learning objectives: students should be able to …

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Learning objectives: Students should be able to …

• Describe how Mendel’s principles of segregation and independent assortment are a consequence of chromosome movement in meiosis.

• Calculate expected frequencies of genotypes and phenotypes in monohybrid, dihybrid, and X-linked crosses.

• Analyze the results of crosses and pedigrees to determine whether phenotypes are autosomal or X-linked, dominant or recessive, linked or on different chromosomes.

• Explain what a dominant allele is and how this applies to incomplete dominance, codominance, gene interactions, and polygenic inheritance.


Chapter 13 outline: Mendel and the gene• Mendel

– how inheritance works

– model organism

– exp with a single trait

– exp with two traits

• Sutton & Boveri (1902)

– meiosis to explain Mendel’s findings

– chromosome theory of inheritance

• Thomas Hunt Morgan

– testing chromosome theory of inheritance

• Nettie Stevens

– sex chromosome discovery

• Outline (cont)

• sex linkage

• crossing over

• incomplete dominance

• codominance

• multiple allelism

• pleiotropic genes

• quantitative traits

• Applying Mendel’s rules to humans

• (controlled experiments not possible)


Key Concepts

In many species, individuals have two alleles of each gene. The principle of segregation states that prior to the formation of eggs and sperm, the alleles of each gene separate so that each egg or sperm cell receives only one of them.

The principle of independent assortment states that alleles of different genes are transmitted to egg cells and sperm cells independently of each other.


Key Concepts

Genes are located on chromosomes. The principle of segregation is explained by the separation of homologous chromosomes in anaphase of meiosis I. The principle of independent assortment applies to genes found on different chromosomes and is explained by chromosomes lining up randomly in metaphase of meiosis I.

There are important exceptions and extensions to the basic patterns of inheritance that Mendel discovered.


Introduction

• In 1865, Gregor Mendel worked out the rules of inheritance through a series of brilliant experiments on garden peas.

• Early in the 20th century, Walter Sutton and Theodor Boveri formulated the chromosome theory of inheritance, which proposes that meiosis causes the patterns of inheritance that Mendel observed.

• Genetics is the branch of biology that focuses on inheritance.


Mendel’s Experimental System

• Gregor Mendel was a 19th-century monk and active member of his city’s Agricultural Society.

• Mendel was interested in heredity. Heredity is the transmission of traits from parents to their offspring. A trait is any characteristic of an individual.


What Question Was Mendel Trying to Answer?

• Mendel was addressing the basic question of why offspring resemble their parents and how transmission of traits occurs.

• In his time, two hypotheses had been formulated to try to answer this question:

1. Blending inheritance – parental traits blend such that their offspring have intermediate traits.

2. Inheritance of acquired characteristics – parental traits are modified and then passed on to their offspring.


Garden Peas: The First Model Organism in Genetics

• Genetics, the branch of biology that focuses on the inheritance of traits, uses model organisms because the conclusions drawn from them can be applied to other species.

• Mendel chose the common garden pea (Pisum sativum) as his model organism because:

– It is easy to grow.

– Its reproductive cycle is short.

– It produces large numbers of seeds.

– Its matings are easy to control.

– Its traits are easily recognizable.


How Did Mendel Arrange Matings?

• Peas normally pollinate themselves, a process called self-fertilization.

• Mendel could prevent this by removing the male reproductive organs containing pollen from each flower. He then used this pollen to fertilize the female reproductive organs of flowers on different plants, thus performing cross-pollination.


What Traits Did Mendel Study?

• Mendel worked with pea varieties that differed in seven easily recognizable traits: seed shape, seed color, pod shape, pod color, flower color, flower and pod position, and stem length.

• An individual’s observable features comprise its phenotype. Mendel’s pea population had two distinct phenotypes for each of the seven traits.

• Mendel worked with pure lines that produced identical offspring when self-pollinated. He used these plants to create hybrids by mating two different pure lines that differed in one or more traits.


Inheritance of a Single Trait

• Mendel's first experiments involved crossing pure lines that differed in just one trait.

• The adults in the cross were the parental generation, the offspring are the F1 generation (for "first filial").


The Monohybrid Cross

• Mendel’s first experimented with crossing plants that differed in only one trait.

• When Mendel crossed plants with round seeds and plants with wrinkled seeds, all of the F1 offspring had round seeds.

– This contradicted the hypothesis of blending inheritance.

– The genetic determinant for wrinkled seeds seemed to have disappeared.

