Mendel and GeneticsTerms and Protocols

Mendel’s ExperimentsProbability

Modern Additions & ModificationsMendelian Genetics and Humans

Terms and Protocols• In any study of genetics there are

some terms you need to absolutely know

• P represents the parental generation• F1 represents the first generation

produced by the parental generation• F2 represents the next generation

(often produced by breeding two members of the F1 generation together)

Terms and Protocols(cont)• Alleles-contrasting forms of a gene

located on homologous chromosomes• Gametes-sperm and egg• Heterozygous- zygote or individual

whose genotype contains 2 different or contrasting forms of an allele (like a gene for blue eyes and a gene for brown eyes)

• Homozygous- zygote or individual with 2 identical genes for a trait.

Terms and Protocols (cont)• Phenotype- the expression of genes or

appearance of an individual (blue eyes)• Genotype-the genes or alleles an

individual has• Dominant- an allele that fully expresses

itself whether it is homozygous or heterozygous

• Recessive-an allele that will only be fully expressed if there are two recessive alleles in the individual’s genotype

Terms and Protocols(cont)• Monohybrid-a type of genetic cross

involving only one trait (ex. Eye color)• Dihybrid – a type of genetic cross involving

two traits(ex. Eye color and hair color)• Testcross- a type of cross in which an

individual of unknown genotype is crossed with a homozygous recessive individual to determine its genotype

• Punnett Square-a problem-solving device used in genetics to predict outcomes of crosses.

Terms and Protocols (cont)• The set-up of a Punnett Square is based on

the events of Meiosis. The genetic composition of each possible gamete that might be made by each parent is placed on each side of the square. Then you put the gametes together to determine the genotype of possible offspring.

• When you are finished you can predict the percentage of offspring with a particular trait by using the ratios you obtain.

Mendel’s Observations• Alternative versions of genes account for

variety in organisms• For each trait, an organism inherits two

alleles, one from each parent• If an individual has two different alleles

(heterozygous)then the dominant allele will be expressed and the recessive will have no noticeable effect.(Law of Dominance and Recessiveness)

• The two alleles separate during meiosis.(Law of Segregation)

Mendel’s Observations (cont)

• Each pair of alleles segregates into gametes independently(Law of Independent Assortment)

Probability in Genetics• Rule of Multiplication-Take the

probability for each individual event and multiply the individual probabilities to get the overall probability that these events will occur together.

• Rule of Addition- take the ways an individual result can occur and add them to get the probability that a particular genotype may result.

• See pages 254-5

Modern Additions & Modifications• Mendel did not know about genes or

the complexities of molecular biology and still made discoveries that are used today to explain how traits are passed from one generation to the next.

• The more we learn, however, the more we realize that the relationship between genotype and phenotype is not a simple one.

Incomplete Dominance• Where hybrids have an appearance

somewhere in between the phenotypes of the two parents.

• Example: Snapdragons that are heterozygous do not make enough red pigment to make their flowers red. They are pink. So only homozygous are red or white. This is called Incomplete Dominance.

Codominance• Occurs when the alleles affect the

phenotype in separate but distinguishable ways.

• Example: M and N Blood Groups• Each allele produces surface molecules

on the red blood cell.• M produces 2 surface molecules and N

produces 2 different surface molecules• If a cell has both M and N the two surface

molecules are different (LMM, LNN, and LMN)

Multiple Alleles• Where there are more than two alleles• Example: Human Blood Types• Type A blood has a certain carbohydrate

on the surface of its red blood cells.• Type B has a different carbohydrate on

the surface of its red blood cells.• Type O has no carbohydrate on the

surface of its red blood cells• Therefore there are 4 distinguishable

blood types

Polygenic Inheritance

• Traits vary in a population along a continuum

• Traits are determined by several genes and the genes have an additive effect

• Example: skin color in humans which is probably controlled by at least three genes

Environmental Effects on Phenotypes

• Phenotype depends on environment as well as genes

• For example, if a person is genetically destined to be tall, poor nutrition will affect how tall they become. Without proper nutrition, growth is impossible.

• Acidity in the soil affects the color of Hydrangea flowers

Mendelian Inheritance in Humans

• There are many traits in humans that follow simple dominant and recessive inheritance patterns. But humans do not produce offspring in large numbers so determining genotypes is not so simple

• Geneticists use pedigrees which focus on particular traits over several generations to determine genotypes of family members

• Examples are on the next slide

Inherited Disorders in Humans

• Recessive disorders: Cystic Fibrosis Tay-Sachs Disease Sickle-Cell Disease• Dominant disorders: Huntington’s Disease

Fetal Testing• Recognition that two carriers for recessive

disorders have a much greater chance of producing offspring with serious disorders has led to the development of fetal testing and genetic counseling

• Amniocentesis uses a sample of amniotic fluid to test for disorders in the fetus

• Chorionic Villi Sampling takes fetal tissue from the placenta for the same purpose