Download Class Notes On Heredity

Notes for Chapter 5 Section 1
First read “Would You Believe…” on pg 104
--For centuries, animals and plants have been bred fro specific traits. Most of these traits
amount to pleasing coloration or measures of strength and endurance that humans find
valuable or beautiful.
-- a trait is a physical characteristic (traits in humans include eye color, hair color, the
ability to roll your tongue into a tube and your earlobes…whether they are attached to
your head or hang free.)
-- in the case of the two lightly gold colored carps, the fisherman selected the baby carps
that were more brightly colored than their parents…The fisherman was practicing
selective breeding
Selective breeding is the crossing of two organisms to bring out a desired trait.
--the trait the fisherman selected for was the bright gold color.
Each time he bred the more brightly colored fish with each other, their babies (offspring)
began to exhibit different traits that he hadn’t planned for.
--These traits were hidden in the DNA of the carp waiting to be expressed.
Reminder: DNA is the molecule that contains all of a cell or organisms information
necessary to make a copy or duplicate of itself. DNA is found in the nucleus of the
eukaryotic cell (eu = true + Karyon = nucleus “true nucleus”).
--overtime the selective breeding of these carps lead to the many different varieties of
goldfish you see on page 104…
--Stargazers: Large bubble eye sacs and a gold and white color
--Calicos: Splotches of gray, gold, white and brown.
--Metallics: Glittery metal finished gold color
--Moors/Lionheads: black fins, fleshy hoods and bright gold color
Look at all of the variations in traits that were available in the DNA of the original carps
that were basically dull in color!
*Homework DNA and traits worksheet: List thirty physical characteristics in humans that
are coded for in DNA. On the back of the worksheet answer these questions
What color are your eyes?
What color are your eyes?
What color are the eyes of your brothers and sisters? This will prepare you for tomorrows
lecture.
DAY 2
We don’t look like a rhinoceros or any other creature except a human being because our
parents are human beings.
--Heredity is the passing of traits (physical characteristics) from parents to offspring.
--Some traits that your parents have, you may not have, but your brothers and sisters may
have them. Some traits that your parents have won’t appear until you yourself have
children.
Heredity or genetic inheritance (The inheriting of genes from your parents) follows a
specific pattern…but, what is that pattern?
150 years ago, Gregor Mendel, an Austrian monk and botanist gave the scientific world
some answers by performing experiments with pea plants.
Mendel noticed that in his garden, some pea plant traits that appeared in one generation
did not show up in the offspring of the following generation of pea plants.
Although he wasn’t aware of it, he was using the scientific method.
1. Ask a question: How are specific traits inherited?
2. Form a hypothesis: Inheritance seems to follow a pattern.
3. Test the hypothesis: by experiments: Cross true breeding pea plants with their
offspring and observe
4. Analyze the results: Identify patterns in inherited traits
5. Draw conclusions: Traits are inherited in predictable patterns.
6. Communicate the results: Unfortunately this is one thing Mendel did not do
effectively
Mendel chose to use garden peas in his experiments because they grow fast (pea
plants are “annuals”, that is they grow from seed and produce seeds in one growing
season), they are self pollinating and they come in many different varieties (flower
color, plant height, seed shape, flower position etc.
Pollination in Flowers.
Look at the model in the front of the class room.
The female reproductive part of a flower is called the pistil, it contains the eggs of the
flower that will later become seeds after fertilization
The male reproductive part of a flower is called the stamen (the anther produces
pollen that is the sperm of a plant)
A self pollinating flower contains both male and female parts. The pollen from one
flower can fertilize the eggs of that same flower. This means the only source of DNA
is coming from the same plant.
Mendel used true breeding plants. When a true breeding plant self pollinates, all of
the offspring will have the same traits as their parents.
Mendel analyzed one trait at a time.
Example: One experiment looked at plant height (Tall vs. Short )
A tall plant self pollinates and all of the offspring are tall plants. It is a true breeding
plant.
