GENETICS BASICS Part 1: WHAT IS HEREDITY? Name________________________________Family members have many of the same characteristics or traits. The passing of traits from parents to

their children is called heredity . All organisms pass information about traits to their children, or offspring.

Mendel’s StudiesGenetics is the study of heredity . More than 100 years

ago, a scientist named Gregor Mendel made important discoveries about heredity. Mendel took seeds from tall pea plants and planted them. Most of the plants grew from the seeds were tall, however some turned out short. Mendel wondered why the short pea plants were unlike their tall parents.

Mendel decided to study pea plants and their seeds. He began by trying to grow plants that would produce only tall plants from their seeds. The flowers of pea plants have both male sex organs and female sex organs. Mendel moved pollen from the male sex organs to the female sex organs on the same

plant. In this way, he carried out self-pollination of each plant. After self-pollination occurred, each flower produced seeds. Mendel planted the seeds and then self-pollinated the plants that grew from those seeds. He did this again and again. Finally, Mendel had seeds that would produce only tall pea plant. These were “pure tall” seeds.

Using the same method of self-pollination, Mendel grew plants that were pure for the trait of shortness. He called the “pure tall” and the “pure short” plants to the P generation. P stands for parent generation. To understand what a generation is, think of your grandparents as one generation. Your parents are part of a second generation, and you are part of a third generation.

The F1 GenerationMendel wondered what type of offspring two different pure plants would produce. He used cross

pollination to find out. Mendel moved the pollen from pure tall pea plants to the female sex organs of pure short pea plants. He also placed pollen from pure short plants on the female sex organs of pure tall plants.

When the cross pollinated flowers produced seeds, Mendel took the seeds and planted them. He called these plants the F1 Generation . The F stands for filial, which means son or daughter. The 1 stands for the first filial generation. To his surprise, Mendel found that all the F1 generation plants were tall. Mendel crossed hundreds of pure short and pure tall plants. He observed the offspring carefully. Each time the results were similar. All of the offspring were tall. What had happened to shortness? Why had this trait disappeared?

Punnett SquaresA punnett square can be used to explain crosses like

those Mendel did. The punnett square to the right shows Mendel’s cross of pure tall plants with pure short pea plants. The TT stands for a pure tall plant and tt stands for pure short plants. The capital letter T stands for tallness and the lowercase t stands for shortness. A gene is the information that a parent passes to its offspring for a trait. Therefore, a pea plant receives, or inherits, two genes for height, one from each parent.

Notice that although all of the pea plants of the F1 pea plants were tall, they also had the gene for shortness. A gene that is hidden when combined with another gene is called a recessive gene . The gene that shows up is called a dominant gene. A capital letter is used to represent a dominant gene. A lower case letter is used to represent a recessive

gene. In the case of pea plants, the gene for tallness is dominant. The gene for shortness is recessive. That explains why all the pea plants in the F1 generation were tall.

Genotypes and PhenotypesAn organism’s combination of genes for a trait is called its genotype . For example the genotype of the

F1 pea plants was Tt. What an organism looks like as a result of its genes is its phenotype . The phenotype of the F1 pea plants was “tall”. An organism has both a genotype and a phenotype for all its traits.

The F2 GenerationMendel produced yet another generation of pea plants by self-

pollinating the F1 generation plants. He called this third group of plants the F2 generation. Mendel found that short pea plants reappeared in the F2 generation. In fact, in the F2 generation, about ¾ of the plants were tall and about ¼ were short. Mendel repeated his experiments many times. He got similar results each time. The F2 generation always included short pea plants.

The punnett square to the left explains Mendel’s results. Short plants reappeared in the F2 generation because they inherited two recessive genes for shortness, one from each tall parent. The recessive genes were hidden in the tall parent plants.

The F2 generation pea plants showed two different phenotypes: tall and short. However, notice in the punnett square that the F2 generation had three different genotypes. All the short plants had the genotype tt. The tall plants, though, had either genotyped TT or Tt.

