Introduction to Genetics

Chapter 11

Genetics, the study of heredity, is a broad area of science that encompasses many of today's fastest-growing fields, including molecular biology and biotechnology.

Modern genetics offers vast potential for eliminating diseases, developing pharmaceuticals, increasing food production, and removing pollutants from the environment.

The science of genetics also raises many important moral, legal, and ethical issues that are widely debated.

What is the ultimate thing you inherit

from your parents?

Your genes which make you who you are!

Who is Gregor Mendel and what did he contribute to biology?

Austrian monk in charge of the monastery garden in Vienna

Who is Gregor Mendel and what did he contribute to biology?

He cross pollinated pea plants to look for patterns.

Traits he studied in pea plants:

The science of genetics was born in 1856… Gregor Mendel performed an historic series of experiments on pea plants, thus discovering the laws of heredity.

In summary:

•Although Mendel published a paper on his work, it was largely ignored by the nineteenth-century scientific community. •Today, he is heralded as the "father of modern genetics".

Mendel's laws of heredity described dominant and recessive traits.

A dominant trait is one that is expressed even in the presence of other genes for the same trait. (you can see it!)

When symbolizing dominant traits, a capital letter is used.

T = tallTT = tall pea plant

Tt = tall pea plant

A recessive trait is one that is "masked" by dominant traits, but is expressed when two recessive genes are present.

When symbolizing recessive traits, lower-case letters are used.

t = shorttt = short pea plant

In the first of Mendel's experiments, a tall pea plant (TT) was crossed with a short pea plant (tt). The result of this cross was two tall pea plants (Tt).

P generation F1 generation

When these second-generation plants were crossed, the result was one tall (TT) homozygote, or organism with two of the same gene for a specific trait; two tall (Tt) heterozygotes, or organisms with two different genes for the same trait; and one short (tt) homozygote.

F1 generation F2 generation

Mendel concluded that tallness in pea plants was a dominant trait, and shortness was a recessive trait.

The offspring of crosses between parents with different traits are called hybrids. (Tt are hybrids because they were crossed with TT and tt)

What is a gene? Genes are the chemical factors that

determine traits… Living organisms get one gene from

mom and one gene from dad… You have a gene or genes that

determine the shape of your ears, nose, feet…everything about you!

What is an allele and where are they found? The different forms of a gene are the alleles. They are found on the chromosomes (which is

where the genes are found) The allele for a tall pea plant is T and the allele

for a short pea plant is t.

In summary: TT, Tt and tt is a gene for height in pea

plants. T is an allele and so is t.

TtThis is an alleleThis also is an

allele!

Both alleles make upa gene!

How to say TT, Tt and tt the correct way:

TT = homozygous tall (homozygous means the alleles are the same)

Tt = heterozygous tall (heterozygous means the alleles are different)

tt = homozygous short (homozygous mean the alleles are the same)

Genetics and Probability The likelihood that a particular event will occur (or

happen) is called probability. The principals of probability can be used to predict

the outcomes of genetic crosses. Gene combinations (what allele you get from your

mom and what allele you get from your dad) that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square.

Punnett squares can be used to predict and compare the genetic variations that at will result from a cross.

In other words, it is a prediction of what the offspring will be by looking at the parents. It is written as a ratio or a

percentage.

Generations…P, F1, and F2

P = is the parents

F1 = is the parent’s first

offspring

F2 = is the F1’s offspring

Let’s work a problem!!!!

1) Write a key

Key:

T=tall

t=short

You can choose your own letters as long as they look different in lower and upper case.

Would S be a good letter?

No!!! Neither would C, K, O, P, U, V, W, X, and Z

2) Write the genotype of the parents as a cross and label them as P (for parent generation)

Key:

T=tall

t=short

TT x ttWhat is genotype?

Genotype is the genetic make up (letters!).

The terms homozygous and heterozygous are used to describe the genotype.

P

3) Show what alleles are available from the gametes of the parents

Key:

T=tall

t=shortTT x ttT T t t

P

4) Draw a Punnett Square (a box with a plus sign in the middle)

Key:

T=tall

t=shortTT x ttT T t t

5) Put the alleles of the parents on the Punnett square (put the first parent’s alleles on the side and the second on top)

Key:

T=tall

t=shortTT x ttT T t t

TT

t t

P

6) Combine the alleles from above and to the side to make a gene in the box and label the box F1

Key:

T=tall

t=shortTT x ttT T t t

TT

t tTt TtTt Tt

Helpful Hint:

Always put the capital letter first then lower case second if there is one!

P

F1

7) Now you will write the genotype and phenotype of the F1 generation in ratio form

Key:

T=tall

t=shortTT x ttT T t t

TT

t tTt TtTt Tt

4 Tt=heterozygous tall

P

F1

What percentage of offspring are tall? Short? Homozygous tall? Homozygous short? Heterozygous tall?