Day 3
TURN TO PAGE 108-109
Mendel then cross pollinated two true breeding plants that had different forms of the
same trait
Example: a tall plants pollen is used to fertilize a short plants flowers (eggs). The
result was that the seeds grown from the first cross produced all tall plants. (not a
mixture of tall and short plants)
--One trait always appeared 100% of the time in the first generation (F1 generation),
Mendel called this the dominant trait because it seemed to dominate over the other
version of the trait he was analyzing.
--The trait that seemed to disappear in the F1 generation was called the recessive trait
because it seemed to recede into the background.
What happened to the recessive trait??
Mendel did a second experiment. He allowed the plants of the F1 generation to self
pollinate. (All of the DNA was coming from one source)
The trait that seemed to disappear in the first generation (F1), reappeared in the
second generation (F2) in a 3 to 1 ratio
3 tall plants to one short plant or…3 dominant traits to one recessive trait.
Mendel repeated this experiment with all seven of the traits he examined…the results
were the same…the F1 generation expressed the dominant trait 100% of the time and
the F2 generation expressed both the dominant and the recessive trait in a 3 to 1 ratio.
PAGE 110 SHOWS ALL SEVEN OF THE TRAITS THAT MENDEL EXAMINED
AND THEIR VARIATIONS.
--Mendel realized that each plant actually had two sets of instructions for each trait
(tall vs. short plants, smooth vs. wrinkled seeds, purple vs. white flowers etc.)
Each parent donates one set of instruction for each trait. These instructions later
became known as genes.
--Genes are segments of DNA that code for a particular trait
The two different forms of a gene that code for the same kind of trait are called alleles
Alleles are alternate forms of a gene that code for the same specific trait
So in the case of Mendel’s experiment we looked at earlier…
The specific gene he examined determined plant height
One allele coded for the plant to be tall…this was the dominant allele
One allele coded for the plant to be short…this was the recessive allele
We use a diagram called the Punnett square to visualize the inherited combination of
alleles. One allele is donated by one parent and the other allele is donated by the other
parent.
We select any letter to represent a gene in the Punnett square.
The capital form of the letter represents the dominant allele
The lower case letter represents the recessive allele
LOOK ON PG 111 AND ON THE CHALKBOARD FOR THIS EXAMPLE
Each of the 4 squares of the Punnett square represents a 25% probability that the
offspring will end up with this inherited combination of alleles
Probability is the mathematic description of the chances that two events will occur
For our example “R” is the dominant trait for brown eye color and “r” is the recessive
trait for blue eye color. What 3 combinations of alleles are we likely to see?
RR = brown eyes, 2 dominant alleles
Rr = brown eyes, 1 dominant and 1 recessive allele
Rr = blue eyes, 2 recessive alleles
Place the genetic contribution of the father on one side of the Punnett square.
The father in this case is “homozygous dominant” which means he can only
contribute a dominant trait to the zygote.
“RR” is known as the genotype, it is the combination of inherited alleles, in this case,
the genotype “RR” = homozygous dominant for brown eye color.
Reminder: A sperm and an egg are known as gametes or sex cells. When an egg is
fertilized the gametes (sperm and egg) fuse and form a zygote. A zygote is another
name for a fertilized egg.
Place the genetic contribution of the mother on the other side of the Punnett square.
The mother in this case is “homozygous recessive” which means she can only
contribute a recessive trait to the zygote. “rr” is known as the genotype, it is the
combination of inherited alleles, in this case, the genotype “” = homozygous recessive
for blue eye color.
Now do the crosses for the F1 generation in your Punnett square (just like using the
distributive property in math)
The results show that all of the offspring have one dominant allele contributed by the
father and one recessive allele contributed by the mother. There should be an “Rr” in
each box of the Punnett square.
This combination of inherited alleles is called “heterozygous” because it has the
possibility of contributing either a dominant allele or a recessive allele to its zygote (or
offspring)
Let’s take a look.
