chapter 6 10.1 studying genetics ... 10.1 mastering concepts how do meiosis, ... copyright © the...

Chapter 6 Patterns of Inheritance

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Genetics Explains and Predicts Inheritance Patterns

Section 10.1

Genetics can explain how these poodles look different.


Section 10.1

Analyzing their genes can also help predict the appearance of their offspring.



Section 10.1

But most genes encode proteins that have nothing to do with outward appearance. The enzymes essential to these poodles’ lives are also the products of genetics.



Section 10.1

Studying genetics also allows scientists to breed superior crops and doctors to track genetic illnesses.



Chromosomes Are Packets of Genetic Information

Section 10.1 Figure 10.1

Recall that DNA is wound tightly into chromosomes.




Cells with only one set of chromosomes, such as sex cells, are haploid.




When two haploid cells fuse during fertilization, a diploid zygote with two full sets of chromosomes is formed.




Most cells of a mature individual are diploid.




Homologous chromosomes have the same genes, but might have different versions (alleles) of those genes.




Diploid cells therefore have two alleles for each gene. These alleles might be identical (gene A) or different (gene B).




Each gene’s locus is its location on a chromosome.


10.1 Mastering Concepts

How do meiosis, fertilization, diploid cells, and haploid cells interact in a sexual life cycle?


© 1996 PhotoDisc, Inc./Getty Images/RF

Mendel Uncovered Basic Laws of Inheritance


Gregor Mendel used pea plants to study heredity.




Hand-pollinating plants allowed Mendel to control plant breeding experiments.




Self-fertilizing and cross-fertilizing in different combinations allowed Mendel to deduce the principles of inheritance.


• In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization

• The true-breeding parents are the P generation

• The hybrid offspring of the P generation are called the F1 generation

• When F1 individuals self-pollinate or cross-pollinate with other F1 hybrids, the F2

generation is produced

© 2011 Pearson Education, Inc.

Figure 14.3-1

P Generation

EXPERIMENT

(true-breedingparents) Purple

flowersWhite

flowers

Figure 14.3-2

P Generation

EXPERIMENT

(true-breedingparents)

F1 Generation(hybrids)

Purpleflowers

Whiteflowers

All plants had purple flowers

Self- or cross-pollination

Figure 14.3-3

P Generation

EXPERIMENT

(true-breedingparents)

F1 Generation(hybrids)

F2 Generation

Purpleflowers

Whiteflowers

All plants had purple flowers

Self- or cross-pollination

705 purple-flowered

plants

224 whiteflowered

plants

The Law of Segregation

• When Mendel crossed contrasting, true-breeding white- and purple-flowered pea plants, all of the F1 hybrids were purple

• When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white

• Mendel discovered a ratio of about three to one, purple to white flowers, in the F2

generation


• Mendel reasoned that only the purple flower factor was affecting flower color in the F1

hybrids

• Mendel called the purple flower color a dominant trait and the white flower color a recessive trait

• The factor for white flowers was not diluted or destroyed because it reappeared in the F2

generation


• Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits

• What Mendel called a “heritable factor” is what we now call a gene

© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.

Table 14.1



True-breeding plants produce offspring identical to themselves.




Dominant alleles exert their effects whenever they are present. Crossing a yellow-seed plant with a green-seed plant always yields

some yellow seeds. Yellow seed color is therefore dominant.




A recessive allele is one whose effect is masked if a dominant allele is also present.

Recessive alleles usually encode nonfunctional proteins.




If yellow seed color is dominant, why are some seeds green when a yellow-seed plant is crossed with a green-seed plant?

We need more information before we can fully answer this question.



Section 10.2 Figures 10.4, 10.5

But the answer has to do with each plant having two alleles for each gene (because of their homologous pairs of chromosomes).



Section 10.2

A genotype represents an individual’s two alleles for one gene. The genotype confers a phenotype, or observable characteristic.

Figures 10.4, 10.5Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.


Section 10.2

• Homozygous dominant individuals have two dominant alleles for a gene.

• Heterozygous individuals have one dominant and one recessive allele.

• Homozygous recessive individuals have two recessive alleles.



Section 10.2

It is possible to look at offspring to determine the genotype of the parent. As we’ll see, Punnett squares help solve these puzzles.


Mendel’s Model

• Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2

offspring

• Four related concepts make up this model

• These concepts can be related to what we now know about genes and chromosomes

© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.

