Chapter 11 Introduction to Genetics / 1
Tabla de contenido
Summary 2
Vocabulary Review 3
Dominance 4
Section Assessment 11-1 4
Workbook Section 11–1 The Work of Gregor Mendel (pages 263–266) 5
Section Review 11-1 7
Homozygous or Heterozygous? 8
11-2 Probability and Punnett Square 8
Enrichment -- Animal Pedigrees—A Canine Family Tree 9
Workbook Section 11–2 Probability and Punnett Squares (pages 267–269) 10
11-2 Reviewing Key Concepts 11
F2 Generation Punnett Square 12
11-3 Exploring Mendelian Genetics 12
Workbook Section 11–3 Exploring Mendelian Genetics (pages 270–274) 13
Section Review 11-3 15
Chapter 11 Assessment 16
Vocabulary Review 18
Summary
11–1 The Work of Gregor Mendel
The scientific study of heredity is called genetics. Gregor Mendel used purebred pea plants in a series of experiments to understand inheritance.
Pea flowers have both male and female parts. Normally, pollen from the male part of the pea flower fertilizes the female egg cells of the same flower. This is called selfpollination. Seeds that come from selfpollination inherit all their characteristics from just one parent.
To carry out his experiments, Mendel had to prevent self-pollination. He did this by cutting away the male parts and then dusting pollen from another plant on the flower. This process is called cross-pollination. The seeds that come from cross-pollination are the offspring of two different parents. Mendel decided to study just a few traits, or characteristics, of the pea plants. He studied seven traits: seed shape, seed color, seed coat color, pod shape, pod color, flower position, and plant height.
First, Mendel crossed two plants with different characters, or forms, for the same trait. For example, one plant was tall and the other was short. Mendel used the seeds produced by this cross to grow plants. These plants were hybrids. Hybrids are the offspring of crosses between parents with different traits.
To Mendel’s surprise, the hybrid plants looked like only one of the parents. He concluded that each trait was controlled by one gene that occurred in two different forms. The different forms of a gene are called alleles. Mendel formed the theory of dominance. He concluded that some alleles are dominant, while others are recessive. Whenever a living thing inherits a dominant allele, that trait is visible. The effects of a recessive allele are not seen if the dominant allele is present.
Mendel wanted to know what happened to the recessive allele. He allowed his hybrid plants to self-pollinate. Some of the plants that were produced showed the recessive trait. The alleles responsible for the recessive characters had not disappeared. Before, the dominant allele had masked the recessive allele, so it was not visible. Mendel concluded that the alleles for the same trait can be separated. He called this segregation. Alleles segregate when sex cells, or gametes, are formed. Each gamete carries only one copy of each gene.
11–2 Probability and Punnett Squares
Mendel used the principles of probability to explain his results. Probability is the likelihood that a particular event will occur. Probability can be used to predict the outcome of genetic crosses because alleles segregate randomly. The gene combinations that might result from a genetic cross can be determined by drawing a Punnett square.
One important rule of probability is that probabilities predict the average outcome of a large number of events. They cannot predict what will happen in a single event. The more organisms examined, the closer the numbers will get to the expected values. This is why Mendel worked with thousands of pea plants.
11–3 Exploring Mendelian Genetics
Mendel wondered whether genes that determine one trait have anything to do with genes that determine another trait. He wanted to know, for example, whether the gene that determines seed shape affects the gene for seed color. To answer this question, he did an experiment. He crossed plants and recorded two traits—seed shape and seed color.
Mendel found that the gene controlling seed shape did not affect the gene controlling seed color. Mendel concluded that genes can segregate independently, or undergo independent assortment, during gamete formation.
Not all genes show simple patterns of dominant and recessive alleles. In incomplete dominance, one allele is not completely dominant over another. In codominance, both alleles contribute to the phenotype. Many genes have more than two alleles and are said to have multiple alleles. Polygenic traits are traits controlled by two or more genes. The characteristics of any organism are not caused only by its genes. Instead, characteristics are determined by the interaction between the genes and the environment
11–5 Linkage and Gene Maps
Some genes are almost always inherited together. These genes belong to the same linkage group. A chromosome is a group of linked genes. It is actually the chromosomes that assort independently during gamete formation, not single genes.
