biology b. people used to think combining two parents’ characteristics was like mixing paint…
TRANSCRIPT
BASIC GENETICS
Biology B
BACK IN THE DAY…
People used to think combining two parents’ characteristics was like mixing paint…
GREGOR MENDEL
Monk and teacher. Experimented with purebred tall and
short peas.
Discovered some of the basic laws of heredity.
Studied seven purebred traits in peas. Presentation to the Science Society
in1866 went unnoticed. He died in 1884 with his work still
unnoticed. His work rediscovered in 1900. Known as the “Father of Genetics”.
MENDEL’S OBSERVATIONS
He developed pure strains of peas for seven different traits (i.e. tall or short, round or wrinkled, yellow or green, etc.)
He crossed these pure strains to produce hybrids.
He crossed thousands of plants and kept careful records for eight years.
MENDEL’S PEAS In peas many traits appear in two forms (i.e. tall or
short, round or wrinkled, yellow or green.) The flower is the reproductive organ and the male
and female are both in the same flower. He crossed pure strains by putting the pollen (male
gamete) from one purebred pea plant on the pistil (female sex organ) of another purebred pea plant to form a hybrid or crossbred.
MENDEL’S EXPERIMENTS
He experimentally crosses different strains to develop hybrids.
He then crossed the hybrids and analyzed the results.
DOMINANT TRAITS RULE
Strong Hereditary traits cover weak traits.
Mendel called stronger traits DOMINANT
Mendal called weaker traits recessive
Dominant traits are represented by capital letters (T) while recessive traits are represented by lower case letters (t).
MENDEL’S RESULTS
Mendel crossed purebred tall plants with purebred short plants and the first generation plants were all tall.
When these tall offspring were crossed the result was a ratio of 3 tall to 1 short.
MENDEL’S LAWS
Law of Inheritance: Inherited traits are determined by “factors” (genes) passed on from the parents.
Law of Dominance: When 2 or more options exist for a trait, one may be dominant.
Law of Segregation: Usually, each parent has 2 genes for each trait (1 from each of their parents) These are separated from each other when gametes (sex cells) are formed
Law of Independent Assortment: Genes for different traits are separated independently of one another.
BUT HOLD ON A MINUTE!!!
The Law of Independent Assortment is WRONG!!! It was a reasonable guess… but now we
know that genes are not inherited as separate pieces… They come in sets!
These sets are CHROMOSOMES – Collections of genes strung together.
CHROMOSOMES
How many do you have?46
Where do they come from?
23 from Mom, 23 from Dad
QUICK DEFINITIONS
Chromosome – structure made up of genes. Humans have 46, 23 from each parent
Gene – a string of DNA controlling one trait
Genotype – the genes one has… ie Bb for eye color Phenotype – what the genes produce. Brown eyes are part of
my phenotype (Physical = Phenotype)
Homozygous – Having two of the same gene for a trait… ie BB
Heterozygous – Having two different genes for a trait… ie Bb
Dominant – A gene that can mask another trait… Brown eyes are dominant to blue
Recessive – a gene that can be masked by a dominant gene