Chapter 11 Notes: Chapter 11 Notes: Mendelian GeneticsMendelian Genetics

Chapter 11-1, 11-3Chapter 11-1, 11-3

• Genetics is the scientific study of heredity Genetics is the scientific study of heredity that involves how genes are passed from that involves how genes are passed from parents to their offspring.parents to their offspring.

The Baldwin brothers

History of GeneticsHistory of Genetics

• Gregor Mendel was Gregor Mendel was an Austrian monk an Austrian monk and scientist who and scientist who was in charge of the was in charge of the monastery garden. monastery garden. Mendel studied Mendel studied garden peas. garden peas.

Pea plants happened to be a good Pea plants happened to be a good choice to study because:choice to study because:

– They are self-pollinating. – He had different pea plants that were true-

breeding. – True-breeding - means that they are

homozygous for that trait. – EX. if the plants self-pollinate they produce

offspring identical to each other and the parents.

Pea plants happened to be a good Pea plants happened to be a good choice to study because:choice to study because:

– He developed a technique of producing seeds from a process called cross-pollination, in which he dusted the pollen of one pea plant onto another plant.

– He was in control of which plants crossed with each other.

Genes and DominanceGenes and Dominance

• A trait is a specific characteristic that A trait is a specific characteristic that varies from one individual to another. varies from one individual to another.

• Mendel studied seven different pea plant Mendel studied seven different pea plant traits including seed shape, seed color, traits including seed shape, seed color, seed coat color, pod shape, pod color, seed coat color, pod shape, pod color, flower position, and plant height. flower position, and plant height.

– Mendel studied two alleles, or different versions, of each trait (wrinkled or smooth pea shape, green or yellow seed color, etc.)

Seed Shape Flower Position

Seed Coat ColorSeed Color Pod Color Plant HeightPod Shape

Round

Wrinkled

Round

Yellow

Green

Gray

White

Smooth

Constricted

Green

Yellow

Axial

Terminal

Tall

Short

Yellow Gray Smooth Green Axial Tall

When discussing generations’ When discussing generations’ traits, we label them as following:traits, we label them as following:

• The true-breeding parental generation is The true-breeding parental generation is called the “P generation”. called the “P generation”.

• The offspring of the two parental plants is The offspring of the two parental plants is called the “F1 generation”. called the “F1 generation”.

• A cross between F1 generation would be A cross between F1 generation would be called “F2 generation.”called “F2 generation.”

Original cross

Cross pollination

(P) Parental Generation (true breeding)

F1 Generation (offspring)

F2 Generation (Cross of F1 Generations)

Mendel’s InvestigationsMendel’s Investigations

• Mendel wanted to cross (or breed) two Mendel wanted to cross (or breed) two plants with different versions of the same plants with different versions of the same trait. He wanted to know if the trait. He wanted to know if the characteristics of the plants were blended characteristics of the plants were blended in the offspring. in the offspring.

Mendel’s InvestigationsMendel’s Investigations

• Mendel saw that when he crossed plants Mendel saw that when he crossed plants with different versions of the same trait (P with different versions of the same trait (P generation), the F1 offspring were NOT generation), the F1 offspring were NOT blended versions of the parents. blended versions of the parents.

• The F1 plants resembled only one of the The F1 plants resembled only one of the parents.parents.

Tall x short all tall…

Mendel concluded:Mendel concluded:

• 1. Biological inheritance 1. Biological inheritance is determined by is determined by “factors” that are passed “factors” that are passed from one generation to from one generation to the next.the next.

• Factors were later Factors were later defined as “genes”-defined as “genes”-– Mendel discovered all of Mendel discovered all of

this without the this without the knowledge of DNA!knowledge of DNA!

Mendel concluded:Mendel concluded:

• In Mendel’s plants, there was one gene for In Mendel’s plants, there was one gene for each trait. each trait. For example, there was one gene for plant For example, there was one gene for plant height. height.

– But, there were two versions of this gene: one for a But, there were two versions of this gene: one for a tall plant and one for a short plant. tall plant and one for a short plant.

Mendel concluded:Mendel concluded:

– Alleles: Different versions of the same gene

• Remember, genes are used to make proteins.• Each allele contains the DNA that codes for a

slightly different version of the same protein• This gives us the different characteristics for

each trait

2. Principal of dominance:2. Principal of dominance:

• Some alleles are dominant and some Some alleles are dominant and some alleles are recessive. alleles are recessive.

– Recessive alleles are able to be maskedRecessive alleles are able to be masked– Dominant alleles mask recessive allelesDominant alleles mask recessive alleles

• The trait that was represented in the F1 The trait that was represented in the F1 generation was the dominant trait.generation was the dominant trait.

