Download Section 1: Mendelʼs Work * Gregor Mendel was a young priest from

Prentice Hall Science Explorer: Cells & Heredity - Chapters 3 & 4 - Notes, Page 1
Section 1: Mendelʼs Work
* Gregor Mendel was a young priest from Central Europe who studied math and science
at the University of Vienna.
* After leaving the university, he returned to his monastery and began teaching at a local
high school.
* As part of his duties, Gregor Mendel also cared for the monasteryʼs garden where he
grew hundreds of pea plants. He noticed that some of the pea plants had different
characteristics than the others and became curious. Apparently he didnʼt have a lot of
meetings at his high school and therefore had some free time think...about plants. : )
* TRAITS are physical characteristics.
* He noticed that sometimes the traits of the offspring pea plants were similar to the
parents and then other times they were different. The passing of traits from parents to
offspring is known as HEREDITY.
* Over the course of ten years, he experimented, studied, and documented thousands
of pea plants, their traits and how they were passed to their offspring. The scientific
study of heredity is called GENETICS. Gregor Mendel formed the basis for genetics
and for that reason he is called the Father of Genetics. (yes, a PUN is intended there
we believe) : )
* To observe how traits were passed from parents to offspring, Mendel had to know
which plants were the parents and which were were the offspring. Pea plants usually
self-pollinate which means the pea plants usually contain the genetic information from
the mother and the father plant on ONE plant.
* In flowers, the PISTIL produces the female sex cell or eggs, while the STAMENS
produce the male sex cells or POLLEN. In many flowers, if the pollen isnʼt moved by
the wind or insects, a single plant will pollinate itself. Pollination is the coming together
of the male and female sex cells in plants.
* Mendel figured out to take the male sex cells (pollen) from stamen of one pea plant
flower and place it on the pistil of another plant, fertilizing the egg cells and creating
offspring that were a mixture of two different parents. By doing this, he created the
process of CROSS-POLLINATION. Doing this with animals has created different
breeds of dogs, cats, cows, etc with the desired traits.
* Mendel crossed plants that had opposite forms of traits. For instance, he crossed short
plants with tall plants. When he crossed these opposite plants, he made sure he used
plants that were PUREBRED, which means an organism that always produces
offspring with the same form of a trait for several generations.
Prentice Hall Science Explorer: Cells & Heredity - Chapters 3 & 4 - Notes, Page 2
* He began by crossing purebred tall plants with purebred short plants. The first
generation that were produced were all TALL. The next generation after that, 75%
were tall while 25% were short. Hmmm.....
* From this, Mendel decided that traits must be controlled by individual factors and that
the factors must exist in pairs and that one of the pairs of these factors comes from the
mother and the other of the pair of factors comes from the father.
* He also reasoned that one of the factors in a pair can mask or hide the trait of another
one. In the pea plants, Tallness covered up shortness.
* The “factors” that control traits are known as GENES. The different forms of a gene
are known as ALLELES.
* DOMINANT ALLELES are ones whose trait always show up in an organism. A
RECESSIVE ALLELE is one whose trait is masked when a dominant allele is present.
The only time a recessive allele shows up in an organism is when both of the gene
pair are recessive alleles. Think of a tall person standing in front of a short person.
Whereʼd they go?
* If an organism has a gene pair for a trait with both a recessive and a dominant allele
together, the organism is considered to be HYBRID. (A hybrid car uses both gas and
electricity to move)
* To make it easier to follow, dominant alleles are represented by a capital letter, while
recessive alleles are represented by the same letter usually, only lower case.
Take a Class Survey activity on page 86
Section 2: Probability and Genetics
* If you tossed a coin 10 times, how many times would you expect it to land with the
head side up? 5 times, right? Is that a guarantee? No. But, the probability that the
coin will land heads up is, “1 in 2” or “1/2” or “50%”. It doesnʼt matter how many
times you do the coin toss, the probability remains the same because there are only 2
possible outcomes...unless the coin lands on its edge. : ) If that happens, YOU win.
* PROBABILITY is the likelihood that a particular event will occur. Mendel discovered
that probability can be used to predict the results of genetic crosses using a tool call a
Punnett Square.
Prentice Hall Science Explorer: Cells & Heredity - Chapters 3 & 4 - Notes, Page 3
* A PUNNETT SQUARE is a chart that shows all the possible combinations of alleles
that can result from the crossing of two parents. It works like mathematical puzzle, but
itʼs easy. Check out the example below. Across the top of the square is the gene pair
for one parent who is hybrid for the trait. Down the left hand side is the gene pair for
the other parent who is also hybrid for the trait. Since both parents are hybrids and the
dominant allele is present, it is the one that will show up and both parents are tall.
