mendelian genetics

MENDELIAN GENETICS

WHERE DOES AN ORGANISM GET ITS UNIQUE CHARACTERISTICS?

An individual’s characteristics are determined by factors that are passed from one parental generation to the next.

Heredity

Every living thing has a set of characteristics inherited from its parent or parents.

The delivery of characteristics from parent to offspring is called heredity.

GENETICS

The scientific study of heredity, known as genetics, is the key to understanding what makes each organism unique.

GREGOR MENDELStudied heredity carefully & objectively using pea plants “Father of Genetics”

Mendel was in charge of the monastery garden, where he was able to do the work that changed biology forever.

Why Peas?

Mendel carried out his work with ordinary garden peas, partly because peas are small and easy to grow. A single pea plant can produce hundreds of offspring.

Why Peas?

By using peas, Mendel was able to carry out, in just one or two growing seasons, experiments that would have been impossible to do with humans or other animals.

WHAT DID MENDEL KNOW?Mendel knew that the male part of each flower makes pollen, which contains sperm—the plant’s male reproductive cells. Similarly, Mendel knew that the female portion of each flower produces reproductive cells called eggs.

Fertilization

During sexual reproduction, male and female reproductive cells join in a process known as fertilization to produce a new cell.

In peas, this new cell develops into a tiny embryo encased within a seed.

PEA PLANTS CAN REPRODUCE THROUGH:1. Self-pollination2. Cross-pollination

SELF-POLLINATION

Pollen from a flower fertilizes an egg cell on the same flower; common in pea plants.

A plant grown from a seed produced by self-pollination inherits all of its characteristics from the single plant that bore it. In effect, it has a single parent.

TRUE-BREEDING STOCK

Mendel’s garden had several stocks of pea plants that were “true breeding” and would always produce offspring with identical traits as themselves. They were self pollinating.Ex. True-breeding stock of pea plants that are tall with green pods always produce tall plants with green pods

CROSS POLLINATION

Mendel decided to “cross” his stocks of these plants—he caused one plant to reproduce with another with a contrasting trait.

CROSS-POLLINATIONPollen from one plant fertilizes eggs from another plant

Offspring have 2 different parents

a.k.a. cross

Process of Cross Pollination

He had to prevent self-pollination, so he cut away the male parts of a flower and then dusted pollen from a different plant onto the female part of that flower.

PROCESS OF CROSS POLLINATION

Cross-pollination allowed Mendel to breed plants with different features and then study the resulting traits.

TRAITA specific characteristic or feature of an individual that may vary from one individual to another.

HYBRID

Offspring produced by parents with different characteristics through cross pollination

Ex. Tall plant X short plant producing tall plants

MENDEL STUDIED 7 TRAITS

-Seed shape-Seed color-Seed coat color

-Pod shape-Pod color-Flower position-Plant height

P GENERATIONMendel’s true breeding generation

“Parents”

P GENERATION CROSSESMendel crossed plants with each of the seven contrasting characteristics and then studied their offspring.

F1 GENERATION

The hybrids resulting from Mendel’s cross pollinating;F stands for filius, meaning son in Latin

F1 PRODUCTION

For each trait studied in Mendel’sexperiments, all the offspring produced had the characteristics of only one of their parents, as shown in the table. Ex> tall x short did not produce medium sized plants, they were all tall

Mendel assumed that each pea plant produced by his cross must contain two possible, or potential traits—one from each parent.

TRANSMISSION OF TRAITS

Mendel questioned how these traits were being passed from one generation to the next.

1ST IMPORTANT DISCOVERY BY MENDEL

Law of Unit Characters -There are units in a cell that are responsible for traits, and these units come in pairs (otherwise known as alleles).

LAW OF UNIT CHARACTERS

Mendel believed each offspring received one allele from each parent, meaning each sperm or egg carries one possible allele.

GENESthe unit that determines traits; it is a segment of DNA that contains information.Ex. If a plant is tall, it has a gene for being tall.

GENES

Each of the traits Mendel studied was controlled by a gene that occurred in two contrasting varieties, one observable and one “masked”.

These gene variations produced different expressions, or forms, of each trait.

