BSCS Chapter 13

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Chapter Introduction

Genes and Chromosomes

13.1 Heredity and Environment

13.2 Mendel and the Idea of Alleles

13.3 Genes and Chromosomes

Mendelian Patterns of Inheritance

13.4 Probability and Genetics

13.5 Inheritance of Alleles

13.6 Sex Determination

Other Patterns of Inheritance

13.7 Multiple Alleles and

Alleles without Dominance

13.8 Linked Genes

13.9 X-Linked Traits

13.10 Nondisjunction

13.11 Multigene Traits

Chapter Highlights

Chapter Animations

Contents

Learning Outcomes

By the end of this chapter you will be able to:

A Explain the roles of heredity and environment in organism development.

B Explain the relationship among alleles, genes, and chromosomes.

C Apply the principles of probability to genetics.

D Explain Mendel’s principles of segregation and independent assortment.

Learning Outcomes 1

Learning Outcomes

By the end of this chapter you will be able to:

E Give examples of how sex is determined in several organisms.

F Explain the role of multiple alleles in inheritance.

G Explain linked genes and recognize how X-linked traits differ from other linked traits.

H Summarize the effects of nondisjunction.

Learning Outcomes 2

Patterns of Inheritance

 What important biological concept is illustrated in this photo?

 What differences among these starfish do you see?

This photo shows a group of starfish in a tide pool in Big Sur, California.

Chapter Introduction 1

Patterns of Inheritance

• Genetics is the branch of biology that seeks to explain inherited variation.

• Some of the differences among individuals are passed from parent to offspring.

• Such inherited features are the building blocks of evolution.

This photo shows a group of starfish in a tide pool in Big Sur, California.

Chapter Introduction 2

End of the Introduction


13.1

Heredity and Environment

• Organisms are products of their heredity and of their surroundings.

Siamese cats have a gene for dark fur, but the enzyme produced by the gene functions best at low temperatures. The cat’s extremities —its ears, paws, and tail

—are cooler than the rest of its body.

13.1

Heredity and Environment 1


13.1


(cont.)

• Studies of twins can help separate the effects of inheritance and the environment.

– Identical twins have the same genetic information while fraternal twins are no more genetically similar than other siblings.

– If identical twins exhibit the same trait more often than fraternal twins, the trait is probably influenced by genetic factors.

– If the trait differs in identical twins, the environment must have a strong influence on it.

13.1



13.1


(cont.)

• According to the once popular theory of “blending”, an individual’s genetic makeup was formed when the parents’ genes mixed at fertilization.

• The result was a sort of averaging of the parents’ genes.

• Genetics no longer includes the theory of blending inheritance, but it was a logical explanation for many visible differences.

13.1



13.2

Mendel and the Idea of Alleles

• In the 1860s, Gregor Mendel used garden peas to study heredity.

• Peas are very easy to grow, are self-fertilizing, and

Mendel could study many generations during his eight-years of experiments.

Gregor Mendel (1822

–1884)

By experimenting with peas in his monastery garden, Mendel developed the fundamental principles of heredity that became the foundation of modern genetics.

13.2

Mendel and the Idea of Alleles 1

Self-fertilization in a pea flower

13.2



13.2


(cont.)

• Mendel concentrated on traits that did not fit the blending theory.

• Mendel found seven different characteristics of pea plants that he could study in an either-or form.

13.2



13.2


(cont.)

• Mendel classified and isolated true-breeding plants which produce offspring identical to themselves generation after generation.

• Mendel then crossbred his plants, classified all the offspring, and looked for patterns of inheritance.

13.2



13.2


(cont.)

• When Mendel mated a plant with round seeds with a plant with wrinkled seeds, the next generation of plants produced only round seeds.

• When those plants were allowed to self-fertilize, however, some plants in the next generation produced round seeds and other plants produced wrinkled seeds.

13.2



13.2


(cont.)

• Mendel demonstrated with pea plants that both parents pass on to their offspring genetic factors that remain separate generation after generation.

• Today the concept of genes has replaced Mendel’s vague idea of factors.

• A gene is now defined as a segment of DNA whose sequence of nucleotides codes for a specific functional product.

13.2



13.2


(cont.)

• Most genes exist in more than one form, or allele .

• Each allele of a particular gene has a different base sequence.

• All organisms have genes that exist as several different alleles.

• Most traits—such as hair color, skin color, nose shape, and handedness —result from the complex interaction of several genes with each other and the environment.

13.2



13.3

Genes and Chromosomes

• The arrangement of genes in chromosomes differs in eukaryotes and prokaryotes.

