Unit 7: Genetics

Individuals that reproduce have many characteristics that make them different from each other. In this unit, you will learn about the mechanisms responsible for these differences.

Lesson 1: Mendel's Pea Plants

Characteristics such as hair color and eye color are inherited from your parents. You may be surprised to learn that our understanding of how these traits are inherited began more than 100 years ago, in a monastery where a curious monk wondered why pea plants have different-colored flowers.

How are traits passed from one generation to another?

Genetics is the study of the way in which heritable characteristics are determined and passed from parents to offspring.

• To understand genetics, we must go back to where it all began—an Augustinian monastery in the middle of the 1800s.

• One monk at that monastery, Gregor Mendel, was in charge of tending the garden. 

• He had never heard of DNA or chromosomes—they had not been discovered yet.

• Mendel loved nature and often thought about the patterns he saw in plants

Mendel’s Curiosity• Gregor Mendel noticed that pea plants had certain identifiable and measurable

features, or characteristics.

• Mendel noted features such as height, flower color, and seed texture. 

• He wondered if these characteristics were carried on from generation to generation.

• To find out, Mendel cross-pollinated pea plants with different characteristics and observed the offspring…then crossed the offspring with one another to see which characteristics showed up in the next generation of plants. 

• He observed that some characteristics, such as flower color, seed shape, and seed color, were heritable, or passed on from parents to offspring regardless of the environment.

• Mendel considered that there must be certain factors that determine the characteristics of pea plants.

Traits & Characteristics• Today, the qualities Mendel observed in his pea plants are referred to

as traits.

• A trait is the specific observable form of an inherited characteristic.

• A characteristic is a broader category within which one can observe any number of specific traits.

• Mendel hypothesized that one factor is dominant over the other.

• A dominant trait shows up, or is expressed, even in the presence of the other factor.

• A recessive trait does not show up even in the presence of the other factor.

Lesson 2: Genes and Alleles

 Genes affect much of how we look, grow, and develop. Let’s take a closer

look at genes and alleles.

Trait’s to GenesFrom Gregor Mendel’s experiments and careful analysis of data, he formed a groundbreaking conclusion:

• Each parent plant gives certain factors to its offspring.

• These factors give instructions for the plant’s traits and the traits seen in the following generations.

• Mendel had discovered the basic unit of inheritance.

• Today, scientists call these units genes.

What Are Traits?• When scientists talk about genes, they are typically referring

to those sections of DNA that control the production of the cell's proteins—and hence all of the activities and characteristics of an organism.

• Depending on the genes a plant has, it may exhibit one or more traits.

• Traits can include tallness, shortness, rough or smooth seed texture, and many other features you can observe.

• Genes also determine many human traits.

• Some traits come from a single gene, whereas others come from combinations of genes.

Who Gets to Be Tall?Whether a pea plant is tall or short depends on which alleles of a gene the plant carries.

• Alleles are alternative forms of a gene.

In any species where there are two parents, the offspring will inherit one form, or allele, of a gene from each parent.

• One parent can pass on a tall-plant allele and the other can pass on a short-plant allele.

• Or, they may both pass on the same allele for a trait.

• The offspring ends up with two alleles—one from each parent.

Dominant and Recessive• One of Mendel’s great discoveries is that, one allele can be

dominant over another.

• The allele that is not dominant is known as the recessive allele.

• Whenever a recessive allele is paired with a dominant allele, the dominant allele will show as a trait

Genes, Alleles, and Proteins• Another way to think about genes and alleles is to remember

from the DNA to RNA to Protein lesson that genes typically code for proteins.

• Many of the most important proteins in our bodies are enzymes, which can speed up chemical reactions.

• Remember that most of the chemical reactions in our bodies would not occur quickly enough for life to occur without enzymes to speed them up.

• K12 Ex. PKU test on newborns.

DNA mRNA protiens

Lesson 3: Inheritance

You have learned that genes carry the information for inherited traits from parent to offspring, but how does this happen?

So, what exactly is DNA—and how is it related to genes, alleles, and Gregor Mendel’s pea plant

experiments?

Inside every cell of every individual is the hereditary material DNA.

• DNA contains all the information an organism needs to grow and develop.  