ALLELES

Different forms of a gene that determine a specific trait

Ex. The gene that determines height in pea plants has 2 possible alleles; one that produces a tall plant & one that produces a short plant

Multiple Allele PossibilitiesSome genes have only 2 alleles while others have dozens of different alleles;Alleles are represented by either an uppercase or a lowercase letter

TWO TYPES OF ALLELES

1. Dominant Allele2. Recessive Allele

1. DOMINANT ALLELE

Allele that will always be expressed when present

DominantExpressed by

Uppercase LettersEx. Dominant

allele for tall: “T”

Dominant

Ex. If a plant has one allele for tall & one for short, then the plant will be tall because the dominant allele is the allele for tall.

2. RECESSIVE ALLELE

Allele that will be expressed only when the dominant allele for that trait is not present.

Represented by lowercase letters

Recessive

Example: Recessive Allele for short “t”, where tall is dominant, or T

EXPRESSING THE RECESSIVE TRAIT

The only way a recessive short plant would be produced was if the dominant allele for tall was not present.

LAW OF DOMINANCE

Mendel’s second important discovery…..Some alleles are dominant and others are recessive. An organism with at least one dominant allele for a particular form of a trait will exhibit that form of the trait. An organism with a recessive allele for a particular form of a trait will exhibit that form only when the dominant allele for the trait is not present.

DOMINANT VS. RECESSIVE

In Mendel’s experiments, the allele for tall plants was dominant and the allele for short plants was recessive.

DOMINANT VS. RECESSIVE

Likewise, the allele for yellow seeds was dominant over the recessive allele for green seeds.

PASSING GENES TO OFFSPRING

Next, Mendel questioned how different forms of a gene, or alleles, were being distributed to offspring.

MASKED ALLELES? Mendel questioned what had happened to the recessive alleles in the F1.By allowing all seven kinds of F1

hybrids to self-pollinate, he would get his answer.

self-pollinating

F2 GENERATIONResults of the self pollination of an F1 hybrid

From the F1 X F1 crosses, Mendel discovered how traits were transferred from generation to generation.

F1 CROSSIn the F2 plants, traits controlled by the recessive alleles reappeared in a 1:4 ratio.

self-pollinating

PASSING ALLELES TO OFFSPRING

Mendel assumed that a dominant allele had masked it’s corresponding recessive allele in the F1 generation. The reappearance of the recessive trait in the F2 indicated that,at some point, the allele for short actuallyseparated from the allele for tall and that only one allele could be passed to offspring produced in the F2.

PASSING ALLELES TO OFFSPRING

Mendel suggested that the alleles for tall and short segregated during the formation of gametes.

How did this separation, or segregation, of alleles occur?

GAMETE

the sex cells -sperm or egg

GAMETES AND ALLELES

Body cells of most sexually reproducing organisms contain two alleles of each gene, one from each parent’s gamete, which can be identical or different.

Therefore, if a gamete contains an allele, it will be present in all body cells produced by that gamete even if it is not expressed.

GAMETES AND ALLELESIf each F1 plant—all of which were tall—inherited an allele for tall from its tall parent and an allele for short from its short parent….

GAMETES AND ALLELESthen when each F1 adult produces gametes, the alleles for each gene must segregate from one another, so each gamete carries only one possible allele for each gene.

They are then capable of

passing that allele on to

offspring.

GAMETES AND ALLELESEach F1 plant in Mendel’s crossproduced two kinds of gametes

—those with the allele for tall(T) and those with the allele for short (t).

GAMETES AND ALLELES

Whenever gametes carrying the t allele united, the F2 plant was short.

When at least one gamete carried the T allele, a tall plant was produced.

The F2 generation has a new combination of alleles.

GAMETES AND ALLELES During gamete formation, alleles, such as

tall or short, separate from each other with each gamete carrying only oneallele for each gene.

LAW OF SEGREGATION

Mendel’s 3rd discovery….States that the alleles for a trait separate when gametes are formed. The allele pairs are then randomly united at fertilization.

HOMOZYGOUS ALLELES

Have 2 same alleles for a trait; the organism will have a pair of identical alleles—either two dominant or two recessive

Homozygous Tall—TT Homozygous short—tt TT—homozygous dominanttt—homozygous recessive

HETEROZYGOUS ALLELES

Have 2 different alleles;the organism will have one dominant & one recessive allele

Tt—heterozygous tall; the plant will be tall, but will carry one dominant & one recessive allele

Naming & labeling all the different kinds of alleles & gametes allowed Mendel to do something called a test-cross to determine how organisms would look after mating—these crosses were later called Punnett Squares .