• Eukaryotic chromosomes are long molecules of

DNA wrapped around proteins.

• Only part of this DNA codes for proteins; noncoding

DNA is not translated.

13.3

Genes and Chromosomes 1


13.3


(cont.)

• Prokaryotes have a single circular chromosome with little associated protein.

• An estimated 90% of prokaryotic DNA is translated.

• Many bacteria also have plasmids—small circles of

DNA that contain additional genes.

A scanning electron micrograph (x79,000) of a bacterial plasmid. These small circular

DNA molecules are common in some types of bacteria. Their ability to transfer genes between cells makes plasmids useful in genetic research.

13.3



13.3


(cont.)

• Homologous chromosomes carry the same genes, although their genes may be present as different alleles.

• Stains help identify homologous chromosomes by binding to specific regions of chromosomes to create unique banding patterns.

• Chromosome painting can also be used to identify all of an organism’s chromosomes.

13.3



13.3


(cont.)

Human chromosomes 7 and X are similar in size and shape. The banding patterns evident in the photographs show more clearly in the drawings. These two chromosomes, which have almost the same length and centromere location, can be distinguished by their banding patterns.

In chromosome painting, fluorescent dyes of different colors are chemically bonded to short pieces of DNA (probes) that bind to genes on different chromosomes. This process stains each homologous pair of chromosomes a different color.

13.3



13.3


(cont.)

• Chromosomes are easiest to study in the condensed form during cell division.

– Chemicals are added to stop cell division during metaphase.

– The cells are treated with water on a microscope slide causing them to swell and their chromosomes to spread apart.

– Stains applied to the resulting chromosome spread produce the banding patterns.

13.3



13.3


(cont.)

In the human chromosome spread, the banding patterns result from differences in stain absorption by different regions of the chromosomes. Banded chromosomes enable biologists to detect missing or extra chromosome parts more easily than uniformly stained chromosomes do. The banding patterns also have made the mapping of genes on chromosomes more accurate.

13.3



13.3


(cont.)

• Stained chromosomes can be photographed under the microscope.

• Individual chromosomes can be cut out of an enlarged photograph and arranged by size and shape to form a display called a karyotype .

• Karyotypes of fetal cells can be used to check for suspected chromosomal abnormalities in developing fetuses.

13.3


In this karyotype of a human cell, the chromosomes are arranged as homologous pairs. The larger groups (for example, pairs 1

–3) include chromosome pairs with similar size and centromere position. A karyotype is prepared by cutting out individual chromosomes from a photograph and matching them, pair by pair. The pair of X chromosomes determines this individual’s sex.

13.3


End of Section 1


13.4

Probability and Genetics

• Diploid organisms usually carry different alleles of many genes.

• Geneticists use probability , a branch of mathematics that predicts the chances that a certain event will occur, to predict the results of matings.

• Probability is usually expressed as a fraction and works when each event is independent.

• The chance that two or more independent events will occur together is the multiplication product of their chances of occurring separately.

13.4

Probability and Genetics 1


13.4

Probability and Genetics

(cont.)

• Geneticists use probability to predict the alleles of the offspring of various crosses, or matings.

• Mathematical tests can help show whether a difference between observed and predicted results is significant.

• Genetic ratios are estimates of probability, not absolute numbers.

• The larger the sample size, the less deviation is expected from predicted ratios.

13.4

Probability and Genetics 2


13.5

Inheritance of Alleles

• The principles of probability enable you to explain the results of Mendel’s experiment.

• Mendel performed a monohybrid cross , or crossed plants that differed in only one trait: seed shape.

• The plants involved in this first cross are called the parental (P) generation .

13.5

Inheritance of Alleles 1


13.5


(cont.)

• The generation of plants that grew from the seeds of the parental crosses is called the first filial , or

F

1

, generation .

• The F

1 generation plants self-fertilized producing the second filial , or F

2

, generation .

13.5



13.5


(cont.)

• Only one trait of each pair was visible in the

F

1 generation.

– The visible trait is the dominant trait.

– The alternative trait, which is not visible in the

F

1 generation, is called the recessive trait.

• In the F

2 of F

1 generation produced by self-fertilization plants, the two traits appear in a ratio of approximately 3:1, dominant to recessive.

13.5



13.5


(cont.)

• Mendel used mathematics to conclude that each true-breeding plant has two identical copies of the factor for a particular trait.

– When gametes formed during meiosis, only one copy of the factor went into each pollen or egg cell.

– At fertilization, the F

1 generation received a factor from each parent.

