mendel experiment ppt

Lesson Overview

11.2 Applying Mendel’s Principles

THINK ABOUT IT

Nothing in life is certain.

If a parent carries two different alleles for a certain gene, we can’t be sure which of those alleles will be inherited by one of the parent’s offspring.

However, even if we can’t predict the exact future, we can do something almost as useful—we can figure out the odds.

Applying Mendel’s Principles

Probability and Punnett Squares

How can we use probability to predict traits?

Punnett squares use mathematical probability to help predict the genotype and phenotype combinations in genetic crosses.

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

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

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.

For example, there are two possible outcomes of a coin flip: The coin may land either heads up or tails up.

The chance, or probability, of either outcome is equal. Therefore, the probability that a single coin flip will land heads up is 1 chance in 2. This amounts to 1/2, or 50 percent.

If you flip a coin three times in a row, what is the probability that it will land heads up every time?

Each coin flip is an independent event, with a one chance in two probability of landing heads up.

Therefore, the probability of flipping three heads in a row is:

1/2 × 1/2 × 1/2 = 1/8

As you can see, you have 1 chance in 8 of flipping heads three times in a row.

Past outcomes do not affect future ones. Just because you’ve flipped 3 heads in a row does not mean that you’re more likely to have a coin land tails up on the next flip.

Using Segregation to Predict Outcomes

The way in which alleles segregate during gamete formation is every bit as random as a coin flip.

Therefore, the principles of probability can be used to predict the outcomes of genetic crosses.

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

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

Because the t allele is recessive, the only way to produce a short ( tt ) plant is for two gametes carrying the t allele to combine.

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

There are two gametes, so the probability of both gametes carrying the � t allele is:

Roughly one fourth of the F 2 offspring should be short, and the remaining three fourths should be tall.

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

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

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

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

In the F 1 cross, both the TT and Tt allele combinations resulted in tall pea plants. The tt allele combination produced a short pea plant.

Organisms that have two identical alleles for a particular gene— TT or tt in this example—are said to be homozygous .

Organisms that have two different alleles for the same gene—such as Tt— are heterozygous .

Probabilities Predict Averages

Probabilities predict the average outcome of a large number of events.

The larger the number of offspring, the closer the results will be to the predicted values.

If an F 2 generation contains just three or four offspring, it may not match Mendel’s ratios.

When an F 2 generation contains hundreds or thousands of individuals, the ratios usually come very close to matching Mendel’s predictions.

Genotype and Phenotype

Every organism has a genetic makeup as well as a set of observable characteristics.

All of the tall pea plants had the same phenotype , or physical traits.

They did not, however, have the same genotype , or genetic makeup.

There are three different genotypes among the F2 plants: Tt, TT, and tt.

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.

Using Punnett Squares

One of the best ways to predict the outcome of a genetic cross is by drawing a simple diagram known as a Punnett square.

Punnett squares allow you to predict the genotype and phenotype combinations in genetic crosses using mathematical probability.

How To Make a Punnett Square for a One-Factor Cross

Write the genotypes of the two organisms that will serve as parents in a cross.

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

How To Make a Punnett Square

Determine what alleles would be found in all of the possible gametes that each parent could produce.

Draw a table with enough spaces for each pair of gametes from each parent.

Enter the genotypes of the gametes produced by both parents on the top and left sides of the table.

Fill in the table by combining the gametes’ genotypes.

Determine the genotypes and phenotypes of each offspring.

Calculate the percentage of each. In this example, three fourths of the chicks will have large beaks, but only one in two will be heterozygous.

Independent Assortment

How do alleles segregate when more than one gene is involved?

The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes.

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. Single-gene crosses are monohybrid crosses.

The Two-Factor Cross: F1

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

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

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

All of the F 1 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 F 1 offspring was RrYy, heterozygous for both seed shape and seed color.

The Two-Factor Cross: F2

Mendel then crossed the F 1 plants to produce F 2 offspring.

Mendel observed that 315 of the F 2 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 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.

Mendel’s experimental results were very close to the 9:3:3:1 ratio that the Punnett square shown predicts.

Mendel had discovered the principle of independent assortment. The principle of independent assortment states that genes for different traits can segregate independently during gamete formation.

A Summary of Mendel’s Principles

What did Mendel contribute to our understanding of genetics?

Mendel’s principles of heredity, observed through patterns of inheritance, form the basis of modern genetics.

The inheritance of biological characteristics is determined by individual units called genes, which are passed from parents to offspring.

Where two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.