PUNNETT SQUARES

Whenever Mendel performeda cross with pea plants, he carefully categorized and counted the offspring.

For example, whenever hecrossed two plants that were hybrid for stem height (Tt), aboutthree fourths of the resulting plantswere tall and about one fourth were short.

PROBABILITY

Mendel realized that the principles of probability could be used to explain the results of his genetic crosses.

Probability is the likelihood that a particular event will occur.

Ex> flipping a coin

PUNNETT SQUARE a chart that illustrates Mendel’s test-crosses between organisms; it is a determination of what traits may result after two parent alleles have crossed.

Punnett squares allow you to predict the allele combinations and physical traits in genetic crosses using mathematical probability.

USING SEGREGATION TO PREDICT OUTCOMES

Mendel’s cross produced a mixture of tall and short plants in the F2.

USING SEGREGATION TO PREDICT OUTCOMES

If each F1 plant had one tall allele and one short allele (Tt), then 1/2 of the gametes they produced would carry the short allele (t).

USING SEGREGATION TO PREDICT OUTCOMESBecause the t allele is recessive, the only way to produce a short (tt) plant is for two gametes carrying the t allele to combine.

USING SEGREGATION TO PREDICT OUTCOMES

Each F2 gamete has a one in two, or 1/2, chance of carrying the t allele.

USING SEGREGATION TO PREDICT OUTCOMESThere are two gametes, so the probability of both gametes carrying the t allele is:

½ x ½ = ¼

USING SEGREGATION TO PREDICT OUTCOMES

Roughly one fourth of the F2 offspring should be short, and the remaining three fourths should be tall.

USING SEGREGATION TO PREDICT OUTCOMES

This predicted ratio—3 dominant to 1 recessive—showed up consistently in Mendel’s test crosses.

USING SEGREGATION TO PREDICT OUTCOMES

For each of his seven traits studied, about 3/4 of the plants showed the trait controlled by the dominant allele.

USING SEGREGATION TO PREDICT OUTCOMES

About 1/4 of the plants showed the trait controlled by the recessive allele.

USING SEGREGATION TO PREDICT OUTCOMES

Not all organisms with the same characteristics have the same combinations of alleles.

USING SEGREGATION TO PREDICT OUTCOMES

In the F1 cross, both the TT and Tt allele combinations resulted in tall pea plants. The tt allele combination produced a short pea plant.

PHENOTYPEThe form of the trait that an organism displaysEx>A plant can express a phenotype for either tall or short; it may be homozygous dominant or heterozygous, both tall, or homozygous recessive, short

PHENOTYPE

Think P for physical-observable traits the organism has

GENOTYPEAn organism’s genetic compositionIt will specify the actual alleles that make up the genetic trait.

Genotype affects Phenotype

A plant will express a phenotype, either tall or short, linked to it’s genotype, either TT, Tt, or tt.

GENOTYPE

Genotype is more specific than the phenotype. It tells which type of tall or short plant; it will determine if a plant is homozygous tall or short, or heterozygous tall

The genotype of an organism is inherited, whereas the phenotype is formed as a result of both the environment and the genotype.

Two organisms may have the same phenotype but different genotypes.

MONOHYBRID CROSS

a genetic cross examining ONE TRAIT (Ex> The trait for tall vs. short)

HOW TO MAKE A PUNNETT SQUARE

1. Identify the genotypes of the two organisms that will serve as parents in the cross.

In this example we will cross a male and female bird that are heterozygous for big beaks. They each have genotypes of Bb.Bb x Bb

2. Draw a table with space for each pair of gametes from each parent on two sides- 4 units. Enter the genotypes of the gametes produced by both parents on the top and left sides of the table.

3. Fill in the table by combining the gametes to form genotypes.

B b

B

b

BB Bb

Bb

bb

4. Determine the genotypes and phenotypes of each offspring. Calculate the percentage of each. In this example, three fourths of the chicks will have big beaks.

Homo Dominant- big beak

Hetero Dominant-big beak

Hetero Dominant-big beak

Homo Recessive-small beak

B b

B

b

BB Bb

Bb bb

¼ small beak or 25%¾ big beak or 75%

2/4 hetero, or 50%¼ homo dominant, or 25%¼ homo recessive, or 25%

EXAMPLE OF A MONOHYBRID CROSS

In pea plants, round seeds ( R ) are dominant to wrinkled ( r ). In a genetic cross of two plants that are homozygous for differing seed shape traits, what are the phenotypic and genotypic ratios?

