The Work of Gregor Mendel

Chapter 11

In Your Notebook

• Fill out the Genetic Profile• Which character traits were you like

the majority of the class? Name at least 3.

• Which character traits were you in the minority? Name at least 2.

• Where did you get these traits?

Who Is Gregor Mendel?

• The father of modern genetics• Born in 1822 in Czech Republic• Became priest• Studied science & math at U. of Vienna• Worked in monastery – taught high school• Was in charge of monastery gardens

Mendel’s Experiments

• Used garden peas• Small• Easy to grow• Produce a lot of offspring

• Wanted to know how an organism gets its unique characteristics• Heredity – passing of characteristics

from parent to offspring

Role of Fertilization

• Male part of flower makes pollen (sperm)• Female part produces eggs• Fertilization

• Joining of male and female reproductive cells to produce a new cell

Types of Fertilization• Self Pollination

• Sperm cells fertilize egg cells within same flower

• One parent• Offspring get all traits from one parent

• Cross Pollination• Two parents• Cut off male parts so it couldn’t self

pollinate then dusted plant with other pollen

Mendel’s Task

• Studied seven traits• Self pollinated plants to get pure stock

• He always knew outcome• Trait – specific characteristic (seed color)

• Then cross pollinated to see what would happen• Hybrid – offspring of crosses between

parents with different traits

Genes and Alleles Vocabulary• P = Parental generation• F1 = First Filial = kids• F2 = Second Filial = grandkids• Gene

• Factors that are passed from parent to offspring

• Alleles• Different forms of a gene• If studying trait of plant height one form of the

gene produced tall plants and the other form of the gene produced short plants

Mendel’s Traits First experiment – cross pollinated plants -

for each trait all the offspring had characteristics of only one parent – one trait disappeared

Mendel’s Conclusions

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

• Principle of Dominance• Some alleles are dominant and some

are recessive• Since F1 generation plants were all tall

he knew the tall allele was dominant and the short allele recessive

In Your Notebook

• Write the dominant allele for seed shape.

• Write the dominant allele for pod shape.

• Write the recessive allele for flower position.

• Write the recessive allele for seed coat.

F2 Generation

• Mendel wanted to know what happened to the disappearing trait

• Self pollinated F1 plants• Recessive trait showed

up again

Explaining The Results

• Decided reappearance of trait meant that, at some point, the allele for shortness had separated from the allele for tallness

• Must have happened during formation of gametes (sperm and egg cells)

• Law of Segregation• During gamete formation the alleles for

each gene segregate from each other so that each gamete carries only one allele for each gene

Explaining The Results

• Dominant • capital letter (T)

• Recessive • lower case letter (t)

In Your Notebook

• You are Gregor Mendel• Tell me your story.

• Who are you?• What experiments did you do?• What were your major findings?

• At least three paragraphs with at least three sentences in each paragraph.

11.2 What Are The Odds?

• If you toss a coin, what is the probability of getting heads? Tails? If you toss a coin ten times, how many heads and how many tails would you to get? Working with a partner, have one person toss a coin ten times, while the other person tallies the results on a sheet of paper.

Then, switch tasks to produce a separate tally of the second set of ten tosses.

In Your Notebook• 1. Assuming that you expect five heads and five tails

in ten tosses, how do the results of your tosses compare? How about the results of your partner’s tosses?

• 2. Add your results to those of your partner to produce a total of 20 tosses. Assuming that you expect ten heads and ten tails in 20 tosses, how close are these results to what was expected?

• 3. If you compiled the results for the whole class, what results would you expect?

• 4. How do the expected results differ from the observed results?

Probability and Punnett Squares• Probability

• The likelihood that a certain event will occur

• Predicts the average outcome of a large number of events

• The larger the sample size, the more likely you are to match the expected results• Like tossing a coin twice and expecting one

head and one tail vs. tossing it one hundred times

Vocabulary Words• Genotype

• The genetic make-up of an individual• These are the letters

• Phenotype• The physical traits of an individual• Tall, short, round, purple, etc.

