More Mendelian genetics

Real Biologists of Genius

• We salute you Mr. Gregor Mendel. An Austrian monk with a love for peas, you published data that showed blending inheritance was incorrect and introduced hereditary factors occurring in discrete pairs.

Mendelian Genetics

• Mendel knew that his 'factors' were discrete and non-blending. 

• He also knew much more about the behavior of these units of inheritance.

• So let’s revisit his peas!

Law of Segregation

• Mendel's First Law (Law of Segregation): Mendel determined that each individual has two copies of each gene (e.g., Pp).

• These copies are called alleles.  If both alleles are the same, then the individual is homozygous (e.g., PP or pp). 

• If the two alleles are different, then the individual is heterozygous (e.g., Pp).

• When an individual creates gametes (sex cells: egg or sperm in humans, egg or pollen grain in plants), only one of each allele is packaged in the gamete. 

• Mendel determined that which allele appears in the gamete is random, with each allele having a 50% chance.  This rule is the Law of Segregation.

Flower color

• Pea flowers are either purple or white.

• Peas fertilize themselves, so

• white white and purple purple.

• called true-breeding• But…

• …if you cross a true-breeding purple with a true-breeding white…

• …all of the offspring have purple flowers.

• Hence Mendel said that purple was dominant to white.

• PP: purple• pp: white• Pp: purple!

Terms to understand

• gene: stretch of DNA that codes for a particular trait. (e.g., flower color)

• allele: a particular variant of a gene (e.g., purple)

• genotype: what alleles an individual has for a particular trait or set of traits (e.g., Pp)

• phenotype: the expression of the genes; what the individual looks like (e.g., purple)

• dominant trait: an allele that is expressed no matter what the other allele is (e.g., purple flower color being dominant to white flower color in pea plants)

• recessive trait: an allele that is only expressed if it is the only allele present (i.e., both alleles are the same) (e.g., white flower being recessive to purple flower color)

Terms to understand

• homozygous: has 2 copies of the same allele for a given trait (e.g., PP or pp)

• heterozygous: has 1 copy of each of two alleles for a given trait (e.g., Pp)

• F1 generation: the kids of the parents

• F2 generation: the grandkids of the parents (kids of F1)

• gamete: sex cell (egg or sperm); only has ONE allele for each gene since it only has one homologous chromosome (either the one you received from Mom or the one you received from Dad)

• True-breeding: homozygous for the trait.

Forming gametes

• How many different gametes can PP make?

• 1• P• How many different

gametes can Pp make?

• 2• P or p

• When forming gametes, you always need one allele for each gene.

• How many different gametes can PPTt make?

• 2• PT or Pt

Determining the number of different gametes possible

• AaBBCc?• 2 x 1 x 2 = 4• AaBbCC?• 2 x 2 x 1 = 4• AaBbCcDd?• 2 x 2 x 2 x 2 = 16• AAbbCCddEE?• 1 x 1 x 1 x 1 x 1 = 1• What is it?• AbCdE

• Which of the following gametes can this parent (AABbCCDdeeFf) make? a. AAbCEf b. ABCDEF c. abcdef d. ABCdef

• d is the answer.• What is the chance of

that parent producing that gamete?

• 1/8 Why?

Determining the number of different alleles

• AaBBCc?• 2 + 1 + 2 = 5 alleles• AaBbCC?• 2 + 2 + 1 = 5 alleles• AaBbCcDd?• 2 + 2 + 2 + 2 = 8• AAbbCCddEE?• 1 + 1 + 1 + 1 + 1 = 5

How many different genes are shown at right?

• 3, 3, 4, and 5 (top to bottom)

Other terms not on the handout

• Incomplete dominance: in this case, the presence of a single gene to code for a particular protein (enzyme) is insufficient to produce the full trait.

• Why?• Because you don’t have

enough of the enzyme to fully express the trait!

Ex. In snapdragons, • RR = red, • rr = white,• Rr = pink!

Incomplete Dominance

Co-dominant alleles: Human ABO blood type

• There are 2 dominant alleles (A and B) and one recessive (O).

• A and B alleles determine sugars present on cell membrane of red blood cells.

• If you have A, then you produce type A sugars.

• If you have B, then you produce type B sugars.

• If you have O, then you produce no sugars.

Possible PossibleGenotypes Phenotypes

AA type A AO type A

BB type B BO type B AB type AB OO type O

Transfusions

• When you need a blood transfusion, they try to match blood types.

• If you give type A blood to someone without type A blood, they have no type A blood sugars on their own red blood cells so their immune system will attack the transfused blood because it recognizes that it is foreign. 

• While they try to give type A blood to a person with blood type A, type O could also be used.

• Why? Because there are no blood sugars in type O blood that the type A person’s body hasn’t seen.

• Therefore, type O is called the universal donor and type AB is the universal recipient.

What about positive and negative?

• That’s a different gene.• The Rh factor is another

sugar on red blood cells.• It’s called Rh for Rhesus, as

it was first found in a Rhesus monkey.

• You are Rh positive if you have the blood sugar, but Rh negative if you do not.

• Thus the ultimate donor is?• O negative• Ultimate recipient?• AB positive

What are the relative frequencies of these blood types in humans?

• O Positive 37% • O Negative 6% • A Positive 34% • A Negative 6% • B Positive 10% • B Negative 2% • AB Positive 4% • AB Negative 1%

Some More Terms

• Monohybrid cross: cross between two monohybrids (only a single trait is tracked) (e.g., Pp x Pp)

• Dihybrid cross: cross between two dihybrids (e.g., PpYy x PpYy).

Dihybrid Cross

Some More Terms

• Pleiotropic: when a single gene determines more than one phenotype for an organism (gene that lengthens bones lengthens legs and arms).

• Gene for sickle cell affects vulnerability to malaria and sickle cell anemia.

Polygenic traits

• A trait that is affected by multiple genes

• These traits are not discrete (yes or no) but show continuous variation.

• E.g. skin color, height, etc.

Test Cross

• Test cross: When a single trait is being studied, a test cross is a cross between an individual with the dominant phenotype but of unknown genotype (homozygous or heterozygous) with a homozygous recessive individual. If the unknown is heterozygous, then approximately 50% of the offspring should display the recessive phenotype.