Chapter 13: Mendel and the Basics of Genetics

Terminology

• Chromosome: structure containing a single, linear DNA molecule that physically transmits hereditary info from one generation to the next.

• Chromatin: the DNA and protein of the chromosome

• Nucleosome: a bead-like structure made of a single DNA molecule spooled around a histone protein core (necessary for fit in the nucleus)

Terminology

• Gene: sequence of DNA nucleotides (sugar, phosphate, nitrogenous base) on a chromosome that encodes a specific protein

• Alleles: alternative forms of a gene (Ex. brown vs. black for hair color)

• Codon: 3 DNA nucleotides in sequence that code for a particular amino acid (combine to form proteins)

Gregor Mendel

• Austrian Monk – Background in mathematics and botany– 1831 to 1853 – Studied inheritance patterns in peas.

• Discovered that:– Heritable factors called traits were passed on from

one generation to the next.– Traits are carried on chromosomes in structures

called genes.• Different forms of the gene are called alleles.• Autosomes: 22 matched chromosomes• Sex Chromosomes: pair #23 (XX, XY)

The Experiment

• Mendel cross pollinated pea plants and followed their traits.

• Stamen – male organ of the pea plant– Produces pollen - the male gamete– Pollen produced at the terminus of the stamen in a structure

called the anther.

• Carpel – female organ of the pea plant (pistil plus ovary)– Produces the ova – female gamete.

• Union of pollen and ova creates a fertilized egg called the zygote which develops into a seed and eventually grows into an embryo.

– Mendel used True Breeders• Self pollinating plants that produced clones of themselves.

What did Mendel Find?

• Mendel found that in the F1 generation (first filial) all of the flowers were the same color.

• There was no blending of parental characteristics.• Only one trait was expressed in the hybrid offspring.

• When he crossed two flowers from the F1 generation he found a 3:1 ratio of purple to white flowers.

• This indicated that one trait was dominant over the other trait.

Mendel’s Two Laws of Heredity

• #1. Law of Segregation: members of each pair of alleles separate when gametes are formed (get ONE allele from mom and ONE allele from dad for each trait)

• #2. Law of Independent Assortment: when two or more pairs of alleles are located on different chromosomes or far apart on the same chromosome, they separate independently of one another during gamete formation (just cause you got mom’s eye color does not mean you will also get her hair color, even if found on her same chromosome)

Mendel Made Four Assumptions

• 1. There are alternative forms of heritable factors called alleles • 2. The offspring inherit an allele from each parent.• 3. The alleles separate during meiosis (Law of Segregation)• 4.One allele can be masked by the presence of another allele. • Phenotype

– Physical characteristics• Purple flowers

• Genotype– Genetic composition

• The genes on that particular locus on the chromosome. – Locus is location of the gene on the chromosome.– Heterozygous

» Different alleles Aa– Homozygous

» Same alleles AA

Crosses and Following Traits• Test Cross

– Used to find the genotype of an unknown individual– Cross with a homozygous recessive to reveal the unknown genotype.– If all white, unknown must be homo recessive– If get some white, unknown must be hetero

• Monohybrid Cross– One trait is followed (generations)

• Dihybrid Cross– Two traits are followed (generations)– According to the laws of probability and if all alleles segregate independently

then when crossing two hetero parents (for both traits) there should be a 9:3:3:1 phenotypic ratio in the offspring.

• 9 dominant/ dominant• 3 dominant /recessive• 3 recessive/dominant• 1 recessive/ recessiveEXPECTED, BUT NOT NECESSARILY OBSERVED!

Not All Traits Exhibit Dominance and Recessive Characteristics

• Incomplete Dominance– The F1 hybrids express a phenotype that is

somewhere in between the phenotypes of the parents.

• Snap dragons only express half the red pigment and appear pink.

• Human hypercholesterolemia: the heterozygote have ½ the LDL receptors and thus have high cholesterol.

Incomplete Dominance

• Codominance:– Both alleles are equally expressed

Ex. Black feathered chickens and white feathered chickens when mated can produce chicks with BOTH white feathers and black feathers.

Multiple Alleles

• More than two alleles may exist for one trait (not SOLELY a dominant and a recessive)• Both alleles may be codominant

– Both alleles are equally expressedEx. Blood types are determined by proteins present on

the surface of the blood cell. – Types:

» IA i or IA IA A blood type

» IB i or IB IB B blood type

» IA IB AB blood type (universal receiver)

» i i O blood type (universal donor) BOTH PARENTS MUST BE TYPE O or at least heterozygotes for type A or B TO HAVE A TYPE O CHILD

Rh factor

• Rh factor is an additional antigen found on the red blood cells and it is a separate gene from the ABO gene.

• If a person has two (+) genes for Rh or one (+) and one (-) they will test (+).

• A person will be (-) ONLY if they have two (-) because Rh(+) is dominant.

• Positive means you have the Rh antigen, negative means you don’t.