– Only one of these factors went into each gamete formed by the F

1 plants.

13.5



13.5


(cont.)

• Mendel called this separation of factors the principle of segregation .

• The alleles (the modern term for factors) of each gene separate during meiosis when homologous chromosomes are divided among the gametes.

13.5


This example of a monohybrid cross demonstrates

Mendel’s principle of segregation. Each plant has two factors

(alleles) for each trait, which segregate during the formation of gametes.

Fertilization restores paired alleles. The grid, or Punnett square, shows how gametes of the F

1 generation combine to form the genotypes of the F

2 generation.

13.5



13.5


(cont.)

• The allele for a dominant trait is commonly represented by a capital letter, and the allele for the recessive trait is represented by the lowercase letter.

• Most multicellular organisms are diploid, so their genotype , or genetic makeup, has two alleles for each gene.

– In Mendel’s experiment, round-seeded plant in the

P generation would have been RR.

– The wrinkled-seeded parental plant had the genotype rr.

13.5



13.5


(cont.)

• If both alleles are the same ( RR or rr ), the genotype is homozygous .

• If the alleles are different ( Rr ), the genotype is heterozygous .

• The genotype of each individual is responsible for its phenotype —its appearance or observable characteristics.

• Because the round-seeded phenotype is dominant, both the homozygous RR and the heterozygous Rr genotypes produce plants with round seeds.

13.5



13.5


(cont.)

• A Punnett square is useful for calculating probable ratios of genotypes and phenotypes.

• All the possible genotypes of the gametes one parent can produce are at the top of the square, and the genotypes of the other parent’s gametes are at the side.

• The square is then filled in like a multiplication table with the genotypes that would result from the union of those gametes.

13.5



13.5


(cont.)

• To follow the inheritance of two characteristics at once, Mendel made dihybrid crosses —crosses between individuals that differ in two traits.

• The ratio 9:3:3:1 is characteristic of the F

2 in a dihybrid cross. generation

• The results of dihybrid crosses formed the basis for

Mendel’s principle of independent assortment .

Alleles for one characteristic assort, or divide up among the gametes during meiosis, independently of alleles for other characteristics.

13.5


A dihybrid cross illustrates the principle of independent assortment. Note that there are 12 round to every 4 wrinkled seeds and 12 yellow to every 4 green seeds. Each trait independently shows the

3:1 ratio typical of a monohybrid cross.

R = round seed; r = wrinkled seed;

Y = yellow embryo; y = green embryo.

13.5



13.6

Sex Determination

• Chromosomes come in matching pairs except for the sex chromosomes, which may be different.

• This sex chromosomes determine the sex of the individual.

• In humans, other mammals, and the fruit fly

Drosophila melanogaster, the sex chromosomes are labeled X and Y .

• Females have two X chromosomes; males have one

X chromosome and one Y chromosome.

13.6

Sex Determination 1

Sex determination

13.6

Sex Determination 2


13.6

Sex Determination

(cont.)

• Some insects, such as grasshoppers, crickets, and roaches, females have two X chromosomes, males have one; there is no Y chromosome.

• Birds, some fish, and some insects have what is called a Z-W system where the male has two matching sex chromosomes (ZZ), and the female has one Z chromosome and one W chromosome.

13.6

Sex Determination 3


13.6

Sex Determination

(cont.)

• Some plants, such as spinach and date palms, have separate female and male plants and have sex chromosomes that follow the X-Y system of sex determination.

• Most plants and some animals have no sex chromosomes.

13.6

Sex Determination 4

End of Section 2


13.7

Multiple Alleles and Alleles without Dominance

• Some genes do not follow an either dominant or recessive pattern.

• A type of inheritance is known as incomplete dominance occurs when the phenotype is intermediate between those of the parents .

13.7

Multiple Alleles and Alleles without Dominance 1

Incomplete dominance of flower color in snapdragons

13.7



13.7


• Human blood type depends on the presence or absence of type A or type B carbohydrates on the

(cont.) surface of red blood cells.

– The alleles I A or I B code for different forms of an enzyme that add different sugars to the carbohydrate bound to the plasma membrane.

– When a person has the I A I B genotype, both types of carbohydrates are produced and the individual’s blood type is AB.

• When alleles are codominant , both phenotypes appear in heterozygous individuals.

13.7



13.7


(cont.)

• Blood types also involve multiple alleles .

– In addition to the I A and I B alleles, the allele i codes for no active enzyme.

– An ii genotype produces type O blood.

– The symbols I A , I B , and i are used to show that the A and B traits are codominant and the O trait is recessive.