• Mendel allowed the F1 progeny to self-pollinate.

– The wrinkled seed trait reappeared in the next F2 generation.


Dominant and Recessive Traits

• Mendel called the genetic determinant for wrinkled seeds recessive and the determinant for round seeds dominant.

– In modern genetics, the terms dominant and recessive identify only which phenotype is observed in individuals carrying two different genetic determinants.

• Mendel repeated these experiments with each of the other traits. In each case, the dominant trait was present in a 3:1 ratio over the recessive trait in the F2 generation.


A Reciprocal Cross

• Mendel wanted to determine if gender influenced inheritance.

• He performed a reciprocal cross, in which the mother's phenotype in the first cross is the father's phenotype in the second cross, and the father's phenotype in the first cross is the mother's phenotype in the second cross.

• The results of the two crosses were identical. This established that it does not matter whether the genetic determinants for seed shape are located in the male or female parent.


Is inheritance of seed shape related to sex?


Particulate Inheritance

• To explain these results, Mendel proposed a hypothesis called particulate inheritance, which suggests that hereditary determinants maintain their integrity from generation to generation.

– This directly contradicts both the blending inheritance and inheritance of acquired characteristics hypotheses.


Genes, Alleles, and Genotypes

• Hereditary determinants for a trait are now called genes.

• Mendel also proposed that each individual has two versions of each gene. Today these different versions of a gene are called alleles. Different alleles are responsible for the variation in the traits that Mendel studied.

• The alleles found in an individual are called its genotype. An individual’s genotype has a profound effect on its phenotype.


The Principle of Segregation

Mendel developed the principle of segregation: the two members of each gene pair must segregate—that is, separate—into different gamete cells during the formation of eggs and sperm in the parents.


Genetic Notational Convention

• Mendel used a letter to indicate the gene for a particular trait. For example, R represented the gene for seed shape. He used uppercase (R) to show a dominant allele and lowercase (r) for a recessive allele.

• Individuals have two alleles of each gene.

– Individuals with two copies of the same allele (RR or rr) for a gene are said to be homozygous.

– Those with different alleles (Rr) are heterozygous.


Crossing Pure Lines

• Pure-line individuals always produce offspring with the same phenotype because they are homozygous—no other allele is present.

• A mating between two pure lines that differ in one trait (RR and rr) results in offspring that all have a heterozygous genotype (Rr) and a dominant phenotype.


The Monohybrid Cross

• A mating of two heterozygous parents results in offspring that are ¼ RR, ½ Rr, and ¼ rr, which produces a 3:1 ratio of phenotypes.


Testing the Model

• Mendel's genetic model—a set of hypotheses that explains how a particular trait is inherited—explains the results of these crosses.

• A Punnett square is now used to predict the genotypes and phenotypes of the offspring from a cross.


Producing a Punnett Square

1. Write the gamete genotypes for one parent along the top of the diagram.

2. Write the gamete genotypes for the other parent down the left side of the diagram.

3. Draw empty boxes under the row and to the right of the column of gametes.

4. Fill in each box with the genotypes written at the top of the corresponding column and at the left of the corresponding row.

5. Predict the ratios of each possible offspring genotype and phenotype by tallying the resulting genotypes in all the boxes.


Single Trait Cross


Mendel’s Experiments with Two Traits

• Mendel used dihybrid crosses—matings between parents that are both heterozygous for two traits—to determine whether the principle of segregation holds true if parents differ in more than one trait.

• Mendel’s experiments tested two contrasting hypotheses:

1. Independent assortment, in which alleles of different genes are transmitted independently of each other.

2. Dependent assortment, wherein the transmission of one allele depends upon the transmission of another.


The Principle of Independent Assortment

• Mendel’s results supported the principle of independent assortment.

• The Punnett square that results from a dihybrid cross predicts:

– There should be 9 different offspring genotypes and 4 phenotypes.

– The four possible phenotypes should be present in a ratio of 9:3:3:1.

Based on these data, Mendel accepted the hypothesis that alleles of different genes are transmitted independently of one another. This result became known as the principle of independent assortment.


Using a Testcross to Confirm Predictions

• In a testcross, a parent that is homozygous recessive for a particular trait is mated with a parent that has the dominant phenotype but an unknown genotype.

• Because the genetic contribution of the homozygous recessive parent is known, the genotype of the other parent can be inferred from the results.

• Mendel used the testcross to further confirm the principle of independent assortment.