Rr Rr

Rr Rr

R Rr

r

Genotypes:4/4 Rr

Phenotypes: all round

In humans, the widow’s peak (W) is dominant over straight hairline (w). A heterozygous man for this trait marries a woman who is recessive.

a. List possible genotypes of their offspring.

b. List the phenotypic ratios for their children.

CYSTIC FIBROSIS

Cystic fibrosis is an example of a genetic disease that is controlled by a single gene with two possible alleles- dominant and recessive. Cystic fibrosis is carried by the recessive allele. Anyone with at least one dominant allele will be unaffected.

The gene codes for a specific cell protein that transports chloride ions across the cell membrane. The recessive allele carries the mutation that causes production of a faulty protein.

POLYGENIC TRAITSNot all traits, whether a physical characteristic or a disorder, are governed by a single gene like cystic fibrosis. Height or diabetes are examples of polygenic traits. Most human traits are polygenic, meaning that they are determined not only by different genes, but different genes located on different chromosomes.

Some genes even produce proteins that affect multiple traits.

REMEMBER….

Punnett squares cannot be used to predict the genotype or phenotype of a specific offspring.

INDEPENDENT ASSORTMENT

Every time organisms reproduce, multiple traits are inherited by their offspring, such as eye color, hair color, or height to name a few.

How do the various alleles for different traits segregate and then come together to produce different possible phenotypes?

INDEPENDENT ASSORTMENT

Mendel wondered if the segregation of one pair of alleles affects another pair.

Mendel performed an experiment that followed two different genes as they passed from one generation to the next.

Because it involves two different genes, Mendel’s experiment is known as a two-factor, or dihybrid, cross.

DIHYBRID CROSSCross examining inheritance of 2 traits

THE TWO-FACTOR CROSS: F1

Mendel crossed true-breeding plants that produced only round yellow peas with plants that produced wrinkled green peas in the F1.

THE TWO-FACTOR CROSS: F1

The round yellow peas had the genotype RRYY, which is homozygous dominant.

THE TWO-FACTOR CROSS: F1

The wrinkled green peas had the genotype rryy, which is homozygous recessive.

THE TWO-FACTOR CROSS: F1

All of the F1 offspring produced round yellow peas. These results showed that the alleles for yellow and round peas are dominant over the alleles for green and wrinkled peas.

The Punnett square shows that the genotype of each F1 offspring was RrYy, heterozygous for both seed shape and seed color.

F2

Next, he allowed the F1 plants to self pollinate…..

ROUND OR WRINKLED, GREEN OR YELLOW……

4 possible phenotypes….Round greenRound yellowWrinkled greenWrinkled yellow

ROUND OR WRINKLED, GREEN OR YELLOW……

9 possible genotypes….

Round yellow RRYY, RrYY, RRYy, or RrYy

Round green RRyy or Rryy

ROUND OR WRINKLED, GREEN OR YELLOW……

Wrinkled yellowrrYY or rrYy

Wrinkled green rryy

THE TWO-FACTOR CROSS: F2 Mendel observed that 315 of the F2 seeds were round and yellow, while another 32 seeds were wrinkled and green—the two parental phenotypes.

But 209 seeds had combinations of phenotypes, and therefore combinations of alleles, that were not found in either parent.

THE TWO-FACTOR CROSS: F2

The alleles for seed shape segregated independently of those for seed color.

Genes that segregate independently—such as the genes for seed shape and seed color in pea plants—do not influence each other’s inheritance.

THE LAW OF INDEPENDENT ASSORTMENT

Mendel’s experimental results were very close to a 9:3:3:1 ratio.

Mendel had discovered his 4th law, the Law of Independent Assortment. The Law of Independent Assortment states that genes for different traits can segregate independently during gamete formation.

LAW OF INDEPENDENT ASSORTMENT

Helps account for the many genetic variations observed in plants, animals, and other organisms

DIHYBRID EXAMPLE In pea plants, round ( R )are dominant to wrinkled(r). Also tall plants (T) are dominant to short (t). In a genetic cross of 2 plants in which one plant is heterozygous round, heterozygous tall (RrTt) and a second plant is homozygous recessive wrinkled, homozygous recessive short (rrtt), what are the phenotypic and genotypic ratio?