• DNA: Hereditary material in a double helix shape. Think of this shape as a twisted ladder.• Chromosomes: Large bundles of tightly coiled DNA. Different organisms have different numbers of chromosomes. Humans have 23 pairs in each cell.• Genes: Sections of DNA, or a very small part of a

chromosome. Each gene typically contains the code for a single protein. Altogether, these genes contain the specific patterns of information that control all the activities and characteristics of an organism.

Chromosomes Match Up• Humans have 23 pairs of chromosomes in each cell, or a total

of 46 chromosomes.

• In fact, this is not always true. A specialized type of cell, known as a sex cell, has exactly 23 chromosomes, or half the number of chromosomes you would find in another type of cell.– Sex cells have half the number of chromosomes as other body cells

because during the process of fertilization, one sex cell from each of two parents combines to form a new organism. The resulting offspring then has the chromosomes of two sex cells. In humans, it is 23 pairs, which is the right number for all other cells. 

Homologous Chromosomes• The two chromosomes in a pair are called a homologous

chromosome. – Every cell (other than the sex cells) carries 23 pairs of homologous

chromosomes.

• Alleles Are on Homologous Chromosomes– Because of the homologous chromosomes each allele knows from where

and how they know to match up to one another.

When a Pair of Alleles Is the Same

• If both alleles in a pair of genes are the same, then the organism is said to be homozygous for that trait. – It may help you to remember that the prefix homo means “the same.”

– Organisms that are homozygous for a trait have two of the same alleles for a trait. 

• Because an organism can be homozygous for either a dominant or a recessive trait, we also have to note that when we describe the alleles of an organism.

When a Pair of Alleles Is Different

• If each allele from a pair of genes is different, then the organism is said to be heterozygous for that trait. – It may help you to remember that the prefix hetero means “different.” 

– All heterozygous offspring will exhibit the dominant trait.

• An organism is never “heterozygous recessive” or “heterozygous dominant.”

• It is only heterozygous for a trait, which always means it has two different alleles for a trait.

• A gene may have many alleles but an individual person can have a maximum of two alleles for a particular gene.

Organisms Are Both Homozygous and

Heterozygous• It might seem difficult to understand at first, but organisms

can be heterozygous for one trait, yet be homozygous for another.

• In the case of pea plants, an offspring might inherit the allele for yellow seeds (Y) from both its parents, making it homozygous for that trait. It might also inherit the dominant allele for height (tall, or T) from one parent and the recessive allele (short, or t) from the other, making it heterozygous for the other trait. For this organism, you would represent its alleles like this:

• YY Tt

Lesson 4: Punnett Squares

 Imagine you have a dog that is about to have

puppies, there is a way to predict how offspring may look. Let’s learn more about it.

Punnett Squares• In the early 1900s that another scientist, named Reginald

Punnett, came up with a tool to make predictions about genetic crosses.

• Punnett thought about traits that have one dominant and one recessive allele.

• Remember that each parent passes one allele for a trait to his or her offspring.

• Punnett used this information to organize a set of parent alleles into a simple checkerboard-type chart.

• With this chart, he was able to predict how likely it was that an offspring would inherit the dominant or recessive trait.

Making Predictions:In the first generation, we could predict that all our seeds (100 percent) would be a particular color.

Is that true of the second generation?

• No! In the second generation, all of our predictions are percentages less than 100.

• In fact, if you were to cross two parents that are heterozygous (Yy)for any of the traits Mendel studied—round vs. wrinkled, tall vs. short—you would always come up with the same ratio for the traits of the offspring.

• In other words, for every one YY, there are two Yy and one yy. The ratio is 1 : 2 : 1. In percentages, it is 25 percent, 50 percent, and 25 percent.

• Punnett squares are a useful tool for predicting the outcomes of genetic crosses.

• The alleles for seed color show a distinctive pattern as you move from one generation to the next.

• Some traits, such as seed color, are controlled by a single gene, whereas other traits, such as eye color in humans, are controlled by several genes.

Lesson 7: Similarities Among Organisms

 Let’s explore how species are similar and yet different from one another.

Genetic Differences• A species is a group of organisms that closely resemble one

another and are able to interbreed.

• But why can’t two similar animals produce offspring…giraffe and a horse.

• The answer to this mystery has to do with an organism’s specific genetic makeup…the genetic differences within and among species.

Why Are Organisms Different?• Organisms within a species often have different alleles of a

trait. – K12 EX. you may have blond hair & your mother has brown hair…or a

dog may have a litter of puppies, half of which are white and the other half black.