The location of genes can be mapped to a chromosome. The rate of crossover events is used to find the distance between genes on a chromosome. The farther apart two genes are, the more likely they will be separated by a crossover event.
Vocabulary Review
Labeling Diagrams Use the words listed below to label the Punnett square. Some words may be used twice.
heterozygous parent homozygous offspring
dominant allele heterozygous offspring
recessive allele
Matching In the space provided, write the letter of the definition that best matches each term.
Biology IIChapter 11 Introduction to Genetics / 1
_____ 7. phenotype
_____ 8. gamete
_____ 9. genetics
_____ 10. probability
_____ 11. haploid
_____ 12. gene map
_____ 13. gene
_____ 14. multiple alleles
_____ 15. Trait
a. likelihood that something will happen
b. shows the relative locations of genes on a chromosome
c. physical characteristics of an organism
d. containing one set of chromosomes
e. sex cell
f. chemical factor that determines traits
g. specific characteristic
h. scientific study of heredity
i. gene with more than two alleles
Biology IIChapter 11 Introduction to Genetics / 1
Completion Fill in the blanks with terms from Chapter 11.
16. The process in which two genes segregate independently is called ______.
17. Plants that, if left to self-pollinate, produce offspring identical to themselves are called ______.
18. The offspring of crosses between parents with different traits are called ______.
19. The process during sexual reproduction in which male and female sex cells join is called ______.
20. The process of reduction division in which the number of chromosomes per cell is cut in half is called ______.
Dominance
Mendel’s principle of dominance states that some alleles are dominant and others are recessive. An organism with a dominant allele will show the dominant form of the trait. An organism will only express the recessive form of a trait when a dominant allele is not present.
In the space provided, fill in the genotype of the offspring. The first one has been done for you.
Use the table to answer the questions. Circle the correct answer.
1. What is the dominant shape of a pea pod?
Constricted smooth
2. What is the recessive color of a pea plant’s seed coat?
white gray
Section Assessment 11-1
1- What are dominant and recessive alleles?
______
2- What is segregation? What happens to alleles during segregation?
______
3- What did Mendel conclude determines biological inheritance?
______
4- Determine how Mendel crosspollinated pea plants.
______
5- Why did only about one fourth of Mendel’s F2 plants exhibit the recessive trait?
______
6- Why were true- breeding pea plants important for Mendel’s experiments?
______
Workbook Section 11–1 The Work of Gregor Mendel (pages 263–266)
This section describes how Gregor Mendel studied the inheritance of traits in garden peas and what his conclusions were.
Introduction (page 263)
1. The scientific study of heredity is called ______.
Gregor Mendel’s Peas (pages 263–264)
2. Circle the letter of each sentence that is true about Gregor Mendel’s peas.
a. The male parts of pea flowers produce eggs.
b. When pollen fertilizes an egg cell, a seed for a new plant is formed.
c. Pea plants normally reproduce by self-pollination.
d. Seeds that are produced by self-pollination inherit their characteristics from two
different plants.
3. What does it mean when pea plants are described as being true-breeding?
______
4. To perform his experiments, how did Mendel prevent pea flowers from self-pollinating and control their cross-pollination?
______
Genes and Dominance (pages 264–265)
Match the term with its definition.
Biology IIChapter 11 Introduction to Genetics / 1
Definitions
____ 5. Specific characteristics that vary from one individual to another
____ 6. The offspring of crosses between parents with different traits
____ 7. Chemical factors that determine traits
____ 8. The different forms of a gene
Terms
a. genes
b. hybrids
c. traits
d. alleles
Biology IIChapter 11 Introduction to Genetics / 1
9. State the principle of dominance. ______
______
10. Is the following sentence true or false? An organism with a recessive allele for a particular form of a trait will always exhibit that form ______
11. Circle the letters of the traits controlled by dominant alleles in Mendel’s pea plants.
a. tall b. short c. yellow d. green
Segregation (pages 265–266)
12. How did Mendel find out whether the recessive alleles were still present in the F1 plants?______
______
13. About one fourth of the F2 plants from Mendel’s F1 crosses showed the trait controlled by ______the allele.
14. Circle the letter of each sentence that is true about Mendel’s explanation of the results from his F1 cross.
a. Mendel assumed that a dominant allele had masked the corresponding recessive allele in the F1 generation.