2. Principal of dominance:2. Principal of dominance:

• How many alleles do you have for each How many alleles do you have for each gene? gene?

• Where do they come from? Where do they come from?

Two

One comes from mother and one comes from father.

3. Segregation:3. Segregation:

• ObservationObservation: After seeing that his F1 : After seeing that his F1 plants looked like only one generation of plants looked like only one generation of the P generation plants, Mendel wanted the P generation plants, Mendel wanted to know what happened to the recessive to know what happened to the recessive alleles. alleles.

• QuestionQuestion: Did they disappear? : Did they disappear?

3. Segregation:3. Segregation:

• ExperimentExperiment: Mendel self-pollinated the F1 : Mendel self-pollinated the F1 plants, or crossed the F1 plants with each plants, or crossed the F1 plants with each other, to produce the F2 generation. From other, to produce the F2 generation. From his F1 crosses, Mendel observed:his F1 crosses, Mendel observed:– The versions of the traits coded for by The versions of the traits coded for by

recessive alleles reappeared in the F2 plants.recessive alleles reappeared in the F2 plants.– The recessive trait was still there!The recessive trait was still there!

3. Segregation:3. Segregation:

– About 25% (or ¼) of the F2 plants exhibited About 25% (or ¼) of the F2 plants exhibited the recessive version of the trait. In this case the recessive version of the trait. In this case the recessive phenotype is short. The the recessive phenotype is short. The dominant phenotype, tall, was found in 75% dominant phenotype, tall, was found in 75% (or ¾) of the F2 plants.(or ¾) of the F2 plants.

P generation F1 generation F2 generation

Segregation of alleles during meiosis:Segregation of alleles during meiosis:

• When the F1 plants produce gametes (sex cells) When the F1 plants produce gametes (sex cells) and self-pollinate, the two alleles for the same and self-pollinate, the two alleles for the same gene separate from each other so that each gene separate from each other so that each gamete carries only one copy of each gene. gamete carries only one copy of each gene.

• Remember, gametes are haploid. In the Remember, gametes are haploid. In the example, we use “T” to represent the dominant, example, we use “T” to represent the dominant, tall allele and “t” to represent the recessive, short tall allele and “t” to represent the recessive, short allele.allele.

4. Law of Independent Assortment4. Law of Independent Assortment

• Law of Independent Assortment- genes Law of Independent Assortment- genes for each trait can be inherited for each trait can be inherited independently from each other. For independently from each other. For example example

– not all tall plants have green pea pods andnot all tall plants have green pea pods and– not all people with brown hair have brown not all people with brown hair have brown

eyes.eyes.

Key Terms in Mendelian Genetics:Key Terms in Mendelian Genetics:

• DominantDominant- allele that can mask; - allele that can mask; represented by capital letters (represented by capital letters (B, D, FB, D, F, , etc.)etc.)

• RecessiveRecessive- alleles that can be masked; - alleles that can be masked; represented by lower case letters (represented by lower case letters (b, d, f,b, d, f, etc.)etc.)

Key Terms in Mendelian Genetics:Key Terms in Mendelian Genetics:

• PhenotypePhenotype- observable traits (brown eyes, - observable traits (brown eyes, yellow seed pods)yellow seed pods)

• GenotypeGenotype- actual alleles; describes the - actual alleles; describes the genetic characteristics (BB, dd, Ff)genetic characteristics (BB, dd, Ff)

Phenotype: brown eyes

Genotype: could be BB,

or Bb

Key Terms in Mendelian Genetics:Key Terms in Mendelian Genetics:

• Homozygous (True-Breeding)Homozygous (True-Breeding)- having two - having two identical alleles for the same trait (TT, tt); identical alleles for the same trait (TT, tt); “homo” means same“homo” means same

• HeterozygousHeterozygous- having two different alleles - having two different alleles from the same trait (Tt); “hetero” means from the same trait (Tt); “hetero” means differentdifferent

• Punnett Squares must be taught before Punnett Squares must be taught before continuing continuing

Beyond dominant and recessive allelesBeyond dominant and recessive alleles

• There are some exceptions to Mendel’s There are some exceptions to Mendel’s principles. Luckily, none of these principles. Luckily, none of these exceptions are exhibited in pea plants. exceptions are exhibited in pea plants.

• If so, Mendel would not have been able to If so, Mendel would not have been able to figure out inheritance.figure out inheritance.

• Some alleles are neither dominant nor Some alleles are neither dominant nor recessive.recessive.– Incomplete DominanceIncomplete Dominance– CodominanceCodominance

IncompleteIncomplete dominancedominance::

• situation in which one allele is not situation in which one allele is not completely dominant over another; the completely dominant over another; the phenotype is a “blending” of the two allelesphenotype is a “blending” of the two alleles– Example: In some plants, when a true-Example: In some plants, when a true-

breeding plant with red flowers is crossed with breeding plant with red flowers is crossed with a true-breeding plant with white flowers, pink a true-breeding plant with white flowers, pink flowers are produced. Neither red nor white is flowers are produced. Neither red nor white is dominant over the other.dominant over the other.

