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Ch. 1
Notes for Chapter 1
Metric units of measurement
The
When working with mass, volume, and length, the “metric staircase” is a helpful visual tool, especially when doing conversions between units:
If going from a big to a smaller unit, move decimal
point to RIGHT the number of times you jump down the staircase. Ex.- 1 meter =
_______millimeters Meters to millimeters is 3
jumps. So there’s a decimal point after 1. Move decimal point 3 places to
right. 1. _ _ _ = 1000. So 1 meter = 1000 millimeters
km
hm
dkm
If going from a smaller to a bigger unit, move decimal
point to LEFT the number of times you jump up the staircase. Ex. – 1 millimeter = _______meters Millimeters to meters is 3
jumps. There’s a decimal point after 1. Move decimal point 3 places to left. _ _ _ 1. So 1 millimeter = 0.001 meters
m
dm
cm
mm
You can remember how the order of the units goes by remembering a simple mnemonic, or memory aide: King Henry Died Monday Drinking Chocolate Milk.
You can use the metric staircase for meters (as we did above), volume, or mass. Just be sure to substitute the ‘m’ on the right of the unit with ‘L’ for liters and ‘g’ for grams.
Common SI (Metric) units:
km (kilometer) =
1000 meters
hm (hectometer) =
100 meters
dkm (dekameter) =
10 meters
m (meter) = 1
meter (base unit)
dm (decimeter) = 1/10 of meter
cm (centimeter) = 1/100 of meter
mm (millimeter) = 1/1000 of meter
The Scientific Method is a useful tool for engaging in scientific inquiry. The traditional steps are:
- Observe
- Ask a question
- Research
- Form a hypothesis
- Test the hypothesis
- Record and analyze data
- Form a conclusion
It is also important to repeat your experiment, to exclude the possibility of error in your experimental setup. You also must communicate your results to the rest of the scientific community. In this way, you contribute to building up of knowledge and experience in a particular scientific discipline, and others benefit from your work.
Let’s put these steps into a practical example. You might observe that leaves are starting to change from green to shades of red, brown, and orange during the fall season. You then ask a question: “What is causing the leaves to change color during this time every year?” You then do some background research. You are now in a position to form a hypothesis: The leaves are changing color as a result of chemical reactions occurring. You must test this hypothesis, and then record and analyze data that you obtain. Only now can you form a reasonable conclusion: The leaves are changing color because of a breakdown in chlorophyll, and secondary chemical reactions that cause the green color to disappear and hues of red, brown, and orange to appear in its place.
Experiments are made up of variables, which are factors in an experiment that change. The Independent Variable (IV) is the factor that the experimenter changes on purpose. In an experiment that seeks to determine the effect of differing amounts of water on plant growth, the differing amounts of water would be the IV – the experimenter might give Plant A 10mL of water, Plant B 20mL, Plant C 30mL, and Plant D 40mL. The factor that changes as a result of the purposely-changed factor is called the Dependent Variable (DV). In other words, the experiment changes it. In our plant growth experiment example, plant growth would be the DV. The independent variable will have different levels, or ways that the experimenter changes it – this is referred to as Level of Independent Variable (LIV). In our example, the different ways that he/she changes, or manipulates the IV are applying 10, 20, 30, and 40mL of water to the different plants. Variables that do not change in an experiment are called Constants. In our sample experiment, some constants might be same type of water (distilled), same type of soil, same type of pot, etc.). An experiment will usually have a Control. A control group is used for comparison with the experimental groups. So in our example, Plant E might be the control – it is not given any water. It is a good idea to repeat our experiment, to reduce the possibility of error. So the Number of Repeated Trials (NRT) refers to the number of times that each level of independent variable is tested. In our example, Plants A-D will be given the specified amounts of water (10, 20, 30, and 40mL) of water a total of three times – so the NRT for this experiment would be 3.
u We want to be able to formulate good title and hypothesis statements for our experiment. When you write an experimental title, you are basically stating what the effect of the independent variable is on the dependent variable, and you write it in this format:
The Effect of IV on DV.
So a good title for our plant experiment would be:
The Effect of Amount of Water on Plant Height.
| |
IV DV
We then can write a hypothesis statement for our
experiment. Hypothesis statements follow
an “If, then” format. Basically, in a
hypothesis statement you are making a prediction about how the dependent
variable will change if you make a certain change to the independent
variable: If how you are
changing the IV, then how you predict the DV will change. So a good hypothesis statement for our
experiment would be:
If the amount of water given a plant is increased,
then the height the plant will grow will be increased.