In most sexually reproducing organisms, each adult has two copies of each gene—one from each parent. These genes segregate from each other when gametes are formed.

Alleles for different genes usually segregate independently of each other.

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.

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 be used to study the inheritance of human traits and to calculate the probability of certain traits appearing in the next generation.

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Mendel’s Principles of Inheritance

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MENDELIAN GENETICS

Apr 04, 2012

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MENDELIAN GENETICS. What is genetics? The study of how traits are inherited or how genetic information is passed from one generation to the next. It also explains biological variation. Gregor Mendel. 1850’s Grew up in a farm wanting to garden

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• different characters
• homologous chromosomes figure

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MENDELIAN GENETICS What is genetics? The study of how traits are inherited or how genetic information is passed from one generation to the next. It also explains biological variation

Gregor Mendel • 1850’s Grew up in a farm wanting to garden • Austrian monk (Flunked out of college twice) but became a mathematician • Experimented with garden pea plants • Using pea plants looked at seven different characters (height of plants, seed color, texture, flower color) and found evidence of how parents transmit genes to offspring • Mendel’s statistical analysis provided a model for predicting what the next generation would be like

What was the prevalent believe about inheritance before Mendel? • People believed in “spontaneous generation” and in the “blending of characters” • Blending theory • Problem: • Would expect variation to disappear • Variation in traits persists Ex: Yellow and green parakeets should have all blue babies. This is not what you observe.

The gene theory • An alternative idea is the “gene” idea. Parents pass on discrete individual heritable units: genes

Petal Stamen Carpel Figure 9.2 A Figure 9.2 B • Experimental genetics began in an abbey garden • Modern genetics • Began with Gregor Mendel’s quantitative experiments with pea plants

The Garden Pea Plant • Mendel chose to work with the pea plant because he could control which plant mated with which. Pea plants are • Self-pollinating • True breeding (different alleles not normally introduced) • Can be experimentally cross-pollinated

Mendel crossed pea plants that differed in certain characteristics • And traced traits from generation to generation • Mendel started his experiments with plants that were “true breeding”. 1Removed stamens from purple flower White 2 Transferred pollen from stamens of white flower to carpel of purple flower Stamens Carpel Parents(P) Purple 3 Pollinated carpel matured into pod 4 Planted seeds from pod Offspring(F1) Figure 9.2 C

Purple Flower color White Terminal Axial Flower position Green Yellow Seed color Seed shape Round Wrinkled Pod shape Inflated Constricted Green Yellow Pod color Tall Stem length Dwarf • Mendel hypothesized that there are alternative forms of genes • The units that determine heritable traits Figure 9.2 D

Mendel’s Principles of Genetics • Mendel refuted the “blending theory” of heredity and provided an explanation of how inheritance works without knowing anything about chromosomes or genes. • He figured that traits must be coded for by some kind of inheritable particle which he called “factors” and now we call “genes”. • He said that those genes were transmitted as independent entities from one generation to the next.

Mendel’s insight continued…3. He figured that there must be different versions of these “genes”( we call them now “alleles”)and that every individual has two genes for each trait. (Or we can say that: For each characteristican organism inherits two alleles, one from each parent)He identified one as dominant, the other as recessive.

4. He figured that the two alleles a parent has are separated into different cells when gametes (sex cells) are formed. This actuallyhappens during metaphase of meiosisI ( no one knew about meiosis in those days). This is known as the Law of SegregationWhat are alleles?Different versions of the same gene

Mendel’s Theory of Segregation • An individual inherits a unit of information (allele) about a trait from each parent • During gamete formation, the alleles segregate from each other

P plants Genetic makeup (alleles) pp PP Gametes All p All P F1 plants (hybrids) All Pp 1 2 1 2 P p Gametes Sperm p P F2 plants Phenotypic ratio 3 purple : 1 white Pp P PP Eggs Genotypic ratio 1 PP: 2 Pp: 1 pp Pp p pp • Mendel’s law of segregation • Predicts that allele pairs separate from each other during the production of gametes Figure 9.3 B

P generation (true-breedingparents)  Purple flowers White flowers F1 generation All plants havepurple flowers Fertilizationamong F1 plants(F1 F1) F2 generation 3 4 1 4 of plantshave white flowers of plantshave purple flowers • Mendel’s law of segregation describes the inheritance of a single characteristic • From his experimental data • Mendel deduced that an organism has two genes (alleles) for each inherited characteristic Figure 9.3 A

What is a dominant trait?The trait that shows, the allele that is fullyexpressedWhat is a recessive trait?The alleles that is masked, the gene is there but it doesn’t showWhat is the phenotype?The observable traitsWhat is the genotype?The genetic make up