ONE EXTRA STEP…..

***FOR A DIHYBRID CROSS,YOU MUST DETERMINE WHAT ALLELES WOULD BE FOUND IN ALL OF THE POSSIBLE GAMETES THAT EACH PARENT COULD PRODUCE.

SEGREGATION

RT

Rt

rT

rt

rt rtrt rt

USING MENDEL’S PRINCIPLES

At the beginning of the 1900s, American geneticist Thomas Hunt Morgan decided to use the common fruit fly as a model organism in his genetics experiments.

The fruit fly was an ideal organism for genetics because it could produce plenty of offspring, and it did so quickly in the laboratory.

USING MENDEL’S PRINCIPLES

Before long, Morgan and other biologists had tested every one of Mendel’s principles and learned that they applied not just to pea plants but to other organisms as well.

The basic principles of Mendelian genetics can even be used to study the inheritance of human traits and to calculate the probability of certain traits appearing in the next generation.

MEIOSIS

THINK ABOUT ITAs geneticists in the early 1900s applied Mendel’s laws, they wondered where genes might be located.

They expected genes to be carried on structures inside the cell, but which structures?

What cellular processes could account for segregation and independent assortment, as Mendel had described?

CHROMOSOMES

Chromosomes—the strands of DNA and protein inside the cell nucleus—are the carriers of genes.

The genes are located in specific positions on chromosomes.Each species has a characteristic chromosome number.

DIPLOID CELLSA body cell in an adult fruit fly has eight chromosomes, as shown in the figure.

Four of the chromosomes come from its male parent, and four come from its female parent. Therefore, each species has two alleles of each gene.

These two sets of chromosomes are homologous, meaning that each of the four chromosomes from the male parent has a corresponding chromosome from the female parent. These chromosomes have the same genes, but may have different alleles.

Diploid Cells

A cell that contains both sets of homologous chromosomes is diploid, meaning “two sets.” The diploid number of chromosomes is sometimes represented by the symbol 2N.

For the fruit fly, the diploid number is 8, which can be written as 2N = 8.

For a human the diploid number is 46 or 2N=46.

HAPLOID CELLS

Some cells contain only a single set of chromosomes, and therefore a single set of genes. Such cells are haploid, meaning “one set.” They have one possible allele for each gene.

The gametes of sexually reproducing organisms are haploid.

For fruit fly gametes, the haploid number is 4, which can be written as N = 4. The haploid number in humans is 23 or N=23.

HOW MANY SETS OF GENES DO MULTICELLULAR ORGANISMS INHERIT?

The diploid cells of most adult organisms contain two complete sets of inherited chromosomes, which is two complete sets of genes.

The number of chromosomes in a cell is reduced by meiosis starting from a cell with 1 set of 46 (23 pairs) chromosomes and ending with 4 cells each containing 1 set of 23 chromosomes.

HOW MANY SETS OF GENES DO MULTICELLULAR ORGANISMS INHERIT?

A human egg is haploid (has 23 chromosomes) and a sperm is haploid (has 23 chromosomes).

Upon fertilization, the new baby now has the correct human number of 46 chromosomes in each of its somatic cells. Fertilization of the egg by the sperm restores the diploid number of 46 chromosomes.

Understanding the process of meiosis is fundamental to understanding human health and development.

MITOSISIn a normal cell, there are 2 copies of each chromosome, each of the copies coming from one of the parents. Daughter cells produced by mitosis are exact copies of the mother cell. This is the usual process of division by which cells in our bodies renew themselves.

WHAT IS MEIOSIS?

Meiosis is the production of sperm and egg cells in organisms that reproduce sexually. These cells are “gamete" or “sex" cells. Each cell has to go through the division process twice in order for the cell to end up with half the number of chromosomes.

WHAT IS MEIOSIS?

Meiosis usually involves two distinct divisions, called meiosis I and meiosis II. By the end of meiosis II, the diploid cell becomes four haploid cells.

To become diploid again—haploid gametes produced by each parent will fuse to form a zygote (during fertilization), and offspring receive one copy of each chromosome from each parent

Meiosis and variation

Depending on the recombination events that occurred to produce a gamete, genetic information may be rearranged.

Meiosis and variation

This explains why siblings get different combinations of genes from their parents, which is why they look related but not identical.