• Even though you look different from your parents, you would never confuse another human with a different species of animal. Why?

Why Are Humans Different?• You look different from your parents because of your genes.

• You have inherited some alleles from one parent and other alleles from the other parent.

• The unique combination of alleles is what makes you an individual.

• Even though you have different combinations of alleles from each parent, you still have the same number and kinds of chromosomes as your parents do.

• Humans look different from one another because they have different combinations of alleles.

• Humans look similar and have similar features because they have the same number and kinds of chromosomes.

Differences Among Species

• You’ve probably noticed variation, or differences, among members of a species.

• Members of the same species look different because each organism carries a different combination of alleles in their genes

• Each species carries a certain number of chromosomes and certain kinds of chromosomes. – You may recall that chromosomes are tightly wound bundles of DNA.

– Each species requires its own unique set of chromosomes to grow and develop normally.

– Do you remember how many pairs of chromosomes humans have?

Genes in Different Species• More chromosomes does not necessarily mean more

complexity in a species.

• Do more complex species have more genes? 

• Scientists have recently discovered that humans have about 30,000 genes inside each of their cells.

• Even though this number of genes seems large, it is smaller than what scientists previously thought.

• They originally hypothesized that the more complex a species is, the more genes it would have.

• As it turns out, this is not true.

Internal and External Features of Organisms

• Chromosomes carry the genetic information that tells organisms what internal and external features they will have.

• Each species has a unique set of chromosomes, and a unique set of genes.

• To figure out which species are most closely related on a genetic level, you would have to unwind the species’ chromosomes and compare their genetic codes.

• K12 Ex. Dolphin/Shark

Lesson 8: Chromosomes

 The material in our cells that determines our traits is like a code that must be broken. This code-carrying material is called DNA. In this lesson, you will learn more about DNA.

Unit 8 so far you have learned that:

• chromosomes are located in cells

• genes are sections of chromosomes.

• A pair of genes can contain two different sets of instruction– called alleles, for the same characteristic.

So, what does DNA have to do with genes and chromosomes?

• Each chromosome is made up of a single long strand of DNA that is bundled with special proteins.

• It is DNA (short for deoxyribonucleic acid) that contains the actual genetic instructions in a code made of chemical compounds.

Where Is DNA?• In eukaryotic organisms, DNA is located in the nucleus of the cell.

• In prokaryotic organisms, such as bacteria, DNA is located in a central area of the cell called the nucleoid.

• With few exceptions, the DNA found in each cell of an individual is identical, no matter what part of the body that cell might be in: lungs, skin, bone, teeth, or muscle.

• But, from one individual to the next, DNA is unique—it is one of a kind.

• Unless you have an identical twin, there is nobody alive that has exactly the same DNA as you do

What Does DNA Look Like?

• If you could look at a strand of DNA, you would see something that looks like a ladder twisted around and around.

• This shape is called a double helix.

• To understand how DNA is shaped this way, you must look at the way a molecule of DNA is made

What Is DNA Made Of?• Every molecule of DNA is made up of units called nucleotides.

• Nucleotides are chemicals that have three parts: a sugar, a phosphate, and a base.

• DNA has four different types of nucleotides that are all identical except for the base.

• These four nucleotides are:– adenine (A)

– thymine (T)

– guanine (G)

– cytosine (C)

So, how are the nucleotides arranged to form a ladder?

• Alternating molecules of sugar and phosphate make up the sides of the ladder.

• Attached to each sugar is one of the four bases.

• The two sides of the ladder are connected when each of the four bases pairs up with one of the other bases.

• These base pairs make up the rungs in the DNA ladder – Adenine always pairs with thymine

– guanine always pairs with cytosine.

• The bases in each pair are known as complementary bases.

How Does DNA Work?• How do these simple molecules hold the code for every living

thing in and around us?

• DNA makes RNA, and RNA makes protein.

• Proteins carry out the functions in each of our cells

DNA Makes RNA• RNA is a nucleic acid like DNA.

• RNA stands for ribonucleic acid.

• It has bases like DNA

• RNA looks more like a ladder that has been split in half lengthwise—it has only one side

• Instead of thymine as one of its four bases, RNA has uracil (U), which will also pair up with adenine.

• RNA can copy DNA because the complementary bases can pair with each other.

• The RNA does this along particular segments of a DNA molecule.

