Foundation of GeneticsCh. 10

The Work of Gregor Mendel

Every living things has a set of characteristics that have been

inherited from its parent or parents!!!!

• Genetics- study of heredity

Gregor Mendel

• “Father of Genetics”

• Worked with garden peas to better understand Genetics

Self Pollination in Pea Plants

• A plant has both male and female parts• The male parts produce

pollen (sperm)• The female parts produce

eggs• When pollen fertilizes an egg a

seed is formed• If the egg and pollen are from

the same plant, then the new plant is essentially a clone having only one parent.

• True-breeding- plants that were allowed to self pollinate producing offspring identical to themselves

Cross Pollination in Pea Plants

• Cross pollination involves sex cells from two different plants.

• The sperm from one plant fertilizes the egg of another.

• The seed produced has two parents.

Pea Plant Traits

• A Trait is a characteristic that varies from one individual to another (Ex. Eye color)

• Pea plants were an excellent choice for genetic experimentation because pea plants possess seven traits in two contrasting forms.

• Peas grow and reproduce rapidly in large quantities

Genes and Dominance

• Mendel crossed plants with each of the seven contrasting traits and observed their offspring

• The original pair of plants is designated as the P (parental) generation• True-breeding- these plants were allowed to

self-fertilize for several generations

• The offspring is designated as the F1 (First Filial) generation• Produced by cross pollination

Mendel’s Crosses

• He determined that the ratio of 3:1 ratio that he observed in the F2 generation was in fact a disguised 1:2:1 ratio

1 2 1true-breeding : not true-breeding : true-

breeding

dominant dominant recessive

Mendel’s Conclusion

• Biological inheritance is determined by factors that are passed down from generation to generation.o Genes- factors that determine traits

• Each of the seven traits studied is controlled by one gene that has two forms (pea shape- round or wrinkled)o The different forms of a gene are called alleles

(there are two alleles for every gene)

Principle of Dominance

• Some alleles are dominant and others are recessive

Segregation

• What happened to the recessive trait? • Does it disappear or

are they still present in the F1 plants.

• To answer that questions, Mendel self pollinated the offspring of the F1 generation to produce F2 offspring

Segregation Cont. . .

• How do the alleles separate?• Mendel suggested that segregation

occurs during the formation of sex cells, or gametes. (egg and sperm)

• Gametes carry only a single copy of each gene (1 allele)

Mendel Proposes a Theory

• We now know that genes are carried on chromosomes, with different alleles on the different homologues.

• The location of a gene on a chromosome is called its locus.

Punnett Squares Cont . . .• Punnett Squares- are diagrams that help

determine the results from genetic crosses.• The letters in the squares represent

alleles. (Remember for ever gene, there are two alleles. One allele is inherited from each parent)

• Capital letters represent dominant alleles

• Lowercase letters represent recessive alleles

• Homozygous- organisms that have two identical/same alleles for a trait (Ex. TT or tt) • Organisms that are homozygous for a

trait are true-breeding• Heterozygous- organisms that have two

different alleles for the same trait• Organisms that are heterozygous are

hybrid

Genotype and Phenotype

• Phenotype- physical characteristic (Ex. tall, short, round, wrinkled, etc . . .)

• Genotype- genetic makeup; the alleles that make up the genes (Ex. TT, Tt, or tt)

Probability and Punnett Squares

• Probability- the likelihood that a particular event will occur. • Example #1: The probability that a flipped

coin will come up heads is 1/2 or 50%.• What is the probability of flipping three

heads in a row? • Example #2: The probability that a dice will

land on a 6 is 1/6. (there are 6 sides)• What is the probability of rolling three 6’s?

Probabilities Predict Averages

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

• Probabilities cannot predict the precise outcome of an individual event

• The larger the sample size, the closer the resulting offspring numbers will get to expected values.• Example: If you flip a coin 100 times the

chances of you getting a 50:50 ratio are higher than if you only flip the coin 20 times.

Exploring Mendelian Genetics

• Review: • There are two alleles for every trait (Ex. TT/Tt

= tall, tt = short)• Alleles segregate (separate) during the

formation of gametes (egg and sperm)• The question Mendel wanted to know is “Does

the segregation of alleles for one trait affect the segregation of alleles for another.

Independent Assortment

• Mendel performed an experiment to answer his questions.• Experiment:

• Mendel crossed true-breeding plants that produced only yellow and round peas (YYRR- homozygous dominant) with true-breeding plants that produced only green and wrinkled peas (yyrr- homozygous recessive)

• All of the F1 generation were yellow and round (YyRr- heterozygous) This didn’t indicate whether or not genes assort independently of one another.

The Law of Independent Assortment

The principle of independent assortment states that genes for

different traits can segregate independently during the formation

of gametes

Mendel’s Laws

• Mendel also investigated the inheritance pattern for more than one factor– when crossing individuals who are true-

breeding for two different characters, the F1 individual that results is a dihybrid

– after the dihybrid individuals self-fertilize, there are 16 possible genotypes of offspring

Figure 11.10 Analysis of a dihybrid cross

Mendel's Experiment Cont. . .