2 TYPES OF REPRODUCTION

Sexual reproduction involves the fusion of two gametes (parent cells) to make a genetically different offspring. Two gametes fusing creates a zygote through the process of fertilization.

Asexual Reproduction occurs when one parent donates all chromosomes to an offspring and there is no union of gametes

MEIOSIS CONTAINS 2 SEPARATE DIVISIONS

Meiosis IMeiosis II

PHASES OF MEIOSIS

MEIOSIS IInterphase Just prior to meiosis I, the cell undergoes a round of DNA replication during interphase.

PROPHASE I

Chromatin condenses to chromosomes, the nuclear membrane breaks down, & a spindle forms.Each duplicated chromosome consists of 2 identical sister chromatids, held together by a centromere.

PROPHASE I

The duplicated chromosomes pair up, forming a structure called a tetrad, which contains four chromatids.

<2 homologous chromosomes, each of which is made up of 2 sister chromatids>

PROPHASE IAs homologous chromosomes pair up and form tetrads, they undergo a process called crossing-over.First, the chromatids of the homologous chromosomes cross over one another.

PROPHASE I

Then, the crossed sections of the chromatids are exchanged.

Crossing-over is important because it produces new combinations of alleles in the cell, increasing genetic variation.

METAPHASE I

During metaphase I of

meiosis, paired homologous

chromosomes attach to the spindle and line up across the center of the cell.

A N A P H A S E I

During anaphase I, spindle fibers pull each homologous chromosome pair toward opposite ends of the cell. In mitosis, sister chromatids were separated; here they are still joined.

When anaphase I is complete, the separated chromosomes cluster at opposite ends of the cell.

TELOPHASE I AND CYTOKINESIS

During telophase I, a nuclear membrane forms around each cluster of chromosomes.

Cytokinesis follows telophase I, forming two new cells.

MEIOSIS I SUMMARY

Meiosis I results in two new cells called daughter cells. Because each pair of homologous chromosomes was separated, neither daughter cell has the two complete sets of chromosomes that it would have in a diploid cell.

MEIOSIS I

The two cells produced by meiosis I have sets of chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I. Chromosome number has been halved.

MEIOSIS II

The two cells produced by meiosis I now enter a second meiotic division.

Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II.

PROPHASE II

As the cells enter prophase II, their chromosomes—each consisting of two chromatids—become visible.

The chromosomes do not pair to form tetrads, because the homologous pairs were already separated during meiosis I. The spindle forms.

METAPHASE II

During metaphase of meiosis II, chromosomes line up in the center of each cell. The spindles attach to the centromeres of the sister chromatids.

ANAPHASE IIThe centromere of each chromosome splits, allowing sister chromatids to separate and move to opposite poles of the cell

TELOPHASE IINuclei form around each group of chromosomes in each daughter cell, the spindles break down, and cytokinesis starts to occur.

The two nuclear divisions in meiosis result in four daughter cells forming from an original parent cell, each with half the chromosomes of the parent cell.

TELOPHASE II, AND CYTOKINESIS

In the example shown here, each of the four daughter cells produced in meiosis II receives two chromatids.

TELOPHASE II, AND CYTOKINESIS

These four daughter cells now contain the haploid number (N)—just two chromosomes each.

MEIOSIS EXPLAINS MENDEL'S RESULTSIn Anaphase I, segregation of chromosomes occurs, so that each parent gives one allele for each trait at random to each offspring, regardless of whether the allele is expressed. Genes for different traits are inherited independently.

GAMETES TO ZYGOTESThe haploid cells produced by

meiosis II are gametes, sperm and egg cells.

In female animals, generally only one of the four cells produced by meiosis is involved in reproduction.

SpermatogenesisMeiosis in a male organism, producing 4 sperm cells

OogenesisMeiosis in a female organism, producing 1 egg and 3 inactive polar bodies

OOGENESIS

An egg cell contributes its cytoplasm and organelles to offspring, while a sperm contributes only its chromosomes.

MEIOSIS VS. MITOSIS1. Division-In Meiosis, two cell divisions take place to produce 4 genetically different haploid daughter cells. Mitosis has one division and 2 genetically identical diploid cells produced.

2. Distribution of genes-During meiosis, chromatids undergo a process called crossing over, so they are NOT identical when randomly distributed.

