Heredity Chapter 5
Introduction:
For many years we have been breeding dogs, cats, horses and other animals and plants to
produce offspring with desired traits. (Example: horses that can run fast, flowers that are
prettier than others, dogs that have better mannerisms, etc.) We also try to breed plants
that produce offspring that will produce a greater harvest, or resistant to specific diseases.
Can you think of other plants and animals that are bred or crossed for specific traits?
Write a few down.
Mendel and his pea plants
If you look at yourself and try to compare yourself to others, most likely there is no one
person exactly like you. Even if you have a twin, you are not exactly alike. You may
resemble each other or your parents, but there is no one else exactly like you. That is
what makes us all special and unique.
We will find out in this chapter, why we are not all exactly alike and how we get the traits
we have that cause us to look, act, and behave different from other humans. We will
focus on plants and animal traits to study this.
Gregor Mendel also wanted to know why organisms differed and what caused the
differences to show up in offspring.
Why Don’t You Look Like A (Rhinoceros or any other Organism?)
Well, what do you think the answer to this topic is? If you said because your parents are
not a rhinoceros or some other organism, then you are correct! You are who and what
you are due to your parents. Heredity is the passing of traits, also called genes, from
parents to offspring.
The person given credit for discovering this was Gregor Mendel. He worked with pea
plants and bred pea plants to determine how traits are passed from one generation to
another (parents to offspring). Mendel noticed some pea plants were always tall, some
always short, and some always produced purple flowers, while others always produced
white. Mendel questioned why this happened.
Mendel lived in a monastery during the time he studied the pea plant characteristics. Pea
plants are self-pollinators. In other words, one pea plant flower has both male and female
parts. The male parts produce pollen and the female part produces the egg. In this
situation, pollen from one flower can fertilize the eggs of the same flower or the eggs of
another flower on the same plant. With the help of an insect, wind, or other organism, the
pollen can also be carried to a totally different pea plant where fertilization can occur.
When genes or traits are passed from the parent to the offspring, we say the offspring has
inherited the genes from the parents. Mendel noticed, from his breeding of pea plants,
that sometimes a trait from one generation would not show up in the second, but if he
crossed (bred or mated) pea plants of the second generation, the traits would show back
up in the third generation. Mendel noticed the same occurrences in other plants and
animals also. To simplify all of his observations and try to learn what was occurring, he
decided to work exclusively with the garden pea plant.
To make his research easier, he decided to study one trait at a time. He worked with one
group of pea plants regarding the height of the plant from one generation to the next and
in a separate experiment with different pea plants he worked with the color of flower the
plants produced.
Recall from Chapter 1 (Scientific Method): When doing experiments it is best to test one
variable at a time. This was what Mendel was doing by looking at height in one group of
peas and flower color in a different group.
True-Breeding Plants
When a true-breeding plant pollinates itself, it always produces offspring with the same
trait as the parent plant. Example: a true-breeding plant for the tall characteristic always
produces tall offspring, a true-breeding plant for purple flowers always produces
offspring with purple flowers.
Traits Mendel Looked at with the Pea Plants
Mendel noticed some of the pea plants produced wrinkled peas and some produced round
peas. Mendel crossed a plant that produced wrinkled peas with a plant that produced
round peas (these are the parent generation). The offspring from this cross is called the
first generation. Mendel noticed the first generation plants produced all round peas. So
Mendel concluded that the round trait must hide the wrinkled trait. Mendel called the trait
that appeared; the dominant trait and the one that did not show up he called the recessive
trait. Mendel then allowed the first generation to self-pollinate and the second-generation
plants produced round and wrinkled seed, but he noticed for every round seed there was
only one wrinkled seed.
In other words, the recessive trait showed up again in the second generation.
Mendel tested seven traits he found the pea plants to have and the following shows his
results.
Characteristic
Flower color
Seed color
Seed shape
Pod color
Dominant
purple
yellow
round
green
Recessive
white
green
wrinkled
yellow
Pod shape
Flower position
Plant Height
smooth
along stem
tall
bumpy
tip of stem
short
Out of all of these traits Mendel conducted research on he noticed the result of the
second-generation always had the dominant trait about three times more often than the
recessive trait. So for every time a dominant trait showed up, the recessive trait showed
up one time. This produces a “ratio” of 3:1.
