Chapter 3 and 4

Genetics

Section 1, Mendel's Work

Heredity -- the passing of traits from parents to offspring

It was first studied by Gregor Mendel (1822-1884).

Gregor Mendel was born in 1822 in what is now Czechoslovakia. His parents were very poor, but he faced poverty with courage, tending fruit trees for the lord of the manor. He developed a love for plants that can be see in his later work.

As he grew up, he had to struggle to get an education. He helped to support himself by tutoring other students. He entered an Augustinian monastery when he was 21 years old. They sent him to the University of Vienna to be trained to be a math and science teacher.

Mendel combined his interest in math and botany with research. He investigated how traits are inherited by growing pea plants which grow fast and reproduce by self-pollination. He grew them in the monastery garden. He self-pollinated plants and wrapped them carefully to keep insects from pollinating them by accident. That way he was sure that the seeds would inherit characteristics from only one parent.

He studied the traits of many plants. He discovered that short plants always produced short plants but tall plants were different. About 1/4 of their offspring where short and 3/4 were tall. He realized that there were two types of tall plants: true-breeders (pure) and non-true-breeders. The non-true-breeders contained both tall and short traits.

He then did more experiments. He crossbreed pure short and pure tall plants. Surprisingly he found that all the offspring were tall. However, when these first generation offspring were self-pollinated about 1/4 of the next generation were pure tall, 1/2 were non-true-breeding tall (today we call them hybrid) and 1/4 were pure short. This was his historic discovery!

Like any good scientist Mendel kept careful records of his work on thousands of plants. However, when he reported his results to the local natural history society, they paid little attention. He tried other ways to get people to recognize his work but failed. Finally he published his findings in a small scientific magazine but no one paid attention.

After 12 years of research Mendel was made abbot of the monastery. His new duties and his increasing weight made it impossible for him to continue to work in his gardens.

His work remained unnoticed during his lifetime and he died in 1884 without knowing that he would be famous as the discoverer of the Mendelian laws of inheritance.

In 1900 three other scientists simultaneously found the same results as Mendel. They were very surprised when the checked the literature and found that Mendel had made these same discoveries 30 years earlier!

Mendel did many experiments using different plant traits. Some of them are shown below:

Mendel's plant height experiment:

1. He bred pea plants so they always produced offspring with

heights like the parents. (tall parent ---------> tall offspring )

2. Then he bred a tall pea plant and a short pea plant

together. He found that their offspring were all tall.

3. Then he bred two of these tall offspring together. This time there

were some tall and some short offspring

This lead him to say that the plants had traits for tallness and

shortness.

He called them dominant and recessive traits.

A dominant trait is one that prevents another trait from appearing

or showing in the offspring.

A recessive trait is one that will not appear

when a dominant trait is present.

An organism with a pure trait has all

dominant or all recessive traits.

A pure breed is an organism that always produces offspring with

the same form of a trait as the parent.

Examples:

Today we know that Mendel's "traits" are genes

A dominant gene (represented

by a capital letter)

covers up (or hides) a recessive gene

(represented by a lower case letter.)

Genes are inherited in pairs -- one gene comes from each parent

Different forms of a gene are called alleles. For example: The gene

for height in some plants has two forms (tall or short)

-one allele is for short stems

-the other allele is for tall stems

Examples:

Let the dominant gene for height = H

Let the recessive gene for height = h

all tall offspring

also called hybrids

Hybrid trait -- made of a combination of dominant and recessive

traits (mixed)

pure tall + pure short ------------> hybrid offspring

Example:

Principles of genetics

1. Traits pass from parents to offspring

2. Gene pairs determine traits

3. One gene of a pair comes from the gamete of each parent

( A gamete is the sex cell of the parent -- either the

sperm or egg)

4. Gene pairs are inherited separately for each trait

Section 2 Probability and Genetices

Probablity is the likelihood that a specific event will happen.

Suppose you were to toss a coin 20 times. Predict how many times the coin would land "heads up"__________ and how many times it would land "heads down" _____________

Now test your prediction by tossing a coin 20 times. Record the number of times the coin lands heads up ________ and heads down _________

Combine the data from the entire class
class data -- heads up __________ heads down _______

The class found that the probabity of heads up was _______ out of ______ tries.

This can also be shown as a fraction or a percent. For example one out of two times is 1/2or 50 percent.

Four out of five times is 4/5 or 80%

These are all Probabilities.

Punnett Squares -- show all possible combinations of a genetic

cross

-- determine the genes of a particular

set of offspring

Genotype -- the gene combination in the chromosomes

Phenotype -- the outside physical trait that shows (what

it looks like)

When the genotype is :

-2 dominant genes it is pure dominant

(two capital letters)

-2 recessive genes it is pure recessive

(two lower case letters)

- 1 dominant and 1 recessive gene it is hybrid

(one capital and one lower case letter)

Examples:

H = dominant (tall plant) h= recessive (short plant)

breed (cross) one pure dominant (HH) with one pure recessive (hh)

Phenotypes:

tall - 100 %

short - 0 %

Genotypes

hybrid - 100%

pure dominant - 0%

pure recessive - 0%

example:

cross a hybrid (Hh) with another hybrid (Hh)

Phenotypes:

tall - 75 %

short - 25%

Genotypes

hybrid - 50%

pure dominant - 25%

pure recessive - 25%

example:

cross a pure dominant (AA) with a hybrid (Aa)

do Punnett square and find genotypes and phenotypes

example:

cross a pure recessive (bb) with a hybrid (Bb)

do Punnett square and find genotypes and phenotypes

Other types of inheritance: (besides dominant and recessive)

Codominance- neither allele is masked (neither dominant

nor recessive)

-offspring show characteristics of both parents

example: p. 93 in text

Incomplete dominance- each allele is equally dominant

-results in a blended or intermediate phenotype

example:

R= red W = white

RR = red

WW = white

RW = pink (blended red and white)

Section 3, The Cell and Inheritance

Chromosomes

- found in nucleus of the cell

- contain genes

- each type of organism has a different

number of chromosomes

-humans have 23 pairs or 46 total

chromosomes in body cells.

