Chapter 14 Mendelian Genetics. Important Terms Character--something that is inherited. Flower color Trait--a variant of a character. Purple flower

Chapter 14Chapter 14

Mendelian GeneticsMendelian Genetics

Important TermsImportant Terms

Character--something that is inherited.Flower color

Trait--a variant of a character.Purple flower vs. white flower

True breeding--produces only one type of offspring.No variation of traits.

Character--something that is inherited.Flower color

Trait--a variant of a character.Purple flower vs. white flower

True breeding--produces only one type of offspring.No variation of traits.


Hybridization--crossing of two variants of a true breeding plants. The hybrid contains genes from both parents which likely come out in the next generation.

Hybridization--crossing of two variants of a true breeding plants. The hybrid contains genes from both parents which likely come out in the next generation.


P generation--Usually true breeding and start the experiment.

F1 generation--1st filial which are hybrid offspring of the parents.

F2 generation--2nd filial which is offspring of the hybrids. This is when we start to see the traits reappear from the P generation.

P generation--Usually true breeding and start the experiment.

F1 generation--1st filial which are hybrid offspring of the parents.

F2 generation--2nd filial which is offspring of the hybrids. This is when we start to see the traits reappear from the P generation.

MendelMendel

By examining the P, F1 and F2 generations, Mendel was able to elucidate the patterns of inheritance.

By examining the P, F1 and F2 generations, Mendel was able to elucidate the patterns of inheritance.

MendelMendel

What made Mendel’s work so good was that he kept excellent records of what he did and the results of his experiments.

What made Mendel’s work so good was that he kept excellent records of what he did and the results of his experiments.

MendelMendel

At the time, people believe in a “blending hypothesis.” They believed that the traits of a particular organism would be blended together.

Mendel’s experiments abolished this notion.

At the time, people believe in a “blending hypothesis.” They believed that the traits of a particular organism would be blended together.

Mendel’s experiments abolished this notion.

MendelMendel

Mendel crossed true-breeding purple flowers and true-breeding white flowers and the offspring (F1) were all purple.

When he crossed the F1 purple flowers, he got purple and white in a 3:1 ratio.

He determined that purple was dominant to white.

Mendel crossed true-breeding purple flowers and true-breeding white flowers and the offspring (F1) were all purple.

When he crossed the F1 purple flowers, he got purple and white in a 3:1 ratio.

He determined that purple was dominant to white.

MendelMendel

The “blending hypothesis” was wiped out because none of the flowers were pale purple.

He also gave rise to the term “heritable factor” which we now call genes. He said heritable factors must somehow determine flower color.

The “blending hypothesis” was wiped out because none of the flowers were pale purple.

He also gave rise to the term “heritable factor” which we now call genes. He said heritable factors must somehow determine flower color.

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Mendel’s 4 Part Model to Explain What He Saw


1. There are alternative versions of genes--alleles.

2. Each organism inherits a copy of an allele from each parent.

3. Some alleles are dominant, while others are recessive.

4. The 2 alleles segregate from one another during meiosis.

1. There are alternative versions of genes--alleles.

2. Each organism inherits a copy of an allele from each parent.

3. Some alleles are dominant, while others are recessive.

4. The 2 alleles segregate from one another during meiosis.

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1. Alternative versions of genes account for variations in inherited characteristics.

1. Alternative versions of genes account for variations in inherited characteristics.

In Today’s TerminologyIn Today’s Terminology

Each gene resides on a specific locus on a specific c-some. The DNA at this locus can vary in its sequence of nucleotides and thus its information.

The sequence of nucleotides, in this case, can change the flower color.

The alleles are due to variations in the DNA.

Each gene resides on a specific locus on a specific c-some. The DNA at this locus can vary in its sequence of nucleotides and thus its information.

The sequence of nucleotides, in this case, can change the flower color.

The alleles are due to variations in the DNA.



2. For each character, an organism inherits 2 alleles, one from each parent.

This was a remarkable deduction from Mendel considering he knew nothing about c-somes or ploidy.

2. For each character, an organism inherits 2 alleles, one from each parent.

This was a remarkable deduction from Mendel considering he knew nothing about c-somes or ploidy.



3. If 2 alleles at a locus differ, then the dominant allele determines the organism’s appearance while the recessive allele gets masked and no noticeable change in the organism’s appearance can be seen.

3. If 2 alleles at a locus differ, then the dominant allele determines the organism’s appearance while the recessive allele gets masked and no noticeable change in the organism’s appearance can be seen.



4. The 2 alleles for a heritable characteristic segregate during gamete formation and end up in different gametes. This makes up what is known as the Law of Segregation.