• Consider thisConsider thisPunnett square:Punnett square:

CodominanceCodominance::

• situation in which both alleles of a gene situation in which both alleles of a gene contribute to the phenotype of the organism; contribute to the phenotype of the organism; both alleles are expressed but NOT blendedboth alleles are expressed but NOT blended

– Example: In cows, the allele for red fur is Example: In cows, the allele for red fur is codominant with the allele for white fur. codominant with the allele for white fur. Heterozygous cows carrying one red and one white Heterozygous cows carrying one red and one white allele have spotted fur, known as roan.allele have spotted fur, known as roan.

• Consider thisConsider thisPunnett square:Punnett square:

• Many traits are controlled by multiple Many traits are controlled by multiple alleles or multiple genes. alleles or multiple genes. – Multiple alleles (more than 2 choices)Multiple alleles (more than 2 choices)– Polygenic (multiple genes control a single Polygenic (multiple genes control a single

trait)trait)

Multiple alleles:Multiple alleles:

• the case where three or more alleles of the case where three or more alleles of the same gene exist. Remember, an the same gene exist. Remember, an organism will have only two of these organism will have only two of these alleles (one from mom and one from alleles (one from mom and one from dad).dad).

– Examples: Coat color in rabbits, blood type Examples: Coat color in rabbits, blood type in humansin humans

Multiple alleles:Multiple alleles:

Polygenic traits:Polygenic traits:

• traits that are determined by alleles from traits that are determined by alleles from more than one gene; these traits usually more than one gene; these traits usually have a range of phenotypeshave a range of phenotypes

– Examples: skin color in humans, height in Examples: skin color in humans, height in humanshumans

Sex DeterminationSex Determination

2.2. There are two types of chromosomes.There are two types of chromosomes.a.a. Autosomes: Of the 46 chromosomes, 44 of them Autosomes: Of the 46 chromosomes, 44 of them

(22 pairs of chromosomes) are called autosomes (22 pairs of chromosomes) are called autosomes (non-sex chromosomes).(non-sex chromosomes).

b.b. Sex chromosomes: The last two chromosomes are Sex chromosomes: The last two chromosomes are called the sex chromosomes because they called the sex chromosomes because they determine the sex of the person. Females have two determine the sex of the person. Females have two X chromosomes (XX) and males have one X and X chromosomes (XX) and males have one X and one Y chromosome (XY).one Y chromosome (XY).

3.3. GametesGametesa.a. All gametes are haploid. In humans, that All gametes are haploid. In humans, that

means each egg cell and each sperm cell means each egg cell and each sperm cell has 1 copy of each chromosome for a total has 1 copy of each chromosome for a total of 23 chromosomes. of 23 chromosomes.

i.i. Egg cells: All human egg cells carry 23 Egg cells: All human egg cells carry 23 chromosomes, one of which is a single X chromosomes, one of which is a single X chromosome. This is written as 23, X. chromosome. This is written as 23, X.

ii.ii. Sperm cells: In males, there are two types of Sperm cells: In males, there are two types of sperm cells- one carries an X chromosome (23, sperm cells- one carries an X chromosome (23, X) and one carries a Y chromosome (23, Y). X) and one carries a Y chromosome (23, Y).

iii.iii. When a sperm and When a sperm and egg cell combine, half egg cell combine, half of the time the of the time the fertilized eggs (also fertilized eggs (also called zygotes) are called zygotes) are female (46, XX) and female (46, XX) and half of the time they half of the time they are male (46, XY).are male (46, XY).

X X

X

Y

XX XX

XY XY

female female

male male

sperm

eggs

• Sex Linked traits: traits that are determined by alleles that are found on the X or Y chromosome.– The Y chromosome

is shorter and does not carry all the same alleles as the X chromosome.

– Females are XX and males are XY. – Females can be homozygous or

heterozygous for a trait carried on the X chromosome, but males (having only one X chromosome) are hemizygous.

Example of a sex-linked Punnett square: Example of a sex-linked Punnett square: – XXBBXXbb (heterozygous female with normal vision) crossed to (heterozygous female with normal vision) crossed to

XXBBY (hemizygous male with normal vision)Y (hemizygous male with normal vision)

XBY

XBY

XbY

XB XB

XB Xb

YXB

XB

Xb

XB Xb

– If they inherit a defective gene from the parent, then they will exhibit the trait because they cannot inherit a second gene to mask it.