13.7



13.7


(cont.)

• If someone receives a transfusion of a different type of blood, antibodies , which are defensive proteins found in the blood, cause the donated red cells to clump together, clogging the blood vessels.

• If a person with type B blood receives a transfusion of type A blood, the anti-A antibodies that are produced attack the A surface carbohydrates and vice versa.

13.7



13.7


(cont.)

13.7



13.8

Linked Genes

• Genes on the same chromosome are said to be linked and are often inherited together.

• Alleles of linked genes do not always stay together.

Crossing-over frequently occurs between homologous chromosomes during the early stages of meiosis. This results in genetic recombination (new combinations of genes). Here the symbols E and e stand for the alleles of one gene, and F and f are the alleles of another gene on the same chromosome.

Because these two genes are close together, crossing-over probably would not occur between them very often.

13.8

Linked Genes 1


13.8

Linked Genes

(cont.)

• The frequency with which linked traits become separated reflects how far apart on the chromosome the genes for those traits are.

• This information can be used to map the positions of genes on chromosomes.

• Important genes that are hard to identify can also be mapped by observing traits produced by nearby marker genes.

13.8

Linked Genes 2


13.8

Linked Genes

(cont.)

A few of the genes that have been mapped to human chromosomes number 7 (left) and 4 (right).

Linked genes are inherited together because they are on the same chromosome.

13.8

Linked Genes 3


13.9

X-Linked Traits

• An understanding of linked genes came from studies of fruit flies by Thomas Hunt Morgan in the 1910s.

Adult fruit flies, Drosophila melanogaster (x10). The top fly is male; the bottom fly is female. Note the difference in their coloring.

13.9

X-Linked Traits 1


13.9

X-Linked Traits

(cont.)

When Morgan crossed a white-eyed male with a normal red-eyed female, all the F

1 flies had red eyes.

The F

2 flies showed the expected

3:1 ratio of red to white eyes, but only the male flies showed the recessive, white-eye trait.

13.9

X-Linked Traits 2


13.9

X-Linked Traits

(cont.)

In the reciprocal cross, a white-eyed female was crossed with a red-eyed male. All male offspring had white eyes because they inherited their X chromosome from their mother.

The F

2 flies contained a 1:1 ratio of white eyes and red eyes.

13.9

X-Linked Traits 3


13.9

X-Linked Traits

(cont.)

• Morgan’s explanation for these results was that the gene for eye color is carried on the X chromosome.

• White eye color was the first known example of an X-linked trait , a trait whose gene is carried only on the X chromosome.

• Well-known examples of human X-linked, recessive alleles include those responsible for red-green color blindness and hemophilia.

13.9

X-Linked Traits 4


13.9

X-Linked Traits

(cont.)

• The British geneticist Mary Lyon suggested that, early in the development of a normal female, one X chromosome becomes inactivated in each body cell.

• According to Lyon’s theory, the cells of a female express a mixture of X-linked traits.

The coloring of this calico female cat is a visible indication of X inactivation. The cat carries two different alleles for coat color on its two X chromosomes. Random X inactivation during embryonic development results in this patchwork-colored coat in some female cats.

13.9

X-Linked Traits 5


13.9

X-Linked Traits

(cont.)

• Lyon proposed that a darkly staining mass called a

Barr body that normally appears in the nucleus of female cells, is the inactive X chromosome.

13.9

X-Linked Traits 6


13.9

X-Linked Traits

(cont.)

• Errors in meiosis cause some females to receive an extra X chromosome resulting in an XXX configuration.

• Cells of these XXX females have two Barr bodies.

• Lyon interpreted this observation to mean that only one X chromosome in a cell remains active.

• Lyon’s X-inactivation hypothesis may help to explain why an extra X chromosome is not as disruptive as an extra copy of another chromosome.

13.9

X-Linked Traits 7


13.10

Nondisjunction

• The failure of homologous chromosomes to separate in meiosis nondisjunction .

• If for some reason the sex chromosomes fail to separate in meiosis the offspring developing from those gametes would have unusual numbers of sex chromosomes.

• Nondisjunction also can occur between the X and Y chromosomes in males.

13.10

Nondisjunction 1


13.10

Nondisjunction

(cont.)

(a), The gene for eye color is carried on the X chromosome. V represents normal red eye color; v represents vermilion. Males inherit their eye-color gene on the X chromosome they receive from their mothers.

13.10

Nondisjunction 2


13.10

Nondisjunction

(cont.)