Mendel’s Brilliant Idea
Mendel realized that the only way this could happen, was if each plant had two sets of
instructions (now known as genes) for each characteristic. Mendel reasoned that each
parent must contribute one gene each and the offspring would end up with two. The
offspring therefore would have two forms of instruction (genes) for the same trait. Each
individual form of a gene is known as alleles.
Proving His Idea was up to the Punnett Square
Mendel did not invent the Punnett Square, it was invented by a man with the last name of
Punnett. A Punnett Square is a visual tool to see all the possible combinations of alleles
that offspring can receive from their parents. When using a Punnett Square the dominant
alleles are given a capital letter and the recessive alleles are assigned a lower case letter.
Example: For flower color, Mendel noted that purple was dominant over the white
recessive gene, therefore in a Punnett square purple would be assigned a “P” and white
would be assigned a “p”.
How is a Punnett Square Set Up?
First, you draw a square that is large enough to divide into equal quarters and the quarters
should be large enough to write the letter of the alleles into.
Try to follow this example on your own paper. We want to breed or cross a true-breeding
purple pea plant “PP” with a true-breeding white pea plant “pp”. So, we would write “PP
x pp”. Now for the square:
Draw a square large enough, maybe 2 inches by 2 inches and the divide it into equal units
of four 1 inch by 1 inch squares. It should resemble the square below
Now we place our parents alleles on the outside of the square. One parent’s alleles will
go across the top and the other will go down the side. Remember we are crossing PP x
pp. You now should have a box that looks like this:
P
P
p
p
Next we bring one parents alleles down and place them into the square and bring the
other parents alleles across and place then into the square like this:
P
P
p
Pp
Pp
Pp
Pp
p
Now we have all the possible
genotypes of the offspring. In
this situation all of the
offspring will have Pp which is
one alleles for the dominant purple and one allele for the recessive white. All of the pea
plants will produce purple flowers, but carry the recessive gene for the white flower. This
situation is called heterozygous (one allele for a different form of the same
characteristic). This is the same type of work Mendel completed.
Now let’s see what happens if we take these offspring (first generation) and cross them
together (Pp x Pp).
P
p
P
PP
Pp
p
Pp
pp
Now, we see the genotype possibilities are PP, Pp, Pp, and pp. For the genotypes, one is
homozygous dominant “PP”, two are heterozygous “Pp”, and one has the possibility of
being homozygous recessive “pp”. for the phenotypes, three have the possibility of being
purple and one has a possibility of being white. So now we have Mendel’s 3:1 ratio. So in
this cross, there is a 75% probability that the pea will produce purple flowers and 25%
probability the offspring plant will be white flowered.
All of these probabilities are random, in other words it is entirely random as to which
alleles the offspring gets and each time we breed organisms with this possibility, there is
the same chance to get the same outcome. Each fertilization or cross we conduct is
independently random each time. Just because we cross a pea plant three times and get
offspring that has purple flowers, does not guarantee we will get a white flowered pea
plant on the fourth cross we complete.
What is probability? Probability is the mathematical chance that an event will happen or
occur. Probability is usually expressed as a fraction or percentage (3/4 or 75%) or a ratio
can be used sometimes (3:1).
Gregor Mendel published his findings in 1865, but his ideas were not given much
attention until after his death about 30 years later. We now often referr to him as the
“father of modern genetics”. Genetics is the study of passing of traits from one generation
to another.
PRACTICE TIME HOMEWORK (1-10): Show your work on a separate sheet of paper
and be prepared to turn in the problems for a grade.
Cross the Following:
1.
A true- breeding purple flowered pea plant with a heterozygous pea plant. You
should have visualized or determined the true-breeding plant as “PP” and the
heterozygous as “Pp”, so you have PP x Pp.
2. Cross a heterozygous purple pea plant with a white pea plant.
3. Use “T” for the tall trait, and “t” for the short trait. Cross a homozygous tall
plant with a homozygous short plant.
4. Cross a heterozygous tall pea plant with a homozygous tall pea plant.
5. “R” is dominant for seed shape in peas, it means round. Wrinkled is the
recessive trait. Cross a homozygous round pea plant with a homozygous wrinkled
pea plant.