Mendel’s ConclusionsMendel also studied other traits of pea plants. Each trait was inherited the same way as plant height. For

example, Mendel obtained only round seeds when he crossed plants pure for round seeds with plants pure for wrinkled seeds. He obtained only green pea pods when he crossed plants pure for green pea pods with those pure for yellow pods.

Mendel concluded that there was information in a plant that caused it to have certain traits. He called this information factors. He reasoned that since traits came in pairs, factors did too. Mendel also thought that one factor of the pair was more powerful than the other factor. The more powerful factor hid the appearance of the weaker factor. Today, we know that Mendel’s factors are genes. Dominant genes hide the appearance of recessive genes. Mendel also thought that the paired factors separated during sexual reproduction. That way, the offspring received half of the factors from one parent and half from the other parent.

Answer the following questions about Part 1:

1. What is heredity?

2. Compare and contrast the terms dominant gene and recessive gene.

3. What is the difference between a genotype and a phenotype?

4. Draw a punnett square to represent Mendel’s cross between pure tall plants and pure short plants.

GENETICS BASICS Part 2: WHAT ARE CHROMOSOMES? Name________________________________Chromosomes are rod-shaped bodies located in the nucleus of a cell. Chromosomes are made of

proteins and a chemical called DNA. Sections of DNA make up an organism’s genes, which determine all the traits of an organism. A chromosome may contain hundreds of genes.

Mitosis and Cell DivisionFrom your study of one-celled organisms, you know that some single celled organisms reproduce by

splitting in half ( asexual reproduction ) . Before doing so, the cell’s chromosomes make a copy of themselves. Then the cell’s nuclear membrane dissolves. The two sets of chromosomes separate and a nucleus forms around each set. The division of a cell into two new identical cells is called mitosis.

During mitosis, the entire cell divides. When the cell splits into two, each new cell gets one of nucleus with a full set of chromosomes. Each set of chromosomes is identical to the parent cell’s chromosomes.Just as some single-celled organisms go through mitosis and cell division, so do the cells that make up the human body. Body cells divide by mitosis and cell division as the body grows and repairs itself. However, to reproduce itself, the body uses a different kind of process.

Sexual ReproductionHumans and most other many-celled organisms reproduce sexually. In sexual reproduction , two cells

called gametes join to form one complete cell. Males produce gametes called sperm cells. Females produce gametes called egg cells. Each gamete has only one half of the chromosomes found in the organism’s body cells. When the gametes join, they form a cell that has a complete set of chromosomes.

Most humans have 46 chromosomes in their body cells. Human sperm and egg cells have only 23 chromosomes. When humans reproduce, a sperm and an egg join to form a cell called a zygote. A zygote has 46 chromosomes and develops into a baby, an adolescent, and, eventually, an adult. Together, the 46 human chromosomes carry 50,000 to 100,000 genes. The mix of chromosomes from both parents during sexual reproduction produces an offspring that is different from either parent.

MeiosisMitosis results in new cells that have a complete set of chromosomes. Gametes for by a different kind of

process called meiosis. Like mitosis, meiosis begins with the copying of chromosomes in the original or “parent” cell. Then, the cell divides into two cells. Each of those new cells divide again to form 4 sex cells, or gametes. Each sex cell contains only half the number of chromosomes of the parent cell.

Sex ChromosomesAs you read before, humans have 46 chromosomes or 23 pairs of chromosomes. Each chromosome that

makes up a pair comes from a different parent. For 22 of the pairs, the two chromosomes look much alike. The chromosome that makes up the 23rd pair are called a sex chromosomes because they determine a person’s sex, or gender. There are two different kinds of sex chromosomes: an X chromosome and a Y chromosome.