• Homozygous• The organism has two identical alleles• TT or tt

• Heterozygous• The organism has two different alleles• Tt

Punnett Squares• Use mathematical probability to help predict the

genotype and phenotype combinations in genetic crosses

• Monohybrid Cross – looks at only one trait

Homozygous x Homozygous

Genotype

White (pp) All = Pp

p p

P Phenotype

Purple All = Purple

(PP) P

Pp Pp

Pp Pp

Flower Color

Homozygous x Heterozygous

Guinea Pig

Homozygous Black = BB Genotype

Heterozygous Black = Bb 50% = BBB b 50% = Bb

B

Phenotype

B 100% Black

BB Bb

BB Bb

Heterozygous x Heterozygous

• Guinea Pig Genotype• Both heterozygous = Bb 25% = BB

50% = Bb

B b 25% = bb

B

Phenotype

b 75% = Black

25% = Brown

BB Bb

Bb bb

Independent Assortment

• Mendel’s next question• Does the segregation of one set of

alleles affect any others?• Does the gene that determines seed

shape affect the gene for seed color?• Will the dominant genes always stay

together?• Used Dihybrid cross

• Looked at two genes

Independent Assortment

Independent Assortment Conclusions

• Genes for different traits can segregate independently during the formation of gametes• The dominant traits don’t always stay together• Genes segregate independently and don’t influence

one another• The only exception would be if the genes are really

close to each other on the chromosome – then they might travel together

• This is why you look different than your siblings even though you have the same parents

A Summary of Mendel’s Work• The inheritance of biological characteristics is

determined by individual units called genes, which are passed from parents to offspring.

• When two or more forms (alleles) of the gene for a single trait exist, some alleles 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.

In Your Notebook• Explain the following statements:

• A tall plant can be homozygous or heterozygous. A short plant must be homozygous.

• One half of the F2 generation is heterozygous and one half is homozygous, but three fourths are tall and one fourth is short.

11.3 Other Patterns of Inheritance

• Discussion• Height in pea plants is controlled by two

alleles; the allele for a tall plant is the dominant allele, while the allele for a short plant is the recessive one.

• What about height in people? Are the factors that determine height more complicated in humans?

In Your Notebook• 1. Make a list of ten people whom you know.

Next to the name of each person, write his or her approximate height in feet and inches.

• 2. What can you observe about the heights of the ten people?

• 3. Do you think height in humans is controlled by two alleles, as it is in pea plants? Explain your answer.

• 4. What other traits in humans show a similar amount of variation between individuals?

Beyond Dominant and Recessive Alleles• All of Mendel’s rules have exceptions• Genetics is more complicated

because there are usually more than two alleles for each trait

• Also complicated because many traits are controlled by more than one gene

Incomplete Dominance• Some alleles are neither dominant or

recessive• The heterozygous phenotype is in

between• A red flower crossed with a

white flower will produce

pink flowers

Codominance• Both alleles are dominant• The heterozygous phenotype doesn’t mix – it shows both

colors equally• Erminette Chickens and Roan Cows

Multiple Alleles

• A gene with two or more alleles• Each individual has only two copies

but within population there may be several different alleles

• Rabbit’s coat color has at least four different alleles

• Human blood type has three• A,B &O

In Your Notebook• Page 320 in your textbook• Read and answer the questions in the Analyzing Data

section• Helpful Hints

• A and B are codominant• O is recessive• The Rh factor is inherited

separately from blood type• Rh+ is dominant

Polygenic Traits• Traits that are produced by the interaction

of several genes• Show wide range of phenotypes• Example: human skin color

In Your Notebook• In your own words, describe multiple

alleles and polygenic traits.

• How are they similar?

• How are they different?

Genes and the Environment• Does the environment have a role in how genes

determine traits?• Genes provide a plan for development, but how that plan

unfolds depends on the environment.• Butterflies hatching in spring have different wing patterns

than those hatching in summer

In Your Notebook• Write a rap or poem using the

following terms:• Incomplete Dominance• Codominance• Multiple Alleles• Polygenic Traits

11.4 Meiosis• Mendel knew genes were carried on

structures in the cells. Our next question is what structures are they carried on?

• Normal human body cells each contain 46 chromosomes. The cell division process that body cells undergo is called mitosis and produces daughter cells that are virtually identical to the parent cell.