- each chromosome has a specific number of genes

-a human chromosome contains hundreds of genes

Meiosis -- cell division which forms gametes

(see p. 99 in text)

In sexual reproduction - half of genes come from each parent

- sexually reproduced offspring are

not identical to either parent

Section 4, The DNA Conncetion

Remember that chromosomes are made of DNA

DNA is made of of four different nitrogen bases

(adenine(A), thymine(T), guanine(G), cytosine(C)

These bases form the rungs of the DNA ladder. A single gene on a chromosome may contain a few hundred to more than a million of these bases. The order of the nitrogen bases determines what type of proteins the cell makes.

Mutations are changes in the DNA. They can be harmful, helpful, or neither.

Book C, Chapter 4, Modern Genetics

Section 1, Human Inheritance

Multiple alleles

Some traits are controlled by a single gene that has more than two alleles. Such a gene is said to have multiple alleles (three or more forms of a gene that control a single trait.)

Because genes are inherited in pairs, a person can only inherit two forms of any allele.

A good example is blood types.

There are four main human blood types: A,B,AB,O

Three alleles control their inheritance. The allele for blood type A and the allele for blood type B are codominant

I^A for blood type A

I^B for blood type B

The allele for blood type O is recessive. It is written i

Blood type Combination of alleles

Blood types are not evenly distributed throughout the human population. O+ is the most common, AB- is the rarest. There are also variations in blood-type distribution within human subpopulations. According to wikipedia.org, the approximate distribution of blood types in the United States are as follows:

Blood type is important when a person receives blood. The chart below shows what types of blood people can receive.

Rh factor is inherited separately from blood type.

Rh positive (Rh+) is dominant over Rh negative (Rh-).

Rh+ people can receive blood type compatable blood that is either Rh+ or Rh-.

Rh- people can only receive blood type compatable blood that is Rh-.

For example:

O+ can donate to A+, B+,AB+, and O+ but receive blood from both O- and O+ while

O- can donate to A+, B+,AB+, and O+ and A-, B-,AB-, and O- but receive blood only from another O-.

Which blood type would be the true universal donor?

O-

Which blood type would be the true universal receiver?

AB+

Sex determination

- females have two X chromosomes

- males have one X chromosome

and one Y chromosome.

- Unlike individual genes, whole chromosomes are

NOT dominant or recessive

Pedigrees

-a chart or "family tree" that shows which members

of a family have a particular trait

If you are doing this from home be sure to see your teacher for the more detailed notes on pedigrees.

Sex linked genes --

--genes found on the X and Y chromosomes

--(sex - linked traits are controlled

by sex- linked genes)

A = no disease gene

a =disease gene

X^A-- dominant allele on the X chromosome

X^a -- recessive allele on the X chromosome

Y^A-- dominant allele on the Y chromosome

Y^a-- recessive allele on the Y chromosome

X^AX^A -- female who does not have the trait (disease)

X^aX^a-- female who has the trait (disease)

X^AX^a --female who does not have the trait but

is a carrier

carrier -- a person who has one recessive allele

(disease) and one dominant allele for a trait (no

disease)

X^AY -- male with one dominant allele on the

X chromosome and no allele for this trait

(disease) on the Y chromosome

- does not have the trait

X^aY--male with one recessive allele on the

X chromosome and no allele for this trait

(disease) on the Y chromosome

-- has the trait (disease)

example:

Section 2 Human Genetic Disorders

A genetic disorder is an abnormal condition that a person inherits through genes or chromosomes. They are caused by mutations or changes in a person's DNA

examples:

1. Cystic fibrosis

- a genetic disorder in which the body produces abnormally thick mucus in the lungs and intestines. This makes in hard for the person to breath and can interfer with digestion

2. Sickle-cell disease

- a genetic disorder that affects the blood. The red blood cells have a sickle (half moon) shape instead of the normal donut shape. They cannot carry enough oxygen and they block the blood vessels causing pain and weakness.

3.Hemophilia

- a genetic disorder in which a person's blood clots slowly or not at all. Minor cuts and bruises can cause major bleeding or death. It is caused by a recessive allele on the X chromosome. Therefore it is a sex-linked disorder.

4. Down Syndrome

- a genetic disorder that results from an extra copy of chromosome #21. People with this disorder have a distinctive physical appearance and some degree of mental retardation.

Section 3, Advances in Genetics

Selective Breeding -- the process of selecting a few organisms with desired traits to serve as parents for the next generation.

1, Inbreeding -- crossing two organisms that have the same or similar sets of traits

examples:

Breeding two fast horses to get a fast offspring.

2. Hybridization -- crossing two organisms that have two different desirable traits

examples:

Breeding a fast horse with a black horse to get a fast, black offspring.

Cloning -- producing offspring genetically identical to a single parent

- in plants cloning can be done by cuttings or runners

- in animals an example is Dolly the sheep or "Copy Cat"

Genetic Engineering -- taking genes from one organism and transferring them directly into the DNA of another organism.

examples:

-"gene splicing" a gene for a protein that controls human blood clotting is inserted into the cells of a cow which then produces that protein in her milk. The milk can then be used to treat human hemophelia.