Intro to Mendelian Genetics

Gregor Mendel
• Austrian monk and gardener
• Significant work done in
1850’s
• Father of genetics (study of heredity)
• Heredity = passing on of traits from parent to offspring
• Traits = characteristics that are inherited (ex. eye color)

Mendel’s First Experiment
P (Parent) Generation
X
Purebred tall (≥ 6 ft.) Purebred short (≤ 2 ft.)
↓ Cross - pollinate

Mendel’s First Experiment
F
1
(filial) Generation all tall
↓ Self - pollinate

Mendel’s First Experiment
F
2
Generation
¾ tall ¼ short

Mendel’s life…
• Mendel tried out different pea plant traits…

Mendel’s life…
• Thousands of times…

Mendel’s life…
• And got the same results!
– In F
1 generation, one trait remained and the other disappeared.
– In F
2 generation, the ratio of plants was ¾ to ¼

What Mendel knew… (1800s)
• Each parent contributes to the traits of a plant/child.
• Something is passed on from parent to child to contribute to the traits.
• Some traits were disappearing, then returning in future generations.

What Mendel didn’t know…
• Chromosomes were being inherited from parents. (1930s)
• DNA specifically is the molecule being that gives us our traits. (1920s – 1950s)
• What DNA is made of. (1953)

Mendel’s Rules and Laws
After 1000s of experiments, Mendel came up with some “rules of heredity”:
1.Rule of Unit Factors
• EACH ORGANISM HAS 2 COPIES OF A GENE
THAT CONTROLS EACH TRAIT; ONE COPY
CAME FROM THE MALE PARENT AND ONE
COPY CAME FROM THE FEMALE PARENT

• each chromosome has genes (a gene is a segment of
DNA that controls a trait – ex. height)
• there can be different forms (versions) of the same gene; these different forms are called alleles
– ex. for the height gene, the alleles are tall and short
• the offspring get 2 copies of the gene and can inherit any combination of two alleles from the parents:
Allele from your mom… tall
Allele from your dad… tall short tall short short short tall

Mendel’s Rules and Laws
2.Rule of Dominance
• ALLELES CAN BE DOMINANT OR RECESSIVE;
THE DOMINANT ALLELE, IF PRESENT,
COMPLETELY MASKS THE RECESSIVE ALLELE
• Dominant allele - remained in the F1 generation (ex. tall)
• Recessive allele - disappeared in F1 (ex. short)
• So we say that tall is dominant to short

• When writing alleles, follow these rules:
– Use the same letter for different forms of the same gene.
– Use uppercase letter for dominant allele.
– Use lowercase letter for recessive allele.
– Always write dominant allele first.
• Example: height _______ = tall
_______ = short
A plant can be:
TT __________
Tt __________ tt __________

Important Vocab
PHENOTYPE = written description of appearance or behavior
Examples: tall and short
BUT, two organisms can look the same but have different gene (or allele) combinations:
GENOTYPE = 2 – letter gene combination
Examples: TT, Tt, tt

There are 3 different types of genotypes:
____________________________________ (TT)
____________________________________ (tt)
FYI: homozygous is the same as PUREBRED heterozygous is the same as HYBRID

JUST A LITTLE PRACTICE
Description
Red hair
GG mm
White eyes
Short toe
Dd
Genotype or
Phenotype?
phenotype genotype genotype phenotype phenotype genotype
Homozygous or
Heterozygous?
N/A
Homozygous (dominant)
Homozygous (recessive)
N/A
N/A
Heterozygous

Mendel’s Rules and Laws
3.Law of Segregation
THE TWO ALLELES THAT CONTROL EACH TRAIL
WILL SEPARATE DURING GAMETE FORMATION

Explains how two tall plants can give offspring that are ¾ tall and ¼ short!
PLANT 1
Tt
Plant 1’s gametes:
X
PLANT 2
Tt
Plant 2’s gametes:
T t
T t
All possible gamete combinations result in the following

An easier way to determine offspring…
• Reginald Punnett (early
1900s)
• Discovered short-hand way to solve genetics problems
• Works based on Mendel’s law of segregation

AND NOW WE…