4. The 2 alleles for a heritable characteristic segregate during gamete formation and end up in different gametes. This makes up what is known as the Law of Segregation.

The Law of SegregationThe Law of Segregation

In terms of chromosomes, the homologous chromosomes are being separated and distributed to different gametes during meiosis.

In terms of chromosomes, the homologous chromosomes are being separated and distributed to different gametes during meiosis.

Law of SegregationLaw of Segregation

If different alleles are present, there is a 50/50 chance that the gamete will receive a copy of one gene vs. another.

If different alleles are present, there is a 50/50 chance that the gamete will receive a copy of one gene vs. another.


If the alleles are the same, each gamete contains the same copy of the gene and it is said to be true-breeding for a particular trait.

If the alleles are the same, each gamete contains the same copy of the gene and it is said to be true-breeding for a particular trait.

The Observed 3:1 RatioThe Observed 3:1 Ratio

Can the segregation of gametes account for the 3:1 ratio Mendel observed?

Using a Punnett square, you find the answer is yes.

Examine the genotypes and the phenotypes.

Can the segregation of gametes account for the 3:1 ratio Mendel observed?

Using a Punnett square, you find the answer is yes.

Examine the genotypes and the phenotypes.

More Useful TermsMore Useful Terms

Homozygous--organisms with identical alleles for a trait in question.

Heterozygous--organisms with different alleles for a trait in question.

Phenotype--the outward appearance of an organism.

Genotype--the genetic makeup of an organism.

Homozygous--organisms with identical alleles for a trait in question.

Heterozygous--organisms with different alleles for a trait in question.

Phenotype--the outward appearance of an organism.

Genotype--the genetic makeup of an organism.

A Test CrossA Test CrossSuppose we have a purple flower and

we want to know if it is homozygous dominant or heterozygous, (recessive will be white).

To do this, cross the organism with a homozygous recessive and observe the offspring. If any white are produced, the trait is said to be heterozygous, and will be produced in a 1:1 ratio.

Suppose we have a purple flower and we want to know if it is homozygous dominant or heterozygous, (recessive will be white).

To do this, cross the organism with a homozygous recessive and observe the offspring. If any white are produced, the trait is said to be heterozygous, and will be produced in a 1:1 ratio.

The Law of SegregationThe Law of Segregation

Applies to all genes on a particular chromosome. Says that all genes segregate independently of each other regardless of what phenotype they are carrying so long as each gamete contains one copy of each trait.

Applies to all genes on a particular chromosome. Says that all genes segregate independently of each other regardless of what phenotype they are carrying so long as each gamete contains one copy of each trait.


Mendel demonstrated this using a dihybrid cross.

He wanted to see if the gametes contained genes of all possible combinations or if certain genes went with certain other genes.

Mendel demonstrated this using a dihybrid cross.

He wanted to see if the gametes contained genes of all possible combinations or if certain genes went with certain other genes.

The CrossThe Cross

Plants producing yellow colored, round seeds were crossed with plants producing green colored, wrinkled seeds.

If they assort independently, a 9:3:3:1 ratio should be produced.

If they don’t assort independently, if they are somehow linked, a different ratio will be observed.

Plants producing yellow colored, round seeds were crossed with plants producing green colored, wrinkled seeds.

If they assort independently, a 9:3:3:1 ratio should be produced.

If they don’t assort independently, if they are somehow linked, a different ratio will be observed.

ConclusionsConclusions

From the cross, Mendel concluded that no matter how many characteristics are observed, they always segregate independently of one another.

From the cross, Mendel concluded that no matter how many characteristics are observed, they always segregate independently of one another.

The Law of Independent Assortment

The Law of Independent Assortment

As a result of Mendel’s breeding experiment with dihybrid crosses, he arrived at what is known as the Law of Independent Assortment which says that all alleles of a gene pair will segregate independently of other pairs during gamete formation.

This law only applies to genes residing on different chromosomes.

As a result of Mendel’s breeding experiment with dihybrid crosses, he arrived at what is known as the Law of Independent Assortment which says that all alleles of a gene pair will segregate independently of other pairs during gamete formation.

This law only applies to genes residing on different chromosomes.

Rules Regarding Probability

Rules Regarding Probability

The probability scale ranges from 0 to 1, zero is not going to happen, 1 is it’s certain to happen.

The probability of all possible outcomes is 1, and all outcomes of a particular event are independent each other--they have no bearing on what has happened or what will happen.

The probability scale ranges from 0 to 1, zero is not going to happen, 1 is it’s certain to happen.

The probability of all possible outcomes is 1, and all outcomes of a particular event are independent each other--they have no bearing on what has happened or what will happen.

2 Rules:2 Rules:

Two rules help us determine the probability of chance events.