• These segments are what we call genes.

RNA Makes Protein• After the RNA copies the information from the DNA segment (the gene)

• it detaches from the DNA and moves out of the nucleus

• The RNA is now essentially carrying a message from the DNA to ribosomes in the cytoplasm, where the message will be "read."

• Because of the RNA’s function in this process, it is called messenger RNA, or mRNA.

• The mRNA will come into contact with transfer RNA (tRNA) and provide the code for amino acid chains that become proteins for the body.

• Each tRNA molecule carries one of the amino acids that will make up the chain.

Lesson 9: Meiosis

Many species produce sex cells by a process in which the daughter cells are not identical to the parent cell or to each other. This process is called meiosis, and it is the reason why every individual human is unique.

Meiosis Produces Gametes• Sperm and eggs are referred to as sex cells, or gametes.

• The process of cell division that forms gametes is called meiosis.

• Meiosis produces daughter cells that each have half the number of chromosomes the parent has.

• In meiosis, a male produces sperm and a female produces eggs.

• Each sperm or egg is a cell with half the usual chromosome number.

• When the gametes come together and fuse, they form a zygote.

• This zygote is the first cell of a new individual. 

• The zygote contains the usual number of chromosomes

Diploid and Haploid• Diploid-having 2 of each type of chromosome, constituting

pairs of homologous chromosomes with the same genes

• Haploid-containing one chromosome of each type

Mitosis and Meiosis• When you are first learning about meiosis, it may be difficult to

understand what makes it different from mitosis.

• Both processes have phases in which chromosomes move apart and new cells form.

• Even the names of the phases are the same. Remember that the main difference between these two processes is their outcomes.

• Mitosis ends with two daughter cells that are identical to the parent cell.

• Meiosis ends with four daughter cells, each with half the genetic material of the parent, and each with a new, unique combination of genes

Stages of Meiosis• Meiosis is divided into two main stages: meiosis I and meiosis

II.

• Before meiosis I, all chromosomes replicate(copy)their DNA… but the chromosomes (pairs) stay attached to each other at their centromeres

• During meiosis I:– homologous chromosomes separate

• During meiosis II: – sister chromosomes separate

Lesson 10: Meiosis and Mitosis

Let’s look at these two types of cell division again to see how they are

different.

Meiosis is:• a process of cell division that organisms use to produce

gametes for sexual reproduction.

• very important to the health and diversity of a species because it helps produce genetic diversity.

• a process that allows a species to gains members with new combinations of genes.

Imagine what the human race would be like without genetic differences: Everyone would be the same.

Mitosis is:• Required by all multi-celled organisms who need cell division

to grow and to heal.

• Produces all the cells in your body from just one cell

Mitosis and Meiosis Are Different

It can be easy to confuse the processes of mitosis and meiosis.

• Their names look and sound very similar.

• They both have phases that have the same names and take place in the same order.

• Both processes happen to the chromosomes inside your cells.

So, what is the difference between the two?• mitosis results in two genetically identical diploid cells

• meiosis results in four genetically different haploid cells

• the process of meiosis has more steps

• mitosis is simply the copying of genetic material, which takes only one cycle

• meiosis requires two cycles of cell division--the first cycle is the copying of genetic material, and the second cycle is the separating of genetic information so that each cell has half the number of chromosomes as a regular cell

Lesson 12: Mutations

The cells in your body are constantly growing, changing, and dividing. It’s amazing to think that so many cells are able to keep growing and dividing for so long. Cells never seem to make a mistake. But do

they?

As you have learned:• the order of the DNA bases is the genetic code of the DNA

molecule

• Each gene is a code for the production of certain proteins, which, in turn, cause certain traits to be expressed

• the replication, or copying, of DNA does not always go smoothly.

Sometimes, changes or mistakes happen during this process

Mutations A mutation is any change in a gene or DNA.

Mutations can be helpful, harmful, or have no effect at all.

Chemicals, viruses, and radiation sometimes cause mutations.

Mistakes occurring during meiosis are another cause of mutations.

Within the cell, specific proteins are able to find and repair many mutations.

• These proteins do not always find the mutations, though.

• A mutation that is not detected may remain in the cell and be passed on when the cell divides.

• If the mutation occurs in an egg or sperm cell, the mutation may be passed on to offspring.

• Scientists think that mutations play an important role in how species change over time.

How Do Mutations Change a Gene?