6. Cross a heterozygous round pea plant with a homozygous round pea plant.
7. Cross a heterozygous round pea plant with a homozygous wrinkled pea plant.
8. Cross a homozygous smooth pod (smooth is dominant, bumpy is recessive) pea
plant with a heterozygous smooth pod pea plant.
9. Cross a homozygous smooth pod pea plant with a homozygous bumpy pea plant,
then cross the first generation and show your second generation results. Tell me
what the original parents (parent generation) genotype is in this scenario.
10. Now, brown hair is dominant over blonde hair in humans. What would the
genotype of a homozygous brown haired person be? What about a heterozygous
brown? What about a homozygous recessive blonde? Now cross a homozygous
brown haired individual with a blonde haired individual. Cross two heterozygous
brown haired individuals. Make sure you show all of your work clearly.
With this information you should be able to cross organisms with different forms of the
same gene at this time. This is referred to a mono-cross. Mono means one. We will learn
how to cross organisms with different forms of the same gene for two different genes,
which are called a di-hybrid cross. Di meaning two.
Chapter 5 Section 2 Notes
Meiosis
Recall: there are two kinds of reproduction, 1) asexual and 2) sexual reproduction.
In asexual reproduction, only one parent is needed for reproduction to occur. This is how
bacteria or prokaryotic cells reproduce. They copy their genetic information and then
divide (binary fission).
In sexual reproduction, you must have two parent cells known as sex cells. In males, the
sex cell is the sperm. In females, the sex cell is the egg. Remember that humans have 23
pairs of chromosomes. When we look at sex cells, each one will have only 23
chromosomes so when an egg and sperm unite, we end up with 23 pairs of chromosomes
again (one from the mother and one from the father). In other words, human sex cells
have half the usual number of chromosomes.
For sex cells to have 23 chromosomes, they must go through a process called meiosis.
Meiosis produces new cells with half the usual number of chromosomes. Simply put,
when sex cells are made, the chromosomes are copied and then the nucleus divides two
times. The resulting cells (egg and sperm) have half the number of chromosomes found
in a normal body cell.
Location, Location, Location
What does location have to do with genes? Well, a man by the name of Walter Sutton
discovered that genes are located on chromosomes. Scientists have actually located
specific genes and their locations on the chromosomes.
RECALL, RECALL, RECALL the steps of mitosis because meiosis is very similar.
Review mitosis for a better understanding (Interphase, Prophase, Metaphase, Anaphase,
and Telophase) know what happens in each phase that we have discussed in the past.
The Process of Meiosis (remember: meiosis is for sex cells).
The following is the phases of meiosis and the major event that happens in each phase.
Interphase: during this phase the chromosomes copy themselves.
Prophase 1: the nuclear membrane disappears and the chromosomes begin to pool into
the center of the cell.
Metaphase 1: the chromosomes line up on the equator or middle of the cell.
Anaphase 1: the copied pairs of chromosomes pull away from each other toward the
“poles” of the cell.
Telophase 1: the nuclear membrane reforms and the cell divide taking one complete pair
of chromosomes into each new cell. (Now we have two cells with 23 pairs of
chromosomes).
Prophase 2: the nuclear membrane disappears and the pairs of chromosomes in each cell
pool in the cell.
Metaphase 2: the pair of chromosomes line up on the equator of each cell.
Anaphase 2: the pairs of chromosomes pull apart and move to the poles of each cell. (23
chromosomes move one direction and 23 move the other direction).
Telophase 2: the two cells form cleavage furrows, nuclear membranes reform and each
cell divides to end up with four cells total and each cell has 23 chromosomes. Each new
cell has half the number of chromosomes present in the original cell.
QUESTION: Does this process resemble anything we studied before? What does it
resemble?
Meiosis review:
1. In a human, how many chromosomes are in the original single cell before meiosis?
2. In a human, how many times do chromosomes make copies of themselves in meiosis?
3. In a human, how many times do cells divide in meiosis?
4. In a human, how many chromosomes are in the cells at the end of meiosis?
5. In a human, how many chromosomes are in the cells at the end of mitosis?
Meiosis Review Answers
1.
