Mendel and the Gene Idea

Mendel and the Gene Idea

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

Pea plants were particularly well suited for use in Mendel's breeding experiments for all of the following reasons except that peas show easily observed variations in a number of characters, such as pea shape and flower color. it is possible to control matings between different pea plants. it is possible to obtain large numbers of progeny from any given cross. peas have an unusually long generation time. many of the observable characters that vary in pea plants are controlled by single genes. Answer: d

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 - Stem length

An individual’s observable features comprise its phenotype
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").

(true-breeding parents) Purple flowers White flowers
P Generation (true-breeding parents) Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers Self- or cross-pollination F2 Generation 705 purple- flowered plants 224 white flowered plants

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.

RESULT PREDICTION

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 cross between homozygous purple-flowered and homozygous white-flowered pea plants results in offspring with purple flowers. This demonstrates the blending model of genetics. true-breeding. dominance. a dihybrid cross. the mistakes made by Mendel. Answer: c

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.

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.

Allele for purple flowers
Pair of homologous chromosomes Locus for flower-color gene Allele for white flowers

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
Write the gamete genotypes for one parent along the top of the diagram. Write the gamete genotypes for the other parent down the left side of the diagram. Draw empty boxes under the row and to the right of the column of gametes. Fill in each box with the genotypes written at the top of the corresponding column and at the left of the corresponding row. Predict the ratios of each possible offspring genotype and phenotype by tallying the resulting genotypes in all the boxes.

P Generation Appearance: Purple flowers White flowers Genetic makeup:
Gametes: P p F1 Generation Appearance: Purple flowers Genetic makeup: Pp Gametes: 1/2 P 1/2 p Sperm from F1 (Pp) plant F2 Generation P p P Eggs from F1 (Pp) plant PP Pp p Pp pp 3 : 1

Phenotype Genotype Purple PP (homozygous) 1 3 Pp (heterozygous) Purple 2 Pp (heterozygous) Purple pp (homozygous) 1 White 1 Ratio 3:1 Ratio 1:2:1

Imagine crossing a pea heterozygous at the loci for flower color (white versus purple) and seed color (yellow versus green) with a second pea homozygous for flower color (white) and seed color (yellow). What types of gametes will the first pea produce? two gamete types: white/white and purple/purple two gamete types: white/yellow and purple/green four gamete types: white/yellow, white/green, purple/yellow, purple/green four gamete types: white/purple, yellow/green,white/white, and purple/purple one gamete type: white/purple/yellow/green Answer: c The purpose of this question is to help students figure out gamete types—a step they often rush past. Students need to realize that gametes are haploid and that each gamete contains one, and only one, allele for each of the traits being studied. The presence of the second pea in the question stem is a distracter. Answer a is wrong because it shows gametes with two alleles for flower color and no alleles for seed color. Answer b is wrong because it shows only two possible gamete types. Answer c is right because it shows all four possible gamete types. Answer d is wrong because it shows gametes that are diploid for one trait and containing an allele for only one locus. Answer e is wrong because it shows a gamete diploid for both loci.

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: Independent assortment, in which alleles of different genes are transmitted independently of each other. 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 (Is it RR or Rr
Using a Testcross to Confirm Predictions (Is it RR or Rr?? OR RRYY or RrYy??) 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.

Dominant phenotype, unknown genotype: PP or Pp?
TECHNIQUE Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp Predictions If purple-flowered parent is PP or If purple-flowered parent is Pp Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs P p Pp Pp pp pp RESULTS or All offspring purple 1/2 offspring purple and 1/2 offspring white

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.

Principle of DEPENDENT ASSORTMENT
W w Recessive allele For seed color Dominant allele For seed color w w W W w w W W W W w w Principle of DEPENDENT 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 ((Relationship between the sex of the progeny and the inheritance))
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.

Genes Can Be Located on the Same Chromosome
The physical association of two or more genes found on the same chromosome is called linkage. Linked genes are predicted to always be transmitted together during gamete formation and thus should violate the principle of independent assortment.

Independent assortment does not apply to linked genes.
Linked genes segregate together EXCEPT when crossing over and genetic recombination happens.

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. However most genes are inherited in a more complex fashion 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.

(a) The three alleles for the ABO blood groups and their carbohydrates
IA IB i Carbohydrate A B none (b) Blood group genotypes and phenotypes Genotype IAIA or IAi IBIB or IBi IAIB ii Red blood cell appearance Phenotype (blood group) A B AB O

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.

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.

AaBbCc AaBbCc Sperm 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 Eggs 1/8 1/8 1/8 1/8 Phenotypes: 1/64 6/64 15/64 20/64 15/64 6/64 1/64 Number of dark-skin alleles: 1 2 3 4 5 6

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.

Key Male Female Affected male Affected female Mating Offspring 1st generation Ff Ff ff Ff 1st generation Ww ww ww Ww 2nd generation 2nd generation FF or Ff ff ff Ff Ff ff Ww ww ww Ww Ww ww 3rd generation 3rd generation ff FF or Ff WW or Ww ww Widow’s peak No widow’s peak Attached earlobe Free earlobe (a) Is a widow’s peak a dominant or recessive trait? b) Is an attached earlobe a dominant or recessive trait?

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.

Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair.
Parents Normal Aa Normal Aa Sperm A a Eggs Aa Normal (carrier) AA Normal A Aa Normal (carrier) aa Albino a

Albinism in humans occurs when both alleles at a locus produce defective enzymes in the biochemical pathway leading to melanin. Given that heterozygotes are normally pigmented, which of the following statements is/are correct? One normal allele produces as much melanin as two normal alleles. Each defective allele produces a little bit of melanin. Two normal alleles are needed for normal melanin production. The two alleles are codominant. The amount of sunlight will not affect skin color of heterozygotes. Answer: a This question is an attempt to help students understand how enzyme action can produce phenotypes. If a person with two defective alleles at a locus in the melanin production pathway is albino, there is no melanin produced. However, a person with only one defective allele is normally pigmented, meaning that the person has a normal amount of melanin. Answer a is correct—one functional allele provides a normal amount of pigment. Answer b is wrong because albinos produce no melanin. Answer c is wrong because it contradicts the statement that heterozygotes are normally pigmented; if this statement were true, heterozygotes would have an intermediate amount of pigment. Answer d is wrong because a heterozygote in a codominance situation will have a different phenotype from both heterozygotes. Answer e is wrong because if heterozygotes have a normal amount of melanin, they will tan (produce more melanin when exposed to uv rays). This question simplifies the inheritance of albinism and its phenotypes.

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.

Achondroplasia is a form of dwarfism caused by a rare dominant allele
Parents Dwarf Dd Normal dd Sperm D d Eggs Dd Dwarf dd Normal d Dd Dwarf dd Normal d

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.

Relationship among alleles of a single gene
Description Example Complete dominance of one allele Heterozygous phenotype same as that of homozygous dominant PP Pp Incomplete dominance of either allele Heterozygous phenotype intermediate between the two homozygous phenotypes CRCR CRCW CWCW Codominance Both phenotypes expressed in heterozygotes IAIB Multiple alleles In the whole population, some genes have more than two alleles ABO blood group alleles IA, IB, i Pleiotropy One gene is able to affect multiple phenotypic characters Sickle-cell disease

Relationship among two or more genes
Description Example Epistasis The phenotypic expression of one gene affects that of another BbEe BbEe BE bE Be be BE bE Be be 9 : 3 : 4 Polygenic inheritance A single phenotypic character is affected by two or more genes AaBbCc AaBbCc

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