1. The multiplication rule.2. The addition rule.

Two rules help us determine the probability of chance events.

1. The multiplication rule.2. The addition rule.

The Multiplication RuleThe Multiplication Rule

To determine the probable outcome of a chance event, multiply the probability of each possible outcome.

Coin example: 1/2 • 1/2 = 1/4

To determine the probable outcome of a chance event, multiply the probability of each possible outcome.

Coin example: 1/2 • 1/2 = 1/4

The Multiplication RuleThe Multiplication RuleAnother example: Suppose we roll

one die followed by another and want to find the probability of rolling a 4 on the first die and rolling an even number on the second die.

P(4) = 1/6P(even) = 3/6

The probability of rolling a 4 and an even is 1/6 • 3/6 = 3/36, or 1/12.

Another example: Suppose we roll one die followed by another and want to find the probability of rolling a 4 on the first die and rolling an even number on the second die.

P(4) = 1/6P(even) = 3/6

The probability of rolling a 4 and an even is 1/6 • 3/6 = 3/36, or 1/12.

The Addition RuleThe Addition Rule

Allows us to determine the probability of any mutually exclusive events by adding together their individual probabilities.

Allows us to determine the probability of any mutually exclusive events by adding together their individual probabilities.

The Addition RuleThe Addition RuleFor instance:Suppose you are going to pull one card

out of a deck.What is the probability of pulling a king or an ace?

P(King) = 4/52P(Ace) = 4/52

The probability of pulling a King or an Ace is 4/52 + 4/52, which is 8/52, or 2/13.

There is a 2 in 13 chance of pulling a King or an Ace.

For instance:Suppose you are going to pull one card

out of a deck.What is the probability of pulling a king or an ace?

P(King) = 4/52P(Ace) = 4/52

The probability of pulling a King or an Ace is 4/52 + 4/52, which is 8/52, or 2/13.

There is a 2 in 13 chance of pulling a King or an Ace.

}Each are mutually exclusive

The Addition RuleThe Addition RuleSo, how does this

apply to us?Use a monohybrid

heterozygous F2 cross to illustrate.

What is the possibility of getting a heterozygous F2 offspring?

1/4 + 1/4 = 1/21/2 of the offspring

should be heterozygous.

So, how does this apply to us?

Use a monohybrid heterozygous F2 cross to illustrate.

What is the possibility of getting a heterozygous F2 offspring?

1/4 + 1/4 = 1/21/2 of the offspring

should be heterozygous.

DominanceDominance

There are varying degrees of dominance. Some characters are completely dominant to others. For instance, purple is completely dominant to white; round is completely dominant to wrinkled.

When you begin looking at things, there are varying forms of dominance.

There are varying degrees of dominance. Some characters are completely dominant to others. For instance, purple is completely dominant to white; round is completely dominant to wrinkled.

When you begin looking at things, there are varying forms of dominance.

Complete DominanceComplete Dominance

Mendel’s peas showed complete dominance. One trait was completely dominant to another (purple to white).

Mendel’s peas showed complete dominance. One trait was completely dominant to another (purple to white).

CodominanceCodominanceAnother extreme is codominance where

an organism has 2 different alleles that affect the phenotype in separate, distinguishable ways. A common example is with cystic fibrosis.

CF causes the patient’s body to produce a thick, sticky mucous that clogs airways and ducts leading from the pancreas to the intestine. This causes a whole host of problems.

Another extreme is codominance where an organism has 2 different alleles that affect the phenotype in separate, distinguishable ways. A common example is with cystic fibrosis.

CF causes the patient’s body to produce a thick, sticky mucous that clogs airways and ducts leading from the pancreas to the intestine. This causes a whole host of problems.

Cystic Fibrosis and Codominance

Cystic Fibrosis and Codominance

The CF gene is found on the long arm of c-some 7.

Codes for CFTR protein.

The CF gene is found on the long arm of c-some 7.

Codes for CFTR protein.

QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.

CFTR FunctionCFTR FunctionCFTR acts as an ion gate which

allows for the movement of Cl- in and out of the cell.

Patients with the CF gene make a dysfunctional protein which keeps the gate closed causing the Cl- to build up. The cell then produces a thick mucous in response to this causing the symptoms of the disease.

CFTR acts as an ion gate which allows for the movement of Cl- in and out of the cell.

Patients with the CF gene make a dysfunctional protein which keeps the gate closed causing the Cl- to build up. The cell then produces a thick mucous in response to this causing the symptoms of the disease.

Codominance at the Molecular Level


Most people have 2 normal copies of the allele for CFTR and make a functional CFTR protein.

People with CF have 2 mutant copies of the allele and produce only dysfunctional CFTR.

Heterozygotes produce one good copy and one bad copy.