• Remember that DNA is made of nucleotides.

• Each nucleotide includes one of four bases: adenine, guanine, cytosine, or thymine (A, G, C, or T).

• Recall also that the sequence of these bases along the DNA molecule forms a code. 

• Segments of DNA, or genes, contain specific code sequences that are used to produce specific proteins. 

• The DNA sequence of a gene determines the protein that is produced.

• A change in one of the bases of the DNA sequence can affect the entire code for a gene

Mutation: Small Scale and Large Scale

• Mutations are generally divided into two types: small-scale mutations-- affect one or a few nucleotides in

the DNA large-scale mutations--affect sections of chromosomes.

3 types of Small-Scale Mutations are:

Substitution

• A substitution occurs when a single nucleotide is exchanged for another. For example, adenine might be exchanged for guanine. This type of mutation can be fixed if a protein that repairs mutations detects it

Deletion

• Another kind of small-scale mutation is a deletion. A deletion occurs when one or more bases are removed or left out of the DNA. With bases missing, the altered genetic code is read and interpreted incorrectly. Unlike substitutions, deletions cannot be fixed.

Insertion

• The third type of small-scale mutation is an insertion. When an insertion happens, one or more bases are added to the DNA. Insertions usually occur when DNA is being replicated. Like substitutions, insertions can be corrected.

Large-Scale Mutations• Not all mutations are restricted to changes in one or a few

nucleotides.

• Large-scale mutations are more serious--large sections of chromosomes are changed.

• Sections of a chromosome may be duplicated. – This may result in extra copies of a gene that may “amplify” the effects

of that gene.

– Conversely, large sections of a chromosome may be deleted, resulting in the loss of entire genes

Consequences of Mutations• You may think that a mutation will always result in damage to

the organism but, on occasion, a mutation will cause an individual organism to develop in a way that is slightly different from the rest of the population.

• Mutations are always random, but they are not always bad.

• EX. Galápagos Marine Iguana

• A karyotype is the complete set of all chromosomes of a cell of any living organism.

Lesson 13: Genetic Engineering

This lesson describes some of the current techniques and methods used to change

the genetic makeup of organisms.

Relationship between genes, alleles, and chromosomes

• DNA determines every genetic trait an organism has.

• Every eukaryotic organism has a complete set of genetic instructions in the chromosomes that are in the nucleus of each of its cells.

Genetic changes that can occur through natural processes

Selective Breeding

• Natural selection- see changes in the traits of a species from generation to generation…occurs when individuals with certain traits survive to reproduce and pass on those traits to their offspring.

• Humans have been intentionally causing similar changes in other species for thousands of years.

• The process of breeding organisms with the most desirable traits is called selective breeding. – EX. Horse, dog & cow breeding, etc.

Genetic Engineering• Selective breeding takes many generations to produce results.

• Since many genes are being shuffled in each breeding experiment, it can take many generations to achieve the desired combination of genes.

• Recently, scientists have begun using genetic engineering to change specific genes to get the results they want.– Ex. Human Insulin production.

Drugs and Vaccines

• Many drugs and vaccines are now produced using genetic engineering techniques.

• producing these drugs in the quantities desired would be impossible without these techniques.

• For many of these drugs, the version produced through genetic engineering is much safer because it is based on a human protein or because it doesn’t have to come from dead bodies.

• Vaccines produced using genetic engineering techniques are also considered safer than more traditional vaccines that contain live or inactivated microorganisms.

Genetic Engineering Cont.• Genetically Modified Organisms (GMO’s)

• Many of our world’s food crops and other crops are destroyed or fail to thrive because of pests, competition with weeds, or poor soil or weather conditions. 

• Genetic engineering can be used to make crops pest- resistant, frost-resistant, or able to grow in salty soil.

• In addition, some types of genetically engineered foods can bring vitamins or vaccines to parts of the world that don’t have access to modern medicine.

**Genetic engineering is a topic that generates much debate. 

Gene Therapy• If a gene is defective, its instructions cannot be carried out by

the cell.

• When this happens, a genetic engineering technique known as gene therapy might be promising in the future.

• In one type of gene therapy, cells with defective genes are replaced with cells with normal genes.

• The goal of gene therapy is to restore the normal function of the cells.

• Gene therapy is still a new procedure, it can be risky and may not be effective.

Fun Science Resource:The Happy Scientist

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