There are of course several different hypothesis statements
that could be written for an experiment such as this. Some are written in such a way that a change
in magnitude in the IV reflects a similar magnitude change in the DV. These are in direct proportion:
If IV increases, then DV increases.
If the amount of water given a plant is increased,
then the plant height will be increased.
If IV decreases, then DV decreases.
If the amount of water given a plant is decreased,
then the plant height will be decreased.
Others are written in such a way that a change in magnitude
in the IV reflects the opposite change in magnitude in the DV. These are in inverse proportion:
If IV increases, then DV decreases.
If the amount of water given to a plant is increased,
then the plant height will be decreased.
If IV decreases, then DV increases.
If the amount of water given a plant is decreased,
then the plant height will be increased.
Let’s take all of this information and
place it on an Experimental Design Diagram:
Experimental
Design Frame/Diagram
Title:
The Effect of Amount of Water on Plant Height
Hypothesis:
If the amount of water given a plant is increased, then the
plant height will be increased.
IV:
Amount of water
Levels
of IV
(LIV)
10mL 20mL
30mL 40mL No
water
(control)
#
of Repeated 3 3 3 3 3
Trials
(NRT)
DV: Plant height
Constants
– C: Type of water, type of soil, type of pot
Notes for Chapter 1 – The World of Life Science
Section 1 – Asking About Life – Science is a process of gathering knowledge about the natural world. Science includes making observations and asking questions about those observations. Life Science is the study of living things. A variety of people may become life scientists for a variety of reasons. Life Science can help solve problems such as disease or pollution, and it can be applied to help living things survive.
Section 2 – Scientific Methods – Scientific methods are the ways in which scientists follow steps to answer questions and solve problems. There are certain steps that scientists use whenever they are engaged in scientific inquiry. First, an observation is made – this is any use of the senses to gather information (for example, noting that the sky is blue or that a cotton ball feels soft). Scientists are then led to ask questions about their observations. After gathering preliminary information, scientists are then ready to form a hypothesis – a possible explanation or answer to a question. A good hypothesis is always testable. In other words, information can be gathered or an experiment can be designed to test the hypothesis. Scientists then make a prediction of what they think will happen before testing the hypothesis. One way to test a hypothesis is to do a controlled experiment, which tests only one factor at a time and consists of a control group and one or more experimental groups. All of the factors for both control and experimental groups are the same except for one. The one factor that differs is called the variable. –Pieces of information obtained through experimentation are called data. After testing a hypothesis, it is important to analyze your results by using calculations, tables, and graphs. Then, after analyzing your results, you should draw conclusions about whether your hypothesis is supported. Finally, communicating your results allows others to check or continue your work.
Section 3 – Scientific Models – A model is a representation
of an object or system. Models are
often used to represent things that are very small or very large. There are three kinds of scientific
models: Physical, mathematical, and
conceptual. A physical model
might be a model space shuttle, or the human eye. Mathematical models are made up of
equations and data, and sometimes use computers.
Section 4 – Tools, Measurement, and Safety – A tool is anything that helps you do a task. Scientists use many tools to help them in their experiments. The application of science for practical purposes is called technology. By using technology, life scientists are able to find information and solve problems in new ways. Some forms of technological tools used in life science are those that enable us to see fine details in things that are too small to be seen with the unaided eye. The compound light microscope is an instrument that magnifies small objects so that they can be easily seen. It has three main parts-a tube with two or more lenses, a stage, and a light. There are other more sophisticated microscopes that do not use light. In electron microscopes, tiny particles called electrons are used to produce magnified images clearer and more detailed than those made with light microscopes.
One way to collect data is to take measurements. But to do this, you need the proper tools, and a common system of measurement used throughout the world. (See metric information posted on 9/19 for detailed information on the International System of Units (SI), or metric system).