• First: alternative versions of genes account for variations in inherited characters

• For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers

• These alternative versions of a gene are now called alleles

• Each gene resides at a specific locus on a specific chromosome


Figure 14.4

Allele for purple flowers

Locus for flower-color gene

Allele for white flowers

Pair ofhomologouschromosomes

• Second: for each character, an organism inherits two alleles, one from each parent

• Mendel made this deduction without knowing about the role of chromosomes

• The two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s P generation

• Alternatively, the two alleles at a locus may differ, as in the F1 hybrids


• Third: if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance

• In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant


• Fourth (now known as the law of segregation): the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

• Thus, an egg or a sperm gets only one of the two alleles that are present in the organism

• This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis


• Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses

• The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup

• A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele



Distinguish between dominant and recessive; heterozygous and homozygous; phenotype and genotype; wild-type and mutant.



Punnett Squares Represent Gamete Formation and Fertilization

Section 10.3

A Punnett squareuses the genotypes of the parents to reveal which alleles the offspring may inherit.

Figure 10.6Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.


Section 10.3

In this example, a female parent that is heterozygous for seed color is crossed with a male parent that is also heterozygous for seed color.



Section 10.3

This is a monohybrid cross since both parents are heterozygous for the one gene being evaluated.



Section 10.3

Genotype Yy indicates that all diploid cells, including germ cells, in these parents have both dominant and recessive seed color alleles.



Section 10.3

When germ cells divide by meiosis, chromosomes (and the alleles on those chromosomes) are randomly distributed among gametes.



Section 10.3

A gamete from the female parent and a gamete from the male parent then unite at fertilization.



Section 10.3

If both gametes carry dominant alleles, the offspring will inherit two dominant alleles.



Section 10.3

If one gamete carries a dominant allele and the other carries a recessive allele, the offspring will be heterozygous.



Section 10.3

If both gametes carry recessive alleles, the offspring will inherit two recessive alleles.



Section 10.3

This Punnett square therefore represents all possible offspring that might result from these parents.



Section 10.3

This Punnett square also shows the relative proportion of the offspring phenotypes and genotypes.



Section 10.3

On average, three offspring will have yellow seeds for every one with green seeds.



Section 10.3

On average, one offspring will have genotype YY for every two with Yy and for every one with yy.


Section 10.3

Punnett squares allow us to determine the genotypes of these yellow-seed pea plants.



Section 10.3

Punnett squares also help us answer this question: If yellow seed color is dominant, why are some seeds green when a yellow-seed plant is crossed with a green-seed plant?



Section 10.3

If a cross between a yellow-seed pea plant (YY or Yy) and a green-seed pea plant (yy) yields all yellow seeds, the yellow-seed parent is homozygous dominant.

Figures 10.5, 10.8



Section 10.3

If the cross yields some green seeds, the yellow-seed parent is heterozygous.



Section 10.3

Punnett squares summarize meiosis and fertilization.

Figure 10.9

Meiosis Explains Mendel’s Law of Segregation


Section 10.3

The two alleles for the Y gene are packaged into separate gametes, which then combine at random.

Figure 10.9



Section 10.3

Can you create a Punnett square representing the information in this figure?

Figure 10.9



Section 10.3

Punnett squares are also useful for tracking the inheritance of genetic disorders, such as cystic fibrosis.

Figure 10.10

Mendel’s Law Applied to Humans


Clicker Question #1

Cystic fibrosis is caused by a recessive allele. If a healthy carrier and an affected individual have a child, what is the chance the child will be affected?

A. 1/4B. 1/3C. 1/2D. 3/4E. 1




What is a monohybrid cross, and what are the genotypic and phenotypic ratios expected in the offspring of the cross?



The Law of Independent Assortment

• Mendel derived the law of segregation by following a single character

• The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one character

• A cross between such heterozygotes is called a monohybrid cross


• Mendel identified his second law of inheritance by following two characters at the same time

• Crossing two true-breeding parents differing in two characters produces dihybrids in the F1

generation, heterozygous for both characters

• A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently


Two genes on different chromosomes can be combined into one large Punnett square.