* Each letter on the top of the square must be copied into the boxes below them, within
the square
* Each letter to the left of the square must be copied into the boxes to the right in order
to end up with a pair of alleles in each of the four inner boxes.
* When looking at the offspring above (the gene pairs found inside each of the four
squares) we see:
- TT, which represents purebred or HOMOZYGOUS DOMINANT offspring which
"
will be tall. (Homozygous means two identical alleles for a trait.)
"
- Tt, which represents hybrid or HETEROZYGOUS offspring which will also be
"
"
tall. (Heterozygous means two different alleles for a trait.)
- tt, which represents purebred or HOMOZYGOUS RECESSIVE offspring which
"
"
will be short
* When considering probability, each box inside the Punnett Square represents 25%.
"
25% X 4 = 100%. With the above Punnett square, we see that we would expect
"
75% of the offspring to be tall and 25% to be short.
Prentice Hall Science Explorer: Cells & Heredity - Chapters 3 & 4 - Notes, Page 4
* An organismʼs PHENOTYPE is its physical appearance or its visible traits. The
"
phenotype of 75% of the offspring from the Punnett Square is TALL.
*An organismʼs GENOTYPE (think genes) is its gene pair or allele combination. 25% of
"
the offspring in the above Punnett square are “tt”.
* Occasionally, for some traits, alleles are neither dominant nor recessive. Instead, both
traits are expressed. This is called CODOMINANCE. In the example below, the black
and white alleles are codominant so both show up and all the offspring end up having w
black AND white feathers.
Prentice Hall Science Explorer: Cells & Heredity - Chapters 3 & 4 - Notes, Page 5
Section 3: The Cell and Inheritance
* 1903 - American geneticist, Walter Sutton, studied the cells of grasshoppers, in
particular, he studied the sex cells, eggs and sperm and how they formed.
* GAMETES is the name given to the sex or reproductive cells.
* While observing the formation of the eggs in female grasshoppers and sperm in male
grasshoppers, he noticed that the number of chromosome changed between number
in the egg and sperm as compared to after fertilization took place and the number
found in the ZYGOTE that was formed.
* He knew grasshoppers had 24 chromosomes in each of their body cells and those
chromosomes were always in pairs, 12 pairs.(watch the math between “pairs” and
individual chromosomes) One of the chromosomes in that pair came from the father
while the other one in each of the pair come from the mother.
* However, when grasshoppers produced their sex cells, each sex cell only had only 12
chromosomes, exactly half of the rest of the grasshopperʼs cells. These cells, with half
the number of the chromosomes, were later called HAPLOID cells.
* When the egg and the sperm came together during FERTILIZATION, the resulting cell,
a ZYGOTE, had 24 chromosomes again, like the rest of the grasshopperʼs cells.
* All of the these clues led Walter Sutton to realize that Chromosomes must be the key
to offspring having half their genetic information from their mother and half from the
father....2 + 2 = 4!
* However, the question came up, how do the sex cells/Gametes end up with only half
the number of the chromosomes as the rest of the body cells? The answer is the
process of MEIOSIS.
* You recall, MITOSIS is the process of a cell duplicating all its genetic information and
and then dividing into two new, IDENTICAL cells than have the exact same number of
chromosomes as the parent cell they came from. “MITOSIS, makes MY bodyʼs cells!”
* MEIOSIS is the process by which four sex cells/gametes (sperm and egg) are
produced from one single cell. “MEIOSIS made ME!” These sex cells have exactly half
the number of chromosomes as the cells from which they came. (They are HAPLOID
((think “half”)) while body cells, that contain a full set of chromosomes, are called
DIPLOID.)
* In the process of meiosis, a single cell makes two exact copies of itself, including all
chromosomes, full of all the genetic information. Then those two cells split their
chromosome pairs in half, then each half set of chromosomes goes on to become a
Prentice Hall Science Explorer: Cells & Heredity - Chapters 3 & 4 - Notes, Page 6
sex cell/gamete, sperm in males and egg in females. Which of that pair of split
chromosome pair goes to which egg or sperm is like flipping a coin.
* For this reason, when an offspring is created through fertilization, it receives half of its
genetic information from its mother and half from its father. This is why you may have
some traits of your mother AND some traits of your father.
* Obviously, since Suttonʼs time, research has continued on the chromosomes of
different organisms, especially humans.
