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Why are offspring from the same parents different? How can offspring show traits not seen in either of their parents?

You have learned that each person has two sets of chromosomes, one set inherited from his or her mother and one from his or her father. The chromosomes are made of DNA, and they contain the genes that determine the traits of the individual. So if two sisters each receive half of their chromosomes (with genes) from their father and half from their mother, why are they different? For example, how can one sister have red hair and one sister have brown hair, especially when neither parent has red hair? The answers to these questions are found in the study of classical, or Mendelian, genetics. The word classical refers to the fact that these questions were answered before scientists knew anything about chromosomes, genes, or DNA. The word Mendelian refers to the scientist who first answered these questions a long time ago. You will learn about his famous experiments in this section.

“Every family, aristocratic or not, inherits in their genes a record of who their ancestors were and where they came from.”

-The Language of Genes

Steve Jones

Long before there was a science of genetics, people did controlled crosses, or breedings, of domesticated plants and animals. It was obvious that specific traits of individual plants or animals could be described, and these traits could be inherited by the offspring of those individual plants or animals. Sometimes a trait of one parent would be expressed in all of its offspring whether or not its mate shared the same trait. Sometimes the traits of the offspring were intermediate between those of the parents, and sometimes the trait of a parent did not show up in its offspring, but reappeared in its grandchildren. All of this underlying information was the special knowledge of plant and animal breeders until the rules underlying the inheritance of traits were discovered by an Austrian monk, Gregor Mendel, in the middle of the 1800s. This was the beginning of the science of genetics.

Gregor Mendel discovered that traits are passed from parents to offspring as “particles of inheritance” (factors) that we now call genes. For each trait, the offspring inherited one particle from each parent. Some particles of inheritance appeared to be dominant in that they were likely to be expressed in the offspring if they were expressed in either parent. Other particles appeared to be recessive in that they were unlikely to be seen in the offspring if the other parent expressed the dominant trait. For example, if two human parents have blue eyes, their children will have blue eyes. However, if one parent comes from a family line that has always had brown eyes, all of the children will have brown eyes, even though one parent has blue eyes. Brown eye color is a dominant trait. However, sometimes two parents with brown eyes can have a child with blue eyes. Gregor Mendel described rules of particulate inheritance that explain how these patterns of inheritance of traits or heredity occur.

The Chromosome Theory of Heredity

Gregor Mendel did not know what a gene was. He did not even know what a chromosome was. He did not know about mitosis and meiosis, either. Right now, you know much more about these things than Gregor Mendel ever did. Mendel guessed that there were things like genes. He then did experiments in which he showed how these particles of inheritance passed from generation to generation. He found out that he could explain the results of his experiments by assuming that each individual inherits two particles (a pair of genes) for each trait, but before reproducing, the individual produced gamete cells that only contained one gene from each pair of genes. This is the process we know of as meiosis. As a result of meiosis, the offspring receives one gene of each of its pairs of genes from its mother and one from its father. However, the gene it receives from its mother could have come through its mother either from its grandmother or its grandfather. The gene it receives from its father could have come from the father's mother or father. This is true for each gene. Thus each offspring receives a unique set of genes. This is why the offspring of the same two parents can be quite different. The genes remain unchanged from generation to generation, but they appear in different combinations. Mendel's rules showed how to calculate the probabilities that traits would appear in offspring.

Hypotheses of gene behavior Observations of chromosome behavior
1. A gamete cell has half the number of genes that a body cell has. 1. Gamete cells have half the number of chromosomes that body cells have.
2. The gene pairs separate during meiosis. 2. Chromosome pairs separate during meiosis.
3. In fertilization, sex cells unite, restoring the original number of genes. 3. In fertilization, sex cells unite, restoring the original number of chromosomes.
4. The individual genes remain unchanged from one generation to the next. 4. Individual chromosomes retain their structure from one generation to the next.
5. The number of possible gene combinations can be calculated. 5. The number of possible chromosome combinations can be calculated.

Figure 6.1 This chart compares gene behavior and chromosome behavior.

Many years after Mendel, scientists discovered chromosomes and how chromosomes behave in mitosis and meiosis. It was clear that the behavior of chromosomes was very similar to Mendel's hypothesis for how genes had to behave to explain the inheritance of traits. This similarity was strong evidence that genes were found on chromosomes, even though no one had ever seen a gene. Figure 6.1 compares the hypotheses of gene behavior based on Mendel's observations with the observations about how chromosomes behave.