Length, volume, mass, and temperature are types of measurement used in science. The meter is the basic SI unit of length. Mass is the amount of matter in an object, and the kilogram (kg) is the basic unit for mass. The kilogram is used to describe the mass of large objects, and the gram is used to measure the mass of smaller objects. Volume is the amount of space that something occupies. Liquid volume is expressed in liters (L). Liters are based on the meter. A cubic meter is equal to 1,000 L. Volumes of solid objects are expressed in cubic meters. If you measure the mass and volume of an object, you have enough information to measure its density – the amount of matter in a given volume. Density is called a derived quantity because it is found by combining the two basic quantities of mass and volume. The equation that relates density to mass and volume is:
m
D = --------
V
The temperature of a substance is a measurement of hot or cold the substance is. Degrees Fahrenheit and degrees Celsius are often used to describe temperature. The SI unit for temperature is the Kelvin (K).
Area is a measure of how much surface an object has, and can be calculated from measurements such as length and width.
You will frequently encounter different safety symbols and rules when engaging in scientific investigations. Always pay attention to any safety labels on the sides of chemicals or other equipment – these alert you to what precautions you need to take, such as wearing goggles or gloves.
Notes for Chapter 2– It’s Alive!! Or Is It?
Section 1 – Characteristics of Living Things –Living things can be said to share six basic characteristics. These include (1) Living things have cells – A cell is a membrane covered structure that contains all of the materials necessary for life. Some organisms are made of one cell, and are called unicellular. More complex organisms are made of billions, even trillions of cells, and are called multicellular. (2) Living things sense and respond to change – All organisms have the ability to sense change in their environment and to respond to that change. A change that affects the activity of the organism is called a stimulus. Stimuli can be chemicals, gravity, light, sounds, hunger – anything that causes an organism to respond in some way. Although an organism’s outside environment can change, conditions inside the organism’s body must remain the same. The maintenance of a stable internal environment is called homeostasis. (3) Living things reproduce – Organisms make other organisms similar to themselves. They do so in two ways. In sexual reproduction, two parents contribute sex cells that unite, producing offspring that share traits from both parents. In asexual reproduction, a single parent produces offspring that are identical to the parent. (4) Living things have DNA – DNA is present in the cells of all living things, and controls the structure and function of cells. When organisms reproduce, they pass copies of their DNA to their offspring. This passing on of traits from one generation to the next is called heredity. (5) Living things use energy – Organisms use energy to carry out the activities of life – things like making food, breaking down food, moving materials into and out of cells, and building cells. An organism’s metabolism is the total of all the chemical activities that the organism performs. (6) Living things grow and develop – all living things – whether single-celled or multi-celled – grow during periods of their lives. In single-celled organisms, the cell gets larger and divides, making other organisms. In multi-celled organisms, the number of cells gets larger, and the organism gets bigger.
Section 2 – The Necessities of Life – Every organism has the same basic needs – water, air, a place to live, and food. Some organisms, such as plants, are producers, meaning they can make their own food by using energy (either solar or chemical) from its surroundings. Other organisms are consumers because they eat (consume) other organisms to get food. Other organisms are decomposers, meaning that they get energy by breaking down the remains of dead plants and animals.
All organisms, whether they are producers, consumers, or decomposers, need to break down food in order to use the nutrients in it. These nutrients are made of molecules (substance made when two or more atoms combine). Molecules found in living things are usually made up of combinations of six elements – carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. These elements combine to firm proteins, carbohydrates, lipids, ATP, and nucleic acids. Proteins are large molecules that are made up of smaller molecules called amino acids. Organisms break down the proteins in food in order to supply their cells with amino acids. Carbohydrates are molecules made up of sugars. Cells use carbohydrates as a source of energy and for energy storage. There are two kinds of carbohydrates – simple and complex. Simple carbohydrates are made up of one sugar molecule or a few linked together (ex. Table sugar). When an organism has more sugar than it needs, its extra sugar is stored as complex carbohydrates. Lipids are compounds that cannot mix with water. Having important cellular functions, lipids, like carbohydrates, store energy. Some lipids form the membranes of cells; these lipids are called phospholipids. This special form of lipids is made up two main parts, a head and a tail (see p. 46). The head of a phospholipid molecule is attracted to water, while the tail is not. Fats and oils are lipids that store energy. When an organism has used up most of its carbohydrates, it can get energy from these lipids. ATP is another important molecule. It is the main energy-carrying molecule in the cell. The energy in carbohydrates and lipids must be transferred to ATP, which then provides fuel for cellular activities. Nucleic acids are called the blueprints of life because they have all the information needed for a cell to make proteins. Nucleic acids are made up of large molecules called nucleotides. A nucleic acid may have thousands of nucleotides. The order of these nucleotides stores information. DNA is a nucleic acid, and is like a recipe book for how to make proteins.
