Mendel’s Second Mendel’s Second ExperimentExperiment

Inheritance of Two TraitsInheritance of Two Traits

Dihybrid CrossDihybrid Cross

Monohybrid ReviewMonohybrid Review

How many traits were being looked How many traits were being looked at during the Mendel’s monohybrid at during the Mendel’s monohybrid cross?cross?– ____________________________________________________________________

What was being crossed in Mendel’s What was being crossed in Mendel’s monohybrid cross?monohybrid cross?

__________________________________________________________________ __________________________________________________________________

Back Track for a SecondBack Track for a Second

What exactly was a purebred species What exactly was a purebred species in terms of genotype?in terms of genotype?– It was a species that had the same It was a species that had the same

alleles in its genetic makeup (either BB alleles in its genetic makeup (either BB or bb)or bb)

What about a hybrid species in terms What about a hybrid species in terms of genotype?of genotype?– It was a species that had different It was a species that had different

alleles in its genetic makeup (like Bb)alleles in its genetic makeup (like Bb)

P X PP X P

When the two When the two parents were parents were crossed, TT X tt, crossed, TT X tt, what resulted?what resulted?

The Results were The Results were four heterozygous four heterozygous offspring.offspring.

These are the F1 These are the F1 generationgeneration

F1 X F1F1 X F1 When the two F1 generation When the two F1 generation

were crossed, TT X tt, what were crossed, TT X tt, what resulted?resulted?

The Results were three The Results were three dominant and one recessive dominant and one recessive in a 3:1 phenotypic ratio but in a 3:1 phenotypic ratio but in a 1:2:1 genotypic ratioin a 1:2:1 genotypic ratio

These are the F2 generationThese are the F2 generation

The Law of SegregationThe Law of Segregation

What does the Law of Segregation What does the Law of Segregation state?state?– It states that inherited traits are It states that inherited traits are

determined by pairs of “factors”determined by pairs of “factors”– These factors segregate (separate) in These factors segregate (separate) in

the gametesthe gametes

What happened next?What happened next?

Mendel completed the monohybrid Mendel completed the monohybrid cross and started to ask and cross and started to ask and formulate new questionsformulate new questions

What do you think he was thinking of What do you think he was thinking of next?next?– He wanted to know if the inheritance of He wanted to know if the inheritance of

one characteristic influenced the one characteristic influenced the inheritance of a different characteristicinheritance of a different characteristic

What did he try next?What did he try next?

He started all over againHe started all over again He produced purebred plants just like He produced purebred plants just like

last time with the traits he wanted to last time with the traits he wanted to examineexamine

He bread pea plants that were round He bread pea plants that were round and yellow pure plants (dominant for and yellow pure plants (dominant for both) and then he did the same for both) and then he did the same for wrinkled and green (recessive for wrinkled and green (recessive for both)both)

What did he try next?What did he try next?

He then crossed them againHe then crossed them again The P X P ended up yielding what?The P X P ended up yielding what?

– The F1 ended up being all yellow and all The F1 ended up being all yellow and all round, just like last timeround, just like last time

– There seems to be some similarityThere seems to be some similarity

P X P Dihybrid CrossP X P Dihybrid Cross

RYRY RYRY RYRY RYRY

ryry RrYyRrYy RrYyRrYy RrYyRrYy RrYyRrYy

ryry RrYyRrYy RrYyRrYy RrYyRrYy RrYyRrYy

ryry RrYyRrYy RrYyRrYy RrYyRrYy RrYyRrYy

ryry RrYyRrYy RrYyRrYy RrYyRrYy RrYyRrYy

Then What?Then What?

Just like last time he crossed the two Just like last time he crossed the two F1 ProgenyF1 Progeny

What do you think he got?What do you think he got?– He got 9 Yellow and roundHe got 9 Yellow and round– He got 3 Green and roundHe got 3 Green and round– He got 3 Yellow and WrinkledHe got 3 Yellow and Wrinkled– He got 1 Green and WrinkledHe got 1 Green and Wrinkled

What’s The Ratio?What’s The Ratio?

The ratio that he got was?The ratio that he got was?– 9:3:3:19:3:3:1

How Many?How Many?

How many plants do you think that How many plants do you think that Mendel used for this experimentMendel used for this experiment– 551 pea plants551 pea plants– And of that 551 this was the ratio that he And of that 551 this was the ratio that he

gotgot

320 round and yellow320 round and yellow 104 round and green104 round and green 101 wrinkled and yellow101 wrinkled and yellow 26 wrinkled and green26 wrinkled and green

What did this tell him?What did this tell him?

The ratio of 9:3:3:1 could be explained The ratio of 9:3:3:1 could be explained if the alleles from one trait were if the alleles from one trait were inherited independently of the alleles inherited independently of the alleles for another traitfor another trait

The Law of Independent The Law of Independent AssortmentAssortment

The Law of Independent The Law of Independent AssortmentAssortment

The second law of inheritance The second law of inheritance states:______________________________states:____________________________________________________________________________________________________________________________________________________________________

Different pairs of alleles are passed Different pairs of alleles are passed to the offspring independently of to the offspring independently of each othereach other

The Law of Independent The Law of Independent AssortmentAssortment

This means that offspring may have This means that offspring may have new combinations of alleles that are new combinations of alleles that are not present in either parentnot present in either parent