The genotype that resulted from our first cross is “Rr” in all four boxes of the Punnett
square.
The genotype of all four offspring is heterozygous (one dominant allele and one
recessive allele)
The phenotype for all of the offspring is that they will all have brown eyes. This is
because if even one capital letter is present in the genotype then the dominant trait
will be expressed
Reminder: The genotype is the inherited combination of alleles
The phenotype the physical appearance determined by the genotype.
Now take one of the offspring in the first cross and make another Punnett square. This
will be the genetic contribution of the father (“Rr”). Suppose this man marries a woman
with the same genotype “heterozygous”(“Rr”). Place her genetic contribution on the other
side of the square and do the cross.
What is the result?
Reminder: You should always construct a chart next to your Punnett square to keep track
of all your genotypes and phenotypes.
Your results should look like this
Genotypes
1 Homozygous dominant = RR
2 Heterozygous = Rr
1 Homozygous recessive = rr
Phenotypes
3 offspring with brown eyes
1 offspring with blue eyes
a 3 to 1 ratio
Your phenotype depends on your genotype.
IT WOULD BE A GOOD IDEA TO REVIEW MITOSIS BEFORE GOING ON TO
MEIOSIS…LOOK AT YOUR NOTES AND THE POWER POINT PRESENTATION
AVAILABLE ON THE CLASS WEB PAGE
DAY 4
MITOSIS REVIEW
EARLY INTERPHASE: DNA is not visible, cell machinery (organelles) and DNA are
being duplicated
LATE INTERPHASE: DNA condenses into spaghetti like structure called chromatin
PROPHASE: Duplicated chromosomes become visible as sister chromatids (each part of
the X is a chromatid). Centrioles migrate to opposite sides of the cell and form the
spindle apparatus
METAPHASE: Sister chromatids (chromosomes) line up in the middle of the cell (the
equator of the cell) and attach to the spindles by their centromeres.
ANAPHASE: The chromosomes split at the centromeres. The chromatids travel to
opposite poles (sides) of the cell.
TELOPHASE: Nuclear envelopes form around the two new sets of chromosomes to form
two new nuclei. The spindle apparatus breaks down
CYTOKINESIS: The cell divides up the cytoplasm and organelles between the two new
cells and then separate into two identical cells.
A QUICK WORD ABOUT MEIOSIS
MEIOSIS IS JUST LIKE MITOSIS EXCEPT THECHROMOSOMES DO NOT
DUPLICATE AGAIN AND THE STEPS PROPHASE, METAPASE, ANAPHASE,
TELOPHASE AND CYTOKINESIS ARE REPEATED. THIS WILL RESULT IN THE
FORMING OF 4 CELLS EACH WITH ONE HALF THE NUMBER OF
CHROMOSOMES AS THE ORIGINAL CELL. THE CELLS THAT HAVE HALF
THE NUMBER OF CHROMOSOMES AS THE ORIGINAL CELL ARE CALLED
SEX CELLS OR GAMETES.
MEIOSIS MEANS REDUCTION.
DAY 5
CHROMOSOMES: What they are made of and what they do.
During most of interphase DNA is not visible…How does it become visible?
--DNA condenses around proteins called histones. DNA winds around the histone
proteins like thread around a spool forming chromatin, a spaghetti like structure.
--The spools of DNA and histone proteins then pull closer together and form
chromosomes, ziti like structures
A chromosome is a combination of DNA and histone proteins
LOOK AT PG 114
A karyotype is a picture profile of all of the chromosomes in the nucleus of an organism
Look at the chromosomes in the karyotype on pg 114
The chromosomes are grouped in homologous pairs.
Homologous pairs: Two chromosomes that look alike in size and carry similar genes.
Note how these chromosomes on pg 114 have a pattern of bands. These bands are actual
genes.