If the two alleles of an inherited pair differ • Then one determines the organism’s appearance and is called the dominant allele ( use capital letters) • The other allele • Has no noticeable effect on the organism’s appearance and is called the recessive allele

Vocabulary • When you mate two contrasting true breeding plants you get a Hybrid. • The true breeding parents are called the “P” (parent) generation • The hybrid offspring of the P generation are called the F1 generation • When two F1 individuals self pollinate you get the F2 generation

F1 Results of One Monohybrid Cross

F2 Results of Monohybrid Cross

Mendel’s Monohybrid Cross Results 5,474 round 1,850 wrinkled 6,022 yellow 2,001 green 882 inflated 299 wrinkled 428 green 152 yellow F2 plants showed dominant-to-recessive ratio that averaged 3:1 705 purple 224 white 651 long stem 207 at tip 787 tall 277 dwarf

Female gametes A a A AA Aa Male gametes a Aa aa Punnett Square of a Monohybrid Cross Dominant phenotype can arise 3 ways, recessive only one

A Test cross • In a pea plant with purple flowers the genotype is not obvious. Could be homozygous or heterozygous • Why do a test cross? It allows us to determine the genotype of an organism with a dominant phenotype but unknown genotype

Test Cross • You cross an individual that shows the dominant phenotype with an individual with recessive phenotype ( one who is homozygous recessive for that trait) • Examining offspring allows you to determine the genotype of the dominant individual

Punnett Squares of Test Crosses Homozygous recessive Homozygous recessive a a a a A A Aa Aa Aa Aa A a aa Aa aa Aa Two phenotypes All dominant phenotype

 Testcross: Genotypes bb B_ Two possibilities for the black dog: BB or Bb Gametes B b B b Bb b bb Bb 1 black : 1 chocolate All black Offspring Geneticists use the testcross to determine unknown genotypes • The offspring of a testcross, a mating between an individual of unknown genotype and a homozygous recessive individual • Can reveal the unknown’s genotype Figure 9.6

Dominantallele Gene loci a B P a b P Recessiveallele Genotype: PP aa Bb Heterozygous Homozygousfor thedominant allele Homozygousfor therecessive allele Homologous chromosomes bear the two alleles for each characteristic • Alternative forms of a gene • Reside at the same locus on homologous chromosomes Figure 9.4

Web sites to check • http://gslc.genetics.utah.edu/units/basics/tour/inheritance.swf • http://science.nhmccd.edu/biol/genetics.html • http://library.thinkquest.org/20465/games.html

Mendel’s two Laws • 1. Law of segregation The two alleles for a trait segregate during gameteformation and only one allele for a trait is carriedin a gamete. The gametes combine at random (In other words:A cell contains two copies of a particular gene, they separate when a gamete is made). • 2. Law of Independent Assortment Alleles from one trait behave independently fromalleles for another trait. Traits are inherited independently from one another

Independent Assortment • Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait • Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis

The law of independent assortment is revealed by tracking two characteristics at once • By looking at two characteristics at once • Mendel tried to determine how two characteristics were inherited

Hypothesis: Independent assortment Hypothesis: Dependent assortment RRYY P generation rryy RRYY rryy ry ry Gametes Gametes RY  RY RrYy RrYy F1 generation Sperm Sperm 1 4 1 4 1 4 1 4 ry ry RY RY 1 2 1 2 ry RY 1 4 RY 1 2 RY RrYY RRYY RRYy RrYy F2 generation Eggs 1 4 ry 1 2 ry rrYY rrYy RrYy RrYY Eggs Yellowround 9 16 1 4 Ry RrYy RRyy RRYy Rryy Greenround 3 16 1 4 ry Yellowwrinkled Actual resultscontradict hypothesis 3 16 rryy RrYy rrYy Rryy Greenwrinkled 1 16 Actual resultssupport hypothesis • Mendel’s law of independent assortment • States that alleles of a pair segregate independently of other allele pairs during gamete formation Figure 9.5 A

Blind Blind Phenotypes Genotypes Black coat, normal vision B_N_ Black coat, blind (PRA) B_nn Chocolate coat, normal vision bbN_ Chocolate coat, blind (PRA) bbnn Mating of heterozygotes (black, normal vision) BbNn  BbNn 9 black coat, normal vision 3 black coat, blind (PRA) 1 chocolate coat, blind (PRA) 3 chocolate coat, normal vision Phenotypic ratio of offspring Figure 9.5 B • An example of independent assortment