In Your Notebook• 1. How many chromosomes would a sperm or an egg

contain if either one resulted from the process of mitosis?

• 2. If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? Do you think this would create any problems in the developing embryo?

• 3. In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have?

Chromosome Number• How many sets of genes do multicellular

organisms inherit?• Two – One from mom and one from dad

• The Rules• An organism with two parents must inherit a

single copy of every gene from each parent• When that organism produces gametes, those

two sets of genes must be separated so that each gamete (egg or sperm) has just one set of genes

Diploid Cells• Diploid

• Two sets (2N)• The diploid cells of most adult organisms contain

two complete sets of inherited chromosomes and two complete sets of genes

• All of your cells are diploid except for sperm and egg cells

• Homologous Chromosomes• A matching pair of chromosomes• The #1 chromosome from your dad and the #1

chromosome from your mom are homologous

Haploid Cells• Cells with only one set of chromosomes (1N)• These are the sperm and egg cells• Why do they only have one set?• If diploid number is 8 then haploid number is

4• Human diploid number is 46 so haploid is 23

In Your Notebook• Page 327• Read the Analyzing Data Section and

answer the questions in your notebook

Phases of Meiosis• How are haploid cells produced from

diploid cells?• Meiosis

• The process in which the number of chromosomes per cell is cut in half

• Homologous chromosomes are separated

• Two phases• Meiosis I and Meiosis II

Meiosis I

• The cell undergoes a round of chromosome replication during interphase – just like mitosis

• Now you have two identical chromatids joined at the center

Prophase I• Each replicated chromosome pairs with its

corresponding homologous chromosome forming tetrad

• A tetrad has four chromatids• Crossing over occurs here and alleles get exchanged

Metaphase I and Anaphase I• Metaphase I

• The chromosomes line

up in the middle• Anaphase I

• Spindle fibers pull each

homologous chromosome

pair toward opposite end

of the cell

Telophase I and Cytokinesis• Telophase I

• Nuclear membrane forms

around each cluster of

chromosomes• Cytokinesis

• Cytoplasm splits and two

new cells are formed

Meiosis II• After meiosis I you now have two

daughter cells that are slightly different from the parent cell • because crossing over occurred

• Now these cells undergo division again

• The big difference is they don’t go through interphase so they don’t replicate their DNA

Prophase II and Metaphase II• Prophase II

• Chromosomes become

visible• Metaphase II

• Chromosomes line

up in middle of cell

Anaphase II and Telophase II

• Anaphase II• Paired chromatids

separate• Start moving to

opposite poles• Telophase II

• Chromatids reach poles• Nuclear membrane

reforms

Cytokinesis II• Cytokinesis II

• Four haploid daughter cells• Sperm

• Four viable spermatids• Egg

• One viable egg• Three polar bodies• Cytoplasm splits unevenly because if egg gets

fertilized the nutrients are needed to get through first few cell divisions before egg is implanted in uterus

Vocabulary Words• Gametes

• Haploid cells produced by meiosis II• Zygote

• Formed when egg is fertilized by sperm• Undergoes cell division by mitosis• Eventually forms new organism

Comparing Meiosis and Mitosis• Difference #1

• Mitosis – Asexual• Meiosis – First step of sexual

• Difference #2• Mitosis – Each daughter cell receives

one complete set of chromosomes• Meiosis – Each daughter cell receives

½ set of chromosomes

Comparing Meiosis and Mitosis• Difference #3

• Mitosis – does not change the chromosome number of the original cell

• Meiosis – reduces chromosome number by ½

• Difference #4• Mitosis – produces two genetically identical

diploid cells• Meiosis – produces four genetically different

haploid cells

Gene Linkage• Read “Gene Linkage” and “Gene Mapping on

page 328-329• Alleles of different genes tend to be inherited

together from one generation to the next when those genes are located on the same chromosome

In Your Notebook• Describe the main result of meiosis• Compare the chromosomes of a diploid

cell to a collection of shoes in a closet. How are they similar? What would make the shoe collection comparable to the chromosomes of a haploid cell?

• If two genes are on the same chromosome but usually assort independently, what does that tell you about how close together they are?