Notes
for Chapter 3 – Cells: The Basic Units of Life
Section 1 – The Diversity of Cells – Cells are the smallest unit that can perform all
life processes. They are membrane
covered and have DNA and cytoplasm.
The Cell Theory
• All living things are made of one or more
cells
• Cells are the basic units of structure and
function in living things
• All cells come from existing cells
Cell size - Most cells cannot be seen without a
microscope. The yolk of a chicken egg is
an exception – it is one big cell.
• If cells get too large, then the cell’s surface area will not be large enough to take in enough food or pump out wastes
Parts of a Cell –
The Cell Membrane and Cytoplasm
• The cell membrane
is a phospholipid layer covering a cell’s
surface-acts as a doorway to the cell
• It is a protective layer that covers the
cell’s surface
• Fluid and almost all its contents inside cell is called cytoplasm
Parts of a Cell -
Organelles
• Inside a cell’s cytoplasm are small bodies that are specialized to perform a specific function – these are the organelles
Parts of a Cell –
Nucleus
• In eukaryotic cells, the nucleus is an organelle that contains the cell’s
DNA.
• Control center, or ‘brain’ of cell. Has a role in processes such as growth,
metabolism, and reproduction
• Genetic material DNA is enclosed inside the
nucleus
Two Kinds of
Cells – Prokaryotes and Eukaryotes
• The two basic types of cells are those without
a nucleus (prokaryotes) and with a nucleus (eukaryotes)
• Prokaryotes are single-celled organisms that do not have a nucleus of membrane-bound
organelles
• There are two types of prokaryotes – eubacteria (or just bacteria), and archaebacteria
• Eukaryotes are organisms made up of cells that have a membrane-enclosed nucleus
• Eukaryotes include animals, plants, and fungi
– but not eubacteria or archaebacteria
Section 2 –
Eukaryotic Cells - Some
eukaryotic cells have cell walls. A cell wall is a rigid structure that gives support
to a cell.
Cell Membrane -
• The cell membrane
is a protective layer that encloses a cell
• Contains proteins, lipids, and phospholipids
(lipid that contains phosphorus)
Cytoskeleton –
• The cytoskeleton is a web of proteins in the
cytoplasm
• Keeps the cells’ membrane from collapsing
• Made up of three types of proteins – one is a
hollow tube, other two are long, stringy fibers
Nucleus –
• The nucleus
is a large organelle in a eukaryotic cell
• Contains the cell’s DNA (genetic material)
• DNA contains information
Ribosomes -
• Ribosomes are
small grain-like body made of RNA
• Found in ER or free floating in cytoplasm
• produced in nucleolus
• Place where proteins are made
Endoplasmic
Reticulum –
• The endoplasmic
reticulum is a system of membranes found in a cell’s cytoplasm
• Tubular passage-way that lead out from
nuclear membrane--spreads throughout cell
• Carries proteins from one part of cell to
another
Mitochondrion -
• Mitochondrion is the main power source of a cell; it’s power house
• Rod-shaped
• located in cytoplasm
• Where food molecules are broken down to make
energy
Chloroplasts –
• Chloroplasts are organelles in plant and
algae cells
• Photosynthesis takes place in chloroplasts
• Chloroplasts are green because they contain chlorophyll,
a green pigment
Golgi Complex -
• The Golgi complex
packages and distributes proteins to be transported out of the cell
Cell Compartments -
• A vesicle
is a small cavity or sac that contains materials in a eukaryotic cell
• A vacuole is a large vesicle. Some vacuoles act like large lysosomes; others store water and other liquids
Lysosomes -
• Lysosomes are
organelles that contain digestive enzymes
• Destroy worn-out or damaged organelles
• Get rid of waste materials
• Protect cell from foreign invaders
Section 3 – The
Organization of Living Things – Multicellular organisms (those made of many cells) grow by
making more small cells, not by making their cells larger.
• There are several benefits to being multicellular, as opposed to being made up of only one cell
(unicellular). One is larger size
– larger organisms are prey for fewer predators. A second is longer life – the life
span of a multicellular organism is not limited to
the life span of any single cell. A
third advantage is specialization – each type of cell has a specific job, and
specialization makes the organism more efficient. For example, the cardiac muscle cell shown in
Figure 1 on p. 76 is a specialized muscle cell.