* Each normal, human cells contains 46 chromosomes/ 23 pairs of chromosomes.
* Although humans have only 23 pairs of chromosomes, we have more than 50 - 60,000
individual genes and each of these control your traits.
SECTION 4 : The DNA Connection
* The Basics of DNA (not in book)
Your body contains 50 trillion tiny cells, and almost every one of them contains the
complete set of instructions for making you. These instructions are encoded in your
DNA. DNA is a long, ladder-shaped molecule. Each rung on the ladder is made up of a
pair of interlocking units, called bases, that are designated by the four letters in the
DNA alphabet - A, T, G and C. 'A' always pairs with 'T', and 'G' always pairs with 'C'.
* DNA is organized into Chromosomes (not in book)
The long molecules of DNA in your cells are organized into pieces called chromosomes.
Humans have 23 pairs of chromosomes. Other organisms have different numbers of
pairs - for example, chimpanzees have 24 pairs. The number of chromosomes doesn't
determine how complex an organism is - bananas have 11 pairs of chromosomes, while
fruit flies have only 4.
* Chromosome are Organized Into Genes (not in book)
Chromosomes are further organized into short segments of DNA called genes. If you
imagine your DNA as a cookbook, then your genes are the recipes. Written in the DNA
alphabet - A, T, C, and G - the recipes tell your cells how to function and what traits to
express. For example, if you have curly hair, it is because the genes you inherited from
your parents are instructing your hair follicle cells to make curly strands.
Prentice Hall Science Explorer: Cells & Heredity - Chapters 3 & 4 - Notes, Page 7
* Genes Make Proteins (not in book)
Cells use the recipes written in your genes to make proteins - just like you use recipes
from a cookbook to make dinner. Proteins do much of the work in your cells and your
body as a whole. Some proteins give cells their shape and structure. Others help cells
carry out biological processes like digesting food or carrying oxygen in the blood. Using
different combinations of the As, Cs, Ts and Gs, DNA creates the different proteins - just
as you use different combinations of the same ingredients to make different meals.
* Genetic Switches Control the Traits Cells Express (not in book)
Cells come in a dizzying array of types; there are brain cells and blood cells, skin cells
and liver cells and bone cells. But every cell contains the same instructions in the form
of DNA. So how do cells know whether to make an eye or a foot? The answer lies in
intricate systems of genetic switches. Master genes turn other genes on and off, making
sure that the right proteins are made at the right time in the right cells.
Sometimes the copy of this genetic information has mistakes.......
* A MUTATION is when there is a change that occurs in a gene or a chromosome.
* The word MUTATION, comes from the Latin word meaning “change”.
* Mutations can cause a cell to produce an incorrect protein during the process of
Protein Synthesis. As a result, the organismʼs traits, or PHENOTYPE, will be different
from what it normally wouldʼve been.
Two General Types of Mutations:
1.
Some are the result of small changes in an organismʼs hereditary material,
during the DNA copying process.
2.
Other mutations occur when chromosomes donʼt separate correctly during
meiosis, resulting in too many or too few chromosomes such as the extra
chromosome that causes Downʼs Syndrome.
Some mutations are harmful, some are helpful, some are neither harmful nor
helpful.
1. Mutations are harmful if they reduce the organismʼs chance to survive and reproduce.
Examples would be an organism being albino and reducing their camouflage abilities
to protect them from predators or cancer where mutated cells over-run the healthy
cells of the body.
2. Mutations are helpful if they increase the organismʼs chances to survive or reproduce.
Gene a potato resulted in a new variety called the Katahdin, a potato that is resistant
to some diseases.
Prentice Hall Science Explorer: Cells & Heredity - Chapters 3 & 4 - Notes, Page 8
Modern Genetics - Prentice Hall Science Explorer - Chapter 4
Section 1: Human Inheritance
Traits Controlled by Single Genes
* Many traits, like those in pea plants are controlled by a single gene with two alleles,
with those alleles being any combination of “dominant” or “recessive”.
* Many human traits are also controlled by a single gene or pair of alleles. When this
occurs, there a typically on two possible genotypes.
* A widowʼs peak is one of those traits. For this reason, in humans, the only two possible
phenotypes are: HAVE A WIDOWʼS PEAK or NO WIDOWʼS PEAK. There is nothing in
between.
!
Consider: Do you think height is controlled by one pair of alleles in humans?