Phenotype of F1 :

Genotype of F1:

4 TallPhenotype is what the gene looks like or appears to be.

8) Now lets cross two pea plants from the F1 generation to show the probability of the genotype and phenotype of the F2 generation …you do not have to write the key over since it will be the same…simply choose two from the F1 to cross (they are the same so it should be simple)

Key:

T=tall

t=shortTT x ttT T t t

TT

t tTt TtTt Tt

Genotype of F1:

4 Tt=heterozygous tall

Phenotype:

4 Tall

P

F1

9) Run through the same steps as before

Key:

T=tall

t=shortTt x TtT t T t

Tt

T tTT TtTt tt

F1

F2

Genotype of F2:

What percentage of offspring are tall? Short? Homozygous tall? Homozygous short? Heterozygous tall?

1 short

3 tall

Phenotype of F2:

1 TT = homozygous tall

1 tt = homozygous short

2 Tt = heterozygous tall

Now try one on your own!

                       

 

     A is a dominant characteristic.

                     

 

     a is a recessive characteristic.

This bird has two genes for orange feathers.

  Its genotype is AA.

This bird has two genes for blue feathers.

  Its genotype is aa.

Key:

A=orange feathers

a=blue feathers

Cross a homozygous orange feathered bird with a heterozygous orange feathered bird.

Key:

A=orange feathers

a=blue feathers

Cross a homozygous orange feathered bird with a heterozygous orange feathered bird.

AA x AaA A A a

P

AA

A aAA

AA

Aa

AaF1

Genotype of F1:

2 AA = homozygous orange

2 Aa = heterozygous orange

Phenotype of F1:

4 orange

What percentage of offspring have orange feathers? blue? Homozygous orange? Homozygous blue? Heterozygous orange?

How about one more!!! (the answer is yes since you do not have a choice…just pretend you’re getting a choice…then it is much more fun!)

Key:

A=orange feathers

a=blue feathers

Cross a heterozygous orange feathered bird with a homozygous blue feathered bird.

Aa x aaA a a a

P

Aa

aa

Aa

F1

Genotype of F1:

2 Aa = heterozygous orange

2 aa = homozygous blue

Phenotype of F1:

2 orange

2 blue

What percentage of offspring have orange feathers? blue? Homozygous orange? Homozygous blue? Heterozygous orange?

a

aAa

aa

Monohybrid CrossesCrosses that involve only one characteristic (the type we’ve worked today!)

And finally…

1. In aliens from space, some are tall and some are short. Short is dominant over tall. If an alien that is homozygous short is crossed with an alien that is homozygous tall, what is the phenotype and genotype of the F1 and F2 offspring?

Monohybrid Cross Monohybrid Cross ProblemsProblems

(your homework problems!)

2. In aliens from space, having antennae is dominant over having horns. If an alien that is heterozygous with antennae is crossed with an alien that is homozygous horns, what percentage of chance will the offspring have horns? What percentage will have antennae?

3. In aliens from space, having antennae is dominant over having horns. What is the genotype and phenotype if the parents are heterozygous?

Monohybrid Cross Monohybrid Cross ProblemsProblems (your homework problems!) Due at the first of next class Please work the problems on your

notebook paper Remember communication is the key, so

ask if you need help! Good Luck…. Now WORK!!!!

1. In aliens from space, some are tall and some are short. Short is dominant over tall. If an alien that is homozygous short is crossed with an alien that is homozygous tall, what is the phenotype and genotype of the F1 and F2 offspring?

Answers to homework problems!

Key:

T=short

t=tall

TT x ttT T t t

TT

t tTt TtTt Tt

P

F1

Genotype of F1:

1. In aliens from space, some are tall and some are short. Short is dominant over tall. If an alien that is homozygous short is crossed with an alien that is homozygous tall, what is the phenotype and genotype of the F1 and F2 offspring?

4 short

4 Tt = heterozygous short

Phenotype of F1:

Key:

T=short

t=tall

Tt x TtT t T t

Tt

T tTT TtTt tt

F1

F2

Genotype of F2:

1. In aliens from space, some are tall and some are short. Short is dominant over tall. If an alien that is homozygous short is crossed with an alien that is homozygous tall, what is the phenotype and genotype of the F1 and F2 offspring?

3 short

1 TT = homozygous short

Phenotype of F2:

2 Tt = heterozygous short

1 tt = homozygous tall

1 tall

2. In aliens from space, having antennae is dominant over having horns. If an alien that is heterozygous with antennae is crossed with an alien that is homozygous horns, what percentage of chance will the offspring have horns? What percentage will have antennae?

Key:

A=antennae

a = horns

Aa x aaA a a a

Aa

a aAa Aaaa

aa

P

F1

Genotype of F1 :

2. In aliens from space, having antennae is dominant over having horns. If an alien that is heterozygous with antennae is crossed with an alien that is homozygous horns, what percentage of chance will the offspring have horns? What percentage will have antennae?

2 Aa = heterozygous antennae

2 aa = homozygous horns

2 horns

2 antennae

Phenotype of F1:

% with horns?50%

% with antennae?50%

3. In aliens from space, having antennae is dominant over having horns. What is the genotype and phenotype if the parents are heterozygous?

Key:A=antennaea = horns

Aa x AaT t T t

Aa

A aAA AaAa a

a

P

F1

Genotype of F1:

3. In aliens from space, having antennae is dominant over having horns. What is the genotype and phenotype if the parents are heterozygous?

3 antennae

1 AA = homozygous antennae

Phenotype of F1:

2 Aa = heterozygous antennae

1 aa = homozygous horns

1 horns