• A tissue is
a group of cells that work together to perform a specific job. Animals have four basic types of tissue:
nerve, muscle, connective, and protective.
Plants have three types of tissue: transport, protective, and ground.
• An organ is a structure that is made up of two or more tissues working together
to perform a specific function. Some
common animal organs are the heart and stomach.
Examples of plant organs are leaves, stems, and roots.
• An organ system is a group of organs working together to perform a particular
function. Each organ system has a
specific job to do in the body.
• An organism is anything that can perform life processes by itself. An organism made up of a single cell is
called a unicellular organism.
Bacteria, most protists, and some kinds of
fungi are unicellular. Multicellular organisms are made up of many cells, and a multicellular organism has specialized cells that depend o
each other for the organism to survive.
• In organisms, structure and function
are related. Structure is the arrangement of parts in an organism. It includes the shape of a part and the
material of which the part is made. Function is the job the part does.
Notes
for Chapter 4 – The Cell in Action
Section 1 – Exchange With the Environment –
Cells must exchange materials with their environment.
Diffusion – movement of a substance from an area of high concentration to low concentration (molecules move from crowded areas to less crowded areas); this process does not require energy
Ex. When the smell of baking cookies spreads throughout the house.
Osmosis – diffusion of water through a semipermeable (only certain substances can pass through) membrane. Look @ Figure 2 on p. 91.
The cell has different ways of moving small and large particles into and out of the cell.
Moving Small
Particles…
Passive transport – small particles pass through channels in the membrane and energy is NOT required. Ex. Osmosis & diffusion
Active transport – small particles pass through channels in the membrane from low concentration to high concentration. This requires energy.
Moving Large
Particles…
Endocytosis – active transport process where cell surrounds large particle (ex. Protein) and encloses it to bring the particle into the cell. See Figure 4 on p. 92.
Exocytosis – active transport process in which a vesicle forms around a large particle within the cell. Vesicle travels to and fuses w/ cell membrane to release the particle. See Figure 5 on p. 93.
Section 2 – Cell Energy –
Almost ALL energy that fuels life comes from the Sun
Plants capture sun energy and convert it to food during the process of:
Chlorophyll is a green pigment that is found in chloroplasts and it is crucial to photosynthesis.
Plants use the energy from the sun to convert carbon dioxide, CO2 and water, H2O, into glucose – food.
Glucose is a carbohydrate and can be stored by the plant. Oxygen (O2) is another product of photosynthesis.
CO2 + H2O ----à glucose + O2, in the presence of sunlight and chlorophyll
Cellular
Respiration
- Process in which animal cells use oxygen to break down food.
Cellular respiration (CR) does NOT mean the cell is breathing! However, breathing supplies our cells with oxygen to perform cellular respiration. AND removes CO2 – waste product of CR.
In CR, food is broken down into CO2 and H2O and energy is released. Energy is used to maintain body T and produce ATP.
Cellular respiration occurs in the cell membranes of prokaryotic cells and in the mitochondria of eukaryotic cells.
Glucose + water à CO2 + water + energy
So, Photosynthesis produces oxygen and glucose and uses carbon dioxide and water while CR produces carbon dioxide and water and uses oxygen and glucose.
The two processes support each other.
Fermentation
Fermentation is a process that enables organisms to perform CR without oxygen.
Two types:
1) When your muscle cells run out of oxygen – byproduct is lactic acid and causes muscle soreness.
2) When yeast perform CR without oxygen – byproduct is carbon dioxide.
Section 3 – The Cell Cycle –
• The life cycle of a cell is called the cell cycle.
• The cell cycle begins when the cell is formed and ends when the cell divides and forms new cells.
• Before cell division can occur, it must make a copy of its DNA.
• The DNA of a cell is organized into structures called chromosomes.
• Copying chromosomes that each new cell will be an exact copy of its parent cell.
How Does a Cell
Make More Cells?
• Well, it depends on whether the cell is prokaryotic (no nucleus), or eukaryotic (has a nucleus).
• Prokaryotic cells are less complex than eukaryotic cells.
• Bacteria (which are prokaryotes), have ribosomes and a single circular DNA molecule.
• Cell division in bacteria is called binary fission (“splitting into two parts”). This results in two cells that each contain one copy of the circle of DNA.
• Eukaryotic cells are more complex than prokaryotic cells are – the chromosomes of eukaryotic cells contain more DNA than those of prokaryotic cells.