MEIOSIS VS. MITOSIS3. In mitosis and meiosis, chromosomes

duplicate only ONCE, even though they divide twice in meiosis.

4. During meiosis, chromosomes are distributed into daughter cells randomly , not identically as in mitosis

5. Reproduction-Mitosis is a form of asexual reproduction, whereas meiosis is an early step in sexual reproduction.

OTHER DIFFERENCES WHEN COMPARING MEIOSIS AND MITOSIS

6. Segregation-In meiosis, homologs move to separate daughter cells. As a result, the two alleles for each gene segregate from each other. In mitosis, the alleles remain together.

IDENTICAL TWINS

A single egg is fertilized to form one zygote.The zygote divides to form two separate embryos.

Meiosis Stages

Unique Features of Meiosis

GENE LINKAGE AND GENE MAPS

Why do some pairs of traits seem to be inherited together?

GENE LINKAGE

Thomas Hunt Morgan’s research on fruit flies led him to the principle of gene linkage.Morgan discovered that many fruit fly genes appeared to be “linked” together in ways that seemed to violate the principle of independent assortment.

GENE LINKAGE

For example, Morgan used a fly with reddish-orange eyes and miniature wings in a series of test crosses.

His results showed that the genes for those two traits were almost always inherited together.

GENE LINKAGE

Morgan’s findings led to two remarkable conclusions:

First, each chromosome is actually a group of linked genes.

Second, chromosomes assort independently, not individual genes.

Alleles of different genes tend to be inherited together when those genes are located on the same chromosome.

EXCEPTIONS TO THE RULES

Mendelian Genetics

MENDEL IS THE TIP OF THE “GENETICS ICEBERG”

As people have studied genetics, they have realized that the inheritance of traits is much more complex than Mendel’s work with peas indicated.

Mendelian Genetics

The Rest of Genetics

THINK ABOUT IT

Mendel’s principles offer a set of rules with which to predict various patterns of inheritance.

There are exceptions to every rule, and exceptions to the exceptions.

What happens if one allele is not completely dominant over another? What if a gene has several alleles?

What are some exceptions to Mendel’s principles?

Some alleles are neither dominant nor recessive.

Many genes exist in several different forms, and therefore have multiple alleles.

Many traits are produced by the interaction of several genes.

BEYOND DOMINANT AND RECESSIVE ALLELES

Despite the importance of Mendel’s work, there are important exceptions to most of his principles.

In most organisms, genetics is more complicated, because the majority of genes have more than two alleles.

Mendel’s principles alone cannot predict traits that are controlled by multiple alleles or multiple genes.

Incomplete DominanceCodominancePleiotropyEpistasisPolygenic TraitsMultiple Alleles

Some of the Many Exceptions to the Rules

1.INCOMPLETE DOMINANCECases in which one allele is not completely dominant over another are called incomplete dominance.

In incomplete dominance, the heterozygous phenotype lies somewhere between the two homozygous phenotypes.

Third phenotype produced that is a blending of the parental traits. (2 alleles produce 3 phenotypes.)

Example: straight hair, wavy, curly

Red, pink, white flowers

EXAMPLE

A cross between two four o’clock plants shows a common exception to Mendel’s principles.

The F1 generation produced by a cross between red-flowered (RR) and white-flowered (WW) plants consists of pink-colored flowers (RW), as shown.

Incomplete Dominance

TYPE OF DOMINANCE?

Incomplete Dominance!

2. CODOMINANCE

Cases in which the phenotypes produced by both alleles that are present are clearly expressed are called codominance.

For example, in certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers.

Heterozygous chickens have a color described as “erminette,” speckled with black and white feathers. The heterozygous organism displays both phenotypes.

Both alleles contribute to the phenotype.Example: In some chickens

Black Chicken x White Chicken Speckled Chicken

WHICH TYPE OF DOMINANCE…????

Codominance!

ABO BLOOD GROUP

Your blood type is determined by the presence or absence of a carbohydrate group attached to a protein on the surface of red blood cells. Type A and Type B blood each have a different carbohydrate group. Type O blood has no carbohydrate group, and is recessive. Type AB blood has both carbohydrate groups present, displaying codominance of Type A and Type B.

In rare cases, a gene can prevent the carbohydrate group from being produced at all, meaning that two AB parents can produce a Type O child.