The Chromosome Theory of Inheritance

• The chromosome theory of inheritance arose out of Sutton and Boveri’s careful observations of meiosis. It states that chromosomes are composed of Mendel’s hereditary determinants, or what we now call genes.

The physical separation of alleles during anaphase of meiosis I is responsible for Mendel’s principle of segregation.


The Chromosome Theory of Inheritance

• The genes for different traits assort independently of one another at meiosis I because they are located on different nonhomologous chromosomes, which themselves assort independently.

• This phenomenon explains Mendel’s principle of independent assortment.


Testing the Chromosome Theory

• Early in the 20th century, Thomas Hunt Morgan adopted fruit flies (Drosophila melanogaster) as a model organism for genetic research.

• Morgan’s first goal was to identify different phenotypes.

– He called the most common phenotype for each trait wild-type.

– He then inferred that phenotypes that differed from the wild-type resulted from a mutation, or a change in a gene.

– Individuals with traits attributable to mutation are known as mutants.


Thomas Hunt Morgan’s Experiments

• Morgan identified red eyes as the wild-type for eye color, and white eyes as a mutation.

• When he mated a wild-type female fly with a mutant male fly, all of the F1 progeny had red eyes.

• However, when Morgan did the reciprocal cross, the F1 females had red eyes but the F1 males had white eyes.

• These experiments suggest a relationship between the sex of the progeny and the inheritance of eye color in Drosophila.


The Discovery of Sex Chromosomes

• Nettie Stevens analyzed beetle karyotypes and found that females’ diploid cells contain 20 large chromosomes; but males’ diploid cells have 19 large and 1 small (Y) chromosomes.

– Y chromosomes pair with the large X chromosome during meiosis I.

• X and Y chromosomes are now called sex chromosomes—they determine the sex of the offspring.

– In beetles, females have two X chromosomes while males have an X and Y.

– Other species have other systems.


Sex Linkage and the Chromosome Theory

• Sex chromosomes pair during meiosis I and then segregate during meiosis II.

– This results in gametes with either an X or a Y chromosome.

• Females produce all X gametes.

• Males produce half X gametes and half Y gametes.


X-Linked Inheritance

• Morgan put together his experimental results with Stevens’ observations on sex chromosomes, and proposed that the gene for white eye color in fruit flies is located on the X chromosome and that the Y chromosome does not carry an allele of this gene.

• Morgan's hypothesis is called X-linked inheritance (or X-linkage). Females (XX) would then have two copies of the gene and males (XY) would have only one.


X-Linked Inheritance and the Chromosome Theory

• The various inheritance patterns that can occur when genes are carried on the sex chromosomes, such that females and males have different numbers of alleles of that gene, is termed sex-linked inheritance or sex-linkage.

• Non-sex chromosomes are called autosomes. Genes on autosomes are said to show autosomal inheritance.

• The discovery of X-linked inheritance convinced most biologists that the chromosome theory of inheritance was correct.


Extending Mendel’s Rules

• Once Mendel’s work was rediscovered, researchers began to analyze traits and alleles whose inheritance was more complicated.


Genes Can Be Located on the Same Chromosome

• The physical association of two or more genes found on the same chromosome is called linkage.

• Note that the terms linkage and sex-linkage differ in meaning. If a single gene is sex-linked, it means that it is found on a sex chromosome.

• Linked genes are predicted to always be transmitted together during gamete formation and thus should violate the principle of independent assortment.


The First Studies of Linked Genes

• To determine whether linked genes behave as predicted, Morgan performed an experiment using Drosophila, in which he mated two flies that were heterozygous for two sex-linked traits.

• The results of this experiment included some fruit flies with novel phenotypes. Morgan referred to these flies as recombinant because the combination of alleles on their X chromosome was different from the combinations of alleles present in the parental generation.


The First Studies of Linked Genes

• Morgan proposed that gametes with new, recombinant genotypes were generated when crossing over occurred during prophase of meiosis I in the females.

• Linked genes are inherited together unless crossing over occurs. When crossing over takes place, genetic recombination occurs.


Linkage Mapping

• Data on the percentage of recombinant offspring can be used to estimate the location of genes, relative to one another, on the same chromosome.

• Data on the frequency of crossing over can be used to create a genetic map—a diagram showing the relative positions of genes along a particular chromosome.

• Morgan proposed that genes are more likely to cross over when they are far apart from each other than when they are close together.


Extending Mendel’s Rules

• By studying a simple genetic system, Mendel discovered the most fundamental rules of inheritance.

• Most genes are inherited in a more complex fashion, however, than were the traits Mendel studied in garden peas.