A human female has two X chromosomes (XX). A human male has one X chromosome and one Y chromosome (XY). Parents pass one of their sex chromosomes to their offspring. A male can pass an X chromosome or a Y chromosome. A female can pass only one of her X chromosomes since she has no Y to pass on.

The Punnett square to the right shows how human sex chromosomes determine the sex of an offspring. An offspring that inherits an X chromosome from both parents will be a female. An offspring that inherits an X from its mother and a Y from its father will be a male. Notice that there is always a 50/50 chance of having a boy or girl baby.

Experiments with Fruit FliesRemember that Mendel used pea plants in his study of heredity. About 50 years later, another scientist,

Thomas Morgan, used fruit flies to learn about chromosomes and genes. Fruit flies are useful to study because they only have four pairs of chromosomes, the chromosomes are large, and the flies reproduce very quickly. In addition, it is easy to tell the males and females apart.

Sex-Linked TraitsFruit flies usually have red eyes but Morgan noticed that one of the males had white eyes. Morgan

mated the white-eyed male with a red-eyed female, He called the offspring the F1 generation. All of the offspring in the F1 generation had red eyes. Morgan concluded that the gene for red eyes was dominant in fruit flies.

Next, Morgan mated the flies from the F1 generation to make an F2 generation. This time, some of the offspring had red eyes and some had white eyes. One thing Morgan noticed was that all the flies that had white eyes were male...none were female. Morgan concluded that white eye color in fruit flies was linked to the sex of the fly. Traits that are linked to the gender of an organism are called sex-linked traits .

As in humans, a female fruit fly has two X chromosomes and a male has one X and one Y chromosome. Morgan found that the gene for eye color in fruit flies is on the X chromosome. There is no gene for eye color on the Y chromosome. That explains why eye color in fruit flies is a sex-linked trait.

The punnett squares on the right show Morgan’s experiments. Notice that only the X chromosome carries a gene for eye color. The genotypes XRXR and XRXr represent a red-eyed female. The genotype XRY represents a red-eyed male. White eyed males have the genotype XrY.

Notice that a female fly with the genotype XRXr has red eyes even though it carries a gene for white eyes. An organism that carries a gene but does not show the gene’s effects is called a carrier.

Answer the following questions about part 2:

1. What are chromosomes?

2. Define mitosis and meiosis

3. What does it mean to be a carrier of a trait or gene?

4. What are the sex chromosomes of a human female and a human male?

5. Why is eye color a sex-linked trait in fruit flies?

GENETICS BASICS Part 3: HOW IS HEREDITY STUDIES IN HUMANS? Name________________________Human genetics is the study of how humans inherit traits . Human genetics is more complicated than the genetics of fruit flies for a number of reasons. First, humans have more chromosomes than fruit flies. Second, humans do not reproduce as quickly as flies. Third, humans cannot be used in laboratory experiments. Scientists study heredity in humans by studying identical twins. They also look at families to learn how genetic diseases are passed from one generation to the next.

Different Kinds of TwinsThere are two different types of twins: identical and fraternal. Identical twins form from the same zygote and have identical genes. Fraternal twins form from two different zygotes (two eggs were separately fertilized by two separate sperm at the same time). Fraternal twins do not have identical genes.

Heredity and EnvironmentYou are born with certain genes that determine your characteristics such as skin color, eye color, body

shape, and others. Your environment, though, also may affect your characteristics. Your family, the air you breathe, and everything else in your surroundings make up your environment. To find out how environment affect a person’s characteristics, scientists study identical twins who have been separated since birth. Both twins have the same genes but they grew up in different environments. Scientists look for differences.

The Influence of the EnvironmentFood, sunlight, air, and other parts of the environment can affect a person’s characteristics. For

example, a person who doesn’t have good nutrition may not grow tall, even though he or she may have the gene for tallness. A person who avoids sunlight may not form freckles, even though they have the freckle gene. The environment also affects a developing baby. Studies show that babies born to women who smoke cigarettes weigh less than babies born to women who are non-smokers.