Pedigrees: Tracing Heredity

To understand genetics, it is necessary to do what Mendel did, that is to trace the inheritance of traits through generations. A pedigree is one of the tools that geneticists use to study and trace how traits and variations are passed from parents to offspring in a family. A pedigree is a family tree that shows relationships among members of a family. Animal breeders keep pedigrees to trace the characteristics of the animals with which they work. You may have seen a pedigree for a cat, dog, or horse, or you may have seen your own family tree. It is useful for geneticists to create a pedigree for a particular trait. We learned that traits have variations. That means that traits are determined by genes that have slight differences. Different forms of a gene are called alleles. A gene pair is made up of a pair of alleles.

There are rules to follow when constructing a pedigree. These rules make it easier for geneticists to understand the pedigrees constructed by other geneticists. Figure 6.2 shows standard symbols used in constructing pedigrees. Members of the same generation are all on the same level. Older generations are on higher levels and younger generations are on lower levels. When placing the offspring of a couple on the pedigree, the oldest offspring is drawn first beginning on the left. Specific alleles can be indicated by shading or coloring the individual symbols.

Figure 6.2 Using these symbols, you can make a chart of your family and ancestors.

Pedigrees can help you learn how a trait is inherited. They can help you learn if an allele is a dominant one or recessive one. With this kind of information about a trait, you can predict the probability that it will be expressed in the offspring of a crossing of two individuals. This ability to predict is very important for plant and animal breeders. It is also important for genetic counselors who advise potential parents about the chances their offspring could have a particular genetic disease. In the activity that follows, you look at a pedigree to determine if a particular allele is dominant, and to figure out the rules of inheritance of this trait.

Family Pedigree One Look at the pedigree of a family in Figure 6.3. The variation illustrated on the pedigree is red hair. The symbol for a person with red hair is filled in to show that she or he has the variation being considered. Notice that the pedigree of the family shows four generations. A geneticist needs at least three generations for the pedigree to be useful.

Figure 6.3 This pedigree shows four generations. The shaded circles and squares represent individuals with red hair.

Study the pedigree of the family closely and see what you can learn. Assume this is your family. The arrow points to you.

a. Are you male or female?

b. How many brothers do you have?

c. How many uncles do you have?

d. How many females have red hair?

e. Is red hair due to a dominant or recessive allele?

It is possible to construct pedigrees or family trees like the one above for many different traits. Such pedigrees help to sort out which traits of humans are determined by dominant alleles and which are controlled by recessive alleles. Figure 6.4 lists many traits that are known to be dominant and many related alleles that are recessive. The construction of pedigrees is a valuable tool for animal and plant breeders. It enables them to keep track of desirable and undesirable traits, determine if they are dominant or recessive, and predict what crosses will favor their expression and which crosses will minimize their expression.

Family Pedigree Two Draw a pedigree for a man and a woman who are married. They have a son and a daughter. Both the son and the daughter are married. The son and his wife have a son. The daughter and her husband have two children-a daughter and a son. Compare your pedigree with others in your class.

Dominant Recessive
dark hair blonde hair
nonred hair red hair
curly straight hair
abundant hair little hair
early baldness normal hair loss
brown eyes blue eyes
hazel or green eyes blue or gray eyes
free ear lobes attached ear lobes
broad lips thin lips
large eyes small eyes
long lashes short lashes
broad nostrils narrow nostrils
high, narrow bridge of nose low broad bridge
Roman nose straight nose
Short stature tall stature
blood groups A, B, AB blood group O
tasters of PTC nontasters of PTC
absence of hair on knuckles hair on knuckles

Figure 6.4 These are some human traits that are genetically determined and are dominant or recessive.

The Genetics of Coat Color in Labrador Retrievers

Imagine that you have a kennel and breed Labrador retrievers. You have bred black Labrador retrievers for years and all of your puppies have always been black. Suppose you have a friend who breeds chocolate Labrador retrievers and her puppies have always been chocolate. The two of you decide to cross your best dogs. The litter of your black female crossed with her chocolate male includes four black males and three black females. The litter of your friend's chocolate female crossed with your black male includes three black males and five black females. Draw the pedigrees for these two crosses. What can you conclude about the dominance of coat-color alleles in Labrador retrievers?

Let's figure out the patterns of inheritance of these alleles. First, you need to know a few more terms and tools that are used by geneticists. For any cross, the parents are designated as the P generation. Their offspring are designated as the \mathrm{F}_1 generation. Alleles are labeled with letters. A capital letter means the allele is dominant. A lowercase letter means the allele is recessive. The genetic makeup of an individual is its genotype. The genotype is represented by two letters for each trait, indicating two alleles. Gamete cells are labeled with only one letter because they carry only one allele for the trait.

Now for the pedigrees you constructed for the two crossings of black and chocolate Labrador retrievers, you can write the animal's coat-color genotype into each symbol. Starting with the parents, the black mother and black father would be BB. The chocolate mother and father would be bb. What would the genotypes of the sperm and ova of these dogs be? What would the genotypes of the puppies be? Remember that through meiosis, cells with two alleles for each trait produce gamete cells that have only one allele for each trait. Determine each of the following gamete cell genotypes.

