Mendelian Genetics:

This is where it all starts

Disclaimer!!!!!!

• Are we talking either mitosis or meiosis?

• For me or offspring?

• Mendel was around in 1822. There was not a lot of technology, but what he discovered is the backbone of all genetics.

The Man: Gregor Mendel

• Austrian priest who studied math and science.

• Loved his peas (male and female parts).

• Peas were the perfect species for his experiment.

Why peas?Why peas?

•Easy to see offspringEasy to see offspring

•Easy to replicateEasy to replicate

•They automatically They automatically true-true-breed breed but willbut will cross-breed cross-breed

The man: Gregor Mendel

• True-breeding- self pollinating• Mendel forced cross-pollinating• He was curious to see how different

genes and traits would interact.• Genes- segment of DNA which codes for

something• Trait- a characteristic that is expressed

from the original DNA– varies from person to person

So what’d he do?The setup

• Well first, the categories he used seed shape, seed color, seed coat color, pod shape, pod color, flower position, and plant height.

• 1st generation – Parental (P)

• 2nd generation– First filial “first son” (F1)

• 3rd generation?

The offspring, are called hybrids

because they come from the crossing of

two different individuals (gene

pools)

The principle of dominance

• Some traits are dominant, others are recessive.

• Dominant traits determine phenotype- physical characteristics– Example cards and trumps

Ace trumps (dominant) the king (recessive)

Mendel wants to know

After making sure that the plants self pollinate, and make the plant lines are pure:

• Will recessive traits disappear or are they still there?

• Which trait is recessive?

• For our purpose- we’re going to just use the tall vs. short trait

• Disclaimer- two copies of each trait per individual

• T= tall (dominant) and t=short (recessive)• Here’s the cross:

– If a dominant tall plant (TT) and recessive short plant (tt) were crossed or mated what would their offspring be?- Tt

– Remember what type of assortment we are talking.

Recap:

• What’s a gene?• What’s a trait?• Is a dominant gene capital or lowercase?• What’s an easy experiment to see what

gene is dominant?• Who is the 1st generation?• Who is the 2nd generation?• Why is Mendel important?

Now to bigger crossesKey terms:• Gamete- 1 sex cell• Zygote- fertilized sex cell• Homozygous- multiple copies of same

gene (TT or tt)• Heterozygous- different copies of a gene

(Tt)• Genotype- genetic type (Tt) vs. tall• Phenotype- physical characteristics- tall

Word problem:So let’s say that one Tt crosses with a Tt

what are all their options for kids and what are the probabilities for each? Let’s use a Punnet square. Hint separate the chromosomes in the zygotes.

Male chromosomes

Female chromosomes

T t

T

t

Male chromosomes

Female chromosomes

T t

T

t

TT Tt

Tt tt

The totals are:

•1TT (homozygous dominant)

•2Tt (heterozygous)

•1tt (homozygous recessive)

•Or 1:2:1 ratio 3:1

Let’s try another

• Let’s cross a male Tt with a female tt.

Male chromosomes

Female chromosomes

T t

t

t

Tt tt

Tt tt

The totals are:

•0TT (homozygous dominant)

•2Tt (heterozygous)

•2tt (homozygous recessive)

•Or 0:2:2 ratio

Let’s try another

• Let’s cross a male Tt with a female TT.

Male chromosomes

Female chromosomes

T t

T

T

TT Tt

Tt

The totals are:

•2TT (homozygous dominant)

•2Tt (heterozygous)

•0tt (homozygous recessive)

•Or 2:2:0 ratio

TT

Another new example

Let’s see if you can do this on your own now. A rat male with straight hair (Ss) crossed with a rat female with curly hair (ss).

1. Is either animal homozygous?

2. Which trait is dominant, straight or curly?

3. Setup a punnet square and identify the breakdown ratio

Another new example

1. Is either animal homozygous, which one?

2. Which trait is dominant, straight or curly?

3. Setup a punnet square and identify the breakdown ratio

1. Yes, curly haired

2. Straight

3. S s

s

s

Ss

Ss ss

ss

0:2:2 ratio

Last one…..

A dude homozygous dominant long-haired lion mates with heterozygous lion.

1. Provide the appropriate abbreviations

2. Identify the genotypes

3. Setup the Punnet square correctly

4. Provide the expected outcome for the Punnet square

Last one…..

1. Provide the appropriate abbreviations

2. Identify the genotypes

3. Setup the punnet square correctly

4. Provide the expected outcome for the punnet square

1. L= long hair, l =short hair

2. LL crosses with Ll

3. L L

L

l

LL

Ll Ll

LL

2:2:0 ratio

And now……

The di-hybrid cross

A BbRr cheetah crosses with a bbRR. Please setup the punnet square and predict

all possible ratios

And now……

• A BbRr cheetah crosses with a

bbRR. • Please setup the

punnet square and predict all possible ratios

The easy way

1. Separate same letters or genes for allele

2. Swap partners so all possiblities are present

3. Now take those new pairs and make new punett squares

4. By the way- how many letters per box

• Step one: BbRr becomes BR, bR, Br, br

• Step two: bbRR becomes bR, bR, bR,bR

• Now for the punnet square

bR

bR

bR

bR

Br bR brBR

BbRR