Chapter 6 PDF

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Nelson grade 11 biology...

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CHAPTER

6

Complex Patterns of Inheritance

Specific Expectations In this chapter, you will learn how to . . .

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D1.1 analyze, on the basis of research, some of the social and ethical implications of research in genetics and genomics (6.3)

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D1.2 evaluate, on the basis of research, the importance of some recent contributions to the knowledge, techniques, and technologies related to genetic processes (6.3)

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D 2.1 use appropriate terminology related to genetic processes (6.1, 6.2, 6.3)

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D2.3 use the Punnett square method to solve basic genetics problems involving monohybrid crosses, incomplete dominance, codominance, dihybrid crosses, and sex-linked genes (6.1, 6.2)

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D3.3 explain the concepts of genotype, phenotype, dominance, incomplete dominance, codominance, recessiveness, and sex linkage according to Mendelian laws of inheritance (6.1, 6.2)

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D3.4 describe some genetic disorders caused by chromosomal abnormalities or other genetic mutations in terms of chromosomes affected, physical effects, and treatments (6.1, 6.2)

The inherited traits of an individual are the result of a complex array of genetic interactions. As genetics research continues to advance, we have a better understanding of these complexities. A significant advancement is the Human Genome Project. In 2003, a team of over 2000 researchers, working in laboratory groups around the world, completed the Human Genome Project. For this project, numerous images like the one shown here were analyzed. This photo shows the products of chemical reactions that are used to identify the nucleotide sequence of a piece of DNA. Scientists used these to determine, base by base, the DNA sequence of the human genome. Other goals of the Human Genome Project included identifying all of the human genes and making them available for study. Because such scientific goals have consequences for society, there are also groups of researchers that explore and monitor the ethical and social impacts of these scientific achievements.

240 MHR • Unit 2 Genetic Processes

Launch Activity Assembling a Mini-Genome The 46 chromosomes that make up our genome contain over 3 000 000 000 base pairs. Each chemical reaction that is used to determine the sequence of DNA can only provide the sequence of a few hundred bases at a time. Therefore, to determine the DNA sequence of the human genome, scientists all over the world worked together to analyze millions of DNA sequencing reactions. They then assembled the DNA sequence of the human genome by piecing together the much smaller fragments of sequences. In this activity, you will model how scientists did this.

output

ultraviolet light gel gel

detector computer

lanes

T AT AAAACATTTTAAAAG C TAG TACCCAG TACC TTC TAG TTCC AAAGCCCAATG TTG TTCAC TA TGG TTCAC AA TGGGACC A 140 150 160 170 180 190 200 210 220

The products of a DNA sequencing reaction are modified so they are visible under ultraviolet light. They are then separated in each lane of a gel-like material. The information in each lane is sent to a computer, which provides output in the form of a printout of the sequence of bases in a piece of DNA. Recall that nucleotides are often identified by their bases. For these data, red bands represent thymines, green bands represent adenines, blue bands represent cytosines, and black bands represent guanines.

Materials • paper DNA fragments • tape

Procedure 1. Obtain the sequence of DNA that you are to work with from your teacher. 2. With your classmates, construct one continuous segment of sequenced DNA from your individual fragments by matching overlapping sections and taping them into place.

Questions 1. How did you decide how to match and link the fragments together? 2. How important was it to collaborate and discuss your results with other class members in order to obtain the full sequence? 3. How important do you think it was for scientists to develop a systematic and organized approach to sequencing the human genome? How do you think computers played a role? Chapter 6 Complex Patterns of Inheritance • MHR 241

SECTION

6.1 Key Terms incomplete dominance codominance heterozygous advantage continuous variation polygenic trait

Beyond Mendel’s Observations of Inheritance Much of today’s genetics research uses sophisticated technologies to study cellular processes at the level of individual molecules and atoms. In addition, international research collaborations and multi-million-dollar budgets are now common. Th ink of what a stark contrast this is to Mendel’s experiments. It is astounding that Mendel’s basic and, at times, simple observations led him to infer patterns of inheritance that still form the basis of our current understanding of heredity. As more sophisticated experimental technologies became available, scientists realized that patterns of inheritance are more complicated than what Mendel proposed. Some patterns result in phenotypes that are between dominant and recessive phenotypes. Other patterns result in phenotypes that are created when both alleles for a trait are equally expressed.