b. The trait controlled by the recessive allele never showed up in any F2 plants.
c. The allele for shortness was always inherited with the allele for tallness.
d. At some point, the allele for shortness was segregated, or separated, from the allele for tallness.
15. What are gametes? ______
16. Complete the following diagram to show how alleles segregate during the formation of gametes.
17. In the diagram above, the dominant allele is represented by ______and the recessive allele is represented by ______.
Section Review 11-1
Reviewing Key Concepts
Short Answer On the lines provided, answer the following questions.
1. State Mendel’s principle of dominance.
______
2. Which two combinations of alleles could produce a trait controlled by a dominant allele?
______
3. What combination of alleles could produce a trait controlled by a recessive allele?
______
4. Explain segregation of alleles, using pea plant traits in your example.
______
Reviewing Key Skills
5. Applying Concepts Explain how Mendel’s experiments would have been different if he had not worked with true-breeding plants.
______
6. Comparing and Contrasting Explain the difference between crosspollination and self-pollination in plants.
______
7. Calculating One fourth of the plants resulting from a certain cross are expected to show a trait controlled by a recessive allele. If 675 plants resulting from the cross display a trait controlled by a dominant allele, how many plants will show the trait controlled by the recessive allele?
______
8. Applying Concepts If one of the plants used in the F1 cross had TT alleles and was combined with a plant with Tt alleles, would the trait controlled by the recessive allele have been produced in the resulting F2 generation? Explain your answer.
______
Homozygous or Heterozygous?
Homozygous organisms have two identical alleles for a particular trait. Heterozygous organisms have two different alleles for the same trait.
Color the homozygous recessive plant yellow. Color the homozygous dominant plant red. Color the heterozygous plant orange. In the space provided, fill in the missing genotype.
Use the diagram to answer the questions.
1. Does the tall offspring plant look like the tall parent plant? Explain.
______
2. Why did you color the offspring plant a different color than the tall parent plant?
______
11-2 Probability and Punnett Square
1- How are the principles of probability used to predict the outcomes of genetic crosses?
______
2- How are Punnett squares used?
______
3- What is probability?
______
4- Define the terms genotype and phenotype.
______
5- A F1 plant that is homozygous for shortness is crossed with a heterozygous F1 plant. What is the probability that a seed from the cross will produce a tall plant? Use a Punnett square to explain your answer and to compare the probable genetic variations in the F2 plants.
______
Enrichment -- Animal Pedigrees—A Canine Family Tree
Where did the dog come from? If you look at the hypothetical family tree shown here, you can see that the canine group began evolving about 30 million years ago from a tiny treeclimbing animal, Miacis.
The dog appears to be the first animal domesticated by humans. DNA evidence indicates that dogs were initially domesticated from wolves about 100,000 years ago. Tribes used dogs for help with hunting, for protection at night, and for companionship.
After dogs were domesticated, they were selectively bred for particular characteristics. Just as other domesticated animals have been bred, dogs were bred so that they had the characteristics most desired by humans. In different parts of the world, the choice of desirable characteristics varied, resulting in the diversity we see today.
As dogs continued to live as human companions, many new breeds were developed for specific tasks, such as hunting, guarding, working, and companionship.
The basic principles of dog breeding are those that apply to any animal. Breeding pairs are selected for the desirable genes they will pass on. Some characteristics involve single-gene inheritance. However, many characteristics, such as coat color, depend on more than one gene. A cross between two heterozygous parents can produce a puppy with blue eyes even though both parents have brown eyes. In German Shepherds, the alleles for white fur and dark eyes are recessive. Two black or two tan parent dogs can still produce a white puppy.