Genotype of gamete cells of the chocolate mother =\underline{\;\;\;\;\;\;\;\;\mathrm{b}\;\;\;\;\;\;\;\;}

Genotype of gamete cells of the chocolate father =\underline{\;\;\;\;\;\;\;\;\mathrm{b}\;\;\;\;\;\;\;\;}

Genotype of gamete cells of the black mother = \underline{\;\;\;\;\;\;\;\;\mathrm{B}\;\;\;\;\;\;\;\;}

Genotype of gamete cells of the black father = \underline{\;\;\;\;\;\;\;\;\mathrm{B}\;\;\;\;\;\;\;\;}

Genotype of the puppies of the chocolate mother and the black father cross = \underline{\;\;\;\;\;\;\;\;\mathrm{Bb}\;\;\;\;\;\;\;\;}

Genotype of the puppies of the black mother and the chocolate father cross = \underline{\;\;\;\;\;\;\;\;\mathrm{Bb}\;\;\;\;\;\;\;\;}

Another way to represent the genetics of a particular cross is to use the allele letters and the symbol for male and female (male = , female = ). Then, arrange them in a table that allows you to write down all the possible combinations of the gamete cells of the mother and the father. The square, called a Punnett Square, in Figure 6.6 shows the outcome of a cross between the pure black male and the pure chocolate female.

Pure black Pure chocolate
Alleles in body cells BB bb
Allele in gamete cell B b

Figure 6.5 Alleles of a pure black male and a pure chocolate female.

Figure 6.6 A Punnett Square shows the outcome of a cross between pure black and pure chocolate Labrador retrievers.

This table shows that the genotype of the mother's gamete cells could only be b and the genotype of the father's gamete cells could be only B. Therefore, the genotypes of the puppies could be only Bb. Thus, each puppy would be black, but would carry a recessive allele for chocolate coat color.

You and your friend like your new puppies so much that you decide to keep a few from each litter to breed when they grow up. You keep one male from your friend's litter and one female from your litter to breed. These puppies are the \mathrm{F}_1 generation. When they grow up and you breed them to each other, they will create an \mathrm{F}_2 generation. Let's see if you can predict what coat colors you will get if you breed your \mathrm{F}_1 generation and produce \mathrm{F}_2 puppies. We can use the same type of table that is shown above to figure out this problem. First, we have to determine the possible genotypes of the gamete cells of each parent and then the genotypes of the resulting puppies.

In each of the four boxes, combine the allele a puppy would receive from its mother and the one it would receive from its father. Each box will represent a possible genotype for a puppy resulting from this cross. How many will be black? How many will be chocolate?

This set of boxes used to show the possible outcome of a cross is called a Punnett Square. It makes it easy to keep track of all possible combinations of alleles.

Figure 6.7 A Punnett Square shows the outcome of a cross of the \mathrm{F}_1 generation of Labrador retriever puppies.

Try two more crosses. Cross the \mathrm{F}_1 genotype with each of the P genotypes. How many black and how many chocolate puppies would you expect from each of these crosses? Your squares only leave room for the four possible genotypes, but what if each litter contained 8 puppies? How many chocolate and how many black puppies would you expect?

If your breeding produce only 4 puppies, how many would you expect would be black and how many would you expect to be chocolate? What is the ratio of black to chocolate puppies? If you did two breedings of your \mathrm{F}_1 dogs and produced a total of 16 puppies, how many would you expect to be black and how many to be chocolate?

Activity 6-1: Expression: Dominant and Recessive

Introduction

How is gene expression influenced by dominant and recessive alleles? In this activity you use red and white beans to represent dominant and recessive alleles to simulate allele combinations and gene expression. Then you observe the outcome of these different combinations by recording the resulting physical characteristics of the offspring.

Materials

  • 20 white beans
  • 20 red beans
  • 2 jars or cups
  • Pen/pencil
  • Activity Report

Procedure

Step 1 Put 10 white beans and 10 red beans into each of 2 jars representing parents, and mix them up.

Step 2 Without looking, pick a bean from each jar.

Step 3 Record the color of each bean on your Activity Report.

Step 4 Put the beans back into the same jars that they came from, mix the beans, and pick again.

Step 5 Repeat Steps 2 through 4 fifteen times, replacing the beans every time.

Step 6 On your Activity Report, fill in the gene pattern (genotype) column by writing down the allele-red bean = R (dominant) and white bean = r (recessive). For example, if you picked a red bean and a white bean, you would write Rr for the gene pattern.

