synapsis in the context of crossing over, where parts of the homologous chromosomes exchange places, leading to recombination.

Highlights

Mendel's Law of Independent Assortment suggests that genes are assorted independently during meiosis, leading to offspring with various genetic combinations.

Linkage is a newer concept that explains deviations from Mendel's law, providing insights into gene mapping and recombination.

Recombination and linkage are key to understanding variations from the expected patterns of inheritance.

The example of fruit flies is used to illustrate the concepts of recombination, showing how traits like eye color and wing type are genetically determined.

Heterozygous individuals serve as a basis for demonstrating how recombination can occur during meiosis.

Nonrecombinant types are those that maintain the original gene configuration through meiosis.

Recombinant types result from crossing over, where genes switch places on homologous chromosomes.

Recombination frequency is a measure of how often genes recombine during meiosis, indicating the likelihood of gene separation.

Centimorgans are units that describe the genetic distance between loci, with lower values indicating closer proximity and higher linkage.

The closer two genes are on a chromosome, the less likely they are to recombine, indicating stronger linkage.

Gene mapping utilizes linkage and recombination frequencies to determine the relative positions of genes on chromosomes.

Linkage is relevant only for genes on the same chromosome, as genes on different chromosomes have a 50% recombination frequency.

The concept of linkage helps in understanding the inheritance patterns and the physical arrangement of genes on chromosomes.

Experimental determination of recombination involves test crosses with homozygous recessive organisms to reveal genotypes.

The phenotypes of offspring from test crosses provide evidence of whether recombination has occurred.

Understanding linkage and recombination is fundamental to the field of genetics, aiding in the identification of gene locations.

Linkage analysis has been instrumental in the isolation and mapping of genes on human chromosomes over the past decades.