Most people have 2 normal copies of the allele for CFTR and make a functional CFTR protein.

People with CF have 2 mutant copies of the allele and produce only dysfunctional CFTR.

Heterozygotes produce one good copy and one bad copy.



These heterozygotes produce enough functional CFTR protein to allow for normal Cl- transport and no adverse effects seen. Thus, even though the genes are codominant, symptoms remain recessive at the physiological level.

These heterozygotes produce enough functional CFTR protein to allow for normal Cl- transport and no adverse effects seen. Thus, even though the genes are codominant, symptoms remain recessive at the physiological level.

Incomplete DominanceIncomplete Dominance

Some alleles exhibit incomplete dominance--certain characteristics fall somewhere in between the phenotypes of the 2 homozygotes.For example: The flowering time of

Mendel’s peas and the color of certain flowers.

Some alleles exhibit incomplete dominance--certain characteristics fall somewhere in between the phenotypes of the 2 homozygotes.For example: The flowering time of

Mendel’s peas and the color of certain flowers.


Mendel knew he had peas that flowered shortly after germination and some that took a long time to flower.

When he crossed them, he found that their offspring produced flowers somewhere in between when the two homozygotes’ flowering time.

Mendel knew he had peas that flowered shortly after germination and some that took a long time to flower.

When he crossed them, he found that their offspring produced flowers somewhere in between when the two homozygotes’ flowering time.


With pink snapdragons, a red and a white will produce a pink flower--incomplete dominance. Why is it not “blending?”

With pink snapdragons, a red and a white will produce a pink flower--incomplete dominance. Why is it not “blending?”

Complete Dominance, Incomplete Dominance and Codominance--A

Summary

Complete Dominance, Incomplete Dominance and Codominance--A

SummaryIf you look at the organismal level

(outward phenotype based on alleles) vs. the biochemical level (the way the metabolism functions) vs. the molecular level (the proteins/enzymes that are made) can play a large role in determining complete dominance, incomplete dominance and codominance.

If you look at the organismal level (outward phenotype based on alleles) vs. the biochemical level (the way the metabolism functions) vs. the molecular level (the proteins/enzymes that are made) can play a large role in determining complete dominance, incomplete dominance and codominance.

Multiple AllelesMultiple Alleles

Thus far we have been talking about 2 alleles that govern certain traits. Often times there are multiple alleles that govern traits within a population.For example:3 alleles which code for 4 different

blood types.

Thus far we have been talking about 2 alleles that govern certain traits. Often times there are multiple alleles that govern traits within a population.For example:3 alleles which code for 4 different

blood types.

Pleiotropy Multiple Phenotypes

Pleiotropy Multiple Phenotypes

Most genes exhibit what is known as pleiotropy which is where one gene has multiple phenotypic effects.

Example: CF and sickle cell anemia

Most genes exhibit what is known as pleiotropy which is where one gene has multiple phenotypic effects.

Example: CF and sickle cell anemia

Gene Masking--EpistasisGene Masking--Epistasis

Epistasis occurs when one gene alters the phenotypic expression of another gene.

This example occurs in the coat color of mice.

Black, brown, albino

Epistasis occurs when one gene alters the phenotypic expression of another gene.

This example occurs in the coat color of mice.

Black, brown, albino

Polygenic InheritancePolygenic InheritanceThe opposite of pleiotrophy (one

gene, many characteristics) is polygenic inheritance which is the case where many genes act on a single characteristic.

For example: skin color is determined by at least 3 separately inherited genes. Variations of the genotype of these individuals produces all of the varieties of skin color we see.

The opposite of pleiotrophy (one gene, many characteristics) is polygenic inheritance which is the case where many genes act on a single characteristic.

For example: skin color is determined by at least 3 separately inherited genes. Variations of the genotype of these individuals produces all of the varieties of skin color we see.

Genetic DisordersGenetic Disorders

Many are recessive traits.Easily propogated because

heterozygotes don’t display outward characteristics--they are carriers.

Tay-Sachs, CF, sickle-cell

Many are recessive traits.Easily propogated because

heterozygotes don’t display outward characteristics--they are carriers.

Tay-Sachs, CF, sickle-cell

Genetic DisordersGenetic Disorders

Some disorders come from dominant alleles.

Dwarfism, Hunington’s Disease.Lethal dominants are much less

common because they are less likely to be passed through the gene pool--for obvious reasons.

Some disorders come from dominant alleles.

Dwarfism, Hunington’s Disease.Lethal dominants are much less

common because they are less likely to be passed through the gene pool--for obvious reasons.

Documents

Chapter 14 Mendelian Genetics. Important Terms Character--something that is inherited. Flower color Trait--a variant of a character. Purple flower