Figure 10.11

Dihybrid Crosses Track the Inheritance of Two Genes at Once

Section 10.4Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

• Using a dihybrid cross, Mendel developed the law of independent assortment

• The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation

• Strictly speaking, this law applies only to genes on different, nonhomologouschromosomes or those far apart on the same chromosome

• Genes located near each other on the same chromosome tend to be inherited together


Based on dihybrid crosses, Mendel proposed the law of independent assortment, which states that the segregation of

alleles for one gene does not influence the segregation of alleles for another gene.

Figure 10.12

Alleles Separate During Meiosis


Clicker Question #2

Why is it impossible for one of the female gametes to have genotype rr?

A. Each germ cell only has one r allele.B. Each gamete can only receive two alleles, and one must be a y.C. The r alleles separate during meiosis.



Tracking two or more genes on one Punnett square is challenging and time-consuming. The product rule simplifies these problems.

Figure 10.13

The Product Rule Replaces Complex Punnett Squares


The chance that two independent events will both occur, equals the product of the individual chances that each event will occur.

Figure 10.13



For example, the probability that an offspring inherits genotype Rr Yy Tt is equal to the probability of Rr (1/2) times

the probability of Yy (1/2) times the probability of Tt (1/2).

Figure 10.13



Clicker Question #3

A male with genotype Qq Bb Dd is crossed with a female with genotype qq bb dd. What proportion of the offspring will be homozygous recessive for all three genes?

A. 1/2B. 1/3C. 1/4D. 1/6E. 1/8




How does the law of independent assortment reflect the events of meiosis?



The product rule cannot be used if genes are linked, because inheriting one allele influences the likelihood

of inheriting a linked allele.

Figure 10.14

Genes on the Same Chromosome Are Linked


However, because of crossing over, linked alleles are not always inherited together.

Figure 10.14



The probability of a crossover event occurring between two linked alleles is proportional to the distance between the genes.

Figure 10.15



The letters below the linkage map of this chromosome represent alleles. The numbers above represent crossover

frequencies relative to y.

Figure 10.15



Crossing over frequently separates y and r but rarely separates y and w. Therefore, even without this diagram, one could infer

that y is nearer to w than to r.

Figure 10.15



Summary of Mendel’s Laws• The law of independent assortment: each pair of

alleles segregates independently of each other pair of alleles during gamete formation

• The law of segregation: the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

• The principle of dominance: if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance


Explain how to use crossover frequencies to make a linkage map.



Figure 10.16

Gene Expression Can Appear to Alter Mendelian Ratios

Section 10.6

So far we’ve discussed genes with two alleles, in which the dominant allele masks the recessive allele. But gene expression does not always follow that pattern.


Figure 10.16

Gene Expression Can Appear to Alter Mendelian Ratios

Section 10.6

So far we’ve discussed genes with two alleles, in which the dominant allele masks the recessive allele. But gene expression does not always follow that pattern.

• Incomplete dominance• Codominance• Pleiotropy


Figure 10.16

Incomplete Dominance and Codominance Add Phenotype Classes

Section 10.6

In incomplete dominance, the heterozygote has an intermediate phenotype.


Figure 10.16


Section 10.6

The recessive allele (r2) still encodes a nonfunctional protein.


Figure 10.16


Section 10.6

The heterozygote is pink because it receives half the dose of the red pigment conferred by the dominant allele.


Figure 10.17


Section 10.6

In codominance, more than one allele encodes a functional protein.


Figure 10.17


Section 10.6

If two dominant alleles are present, both proteins encoded by those alleles will be represented in the phenotype.


Figure 10.17


Section 10.6

In human blood types, both IA and IB are dominant alleles. Genotype IAIB confers red blood cells with both A and B molecules.


Figure 10.17


Section 10.6

The I gene also has a recessive allele, i, which encodes a nonfunctional protein. But the two dominant alleles, IA and IB, make the I gene codominant.


One Gene, Many Phenotypes

Section 10.6

Gene

Protein (enzyme)

Biochemical pathways

B1 B2 B3 Phenotype B

A1 A2 A3 Phenotype AA1 A2 A3 Phenotype A

C1 C2 C3 Phenotype C

+

+

X

In pleiotropy, one gene has multiple effects on the phenotype. For example, a gene might affect more than one biochemical pathway.


Section 10.6

Gene

Protein (enzyme)

Biochemical pathways

B1 B2 B3 Phenotype B

A1 A2 A3 Phenotype AA1 A2 A3 Phenotype A

C1 C2 C3 Phenotype C

+

+

X

In this example, a gene encodes a protein that catalyzes reactions in two biochemical pathways and blocks another.