A Dihybrid Cross - F1 Results purple flowers, tall white flowers, dwarf TRUE- BREEDING PARENTS: AABB x aabb GAMETES: AB AB ab ab AaBb F1 HYBRID OFFSPRING: All purple-flowered, tall

ab ab aB AB AB Ab Ab aB 16 Allele Combinations in F2 1/4 1/4 1/4 1/4 1/4 1/16 1/16 1/16 1/16 AABB AABb AaBB AaBb 1/4 1/16 1/16 1/16 1/16 AABb AAbb AaBb Aabb 1/4 1/16 1/16 1/16 1/16 AaBB AaBb aaBB aaBb 1/4 1/16 1/16 1/16 1/16 AaBb Aabb aaBb aabb

Phenotypic Ratios in F2 Four Phenotypes: • Tall, purple-flowered (9/16) • Tall, white-flowered (3/16) • Dwarf, purple-flowered (3/16) • Dwarf, white-flowered (1/16) AaBbX AaBb

ab ab aB AB AB Ab Ab aB Explanation of Mendel’s Dihybrid Results If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible 1/4 1/4 1/4 1/4 1/4 1/16 1/16 1/16 1/16 AABB AABb AaBB AaBb 1/4 1/16 1/16 1/16 1/16 AABb AAbb AaBb Aabb 1/4 1/16 1/16 1/16 1/16 AaBB AaBb aaBB aaBb 1/4 1/16 1/16 1/16 1/16 AaBb Aabb aaBb aabb

Mendel’s laws reflect the rules of probability • Inheritance follows the rules of probability

F1 genotypes Bbmale Formation of sperm Bbfemale Formation of eggs 1 2 1 2 b B b B B B 1 2 B 1 4 1 4 F2 genotypes 1 2 B b b b b 1 4 1 4 • The rule of multiplication • Calculates the probability of two independent events • The rule of addition • Calculates the probability of an event that can occur in alternate ways Figure 9.7

Dominant Traits Recessive Traits Freckles No freckles Widow’s peak Straight hairline Free earlobe Attached earlobe Genetic traits in humans can be tracked through family pedigrees • The inheritance of many human traits • Follows Mendel’s laws Figure 9.8 A

D ? John Eddy Dd Abigail Linnell D ? Hepzibah Daggett Dd Joshua Lambert dd Jonathan Lambert Dd Elizabeth Eddy D ? Abigail Lambert Dd Dd dd Dd Dd Dd dd Female Male Deaf Hearing • Family pedigrees • Can be used to determine individual genotypes Figure 9.8 B

Parents Normal Dd Normal Dd  Sperm Dd Dd Normal (carrier) DD Normal D Offspring Eggs Dd Normal (carrier) dd Deaf d • Recessive Disorders • Most human genetic disorders are recessive Figure 9.9 A

VARIATIONS ON MENDEL’S LAWS The relationship of genotype to phenotype is rarely simple • Mendel’s principles are valid for all sexually reproducing species • But genotype often does not dictate phenotype in the simple way his laws describe

Genetics is not as simple as Gregor Mendel concluded, (one gene, one trait). • We know now that there is a range of dominance and • that genescan work togetherand interact. • Incomplete dominance: • When the F1 generation have an appearance in between the • phenotypes of the parents. • Ex: pink snapdragons offspring of red and white ones. • Another way to say it is • Inincomplete dominance • Heterozygote phenotype is somewhere between that of two • homozygotes

Flower Color in Snapdragons: Incomplete Dominance Red-flowered plant X White-flowered plant Pink-flowered F1 plants (homozygote) (homozygote) (heterozygotes)

Incomplete dominance in snapdragon color

Flower Color in Snapdragons: Incomplete Dominance • Red flowers - two alleles allow them to make a red pigment • White flowers - two mutant alleles; can’t make red pigment • Pink flowers have one normal and one mutant allele; make a smaller amount of red pigment

Flower Color in Snapdragons: Incomplete Dominance Pink-flowered plant X Pink-flowered plant White-, pink-, and red-flowered plants in a 1:2:1 ratio (heterozygote) (heterozygote)

Incomplete dominance in carnations

Co-Dominance or multiple alleles: • Codominance • Non-identical alleles specify two phenotypes that are both expressed in heterozygotes • Having more than 2 alleles for a given trait and both alleles show in the phenotype. No single one is dominant over the other. • Example: ABO blood types

Genetics of ABO Blood Types: Three Alleles • Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells • Two alleles (IA and IB) are codominant when paired • Third allele (i) is recessive to others

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