Incomplete Dominance incomplete dominance a condition in which neither allele for a gene completely conceals the presence of the other; it results in intermediate expression of a trait

Incomplete dominance describes a condition in which neither of the two alleles for the same gene can completely conceal the presence of the other. As a result, a heterozygote exhibits a phenotype that is somewhere between a dominant phenotype and a recessive phenotype. One example is the flower colour of snapdragons (Antirrhinum majus). As you can see in Figure 6.1, a cross between a true-breeding red-flowered plant and atrue-breeding white-flowered plant produces offspring with pink flowers in the F1 generation. If the F1 plants are allowed to self-fertilize, the F2 generation will include offspring with all three phenotypes—red, pink, and white. The Punnett square in Figure 6.1 predicts that all three phenotypes will be observed in the F2 generation in a ratio of 1:2:1 (red:pink:white), which is what is observed experimentally. In true Mendelian inheritance, we would have predicted a phenotypic ratio of 3:1. Nevertheless, the alleles for flower colour do segregate according to Mendel’s law of independent assortment. When representing incomplete dominance, upper-case and lower-case letters are not usually used to represent the alleles, since neither allele is dominant over the other. One way to represent incomplete dominance is by using superscripts. In the example of snapdragon flower colour, both alleles affect the colour of the flower, C. The two alleles are represented as superscripts, R for red (CR), and W for white (CW). Lower-case letters are only used to represent a recessive allele.

Figure 6.1 When red (C RC R) flowers and white (C WC W) flowers of the snapdragon are crossed, the resulting offspring have an intermediate phenotype, pink flowers (C RC W). In the F2 generation, all three phenotypes are observed.

P generation

F1 generation

F2 generation

gametes R R

CC

CR

self-fertilization of F1 offspring CRCW

242 MHR • Unit 2 Genetic Processes

CRCR

CRCW

CRCW

CWCW

CW CWCW

white

CW

CR

red

×

CR

CW

pink

Incomplete Dominance and Human Disease There are many examples of genetic disorders in humans that exhibit incomplete dominance. For example, there is a genetic disorder, called familial hypercholesterolemia, that prevents tissues from removing low-density lipoproteins (LDL) from the blood and causes very high levels of cholesterol in the bloodstream. In the majority of cases, the disorder is due to a mutation in the LDLR gene. LDL particles transport molecules like cholesterol throughout the body. The mutated version of the LDLR gene no longer produces a protein that interacts with LDL particles and removes them from the bloodstream. This disorder has an autosomal dominant inheritance pattern. So, an individual only requires one allele of the mutated form of the gene to show symptoms of the disorder. However, if the allele for the normal form of the gene is present, symptoms of the disease will not be as severe. People who are homozygous dominant for the trait have six times the normal amount of LDL in their blood and may have a heart attack by the age of 2. Heterozygotes have about twice as much cholesterol in their blood and may have a heart attack by the age of 35. Scientists are now finding that identifying the patterns of inheritance for many traits is not as straightforward as first thought. Today’s more accurate techniques are showing that, in some cases, what had been identified as a dominant inheritance pattern may actually be incomplete dominance. As a result, an individual who is heterozygous for a trait is not exactly the same as an individual who is homozygous dominant for the trait.

