Genetics
Mendelian Inheritance:
● Inheritance of 2 traits (dihybrid crosses)
● 2 traits: phenotype: smooth/yellow, wrinkled/green
- Genotype: SSYY, ssyy
Mendel’s Conclusions from one-trait crosses:
● Two independent units (“gene”) for each trait so for each trait (shape or color) there are
two forms of the gene – we call these two forms of the gene alleles.
● The 2 alleles separate from each other during pollen and egg formation –this is called
segregation (during meiosis)
● And the 2 alleles join in random combinations during fertilization.
● Alleles can be present but not expressed
- The unexpressed trait is recessive
- The expressed trait is dominant
● Phenotype is not equal to genotype: despite identical appearance genotype can be
different
● The reason for independent assortment is that the
● Genes for the traits are located on different chromosomes that segregate independently
- On separate chromosomes
- Inherited independently
New combinations are due to independent assortment vs. Genetic linkage
● The reason for independent assortment is that the genes for the traits are located on
different chromosomes that segregate independently
- On separate chromosomes
- Inherited independently
● This is in contrast to “genetic linkage” where the genes are located on the same
chromosome and can’t separate/segregate
- Traits always inherited together
- One one – the same chromosome
Basic/Simple Mendelian Inheritance is not always seen
● Genes that are linked do not show independent assortment
- Red hair/light complexion
● Many genes have more than 2 alleles
- Blood type - 3 alleles A, B, O; genotypes AA BB AB AO BO OO
- HLA more than 2000 alleles
● Some alleles are codominant
- Blood type AB is an example
● Incomplete dominance
- In peas Red x White = pink flowers
● Epistasis: when the presence of another gene alters the expression of another gene
, ● Epigenetics: when environmental factors have an effect on gene activity
● Many traits due to multiple genes (multigenic)
- heart disease, high blood pressure
● Many traits are multifactorial and interact with the environment
- diabetes (environmental influence on trait – diet)
● Mitochondrial inheritance:
- Genetically ‘semi-autonomous’
- DNA genome, ribosomes (related to bacterial ribosomes)
- Diseases due to mutations in mitochondrial DNA (optic neuropathy,
cardiomyopathy, Leigh syndrome (brain degeneration))
- Exhibits Maternal inheritance because all of our mitochondria are inherited from
the egg (~100,000)
- Sperm: ~50 mitochondria
● Some traits are sex-linked – located on X-chromosome or Y color blindness, hemophilia,
others
Experimental Model Organisms used for Genetic Studies:
● Purpose is to define genes and components involved in processes such as:
- Inheritance patterns of phenotypes, diseases
- Different cell functions
- Development
- Behavior
- Diseases
● An ideal model organism:
- Short life cycle, large number of offspring, controlled crosses, simple phenotypes,
easy and inexpensive to grow
● Most widely used at model is the bacteria: E. coli (prokaryote)
- Easy to grow, rapid life cycle, and we can control their environment
- One of the first model systems we used to study cells and gene regulation
● Bacteriophage T4: more simple
- Viruses that infect bacteria
- Floats in water and lands on surfaces of bacteria, and injects DNA from its head
into the bacteria
- Releases 100s of new viruses into bacteria
- Phage therapy (PT) uses viruses to treat bacterial infections
● Yeast (eukaryote): grows like bacteria, single cell organism
- More similar to human cell (has nucleus)
- Used as model for cell cycle
- Genes for cell division and signaling exchangeable with human
- Human genes can replace the yeast genes
● Human cells: most derived from cancers
Mendelian Inheritance:
● Inheritance of 2 traits (dihybrid crosses)
● 2 traits: phenotype: smooth/yellow, wrinkled/green
- Genotype: SSYY, ssyy
Mendel’s Conclusions from one-trait crosses:
● Two independent units (“gene”) for each trait so for each trait (shape or color) there are
two forms of the gene – we call these two forms of the gene alleles.
● The 2 alleles separate from each other during pollen and egg formation –this is called
segregation (during meiosis)
● And the 2 alleles join in random combinations during fertilization.
● Alleles can be present but not expressed
- The unexpressed trait is recessive
- The expressed trait is dominant
● Phenotype is not equal to genotype: despite identical appearance genotype can be
different
● The reason for independent assortment is that the
● Genes for the traits are located on different chromosomes that segregate independently
- On separate chromosomes
- Inherited independently
New combinations are due to independent assortment vs. Genetic linkage
● The reason for independent assortment is that the genes for the traits are located on
different chromosomes that segregate independently
- On separate chromosomes
- Inherited independently
● This is in contrast to “genetic linkage” where the genes are located on the same
chromosome and can’t separate/segregate
- Traits always inherited together
- One one – the same chromosome
Basic/Simple Mendelian Inheritance is not always seen
● Genes that are linked do not show independent assortment
- Red hair/light complexion
● Many genes have more than 2 alleles
- Blood type - 3 alleles A, B, O; genotypes AA BB AB AO BO OO
- HLA more than 2000 alleles
● Some alleles are codominant
- Blood type AB is an example
● Incomplete dominance
- In peas Red x White = pink flowers
● Epistasis: when the presence of another gene alters the expression of another gene
, ● Epigenetics: when environmental factors have an effect on gene activity
● Many traits due to multiple genes (multigenic)
- heart disease, high blood pressure
● Many traits are multifactorial and interact with the environment
- diabetes (environmental influence on trait – diet)
● Mitochondrial inheritance:
- Genetically ‘semi-autonomous’
- DNA genome, ribosomes (related to bacterial ribosomes)
- Diseases due to mutations in mitochondrial DNA (optic neuropathy,
cardiomyopathy, Leigh syndrome (brain degeneration))
- Exhibits Maternal inheritance because all of our mitochondria are inherited from
the egg (~100,000)
- Sperm: ~50 mitochondria
● Some traits are sex-linked – located on X-chromosome or Y color blindness, hemophilia,
others
Experimental Model Organisms used for Genetic Studies:
● Purpose is to define genes and components involved in processes such as:
- Inheritance patterns of phenotypes, diseases
- Different cell functions
- Development
- Behavior
- Diseases
● An ideal model organism:
- Short life cycle, large number of offspring, controlled crosses, simple phenotypes,
easy and inexpensive to grow
● Most widely used at model is the bacteria: E. coli (prokaryote)
- Easy to grow, rapid life cycle, and we can control their environment
- One of the first model systems we used to study cells and gene regulation
● Bacteriophage T4: more simple
- Viruses that infect bacteria
- Floats in water and lands on surfaces of bacteria, and injects DNA from its head
into the bacteria
- Releases 100s of new viruses into bacteria
- Phage therapy (PT) uses viruses to treat bacterial infections
● Yeast (eukaryote): grows like bacteria, single cell organism
- More similar to human cell (has nucleus)
- Used as model for cell cycle
- Genes for cell division and signaling exchangeable with human
- Human genes can replace the yeast genes
● Human cells: most derived from cancers
