Genetics

Really about growth and development……

Heredity

• Heredity is biological inheritance– The set of characteristics an organism inherits

form its parents

• Genetics is the study of heredity

Gregor Mendel (1822-1884)

• Studied in Vienna• Taught in a High School (Brno, in Czech

Republic)• Worked in a garden• One of the first to study only one trait at a time• One of the first to analyze his data systematically• One of the first to use mathematics to analyze data• Did about 30,000 experiments

Mendel in the garden

• The true breeding stock of plants– Always passes its traits to the next generation

• Reproduction in pea plants– Pollen, male sex cells

– Ovum, female sex cells

– Seed is produced for new plant

– Normal process is self-pollination; one plant is the parent

• Cross fertilization, two plants as parents

Mendel in the garden

• Seven traits

• By limiting what traits he followed he could manage the data

• These traits are obvious, yes or no traits

• Look on page 310 of your textbook now

Mendel in the garden

• Crossing (short for cross fertilization)• Crossed the pollen and eggs of two true

breeding parents• The result was always the trait of only one

parent•  Look at page 310 again•  Understand the nomenclature P and F-1

generation

Mendel in the garden

• Crossing of the plants from F-1

• If Mendel had not done this he would have made no discovery

• The trait of the parent that did not show up in F-1 appeared in ¼ of the F-2

Mendel in the garden

• Merkmal– Some indivisible unit determined which trait

the plant displayed– This word is German for character– We call this a gene today (the unit that

determines traits)

Mendel in the garden

• Alleles– Mendel figured each plant has two copies of

each “character”– One from each parent– Today we say a gene has two alleles

Mendel’s Big Leap

• Dominant and recessive – Why does a plant with two characters for Tall (each

plant has two copies) grow to the same height as a plant with one copy of Tall (each plant got one copy of Tall and one copy of short from the plant with two copies of short)

• Mendel called the copy (allele) for tall the dominant character, the copy for short was the recessive character.

• Today we use the same terminology

Out of the garden

• Phenotype and Genotype• Phenotype is what you see when you look

at an organism, phenotype of the plants is what Mendel was looking at

• Genotype is the genetic composition, genotype is what Mendel was hypothesizing when he talked about characters and copies of characters

Out of the garden

• Today we say that an organism with identical copies of the alleles for a trait is homozygous for that trait (TT or tt, for Tall and short)

• Organisms with a mixed pair of alleles are heterozygous for the trait (Tt for tall)

• In modern terms Mendel crossed two plants, one homozygous for Tall (TT) one homozygous for short (tt) and the offspring (F-1) were all heterozygous (Tt) yet were all Tall.

Out of the garden

• Segregation– Mendel figured out that the two copies of the

characters must separate when the plant produced pollen and eggs

• Today we call this segregation of alleles– TT and tt crossed produce Tt – Tt and Tt crossed produce TT, Tt, Tt and tt

Out of the garden

• Independent assortment– Mendel followed two traits at the same cross– The offspring showed distinct ratios of the

different combination so he reasoned that the characters behave independently

Mendel’s contributions

• Individual units called genes determine biological characteristics

• For each gene, an organism receives one allele from each parent.

• The alleles separate form each other (segregation) when the reproductive cells are formed

• If an organism inherits different alleles for some trait one allele may be dominant over the other

• Some genes segregate independently