Genes are located on chromosomes
Reminder: Alternate forms of the gene that codes for a particular trait are called alleles
One allele is found on one member of the homologous pair donated by the female in her
egg.
One allele is found on the other member of the homologous pair donated by the male in
his sperm.
DAY 6
MEIOSIS AND REPRODUCTION
There are two kinds of reproduction…asexual reproduction and sexual reproduction
In asexual reproduction only one parent cell is needed…it copies itself by mitosis which
we know results in two identical cells.
--bacteria divide by asexual reproduction in a process we learned was called binary
fission (to split into two)
An easy way to remember the difference between asexual reproduction and sexual
reproduction is…
In asexual reproduction “one makes two”
In sexual reproduction “two makes one”
In sexual reproduction, 2 sex cells or gametes join together during fertilization to form a
zygote (fertilized egg)
--female sex cell/gamete is called the egg or ova
--male sex cell/gamete is called the sperm
Sperm + Ova (egg) -------fertilization---- Zygote (fertilized egg)
Sex cells contain ½ the chromosomes found in a regular (somatic) cell.
“somatic = “of the body” so a somatic cell = a body cell
Humans have 46 chromosomes in their somatic cells, their sex cells/gametes have 23 (½
of 46)
23 + 23 ---------fertilization----
Sperm Egg
46
Zygote
Meiosis is the process that reduces the number of chromosomes by one half to form sex
cells/gametes.
Meiosis is Greek for reduction
Meiosis is like is like two rounds of mitosis except the chromosomes are copied once but
the cell divides twice.
Meiosis is divided into 2 steps…Meiosis I and Meiosis II
Meiosis I = IPMATC and Meiosis II = PMATC
(Do you see how it’s like 2 rounds of mitosis?)
The major difference between meiosis and mitosis is during anaphase I and anaphase II
--During anaphase I the sister chromatids don’t separate at the centromeres, instead
homologous pairs separate and move to opposite sides (poles) of the cell
--During anaphase II the chromatids separate at the centromeres and the chromosomes
move to opposite poles (sides) of the cell
Mitosis takes one cell and makes it into 2 identical cells
Meiosis takes one cell and makes it into 4 sex cells with ½ the chromosomes as the
original cell
--In males, one cell becomes 4 sperm cells and all 4 can function as sex cells/gametes
--In females, one cell becomes 1 ova/egg and 3 polar bodies, this is because during
meiosis the cell that will become the egg gets most of the cytoplasm during division
while the polar bodies hardly get any. Only the egg can function as a sex cell/ gamete
REMINDER: Genes are segments of DNA that code for a particular trait
DNA condenses into spaghetti like chromatin
Chromatin condenses into ziti like chromosomes (DNA wrapped around histone proteins)
Therefore genes are located on chromosomes
A duplicated chromosome consists of two sister chromatids(each chromatid is one half of
the X)
Homologous pairs of chromosomes are two chromosomes that are alike in size and the
type of genes they carry.
1 member of the homologous pair was donated by the mother in her egg
The other member of the homologous pair was donated by the father in his sperm
The alternate forms of a gene that code for the same trait but give a different physical
appearance are called alleles, one allele is located on one member of the homologous pair
and the other allele is located on the other member of the homologous pair.
SEX CHROMOSOMES
Sex chromosomes carry genes that will determine if the offspring will be male or female
--the female sex chromosome is called the X chromosome
--The male sex chromosome is called the Y chromosome
If genotype of the zygote is homozygous for the X chromosome the offspring will be
female (the phenotype is female) Genotype = XX = female phenotype
If genotype of the zygote is heterozygous for the X and Y chromosomes the offspring
will be male (the phenotype is male) Genotype = XY = male phenotype
COMPARE THE KARYOTYPES ON PG 114 AND PG 119…WHAT ONE IS
FEMALE?…WHAT ONE IS MALE?
Sex chromosomes are the only homologous pair that are different in size
They are still considered homologous because both are used to determine what sex the
offspring will be.