– Conversely, a healthy male cannot be “hiding” a bad recessive allele because they only have one X chromosome.

Mapping GenesMapping Genes

• It’s easy to imagine that genes on different It’s easy to imagine that genes on different chromosomes assort independently, but what chromosomes assort independently, but what about genes that occur on the same about genes that occur on the same chromosome? Don’t they always appear chromosome? Don’t they always appear together?together?

• Not always due to crossing over. Genes that Not always due to crossing over. Genes that occur together on a chromosome will be occur together on a chromosome will be separated when homologous chromosomes separated when homologous chromosomes exchange genes. exchange genes.

• The frequency of genes occurring together can The frequency of genes occurring together can help us generate a gene map. help us generate a gene map.

• The more often two genes occur together, The more often two genes occur together, the closer they are to each other on the the closer they are to each other on the chromosome. chromosome. – If the genes are never separated by crossing If the genes are never separated by crossing

over, they always occur together. All offspring over, they always occur together. All offspring will look like one of the parents (in reference will look like one of the parents (in reference to the genes in question). to the genes in question).

– If half of the offspring are parental and half If half of the offspring are parental and half are recombinations of the parents (in are recombinations of the parents (in reference to the genes in question), then they reference to the genes in question), then they are said to be independent. This means they are said to be independent. This means they are either on separate chromosomes or they are either on separate chromosomes or they are almost always separated during meiosis.are almost always separated during meiosis.

– You will learn to calculate distances and You will learn to calculate distances and create a map in AP Bio, or in collegecreate a map in AP Bio, or in college

Genetics and the EnvironmentGenetics and the Environment

• Characteristics are determined by both Characteristics are determined by both genes and the environment. genes and the environment.

• External: While genes will influence the External: While genes will influence the height of a plant, the amount of water, sun, height of a plant, the amount of water, sun, and other climate conditions will also affect and other climate conditions will also affect the height.the height.

Genetics and the EnvironmentGenetics and the Environment

• Internal: There are recent findings that Internal: There are recent findings that proteins involved with DNA can turn genes proteins involved with DNA can turn genes on or off based on environmental factors. on or off based on environmental factors. – Certain chemical exposure can turn genes on Certain chemical exposure can turn genes on

or off (make the traits show up or not) for or off (make the traits show up or not) for generations after exposure, but there are no generations after exposure, but there are no changes to the DNA (no mutations). changes to the DNA (no mutations).

– This new understanding of how genes are This new understanding of how genes are expressed is called epigenetics.expressed is called epigenetics.

Blood TypingBlood Typing In humans, blood type is determined by the In humans, blood type is determined by the

Rh blood group and the ABO blood group. Rh blood group and the ABO blood group. The Rh blood group determines if your blood is The Rh blood group determines if your blood is

positive or negative. positive or negative. There are two Rh alleles: the RhThere are two Rh alleles: the Rh++ allele is dominant and the allele is dominant and the

RhRh-- allele is recessive. allele is recessive. Your blood is Your blood is positivepositive if you are Rhif you are Rh++ /Rh /Rh++ or Rh or Rh++/Rh/Rh--. . Your blood is Your blood is negativenegative if you are Rhif you are Rh--/Rh/Rh--

When determining the ABO blood group When determining the ABO blood group there are three alleles: there are three alleles: IIAA, , IIBB, and , and ii. .

The The IIAA and and IIBB alleles are alleles are codominantcodominant.. The The IIAA and and IIBB alleles cause expression of alleles cause expression of carbohydrate chains called antigens on carbohydrate chains called antigens on surface of red blood cells. They help your surface of red blood cells. They help your body identify the cells.body identify the cells.

The The ii allele is recessive to the allele is recessive to the IIAA and and IIBB alleles. The alleles. The ii allele produces O blood allele produces O blood because it does not produce any antigens. because it does not produce any antigens.

If the blood recipient has never been exposed to an If the blood recipient has never been exposed to an antigen (A or B) and that antigen enters the body, it antigen (A or B) and that antigen enters the body, it will will cause an immunecause an immune reaction. This can cause reaction. This can cause death. death.

Blood TransfusionsBlood Transfusions

In emergency rooms, if there is not enough time to In emergency rooms, if there is not enough time to figure out the blood type of the patient, which type figure out the blood type of the patient, which type of blood will the patient receive Type O blood. This of blood will the patient receive Type O blood. This is because these blood cells have no A or B is because these blood cells have no A or B antigens. People with Type O blood are called antigens. People with Type O blood are called

universal donors.universal donors.

Blood TransfusionsBlood Transfusions

People with AB blood can receive any blood type. People with AB blood can receive any blood type. They are considered to be the universal They are considered to be the universal

recipient. recipient.

Blood TransfusionsBlood Transfusions