3. PLEIOTROPY

Occurs when a single gene influences multiple phenotypic traits.

Consequently, a mutation in the gene will have an effect on all traits simultaneously.

This is an example of the ability of a single gene to have multiple effects.

Example: Marfan’s Syndrome

- a dominant disorder caused by a single gene

- the defective gene causes individuals to be tall and skinny with very long fingers, hyper joint mobility, eye and heart defects

MARFANS

4. EPISTASIS

The interaction between two or more genes to control a single phenotype.

Epistasis occurs when more than one gene is needed to control one trait.

One gene alters the expression of another that is independently inherited.

-EXAMPLE: In mice the color is controlled by one

gene with 2 alleles ; the allele for gray (G) is dominant to the allele for black (g)

- Another gene controls an early stage in the development of hair pigment; normal color development (A) is dominant to no color development(a)

- AA, Aa = color production

- aa = no color production = albino mouse

EPISTASIS IN MICE

The gene for color production influences the expression of the gene for fur color

EPISTASIS – BOMBAY PHENOTYPE

-influences the expression of A and B blood types

- the gene involved is responsible for the formation of antigens on blood cells.

The dominant form of the gene allows the formation of antigens

HH or Hh allow antigens to form

The recessive form prevents the formation of antigens

hh means no antigens form

-If an individual is HH or Hh for blood antigen formation then:

AA, AO = A blood

BB, BO = B blood

AB = AB blood

-If an individual is hh for blood antigen formation then they produce the O blood type

5. POLYGENIC TRAITS

Traits that are determined by multiple alleles for a characteristic.

Traits controlled by two or more genes are said to be polygenic traits.

Polygenic means “many genes.”

Often show a wide range of phenotypes. Human examples: Hair, eye and skin color- the variety of skin color in humans comes about partly because more than four different genes probably control this trait.

THE INHERITANCE OF EYE COLOR Only partially understood ; at least 3 different genes with 2 alleles each are responsible for eye color.

– bey 1 bey 2 and gey

These genes code for the formation of the pigment melanin

The function of only 2 of the 3 genes is currently understood

THE INHERITANCE OF EYE COLOR AS IT IS CURRENTLY UNDERSTOOD

One gene known as bey 2 has 2 alleles:

B for brown eyes and b for blue eyes

The other gene known as gey has 2 alleles :

G codes for green eyes and g for blue eyes

THE 2 GENE MODEL- B is dominant to all other alleles-

BBGG, BBGg, BBgg, BbGG

BbGg, Bbgg = Brown eyes

- G is dominant to b

bbGG , bbGg = Green eyes

- bbgg only = blue eyes

6. MULTIPLE ALLELES

A single gene can have many possible alleles.

A gene with more than two possible alleles is said to have multiple alleles.

6. MULTIPLE ALLELES

Many genes have multiple alleles, including the human genes for blood type. This chart shows the percentage of the U.S. population that shares each blood group.

BLOOD GROUPS

There are 3 blood type alleles: IA, IB, and i. Each person has only two of the possible three alleles. Instead of three possible genotypes, there are a larger number of genotypes.

Blood can be classified other ways as well, such as by Rh factor.

GENES AND THE ENVIRONMENT

The characteristics of any organism are not determined solely by the genes that organism inherits.

Genes provide a plan for development, but how that plan unfolds also depends on the environment.

The phenotype of an organism is only partly determined by its genotype.

GENES AND THE ENVIRONMENT

For example, consider the Western white butterfly. Western white butterflies that hatch in the summer have different color patterns on their wings than those hatching in the spring.

Scientific studies revealed that butterflies hatching in springtime had greater levels of pigment in their wings than those hatching in the summer.

In other words, the environment in which the butterflies develop influences the expression of their genes for wing coloration.

GENES AND THE ENVIRONMENT

In order to fly effectively, the body temperature of the Western white butterfly needs to be 28–40°C.

More pigmentation allows a butterfly to reach the warm body temperature faster.

Similarly, in the hot summer months, less pigmentation prevents the butterflies from overheating.

ENVIRONMENT AND GENE EXPRESSION

example: Siamese cats

- Siamese coloring is a partial albinism. Most of the cat is white

-Black fur is only expressed in areas where the temperature is lower than the rest of the body

- if black hair is shaved and the area kept warm then the hair that grows back will be white

- if white hair is shaved and the area is cooled, then the hair will grow back black