• Different kinds of eukaryotes have different numbers of chromosomes. IN humans, there are 46 chromosomes (23 pairs). These pairs made up of similar chromosomes are known as homologous chromosomes.
Making More
Eukaryotic Cells
• The eukaryotic cell cycle includes three stages.
Phase One: Interphase
(1) Chromosomes appear as threadlike chromatin
(2) In animal cells, centrioles appear outside the nucleus
(3) All the chromosomes are duplicated and each chromosome is attached with its sister
Phase Two: Prophase
(1) Each chromosome is made up of two identical chromatids attached at the centromere
(2) The two centrioles begin to move to opposite ends of the cell in an animal cell and a spindle begins to form in plant and animal cells
(3) A nuclear membrane begins to break down and the nucleolus begins to disappear
Phase Three: Metaphase
(1) The chromosomes attach to the spindle
(2) They are attached by the centromere
(3) Each chromatid is still connected to its sister
Phase Four: Anaphase
(1) Sister chromatids separate
(2) The chromatids move to opposite ends of the cell
(3) Chromatids are called chromosomes again
Phase Five: Telophase
(1) Chromosomes begin to uncoil and appear again as chromatin
(2) A nuclear membrane forms around the chromatin at each end of the cell
(3) In each nucleus, a nucleolus reappears
Phase Six: Cytokinesis
(1) The cell membrane moves inward until the cytoplasm is divided equally
(2) Two new daughter cells are produced with their own nucleus and identical chromosomes
(3) In plant cells, each daughter cell also forms a cell wall
Notes
for Chapter 5 – Heredity
Section 1 – Mendel and His Peas –
Heredity is the passing on of traits from parents to offspring. Gregor Mendel conducted key experiments with pea plants in his monastery garden that would launch the science of genetics. In his first experiments, he crossed pea plants to study seven different characteristics (see Table 1, p. 118). He chose pea plants because they were plentiful in the gardens, but also because of their properties. They had the ability to self-pollinate – a self-pollinating plant has both male and female reproductive structures. So pollen from one flower can fertilize the ovule of the same flower. Pea plants can also cross-pollinate – here, one plant fertilizes the ovule of a flower on a different plant. (see Fig. 2, p. 115 for both cases).
He used plants that were true breeding for different traits for each characteristic. When a true breeding plant self-pollinates, all of its offspring will have the same trait as the parent. So a true breeding plant with purple flowers will always have offspring with purple flowers. Mendel, then, crossed plants that had purple flowers with plants that had white flowers (see Fig. 5, p. 117). The offspring of such a cross are called first generation plants. All the first generation plants in this cross had purple flowers.
Mendel got similar results for each cross. One trait was always present in the first generation, and the other trait seemed to disappear. The trait that was present in the first generation was called dominant, and the other trait, that seemed to disappear, was called recessive. Mendel was curious what happened to the recessive trait (which was the trait for white flowers), so he conducted another set of experiments.
In this second round of experiments, Mendel did the same experiment he did initially on each of seven characteristics (flower color, seed color, seed shape, pod color, pod shape, flower position, and plant height). In each case, some of the second generation plants had the recessive trait. Mendel then decided to figure out the ratio of dominant to recessive traits. As Figure 1, p. 118 shows, there is a 3:1 ratio of dominant to recessive trait for all seven characteristics.
Section 2 – Traits and Inheritance –
Mendel knew from his experiments with pea plants that there must be two sets of instructions for each characteristic. Scientists now call these instructions for an inherited trait genes. Each parent gives one set of genes to the offspring. The offspring then has two forms of the same gene for every characteristic-one from each parent. The different forms (often dominant and recessive) of a gene are known as alleles. Dominant alleles are shown with a capital letter, and recessive alleles are shown with a lowercase letter. So in Mendel’s experiments, purple, which was dominant in flower color, was represented with a capital R, and white, which was a recessive trait in flower color, was presented with a lowercase r.
An organism’s appearance is known as its phenotype. So for pea plants, possible phenotypes for flower color would be purple flowers or white flowers.
Both inherited alleles together form an organism’s genotype. Because the allele for purple flowers (P) is dominant, only one P allele is needed for the plant to have purple flowers. Two dominant or two recessive alleles (PP or pp) is said to be homozygous. A plant that has a hybrid genotype (Pp) is known as heterozygous.
