classical and modern genetics. “genetics”: study of how biological information is carried from...

Classical and Modern Genetics


“Genetics”: study of how biological information is carried from one generation to the next

– Classical• Laws of inheritance developed from

observations of Gregor Mendel (1800’s)

– Modern• Studies of how genes pass information from

parent to offspring on the basis of molecular chemistry (1950 to present).


Classical (beginning in the 1800’s)

– Mendel observed traits in pea plants being passed from parent to offspring via “unit of inheritance”

– Observed traits could be dominant or recessive• Dominant traits expressed even if only present in one

parent • Recessive traits expressed only if neither parent has

the dominant trait

– Tested crosses in pea plants• Tested in both 1-trait and 2-trait crosses


When studying genetic crosses must distinguish between genotype and phenotype for a particular trait:– Example: tall (dominant) / short (recessive)

genotype phenotype

The actual pair of genes for a particular trait

Visual expression of a genotype

One gene from each parent Reflects the dominant trait if it is present

Either pure (both genes the same) or hybrid (one of each)

Reflects the recessive trait only if both genes are recessive

TT = pure for tall

Tt = hybrid for tall

tt = short

Looks tall

Looks tall

Looks short


Mendel’s test crosses - Cross-pollinated purebred and hybrid plants to observe offspring

• If parents are pure for a single trait:

– Female: TT

– Male: tt

• All four possible offspring in first generation look “tall” but are hybrid Tt

• If cross hybrids above:

– Female: Tt

– Male: Tt

• 3 of 4 possible offspring in second generation look tall and 1 of 4 looks short. One of tall is pure TT, Two of tall are hybrid Tt, The one short is pure tt.

T T

t Tt Tt

t Tt Tt

T t

T TT Tt

t Tt tt


Modern (beginning in the 1950’s)

– Mendel’s “units of inheritance” recognized as “genes”

– Genes located on chromosomes– Chromosomes present in pairs in

nucleus of eukaryotic cells– Chromosomes contain nucleic acids

which code for all inheritance


Human Chromosomes

– A total of 46 chromosomes (23 pairs)

– All cells have 46 chromosomes except reproductive cells

– Reproductive cells have just 23 – one member of each pair

– 22 pairs are “autosomes” and 1 pair is the “sex chromosome pair”

– Males have the sex chromosome XY, Females have XX


Nucleic Acids– 4th category of organic molecules– Include DNA, RNA, (and ATP)– Large polymers made from chain of

monomers– Monomer unit = nucleotide

• Sugar• Phosphate group• Base


DNA– Double strand in helix formation– Described by Watson and Crick (1953)– DNA nucleotides contain

• Sugar– deoxyribose

• Phosphate group• Base

– A, Adenine– T, Thymine– C, Cytosine– G, Guanine

– Base pairs include• A-T, T-A, C-G, G-C

– Sequence of bases determines genetic code


RNA– Uses code from DNA to direct production of

protein by the cell– Single strand nucleic acid– Nucleotide contains

• Sugar– Ribose

• Phosphate group• Base

– A, Adenine– U, Uracil (instead of thymine as found in DNA)– C, Cytosine– G, Guanine

– Three different types: mRNA, tRNA, rRNA


Transcription of DNA message

– Method by which coded DNA message is read by RNA

• mRNA (messenger RNA) copies a specific DNA sequence and carries it from the nucleus to the ribosomes


Translation of message into a protein

– A “codon” (set of three bases) on the mRNA is read by a tRNA (transfer RNA) which picks up the proper amino acid for the code and brings it to the ribosome

– Protein synthesis (joining the correct amino acids together) occurs at the ribosomes


Definitions– Genetic code

• Correspondence between base pair sequences and amino acids

– Mutation• Error in the coded sequence

– Genome• Complete description of an organisms genetic code

– Mapping• Position of every gene on every chromosome

– Sequencing• Exact order of base pairs on every gene

Human genome has 3 billion base pair sequences!

classical and modern genetics. “genetics”: study of how biological information is carried from...

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