Incomplete Dominance and Codominance

• Alleles of a gene are not always clearly dominant or recessive. In some cases, incomplete dominance occurs, and the heterozygotes have an intermediate phenotype.

• A heterozygous organism that displays the phenotype of both alleles of a single gene is said to display codominance. In this situation, neither allele is dominant or recessive to the other.


An example of

codominance


Multiple Alleles and Polymorphic Traits

• Many genes have more than two alleles, a situation known as multiple allelism.

• When more than two distinct phenotypes are present in a population due to multiple allelism, the trait is called polymorphic.


Pleiotropy

• The alleles Mendel analyzed affected only a single trait. Some genes, however, influence many traits—these genes are said to be pleiotropic.

• An example of pleiotropy in humans is the gene responsible for Marfan syndrome. Although research suggests that just a single gene is involved, individuals with Marfan syndrome exhibit a wide array of phenotypic effects, including increased height and limb length, and potentially severe heart problems.


The Physical Environment Also Affects Phenotype

• Mendel controlled the physical environment of his plants to be sure that it did not affect phenotype when he studied plant height.

• Most phenotypes are strongly influenced by the physical environment in addition to their corresponding genotypes.

• The combined effect of genes and environment is referred to as gene-by-environment interaction.


Gene-by-Environment Interactions

• The human genetic disease phenylketonuria (PKU) is a good example of a gene-by-environment interaction.

– Untreated, this disease causes phenylalanine to accumulate in the body of affected individuals and results in profound mental retardation.

– Individuals placed on a low-phenylalanine diet, however, develop normally.


Interactions with Other Genes Affect Phenotypes

• The expression of many genes is dependent upon the presence or absence of other genes.

• When these types of gene-by-gene interactions occur, the phenotype produced by an allele depends on the action of alleles of other genes.


Quantitative Traits

• Mendel worked with discrete traits, or characteristics that are qualitatively different. In garden peas, seed color is either yellow or green—no intermediate phenotypes exist.

– Traits that are not discrete but instead fall into a continuum are called quantitative traits.

• Nilsson-Ehle proposed that when many genes each contribute a small amount to the value of a quantitative trait, then the population usually exhibits a bell-shaped curve, or normal distribution, for the trait.


Quantitative Traits

• Nilsson-Ehle used wheat to propose why the distribution of kernel color exhibited a normal distribution.

• Transmission of quantitative traits results from polygenic inheritance, in which each gene adds a small amount to the value of the phenotype.


Applying Mendel’s Rules to Humans

• Because experimental crosses cannot be done in humans, pedigrees—family trees—are used to analyze the human crosses that already exist.

• Pedigrees record the genetic relationships among the individuals in a family, along with each person’s sex and phenotype for the trait being studied.


Identifying Human Alleles as Recessive or Dominant

• If a given trait is due to a single gene, the pedigree may reveal whether the trait is due to a dominant or recessive allele and whether the gene responsible is located on a sex chromosome or an autosome.

• To analyze the inheritance of a trait that shows discrete variation, biologists begin by assuming the simplest case: that a single autosomal gene is involved and that the alleles present in the population have a simple dominant-recessive relationship.


Patterns of Inheritance: Autosomal Recessive Traits

• When a phenotype is due to an autosomal recessive allele:

– Individuals with the trait must be homozygous.

– Unaffected parents of an affected individual are likely to be heterozygous carriers for the trait.

– Carriers have the allele and transmit it without exhibiting the phenotype.

• In general, a recessive phenotype should show up in offspring only when both parents have that recessive allele and pass it on to their offspring.


Patterns of Inheritance: Autosomal Dominant Traits

• Autosomal dominant traits are expressed in any individual with at least one dominant allele.

– In other words, individuals who are homozygous or heterozygous for the trait will display the dominant phenotype.

• This is the case with Huntington's disease.


Example: Autosomal dominant trait: Huntington’s disease


Is the Trait Autosomal or Sex-Linked?

• If a trait appears equally often in males and females, it is likely to be autosomal. If males are much more likely to have the trait, it is usually X-linked.

• Hemophilia is an example of an X-linked trait resulting from a recessive allele.

– These traits usually skip generations in a pedigree.

• X-linked dominant traits rarely skip generations.

– These traits are indicated in a pedigree wherein an affected male has all affected daughters but no affected sons.


A child can show the trait without the parent showing the trait

Example: X-linked trait from a recessive allele

learning objectives: students should be able to …

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