Codominance Codominance is a situation in which both alleles are fully expressed. A roan animal is an excellent, visible example of codominance. A roan animal is a heterozygote in which both the base colour and white are fully expressed. If you look closely at the individual hairs on a roan animal, such as the cow in Figure 6.2, you will see a mixture of red hairs and white hairs. One allele is expressed in the white hairs, and the other allele is expressed in the red hairs.

codominance the condition in which both alleles for a trait are equally expressed in a heterozygote; both alleles are dominant

Figure 6.2 A roan cow is the product of a mating between a red cow and a white cow. The red and white hairs may be present in patches, as shown here, or be completely intermingled. Chapter 6 Complex Patterns of Inheritance • MHR 243

heterozygous advantage a survival benefit for individuals who inherit two different alleles for the same trait

Figure 6.3 Normal red blood cells are flat and disk-shaped. Sickle-shaped cells are elongated and “C” shaped.

Sickle Cell Anemia Sickle cell anemia is one of the most thoroughly studied genetic disorders. Although it is often described as being the result of autosomal recessive inheritance, it is actually an example of codominance. Sickle cell anemia is caused by a specific form of the gene that directs the synthesis of hemoglobin. Hemoglobin carries oxygen in the blood. The hemoglobin molecule that is made in individuals with the sickle cell allele leads to a C-shaped (or sickled) red blood cell. These misshaped red blood cells, like the one shown in Figure 6.3, do not transport oxygen effectively because they cannot pass through small blood vessels. Th is leads to blockages and tissue damage. The allele for normal hemoglobin is represented as HbA, and the allele for sickle cell hemoglobin is represented as HbS. As shown in Figure 6.4, individuals who are homozygous (HbSHbS) have sickle cell anemia. Individuals who are heterozygous (HbAHbS) have some normal and some sickled red blood cells. Th ese individuals are said to have the sickle cell trait, but they rarely experience any symptoms. In fact, having the sickle cell trait can be an advantage, because these heterozygotes are more resistant to malaria. Malaria is a life-threatening disease caused by a parasite that is transmitted to humans through mosquito bites. The parasite infects the liver and eventually the red blood cells. Th e sickling of red blood cells is thought to prevent the parasites from infecting the cells. Resistance to malaria is very beneficial in certain parts of Africa, where deadly epidemics can occur. The sickle cell trait is an example of the principle of heterozygous advantage, which describes a situation in which heterozygous individuals have an advantage over both homozygous dominant and homozygous recessive individuals. A

S

Hb A

Hb Hb

A

A

Hb Hb

S

Hb Hb

A

S

Hb Hb

Hb A

A

Hb Hb

A

S

Hb Hb

A

S

Hb Hb

S

sickle cell trait

S

S

S

S

sickle cell trait

sickle cell trait

A

Hb Hb

sickle cell trait

normal

Hb

Hb

sickle cell anemia

Figure 6.4 When a man and a woman are both heterozygous for the sickle cell gene, there is a one in four chance that they will have a child with sickle cell anemia.

Learning Check 1. Distinguish between incomplete dominance and codominance. 2. Why do geneticists use notations like CW and CR to describe incomplete or codominant alleles instead of using W and w or R and r ? 3. A plant that produces white flowers is crossed with a plant that produces purple flowers. Describe the phenotype of the offspring if the inheritance pattern for flower colour is a. incomplete dominance b. codominance

244 MHR • Unit 2 Genetic Processes

4. The frequency of the appearance of the sickle cell allele in human populations is much higher in Africa than in most other areas of the world. What has been proposed to explain this observation? 5. Provide two pieces of evidence that support the idea that some inheritance patterns are more complex than those originally proposed by Mendel. 6. Scientists first thought that sickle cell anemia was inherited as an autosomal recessive allele. What led them to identify the true inheritance pattern of the disease?

Multiple Alleles The traits you have studied so far have all been controlled by one gene with two alleles, such as the flower colour in pea plants. Many traits in humans and other species are the result of the interaction of more than two alleles for one gene. Agene with more than two alleles is said to have multiple alleles. As you know, any individual has only two alleles for each gene—one allele on each homologous chromosome. However, many different alleles for a gene can exist within the population as a whole.

