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

F1 (filial) Generation

all tall

↓ Self - pollinate

Mendel’s First Experiment

F2 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 F1 generation, one trait remained and the other disappeared.

– In F2 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… Allele from your dad…tall

tall

tall

short 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 __________

T

t

tall

tall

short

Important VocabPHENOTYPE = written description of appearance or

behaviorExamples: 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)____________________________________ (Tt)

Homo- means _______________________Hetero- means _______________________ FYI: homozygous is the same as PUREBRED

heterozygous is the same as HYBRID

Homozygous dominant

Homozygous recessive

Heterozygous

same

different

Description Genotype or Phenotype?

Homozygous or Heterozygous?

Red hair

GG

mm

White eyes

Short toe

Dd

JUST A LITTLE PRACTICE

phenotype

phenotype

phenotype

genotype

genotype

genotype

N/A

N/A

N/A

Homozygous (dominant)

Homozygous (recessive)

Heterozygous

Mendel’s Rules and Laws

3.Law of SegregationTHE 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 PLANT 2

Tt X Tt

Plant 1’s gametes: Plant 2’s gametes:

All possible gamete combinations result in the following offspring:____________________ and ____________

T Tt t

TT, Tt, and Tt (3/4 tall) tt (1/4 short)

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…

PRACTICE, PRACTICE,

PRACTICE!