Punnett Squares are
tools used in genetics studies to organize the possible combinations of
offspring from particular parents (see Fig. 2, p. 121 for an example). We can use
Sometimes one trait is not completely dominant over another trait. These traits do not blend together, but each allele has its own degree of influence. This is known as incomplete dominance. Figure 5, p. 124 shows the result of incomplete dominance in true breeding snapdragons – a cross between true breeding red and white snapdragons yields pink flowers, because both alleles of the gene have some degree of influence.
Sometimes one gene can influence more than one trait (for example, the gene that causes the tiger in Fig. 6 p. 124 to have white fur also causes its blue eyes). Additionally, environment needs to be considered in studies of traits, as well as genes. For example, one may have the genes to be tall, but unless this is supplemented with a healthy diet, your full height potential will not be reached.
Section 3 – Meiosis -
In asexual reproduction, only one parent cell is needed. This parent cell divides in a process called mitosis, making two exact copies of itself. Most of the cells in your body reproduce in this way. However, in sexual reproduction, two parent cells join together to form offspring that are different from both parents. The parent cells are called sex cells. Sex cells are different from ordinary body cells. Human body cells have 46, or 23 pairs, of chromosomes. Chromosomes that carry the same set of genes are called homologous chromosomes. Sex cells are different though. They have 23 chromosomes-half the usual number. Each sex cell, then, has only one of the chromosomes from each homologous pair.
Sex cells are made during meiosis – a copying process that produces cells with half the usual number of chromosomes. Each sex cell receives one-half of each homologous pair. A human egg cell has 23 chromosomes, and a sperm cell has 23 chromosomes. The new cell that forms when an egg cell and a sperm cell join has 46 chromosomes.
Genes, which carry sets of instructions for traits, are located on chromosomes. Understanding meiosis was critical to finding the location of genes. The steps of meiosis are illustrated in Figure 3, p. 128-129. During the meiosis process, chromosomes are copied once, and then the nucleus divides twice. The resulting sperm and eggs have half the number of chromosomes of a normal body cell.
The steps in meiosis explain Mendel’s results that he obtained with his pea plants. Each fertilized egg in the first generation had one dominant allele and one recessive allele for seed shape. Only one genotype was possible because all sperm formed by the male parent during meiosis had the wrinkled-seed allele, and all of the female parent’s eggs had the round-seed allele.
Sex chromosomes carry genes that determine sex. In humans, females have two X chromosomes (XX). Human males have one X chromosome and one Y chromosome (XY). Figure 5 p. 131 shows that there is a 50% chance of the human offspring being male and 50% chance of offspring being female.
The genes for certain disorders, such as colorblindness and hemophilia, are carried on the X chromosome. These disorders are called sex-linked disorders. Because the gene for such disorders is recessive, men are more likely to have sex-linked disorders (because males have only one copy of each gene on their one X chromosome – females have two X’s, and so they carry two copies of each gene, and this makes a backup gene available if one becomes damaged).
Genetic disorders such as hemophilia can be traced through a family tree. Couples who are worried that they might pass a disease on to their children may consult a genetic counselor. These genetic counselors make use of a diagram called a pedigree, which is a tool for tracing a trait through generations of a family. By making a pedigree, a counselor can often predict whether a person is a carrier of a hereditary disease. Fig. 7 p. 132 shows a pedigree for the disease called cystic fibrosis, which affects the lungs.
When selective breeding is undertaken, organisms with desired characteristics are mated. Animals such as pets may be the result of selective breeding, as are flowers (see Fig 8 p. 132).
Notes
for Chapter 6 – Genes and DNA
Section
1 – What Does DNA Look Like?
DNA (deoxyribonucleic acid) is the genetic material that determines inherited characteristics. Scientists initially thought that only complex molecules could explain the behavior of genes, which give instructions for building and maintaining cells, and the ability to be copied each time a cell divides. They were surprised that these important functions could be performed by the DNA molecule.
DNA is made of subunits called nucleotides. A nucleotide consists of a sugar, a phosphate, and a base. Except for the base, the nucleotides are identical. The four bases are adenine, thymine, guanine, and cytosine, and are often referred to by the first letter of their name (A, T, G, C). Each of these bases is shaped differently (see Fig. 1, p. 144). In the 1950’s, Erwin Chargaff determined that the amount of adenine in DNA always equals the amount of thymine. He found the same to be true of cytosine and guanine. His findings are known as Chargaff’s Rules.
Other scientists gave us info