(b), Nondisjunction of the X chromosome can produce red-eyed, sterile males and vermilion-eyed females. Bridges explained these rare individuals by suggesting that sometimes the X chromosomes fail to separate during meiosis in egg cell development. When Bridges examined the cells of these exceptional flies, he found the abnormal number of chromosomes that he had predicted. Nondisjunction also can occur in males.

13.10

Nondisjunction 3


13.10

Nondisjunction

(cont.)

• Nondisjunction also occurs in the sex chromosomes and other chromosomes of humans with possibly severe effects.

• Certain syndromes, or typical combinations of symptoms, can result from abnormal numbers of sex chromosomes.

13.10

Nondisjunction 4


13.10

Nondisjunction

(cont.)

• Nondisjunction can produce an individual with three copies of a chromosome in a condition known as trisomy.

• Trisomy of human chromosome 21 (Down syndrome) causes limited mental abilities, short stature, characteristic facial features, and heart defects, although severity varies.

13.10

Nondisjunction 5


13.10

Nondisjunction

(cont.)

(a), Chromosome number 21 identifies this male Down syndrome karyotype.

(b), This girl shows typical facial features of Down syndrome.

13.10

Nondisjunction 6


13.11

Multigene Traits

• Most human traits are multifactorial —they are affected by several genes and environmental factors.

• Multifactorial traits, such as height, are also known as quantitative traits.

• The environment includes both external factors, such as light and temperature, and internal ones, such as the organism’s metabolic activities.

• Most multifactorial traits, such as height, intelligence, color, and the control of metabolic processes, vary in finely graded steps.

13.11

Multigene Traits 1


13.11

Multigene Traits

(cont.)

• A graph of a multifactorial trait usually resembles a bell-shaped curve.

• A discontinuous distribution is typical of characteristics such as sex that exist in only two forms and are affected by few genes.

13.11

Multigene Traits 2


13.11

Multigene Traits

(cont.)

(a), This graph of the heights of students in a high school biology class shows a continuous distribution. Height is a quantitative, or continuous, characteristic. (b), This graph shows the distribution of the sexes in the same class. Sex is a qualitative, or discontinuous, characteristic.

13.11

Multigene Traits 3

End of Section 3

Summary

• The concept of a gene has changed during the past century as research techniques have allowed more detailed analysis.

• A gene is a region of DNA that codes for a product.

• Most genes exist as several alleles that code for different products.

• Studies of twins demonstrate the effects of environment on gene expression.

• Chromosomes are complex structures, each containing many genes.

• Using techniques such as staining to produce banding patterns, scientists can identify homologous chromosomes.

Chapter Highlights 1

Summary

(cont.)

• Homologous chromosomes carry the same genes, but these genes may be different alleles.

• Mendel’s work forms the basis of modern genetics.

• Probability can be used to predict the outcome of breeding experiments.

• Working with Drosophila, Morgan and his students identified many X-linked traits; helped to establish the chromosomal basis of inheritance; and explained nondisjunction, crossingover, and recombination.


Reviewing Key Terms

Match the term on the left with the correct description.

___ allele

___

___

___

___

___ karyotype probability genotype phenotype antibodies a.

the genetic makeup of an organism b.

a display of the image of a set of chromosomes sorted by number c.

the chance that an event will occur d.

one of two or more possible forms of a gene e.

the expression of a genotype in the appearance or function of an organism f.

a blood protein produced in response to an antigen


Reviewing Ideas

1. What is the principle of independent assortment?

Alleles for one characteristic assort, or divide up among the gametes during meiosis, independently of alleles for other characteristics.


Reviewing Ideas

2. What is an X-linked trait? What are some human examples?

An X-linked trait is a trait whose gene is carried only on the X chromosome. More than 300

X-linked traits have been identified in humans.

Well-known examples of human X-linked, recessive alleles included those responsible for red-green color blindness and hemophilia, a disease in which blood does not clot normally.


Using Concepts

3. How are linked genes useful in locating the positions of genes on chromosomes?

The frequency with which linked traits become separated reflects how far apart on the chromosome the genes for those traits are. This information can be used to map the positions of genes on chromosomes.


Using Concepts

4. A person with which blood type would be able to accept all blood type in a transfusion? Why?

A person with blood type AB would be able to accept any of the four blood types. Their blood does not produce anti-A or anti-B antibodies which would cause rejection. Type O is universally accepted among all blood types.


Synthesize

5. Why is an extra X chromosome (XXX) in females not as disruptive as having an extra copy of another chromosome?

Cells of XXX females have two Barr bodies which means that only one X chromosome in a cell remains active.


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