Chapter 12 Mendelian Genetics

Important Terms  Character--something that is inherited.  Flower color  Trait--a variant of a character.  Purple flower vs. white flower  Allele--an alternative version of a gene.  Brown hair or blond hair.  Character--something that is inherited.  Flower color  Trait--a variant of a character.  Purple flower vs. white flower  Allele--an alternative version of a gene.  Brown hair or blond hair.

Important Terms  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.

More 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--blue eyes.  Genotype--the genetic makeup of an organism--QQ.  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--blue eyes.  Genotype--the genetic makeup of an organism--QQ.

Important Terms  True breeding--describes an organism that is homozygous--either dominant or recessive, for example TT, or tt.  Heterozygous--describes the genotype of an organism that has a dominant and a recessive allele, for example Tt.  Monohybrid--cross of one character.  Dihybrid--cross of two characters.  True breeding--describes an organism that is homozygous--either dominant or recessive, for example TT, or tt.  Heterozygous--describes the genotype of an organism that has a dominant and a recessive allele, for example Tt.  Monohybrid--cross of one character.  Dihybrid--cross of two characters.

Important Terms  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.

Important Term: Test Cross  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.  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.

Gregor Mendel  He studied garden peas.  What made Mendel’s work so good was that he kept excellent records of what he did and the results of his experiments.  He studied garden peas.  What made Mendel’s work so good was that he kept excellent records of what he did and the results of his experiments.

Mendel  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.

Mendel  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.

Mendel  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.

Alleles  These heritable factors are what we now call genes.  Minor differences in the DNA account for the different versions of these genes which we call alleles.  These heritable factors are what we now call genes.  Minor differences in the DNA account for the different versions of these genes which we call alleles.

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

Law 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.

Law of Independent Assortment  Mendel demonstrated this using a dihybrid cross.

The 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.

Conclusions  From the cross, Mendel concluded that no matter how many characteristics are observed, they always separate independently of one another when the traits are on different chromosomes.

The Law of Independent Assortment  As a result of the dihybrid cross, Mendel arrived at what is known as the Law of Independent Assortment.  It says that all alleles of a gene pair will separate independently of other alleles from other gene pairs during gamete formation.  The “green/yellow” alleles will separate independently from the “round/wrinkled” gene pair.  As a result of the dihybrid cross, Mendel arrived at what is known as the Law of Independent Assortment.  It says that all alleles of a gene pair will separate independently of other alleles from other gene pairs during gamete formation.  The “green/yellow” alleles will separate independently from the “round/wrinkled” gene pair.

2 Rules of Probability  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 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 Rule  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) = 1/6  P(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) = 1/6  P(even) = 3/6  The probability of rolling a 4 and an even is 1/6 3/6 = 3/36, or 1/12.

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

The Addition Rule  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) = 4/52  P(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) = 4/52  P(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 Rule  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 = 1/2  1/2 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 = 1/2  1/2 of the offspring should be heterozygous.

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

Codominance  This is where both alleles are expressed.  MN blood group is the example.  There are 2 variations of alleles at the gene locus.  MM individuals have “M” proteins on their surface.  NN individuals have “N” proteins on their surface.  MN have both “M” and “N” proteins on their surface.  There is no intermediate phenotype.  This is where both alleles are expressed.  MN blood group is the example.  There are 2 variations of alleles at the gene locus.  MM individuals have “M” proteins on their surface.  NN individuals have “N” proteins on their surface.  MN have both “M” and “N” proteins on their surface.  There is no intermediate phenotype.

Incomplete Dominance  Some alleles exhibit incomplete dominance--certain characteristics fall somewhere in between the phenotypes of the 2 homozygotes.  For example: pink snapdragons.  Some alleles exhibit incomplete dominance--certain characteristics fall somewhere in between the phenotypes of the 2 homozygotes.  For example: pink snapdragons.

Incomplete Dominance  With pink snapdragons, a red and a white will produce a pink flower-- incomplete dominance.  Less red pigment is made and an intermediate phenotype is seen.  Why is it not “blending?”  With pink snapdragons, a red and a white will produce a pink flower-- incomplete dominance.  Less red pigment is made and an intermediate phenotype is seen.  Why is it not “blending?”

Multiple 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.  AB blood is also a case of codominance.  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.  AB blood is also a case of codominance.

Polygenic Inheritance  Polygenic inheritance 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.  Polygenic inheritance 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.

Pleiotropy  Pleiotropy is where one gene has many characteristics features.  CF is caused by a mutation in one gene and causes all sorts of problems-thick mucous in the lungs, digestive problems, excessively salty skin, etc.  Pleiotropy is where one gene has many characteristics features.  CF is caused by a mutation in one gene and causes all sorts of problems-thick mucous in the lungs, digestive problems, excessively salty skin, etc.