Transcripts

Mendel's Law of Independent Assortment

said that every single

Al was assorted to daughter cells

differently during the process of myosis

and so no genes were likely to be

transmitted to The Offspring together

versus with their Al separated uh along

their homologous chromosome and in order

to understand this topic of linkage

which is a newer development that's come

about since we've discovered things

about the process of meiosis and since

we've done Gene mapping it's really

important to understand this idea of

recombination and recombination and

linkage are something that help us

understand where deviations occur from

the mandelian rule of Independent

Assortment so to explain this we use uh

the example of fruit flies in fruit

flies eye color body color and the type

of wing that they have are all

genetically derived and the red eye is

the dominant

phenotype the the brown eye is the

recessive phenotype the yellow brown or

tan body color is the dominant body

color and the black body color is the

recessive and then dropa fruit flies can

have functional wings or they can have a

recessive phenotype known as vestigial

wings and those are essentially wings

that are ineffective for flying but

still have some Wing like structures and

for the purposes of this illustration

we're going to assume that they're all

on the same chromosome and that will

help us understand the nature of

recombination another thing that we'll

do is we'll say that this individual is

heterozygous for all three traits and so

what we have is the do dominant eye

color the recessive body color and the

dominant Wing type on one chromosome and

then on the homologue we're going to

have the recessive eye color the

dominant body color and the recessive

Wing type and so what re combination

means is that if we have these three

together what are the odds that these

three are no longer together what are

the chances of during this process of

crossing over let's just say that this

transfers over and these two pieces

switch what are the odds of us getting

uh capital E for the dominant eye color

and moving these pieces so that we get

the dominant body color as well so in

order to understand this these are the

nonrecombinant types or the

non-recombinant combinations of alals if

we have capital E lowercase b and

capital W or if we get lowercase e

capital B and lowercase w those have not

seen recombination those when those are

passed on through the process of meiosis

and these three are all found in the

same gamt that means that recombination

didn't occur the three that were on this

chromosome are the three that are being

passed on through the gamet however if

crossing over occurs and let's say we

get capital

e um let's say capital B and lowercase w

we can see that that means that this

part has remained together and the E has

switched from one of the chromosomes to

its H homologue that is a recombinant

because the capital E and the capital B

are not found together on the parent

chromosomes similarly if we get uh

lowercase e lowercase b and let's say

lowercase w what that means is that this

little component

crossed over and this component crossed

over here during that crossing over or

synapsis process and once again this is

a recombinant Arrangement so E capital E

being with capital B that's

recombination lowercase e being with

lowercase b that's another example of

recombination lowercase b lowercase e

being with capital W again that's

recombination because these things in

the parent weren't found on the same

Chrome but during the myotic process now

they found a way where they cross over

together and so recombination occurs

because of this crossing over process

and it relates to parts of the homologus

chromosome coming over and switching

places with parts of the other

chromosome and the way to understand

this idea of linkage is to look at the

frequency of recombination what are what

is the frequency that e and B will

recombine where you won't have a capital

E and a lowercase b but instead either a

capital E and capital b or a lowercase e

and lowercase b what is the frequency of

that happening so recombination

frequency is the likelihood in let's

just say 100 myotic events what is the

per percentage of those events that

we'll see recombination rather than

these two alals maintaining their

configuration so let's say with the E

and

B the uh let's say that this happens

93% of the time the capital E and the

lowercase b which is the way it's

arranged on the parental chromosome and

then the rec combination event let's say

uh capital e capital b or lower case

case e lowercase b those are the things

that happen 7% of the time so that's the

re combination frequency there is

7% and what that does is it it serves as

an indicator of How likely these pieces

are to switch over without both of them

moving together and so once you

understand recombination frequency you

end up with a percent value of the

recombinant type that shows up and that

will tell you something know as the

genetic distance the gene map distance

or the centimorgans centimorgans is a

unit that describes the likelihood of

things crossing over uh together versus

separately so the centimorgans tell you

what is the likelihood that these things

will recombine and so here with a 7%

recombination

frequency we have a 7 C

Morgan distance 7 centimorgans means

that 7% % of the time these things will

Rec recombine they will not be inherited

together this e and

B and instead the E and the B will be

inherited in a different Arrangement

than they were on the parental

chromosome one thing that becomes

evident is that The Closer they are the

more likely you are to have these large

chunks Stay Together there are always

some pieces that that cross over during

the this process but if two things are

very close to each other there's a much

greater chance that this will stay

together either this will pass over by

itself or it will not whereas if they're

further apart there's a much greater

likelihood of the crossing over events

segregating these two alals here and

causing them to be separate and so what

you'll see is that if you have a greater

recombination frequency that corresponds

with a greater distance along the

chromosome and the re combination

frequency will never be greater than 50%

if two genes are on opposite or are on

completely separate chromosomes then

their recombination frequency will be

50% they will have just as much a chance

of being inherited together as they will

have a chance of recombining and those

alals being inherited separately with a

new combination of things however for

Al's on the same chromosome we can come

up with a concept known as linkage and

linkage is a way of looking at the

likelihood that two alals will be

inherited together and linkage is

something you really are only looking at

when you have alals on the same

chromosome so let's just say for example

that the distance here is going to be

seven CTI Morgans and we'll say the

distance between e and w here might be

let's say 35

centimorgans what that tells us is that

e and w are a lot further apart on the

chromosome than e and B are and the

likelihood of them crossing over um is

uh crossing over separately and being

recombined it's inversely proportional

to distance and so two genes will be

highly linked for example this e and B

if their odds of recombining are very

very low they're 7% or something like

that if the odds of e and w in this case

the two dominant ones recombining into

something where we have Big E and

lowercase w or small e and uppercase W

if that odd of that happening is 35% so

in 35% of meiosis events you see

crossing over between the E and the W

it's less than 50% so that tells us yes

it's on the same chromosome but it's

further along the chromosome there's a

greater distance and so there's a far

greater chance of the E and W's

recombining at that frequency and so the

basic rule is the more closely linked

these Al are that means that they're

going to be closer on the chromosome and

they're less likely to recombine during

the process of meiosis linkage is

something that is only really relevant

for things that are on the same

chromosome because otherwise you're

looking at a 50% chance due to the fact

that chromosomes will align differently

on that metaphase plate and there's no

desire of them to be on one side or the

other and if you understand linkage and

understand that it's exactly it's

inversely proportional to distance the

closer they are the more more highly

linked they are then you will be able to

understand how this process works and

this is at the heart of something known

as Gene mapping and this is something

how we figured out where a lot of

different alal and genes are located on

our different chromosomes we did Gene

mapping where we did a ton of different

linkage studies trying to find the

recombination frequency of all of these

different alals and the ones that we

found out were linked we knew they were

on the same chromosome and if we found a

high degree of linkage that means they

were a lot closer on the chromosome and

so over the past 20 years or so we've

done a lot of linkage analysis and

helped isolate where these genes are

located on the human chromosomes so

understand how this relates to Gene

mapping how we can use linkage which is

an understanding of whether you're going

to see a recombination event or whether

you're not going to see that we can use

that in order to figure figure out

distances between genes on chromosomes

and once we understand that then we're

much closer to being able to pinpoint on

a chromosome exactly where one gene is

and where its other neighbors are so

this is all Rec combination linkage and

Gene mapping are all about looking at

the likelihood that things switch places

during meiosis and that tells us a

tremendous amount about their location

and the distance between separately

one other thing worth mentioning is how

they experimentally can determine

whether recombination has occurred or

not the way that you figure out whether

there has been recombination or not is

the same way that you look at what's

going on with an unknown genotype you do

a test cross with a homozygous recessive

organism and then it will be very clear

based upon the phenotype of The

Offspring

whether that recombination has occurred

or whether it hasn't the test cross once

again is the way that you can figure out

the genotype of an organism with a known

phenotype but uh perhaps mixed or

unclear genotype