lA

Possible alleles from male

Human Blood Groups Do you know what blood type you are? In humans, a single gene determines a person’s ABO blood type. This gene determines what type of an antigen protein, if any, is attached to the cell membrane of red blood cells. An antigen protein is a molecule that stimulates the body’s immune system. The gene is designated I, and it has three common alleles: IA, IB, and i. As shown in Figure 6.5, the different combinations of the three alleles produce four different phenotypes, which are commonly called blood types A (IAIA homozygotes or IAi heterozygotes), B (IBIB homozygotes or IBi heterozygotes), AB (IAIB heterozygotes), and O (ii homozygotes). The IA allele is responsible for the presence of an A antigen on the red blood cells. The IB allele is responsible for the presence of the B antigen, and the i allele results in no antigen. Of the three alleles that determine blood type, one (i ) is recessive to the other two, and the other two (IA and IB) are codominant.

Possible alleles from female

Rabbit Coat Colour Another example of multiple alleles involves coat colour in rabbits, as shown in Figure 6.6. The gene that controls coat colour in rabbits has four alleles: agouti (C ), chinchilla (cch), Himalayan (ch), and albino (c). In that order, each allele is dominant to all the alleles that follow. The order of dominance sequence can be written as C > cch > ch > c, where the symbol > means is dominant to.

lA

lB

or

i

or

lAlA

lAlB

lAi

lAlB

lBlB

lBi

lAi

lBi

ii

or

lB or

i

blood types

A

AB

B

O

Figure 6.5 Different combinations of the three I alleles result in four different blood types: type A, type B, type AB, and type O.

agouti

Himalayan

chinchilla

albino

Figure 6.6 Rabbits have multiple alleles for coat colour, with four possible phenotypes. Predict the possible genotypes for each rabbit. Chapter 6 Complex Patterns of Inheritance • MHR 245

Clover Leaf Patterns The pattern on the leaves of the clover plant is also controlled by multiple alleles. While a single gene is responsible for clover leaf pattern, there are seven different alleles for the pattern. Varying combinations of these result in 22 different patterns that can be expressed in clover leaves. Patterns for the seven homozygous combinations of alleles are shown in Figure 6.7. Figure 6.7 There are seven different alleles for clover leaf pattern.

v/v

vl/v l

vh/vh

vf/vf

vba/vba

Sample Problem

Using a Punnett Square to Analyze Inheritance of Multiple Alleles Problem If a man has type O blood and a woman has type B blood, what possible blood types could their children have? If this couple has six children, all with type B blood, what could you infer about the woman’s genotype? What Is Required? You are asked to determine all possible blood types of the children and the possible genotype of the mother based on all the children having type B blood. What Is Given? The man has blood type O, the woman has blood type B. Plan Your Strategy Determine the possible genotypes of the man and the woman.

Act on Your Strategy Since the man has blood type O, his genotype must be ii. The woman has blood type B, so her genotype could be either IBIB or IBi.

Make Punnett squares for all the possible combinations of genotypes.

mother IB

IB

i

IBi

IBi

i

IBi

IBi

father

mother IB

i

i

IBi

ii

i

IBi

ii

father

List all the possible genotypes and phenotypes of the children.

The children could have genotype IBi, resulting in type B blood, or genotype ii, resulting in type O blood.

What could be the mother’s genotype based on the children being type B?

The mother`s genotype is most likely IBIB.

Check Your Solution The only genotype that produces type O blood is ii. To have type B blood, the woman must have at least one IB allele. Her second allele could be either IB or i. Since all of the children had to receive an i allele from their father, they must have inherited an IB allele from their mother. Since all of the children have type B blood, the mother is most likely IBIB.

246 MHR • Unit 2 Genetic Processes

vb /vb

vby /vby

Practice Problems 1. If a man has type AB blood and a woman has type A blood, what possible blood types could their children have? 2. A baby has blood type AB. If the baby’s mother has blood type B, what blood type(s) could the father have? 3. A couple just brought home their new baby from the hospital. They begin to suspect that the hospital switched babies, and the baby they brought home is not theirs. They check the hospital records, and find that the man’s blood type is B, the woman’s blood type is AB, and the baby’s blood type is O. Explain why the parents are or are not justified in their concern about this baby. 4. Four children ...