• Other minor antigens include Kell, Lewis A, Lewis B, rho, P etc. and doctors also attempt to match as many of those minor antigens as possible when selecting blood for transfusions.

Blood Types

• Types: Red blood cells: Plasma:– IA i or IA IA A: A antigen B antibodies– IB i or IB IB B: B antigen A antibodies– IA IB AB: A and B antigens NO antibodies– i i O: NO antigens A and B antibodiesUniversal donor: Type O-negative (no antigens) is

compatible with all blood types because it has no antigens for other blood types to recognize with their antibodies

Universal Recipient: Type AB-positive has no antibodies in its plasma so it can accept any type without destroying those foreign cells

Pleiotrophy

• The inheritance of a single gene can have multiple effects on the individual.Ex. Sickle Cell Anemia

Causes defective hemoglobin molecules but that single gene can ALSO cause:

1. heart failure

2. anemia

3. susceptibility to pneumonia

4. kidney failure

5. enlarged spleen

Sickle Cell Anemia

Epistasis

• An allele on one chromosome can affect the expression of the allele on another chromosome.– Example

• Epistasis occurs in mice.• One allele determines the coat color.

– Dominant B for black– Recessive b for brown

• The other allele determines whether or not pigment is deposited at all.

– Dominant C for pigment deposition.– Recessive c for no pigment deposition

Epistasis

Polygenic Inheritance

• More than one copy of the allele determines the degree in which the allele is expressed.

• Pigment deposition in human skin cells.• Dominant alleles cause the pigment melanin to

be deposited.– Multiple copies of the dominant allele cause more

melanin to be deposited.– AABBCC – very dark and aabbcc very light.– AaBbCc is intermediate shade

Environmental Factors

• The environment can have an impact on phenotypic characteristics.– Exercise changes the build of a person.

• The product of a genotype is not rigidly defined by a phenotype but rather a range of possibilities.– This range is known as the norm of the reaction.

• Blood: a particular locus may determine the blood type, however the number of blood cells from one individual to the next varies and this affects physical fitness at certain altitudes.

Ex. In Hydrangea an acidic soil produces a pink flower

Pedigree Charts

• Pedigrees are used to track the inheritance patterns of previous generations to predict the future to determine what characteristics are likely to be inherited in future generations.

• Probabilities and Mendelian genetics are used to determine the genotypes of various family members

• Square=Male Circle=Female

Recessive Disorders

• The allele is recessive and in most cases codes for a protein that is malfunctioning.

• The result can be a syndrome such as– Tay Sachs Disease

• Malfunctioning protein that breaks down lipids in the brain • Higher occurrence in Ashkenazic Jews due to ancestral

heritage.• Infant experiences seizures, blindness and degeneration of

motor and mental performance.

• The heterozygote is the carrier.– May produce enough normal protein to compensate

and the individual is normal.

Penetrance

The proportion of individuals who show the phenotype expected from their genotype.

• 100% means all individuals with genotype show phenotype.

• Tay-Sachs disease shows complete or 100% penetrance as all homozygote for the allele develop disease and die.

Expressivity

• The degree to which a particular gene is expressed in individuals showing the trait.– Example Retinoblastoma ( type of eye tumor)– Not all individuals who inherit the allele

develop the tumor ( incomplete penetrance) and in those who do develop the tumor the severity varies.

Dominant Allele Disorders

• The heterozygote has the disorder.• Dominant Lethal disorder

– Huntington’s disease• Eventual deterioration of the nervous system.• Remains in the gene pool because the onset is in

individuals that are 35 years or older.• Any child born to the parent carrying this allele has

a 50% chance of having the disease.

AchondroplasiaA type of dwarfism

Nondisjuntion

• Non-disjunction:

Failure of the chromosomes to separate properly

• Ex. Extra chromosome #21 = Downs Syndrome

• XXY= Klinefelter

• XYY= Jacob’s Syndrome

• Single X (XO)= Turner Syndrome

Multifactorial Disorders

• Heart disease, cancer, diabetes manic depression, schizophrenia and alcoholism– Increased propensity due to heredity can be

circumvented by diet, exercise and behavior.

• Carriers for certain diseases can be identified through the genetic testing.– Genetic counseling may help couples decide

if they want to have children based on the probability the disorder will be inherited.

Fetal Testing• Amniocentesis

– Amniotic fluid is extracted from the womb during the 14th to 16th week of pregnancy.

– The presence of certain chemicals can determine whether or not a particular genetic disorder is present immediately.

• Cystic Fibrosis– Cells can be grown in vitro for 2 weeks and chromosomal abnormalities

can be determined through a karyotype.• Down syndrome• Tay Sachs

– Chorionic Villis Sampling (CVS)• A needle is inserted into the cervix and placental (fetal) fluid is extracted.

– Karyotype can be done in 24 hours– Can be performed in the 8th week of pregnancy– More risky than amniocentesis– Less available than amniocentesis– Cannot be done to look for abnormalities in amniotic fluid.

What genetic disorder will this patient have?