• Mendel crossed two heterozygous pea plants (YyRr x YyRr)

• Mendel observed the following• Yellow round peas• Yellow wrinkled peas• Green round peas• Green wrinkled peas

• Mendel’s results were very close to the 9:3:3:1 ratio that is expected from completing a punnet square

Summary of Mendel’s Principles

• Genes determine an individuals characteristics• Genes are passed from parent to offspring• Some form of a gene are dominant while others are

recessive• In most sexually reproducing organisms, each adult has two

alleles for every gene. One is inherited from each parent• Alleles are separated during the formation of sex cells

(gametes)• Alleles for different traits segregate independently of one

another

Exceptions to Mendel’s Principles

• Some alleles are neither dominant nor recessive

• Many traits are controlled by multiple alleles or multiple genes

Why Some Traits Don’t Show Mendelian Inheritance

• Often the expression of phenotype is not straightforward

• Continuous variation– characters can show a range of small

differences when multiple genes act jointly to influence a character• this type of inheritance is called polygenic

– Phenotype is the result of several genes

Height is a continuously varying character

Why Some Traits Don’t Show Mendelian Inheritance

• Incomplete dominance– not all alternative alleles are either fully

dominant or fully recessive in heterozygotes• in such cases, the alleles exhibit

incomplete dominance and produce a heterozygous phenotype that is intermediate between those of the parents

Incomplete dominance

Codominance

• Codominance: both alleles for a trait contribute to the phenotype

• Example: Cattle• If a white cow (WW) is

crossed with a red bull (RR), all of the offspring a roan (RW). Roan is a mixture of both red and white hairs.

Multiple Alleles• Codominance- no single blood allele

is dominant• Multiple alleles- Genes that have

more than two allele options that code for a particular trait

Environmental Effects

• The degree of gene expression depends on the environment• Example:

Temperature may affect the pigmentation of the fox

Applying Mendel’s Principles

• Morgan performed genetic studies on fruit flies and learned that Mendel's principles applied not only to plants, but to animals as well.

• Morgan performed experiments using the fruit fly Drosophila melangaster.

Morgan’s Experiment

• Morgan detected a mutant male fly that had white eyes instead of wild type red eyes.

• He crossed a white-eyed male with a red eyed female. All of the F1 were red-eyed.

• He then crossed a male and a female from the F1 and discovered that all of the white-eyed F2 flies were males!!

Sex-Linkage

• The gene causing the white-eye trait in Drosophila resides on the X chromosome only.

• Genes located on the X and Y chromosomes are said to be sex-linked genes.

• Most sex-linked genes are located on the X chromosome and are said to be X-linked.

• Examples of sex-linked disorders in humans:• Colorblindness• Hemophilia• Duchenne Muscular

Dystrophy

Human Heredity

1. How many chromosomes do human body cells (2n) contain? 2. How many chromosomes do human sex cells (n) contain? 3. Out of the 46 chromosomes present in a human somatic cell, how many of

them are sex chromosomes (chromosomes that determine and individuals sex)? • Males have one X and one Y sex chromosome• Females have two X sex chromosomes.

4. Out of the 46 chromosomes present in a human somatic cell, how many of them are autosomes (chromosomes that do not determine sex)?

Nondisjunction of the X chromosome

Down Syndrome

• Individuals with this disorder have three copies of a chromosome- “trisomy”

• Trisomy 21- three copies of chromosome 21

• Characteristics• Sever retardation• Increased

susceptibility to many diseases

• Higher frequency of some birth defects

Sex Chromosome Disorders

• Sometimes individuals are born with an irregular number of sex chromosomes

• Recall: How many sex chromosomes do normal individuals have?2

• Turner’s Syndrome- nondisjunction occurs and a female inherits only one X chromosome (genotype XO)• Sterile because sex organs do not develop at puberty

• Klinefelter’s Syndrome- nondisjunction leads to the inheritances of extra X chromosomes in males (genotype (XXY)• The extra X chromosome interferes with meiosis and prevents these individuals

from reproducing.• No individuals have ever been born without an X chromosome

• This indicates that the X chromosome contains vital genes for normal development

• The Y chromosome contains the sex-determining region necessary to produce male sexual development

Pedigree Chart

• Squares represent males• Circles represent females• Horizontal lines represent a

marriage• Vertical lines represent offspring• Shaded circles or squares

indicates that a person expresses the trait

• Half shaded circles or squares indicate that a person is a carrier

Some Important Genetic Disorders

Hemophilia

• Individuals with hemophilia lack a protein necessary for normal blood clotting.

• Sex-linked• Hemophiliacs can bleed to death from

minor cuts and may suffer internal bleeding from bumps and bruises.

• Can be treated with protein injections

Sickle Cell Disease

• Results from a single DNA base change known as a substitution

• Common disorder found in African Americans

• May be Fatal• Characteristics:

• Bend and twisted shape of red blood cells• Blood tends to get stuck in

the capillaries which stops the blood flow to tissues beyond the blockage

• Produces physical weaknesses and damage to the brain, heart and spleen

Other Disorders

• Tay-Sachs Disease• Deterioration of the Brain• Symptoms generally

develop at eight months• Rarely live past 5yrs of age• Caused by a recessive allele• High incidence in Jews of

Eastern and central Europe• Huntington’s Disease

• Caused by a dominant allele• Progressive dete4rioration

of brain cells• Symptoms usually do not

develop until the affected individuals are more than 30 yrs of age

Genetic Counseling and Therapy

• There are no cures for genetic disorders

• Genetic Counseling- process by which parents at risk of producing children with genetic defects are identified. Genetic counseling can also asses the genetic state of early embryos

• How is this accomplished?• Pedigree analysis• Amniocentesis• Ultrasound• Chorionic villi sampling