Step 7 Fill in the column for physical expression or phenotype. The physical expression or phenotype is what the offspring actually looks like. Use the following rules to fill in the column.

  • RR is red in color.
  • Rr is red in color.
  • rR is red in color.
  • rr is white in color.

Step 8 Put 10 white beans and 10 red beans into one jar and 20 white beans into the other jar. Repeat steps 2 through 7. Record your information on your Activity Report.

Step 9 Complete the Activity Report.

Predicting and Explaining Variations

By working on the problems of inheritance of coat color in Labrador retrievers, you have discovered some basic laws of genetics. These laws were discovered over 100 years ago by Gregor Mendel who did similar experiments on pea plants.

Gregor Mendel was the founder of the formal science of genetics. In the 1860 s, working with common garden peas, he discovered the basic patterns of inheritance. Mendel was lucky that he decided to work with pea plants, because they have several traits that have only two variations or two alleles. Mendel studied stem length, which has two variations- tall and short; pea pod color-green or yellow; pea seed color-green or yellow; and pea shape-round or wrinkled.

Mendel worked with his pea plants for several years, making sure he had true-breeding varieties. True breeding varieties always produced offspring that were identical to themselves generation after generation. When Mendel mated tall pea plants with tall pea plants and planted those seeds, the plants from those seeds were all tall. When he mated short plants with short plants, the seeds from those plants produced all short plants. Then when he mated a tall plant with a short plant, those seeds all produced plants that were tall. Similarly, when he crossed plants that produced wrinkled seeds with ones that produced round seeds, all of the offspring plants produced round seeds. When he crossed yellow pod plants with green pod plants, all of the offspring had green pods. For each trait Mendel studied, the trait that always showed up (was expressed) among offspring of two different pure breeding plants he called the dominant trait. The trait that did not show up he called the recessive trait.

What Do You Think?

The phrase “Every human gene must have an ancestor” is found in the book The Language of Genes. What does the phrase mean to you? Write a story or poem about real or imagined ancestors who might have had one or more variations you carry in your DNA today. Be creative. Have fun imagining who these people might have been and what they might have been like.

Mendel did not use the words gene or allele, but he was able to predict what would happen, and explain what happened when he mated the peas in his garden. He observed that a pair of factors controlled each of the traits he was studying. Factor was the word he used. Today we use the words gene and allele. When a plant had one dominant factor and one recessive factor, the dominant factor was the one that was expressed (observed) in the offspring.

Figure 6.8 Gregor Mendel studied the variations in peas. He learned to predict the appearance of certain characteristics in the offspring, based on whether they were dominant or recessive.

Did You Know?

As geneticists learn more about traits and variations and the genes and alleles that cause them, no one is surprised that there are genes for different traits on the same chromosome. Genes close together on the same chromosome and inherited together are called linked genes. The study of linked genes is one of the most exciting things that geneticists concerned with traits and variations in humans are studying today. By studying linkage, geneticists figure out which genes are on which chromosomes, and how close they are to each other on the chromosome. Using these techniques and others, geneticists are making maps of the chromosomes. These maps show where different genes are located.

Gregor Mendel had another important contribution to the study of genetics. He determined that the factors must somehow separate and recombine when the pea plants produced seeds that would become new plants. He knew this because sometimes when plants showing a dominant trait were crossed, some of their offspring showed the recessive trait. This was just like your experiment in crossing the \mathrm{F}_1 puppies. When Mendel crossed a tall and a short pea plant and got all tall plants that did not mean the short trait was lost. The short trait was just hidden. Mendel had never seen a chromosome and did not know about meiosis. He did conclude, however, that pairs of factors separate and recombine during the crossing of two plants. What Mendel figured out using the peas in his garden is truly amazing. His discoveries form the foundation for modern genetics.

Sometimes a baby with blue eyes is born to two parents with brown eyes. Grand-parents smile and say “blue eyes skip a generation.” Is this myth correct? Can you explain how two brown-eyed parents could have a blue-eyed child?

  • Using Punnett Squares, show the results of crossing a homozygous tall (TT) and a homozygous short (tt) pea plant. Then show the results of back-crossing the \mathrm{F}_1 pea plants to each parental type.
  • Use the Punnett Square technique to show the results of crossing a tall pea plant with two tall alleles (TT) that has two alleles for wrinkled seeds (\mathrm{rr}) with a short pea plant (tt) that has two alleles for round seeds (RR).
  • What do you think is the importance of genetic maps?

Review Questions

  1. What are alleles? Why are they important?
  2. What is the difference between dominant and recessive alleles?
  3. Although Gregor Mendel didn't know about meiosis or mitosis, his discoveries provided the foundation for modern genetics. What did he find?

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Feb 23, 2012

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