One Gene, Many Phenotypes


Polygenic Inheritance

• Quantitative characters are those that vary in the population along a continuum

• Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype

• Skin color in humans is an example of polygenic inheritance


Figure 14.13

Eggs

Sperm

Phenotypes:

Number ofdark-skin alleles: 0 1 2 3 4 5 6

1/81/8

1/81/8

1/81/8

1/81/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/8

1/646/64

15/6420/64

15/646/64

1/64

AaBbCc AaBbCc

Nature and Nurture: The Environmental Impact on Phenotype

• Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype

• The norm of reaction is the phenotypic range of a genotype influenced by the environment

• For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity


Figure 14.14

Figure 14.14a

Figure 14.14b

• Norms of reaction are generally broadest for polygenic characters

• Such characters are called multifactorialbecause genetic and environmental factors collectively influence phenotype



How do incomplete dominance and codominance increase the number of phenotypes?



Sex-Linked Genes Have Unique Inheritance Patterns

Section 10.7

In humans, females have two X chromosomes. Males have one X chromosome and one Y chromosome.



Section 10.7

This Punnett square shows that each fertilization event has a 50% chance of producing a female and a 50% chance of producing a male.



Section 10.7

Which gamete, the sperm or the egg, determines the sex of the offspring?



Section 10.7

The egg will always carry an X chromosome. The sex chromosome in the sperm therefore determines if the offspring is female or male.



Section 10.7

X-linked recessive disorders affect more males than females.



Section 10.7

Females must receive a recessive allele on both X chromosomes to express an X-linked recessive disorder.



Section 10.7

Males only have one X chromosome. To express a recessive disorder, they only need to inherit one X-linked recessive allele.


Clicker Question #4

Hemophilia is a X-linked recessive disorder. If an affected female and an unaffected male have a boy, what is the chance he will have hemophilia?

A. 0B. 1/4C. 1/2D. 3/4E. 1





Table 10.2


Section 10.7

X-inactivation prevents double-dosing of gene products. Each cell in an XX individual, such as these female cats, randomly inactivates

one X chromosome.



Section 10.7

If one X chromosome has an allele for orange fur and the other has an allele for black fur, color patterns emerge when X chromosomes

are randomly inactivated.



Why do males and females express recessive X-linked alleles differently?



Pedigrees Show Modes of Inheritance

Section 10.8

A pedigree depicts family relationships and phenotypes.



Section 10.8

This pedigree tracks an autosomal dominant disorder.



Section 10.8

This pedigree tracks an autosomal recessive disorder.



Section 10.8

This pedigree tracks an X-linked recessive disorder. Note that more males are affected than females.


Clicker Question #5

This pedigree tracks an autosomal dominant disorder. What is the genotype of I-2?

A. homozygous dominantB. heterozygousC. homozygous recessive




How are pedigrees helpful in determining a disorder’s mode of inheritance?



The Environment Can Alter Phenotype

Section 10.9

Many genes are affected by the environment. For example, the enzyme responsible for pigment production in Siamese cat fur is active only in cool body parts.


Some Traits Depend on Multiple Genes

Section 10.9

Skin color is a polygenic trait; it is affected by more than one gene.



How can the environment affect a phenotype?



Investigating Life: Heredity and the Hungry Hordes

Section 10.10

Bollworm larvae devastate cotton crops. But some bollworms are susceptible to Bt toxin. Biologists have inserted the gene encoding this toxin into the cotton genome.



Section 10.10

In a mating between two Bt-resistant bollworms, all of the offspring will also be resistant.



Section 10.10

However, if a resistant bollworm mates with a susceptible bollworm, only some—and sometimes none—of the offspring will be resistant. (Would you guess Bt resistance is conferred by a dominant or a recessive allele?)



Section 10.10

To avoid 100% resistance among bollworms of future generations, farmers must plant some crops without the toxin gene.

Figure 10.24

Crops with the Bt toxin

Crops without the Bt toxin



Section 10.10

This arrangement increases the chance that some susceptible bollworms will remain in the population.

Figure 10.24

Crops with the Bt toxin

Crops without the Bt toxin



Explain the logic of planting non-Bt crop buffer strips around fields planted with Bt crops.



chapter 6 10.1 studying genetics ... 10.1 mastering concepts how do meiosis, ... copyright © the...

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