The environment can directly affect a person’s genes. For example, X-rays and some types of chemicals cause changes in genes. These changes are called mutations. Mutations can cause problems in humans and other organisms. How do mutations occur? Before you can understand mutations better, you need to know more about DNA.

DNA DNA in chromosomes is the material that contains an

organism’s genes. DNA passes the genes from one cell to another during cell division. All the informations needed to carry out life activities is in DNA. All the information that makes you human is in your DNA.

Sometimes there is a change in the order of bases in a DNA molecule. This is another form of mutation. Since mutations cause changes in genes, mutations may affect the traits of an organism. Green eyes are actually the result of a mutation in humans...weird, huh?

Genetic DiseasesA genetic disease is a disease that results from the genes a person inherits . Recessive genes cause most

genetic diseases but some diseases can be dominant. When a disease is recessive, it means a person must inherit the recessive gene from both parents to have the disease. Dominant genes cause other genetic diseases. Genetic diseases result from mutations.

A recessive gene causes one form of diabetes which prevents the body from making insulin which controls the amount of sugar in the bloodstream. Another recessive gene causes a condition called sickle-cell anemia. This disease causes a person’s red blood cells to have a sickle shape rather the normal shape. Blood cells with this sickle shape tend to clog blood vessels makes people with the disease experience weakness as well as an irregular heartbeat.

Answer the following questions about Part 3:

1. What is the difference between identical twins and fraternal twins?

2. Give an example of how both heredity and environment may affect a certain human characteristic (trait).

3. What is DNA?

4. What is a mutation?

5. Define the term “genetic disease” and give an example of a genetic disease.

GENETICS BASICS Part 4: Other Aspects of Genetics Name________________________________Farmers use breeding techniques to grow hardier and better tasting crops. Animal breeders use

breeding techniques to make prizewinning horses. Scientists transfer genes from one organism to another. These are all examples of people affecting traits in organisms.

Useful MutationsA mutations may sound like a bad thing but it has caused many helpful changes in organisms along with

the not so good changes. A mutation that resulted in an animal having white fur would help that animal hide from predators in a snowy environment. The white fur would blend in with the white snow to help them avoid being caught and eaten by a predator.

Selective BreedingFamers and scientists use their knowledge of genetics to produce new varieties of plants and animals.

The method of selecting useful traits and breeding organisms for those traits to show up in the offspring is called selective breeding. For example, at one time a mutation for short legs arose in sheep. Short legs many not seem like a desirable trait in sheep but it actually prevented the sheep from jumping the fences and getting lost. Farmers ended up breeding sheep to have shorter legs until a new breed of shorter legged sheep was produced. Farmers have used selective breeding to produce cows that give large amounts of milk and chickens that lay larger eggs. Selective breeding has also resulted in pink grapefruit and navel oranges.

Genetic EngineeringIn the 1970s, scientists began introducing new genes in to organisms. This process is called genetic

engineering. At first, scientists transferred genes from one species of bacterium to another species of bacterium. Today, scientist are able to transfer genes between entirely different organisms. For example, scientists have transferred human genes into bacteria. The human genes function in the bacteria as they do in human cells. When scientists transferred the gene for human insulin into bacteria, the bacteria produced human insulin. That insulin can be used to treat people who suffer from diabetes.

Scientists have also used genetic engineering to improve crop plants. For example, they have transferred a bacterial gene into plants that makes the plants resistant to insect attacks. Some scientists would like to use genetic engineering to cure genetic diseases. So far, thy have not been able to permanently replace disease-causing genes in humans with normal ones However, scientists have found ways to temporarily introduce normal genes into people who have certain genetic diseases. As a result, the people are free of the diseases for a while.

Answer the following questions about Part 4:

1. Give an example of a mutation that is helpful to an organism?

2. What is selective breeding?

3. Define the term genetic engineering.

4. Describe one way that scientists have used genetic engineering to help humans.