In a human, there are 23 pairs of chromosomes or 46 chromosomes in the
original single cell before Interphase begins. At the end of Interphase, there are 46
pairs of chromosomes or 92 chromosomes. At the end of Telophase 1 there are 23
pairs or 46 chromosomes in each cell. At the end of Telophase 2, there are 23
chromosomes in each cell and we have four cells total.
2.
In a human, chromosomes copy themselves only one time in meiosis.
3.
In a human, cells divide in meiosis two times. (Once in Telophase 1 and once in
Telophase 2).
4.
In a human, there are 23 chromosomes in each of the four cells after the
completion of meiosis.
5.
In a human, there are 23 pairs (46 chromosomes) in each cell at the end of
mitosis.
Male or Female?
In humans we have 23 pairs of chromosomes. Twenty-two of those pairs are called
autosomes, but one pair is called sex chromosomes. The reason they are called sex
chromosomes is because these chromosomes determine if you are a male or female.
The sex chromosomes resemble an “X” or a “Y”. Remember we get one from our father
and one from our mother, so we have two sex chromosomes. If a person has two X’s
(XX), then the person is a female. If the person has an X and a Y (XY), then the person is
a male.
In simple terms, the female can contribute an X sex chromosome and the male may
contribute an X or a Y to the offspring. Experiment: Cross a male and female in a Punnett
square to see the probability of having a male or female offspring. What did you get?
Remember, it is random as to which sex chromosomes become fertilized together so we
can only give probabilities that an offspring will be male or female by using Punnett
squares.
We have become so technologically advanced that we can remove some of the cells from
an embryo prior to birth and look at the sex chromosomes to determine if a male or
female has been conceived. We also have ultrasound tests that can give us a picture of the
fetus and knowing what to look for, we can tell if the child will be a male or female.
Chapter 5 Section 1 Worksheet (You must read to find the answers). (pages 106-110).
1.Why do you think you do not look exactly like anyone else?
2. When traits or genes are passed from parents to offspring, we call this h
.
3. One scientist is known in the realm of science for working with pea plants in order
to understand how traits are passed on from one generation to another.
This scientist is G
M
and was born in 1822 in Austria.
4. Mendel used the Scientific Method to conduct his work. Look at the example below:
Ask a Question: How are traits inherited?
Form a hypothesis: Inheritance has a pattern.
Test the Hypothesis: Cross true breeding plants and offspring.
Analyze the results: Identify patterns in inherited traits.
Draw Conclusions: Traits are inherited in predictable patterns.
As we study Mendel’s work, we will see that the example of the scientific method
worked in this situation very well.
5. Draw and label the parts to the flower on page 107 in your book.
Explain what a true breeding plant is in your own words.
In Mendel’s first experiment, he called the trait that showed up (appeared) the d
trait and the trait that “seemed to disappear” or recede into the background Mendel
called the r
trait.
What characteristics of the pea plant did Mendel look at? (page 109 tells the number and
page 110 tells each characteristic).
In Mendel’s first experiment, he looked at pea seed shapes. What were the two shapes he
wondered about?
What were the results of the first experiment?
What did Mendel do for his second experiment? What were the surprising results? How
many round seeds turned up for every wrinkled seed?
Look at the numbers for the results of the second-generation crosses that Mendel
completed. Do you see a pattern? What is the number of Dominant traits showing up
for every Recessive trait? (Not an exact number with his data, but pretty close in all
crosses).
Chapter 5 Study Sheet 2
Mendel realized that offspring must have t
sets of instructions and that the
offspring must get o
set from each parent.
When each parent donates one set of instructions, we call these instructions ch
. This means the fertilized egg will have two forms of the same gene for every
characteristic (one from each parent). The two forms of a gene are known as
a
.
We now use Punnett Squares to understand Mendel’s concept (a picture is worth a
thousand words). The actual genetic make-up of an organism is known as the organism’s
g
. The appearance of an organism is the organism’s p
when we discuss heredity.
Recall (Dominant trait versus Recessive Trait).
We represent a dominant trait by using c
using l
c
letters.
letters, and a recessive trait by
Terms to recall (Heterozygous and Homozygous)
Use a Punnett Square to cross a homozygous purple flowered pea plant with a
homozygous white flowered pea plant (PP x pp).
Recall from notes and class (distinguish between self pollination and cross pollination).
Recall the terms genotype and phenotype.