Yes/No and explain
Multiple Alleles
* Some human traits are controlled by a single gene, but that gene can have MORE
than two alleles, not just “dominant” or “recessive”. These genes are said to have
MULTIPLE ALLELES. These are like different flavors of pudding. Theyʼre all pudding,
but there are more than two flavors.
* Although a gene may have more than two forms, a human can only carry a pair, or two
alleles in each gene because a gene is made of chromosomes and chromosomes
always come in pairs.
* Human blood types are controlled by a gene that has THREE alleles. The different
combinations of these alleles result in the different types of blood in humans.
Traits Controlled by Many Genes
* Some traits in humans show a large number of phenotypes, not just two or four, but
MANY. As an example there are at least FOUR genes that control height in humans so
there are MANY combinations of genes and their alleles resulting in a great
differentiation in heights in humans.
* Skin color is controlled by many genes.
"
"
Consider: Describe the heights of human if the trait of height was controlled by
"
one gene and only one pair of alleles in that gene.
Male and Female?
* Among the 23 pairs of chromosomes in each body cell of humans, there is one pair
known as the SEX CHROMOSOMES. This pair determine if a person is male or
female.
Prentice Hall Science Explorer: Cells & Heredity - Chapters 3 & 4 - Notes, Page 9
* XX is the chromosome pair code for FEMALES and XY is for males.
* If a Y is present, the offspring will be male, therefore it can be considered that the
sperm gamete determines the sex of an offspring.
"
"
Consider: Use a Punnett square to cross a male and a female. What % of the
"
offspring would we expect to have the genotype “XY”?
Pedigrees
* On important too that geneticists use to trace the inheritance of traits in humans is a
PEDIGREE. A pedigree is a chart or “family tree” that tracks which members of a
family have a particular trait. (Study the example on page 118)
Section 2: Human Genetic Disorders
* A GENETIC DISORDER is an abnormal condition that a person inherits through genes
or chromosomes. These are caused by mutations (changes in a personʼs DNA).
* A familiar genetic disorder is DOWN SYNDROME. Down Syndrome results from the
incomplete separation of a chromosome pair during meiosis. This gives each cell an
extra copy of the 21st pair of chromosomes. The result in humans is a distinctive
physical appearance.
Section 3: Advances in Genetics
Three methods people have used to develop organisms with desirable traits are
SELECTIVE BREEDING, CLONING, and GENETIC ENGINEERING.
1.
SELECTIVE BREEDING is the process of selecting a few organisms with
"
" desired traits to serve as parents of the next generation. This is used
"
"
for plants, vegetables, and even can created our many breeds of dogs.
2. CLONING is creating an offspring that is genetically identical to the parent. In plants
"
"
this is can be simple such as in self-pollination or in taking a “cutting” from
"
"
a plant, putting it in water and letting it grow roots, then planting it.
* In animals, cloning is a much more complicated process since animals typically are a
combination of the genetic information from TWO parents.
* The first genetically cloned animal was DOLLY the sheep who was born in 1996. To
make Dolly, a fertilized egg was removed from one sheep, its nucleus was removed
and was replaced with the nucleus from a cell from a 6 year old sheep. That egg was
then placed into the uterus of a third sheep where it grew. Dolly was born 5 months
later. Dolly is a genetic clone of the 6 year old sheep.
Prentice Hall Science Explorer: Cells & Heredity - Chapters 3 & 4 - Notes, Page 10
3. GENETIC ENGINEERING (sometimes called GENE SPLICING) is the process
where genes from one organism are transferred into the DNA of another organism.
Genetic engineering is used to produce medicines such as insulin for diabetics,
improve food crops, and to try and cure some genetic disorders.
DNA Fingerprinting
* DNA is different for EVERY person except identical twins. Since EVERY cell inside
human bodies contains an identical copy of DNA (besides the gametes/sex cells) AND
individual cells are often left behind by criminals at crime scenes, specialist can pull
the DNA from those cells and compare it to the DNA of suspects in custody or to DNA
from other crime scenes. If the compared DNA is a match, this absolutely ties
criminals to crime scenes and the crimes themselves.
The Human Genome Project
* A GENOME is all the DNA in one cell of an organism.
* The goal of the Human Genome Project (HGP) is to identify the DNA sequence of
every gene in the human genome.
* The 23 pairs of chromosomes that make up the human genome may contain 60 80,000 genes or around 3,000,000,000 DNA base pairs.
* By identifying each of these base pairs, scientists will know the DNA sequence of
every human gene, and thus the amino acid sequence used to make every protein.
* This information may help us understand how we develop from a zygote to an adult
and identify when things go “wrong” and be able to fix it.