Genetics

Mendelian GeneticsComplete, Incomplete &

Codominance

Gregor Mendel

Father of Genetics

Terms

• GENETICS: The study of heredity or inheritance of traits.

• TRAIT: A specific characteristic that can be passed from parent to offspring

• HEREDITY: the passing of traits/genes from one generation to the next

Mendelian Genetics

• Examines the physical characteristics

• Principals and laws come from experiments by Mendel

• Predicts inheritance patterns from generation to generation from generation to generation

Peas

• Self-pollinating• Cheap• Easy to grow• High yield• Edible• 7+ traits that show complete

dominance…

Parental or P Generation

(Original pair of plants)

• Self-pollination (Pollen and egg are derived from the same plant)

• Cross-Pollination (Pollen and egg are derived from different plants)

• Purebred vs. Hybrid

• Mendel Studied Seven Traits That Bred True

Monohybrid crosses

• Mendel crossed a tall plant with a short plant test.

• The first group of offspring were called F1 generation for filial . They should have had blended traits, but didn’t….

P

F1

Proposals• Law of Dominance: when two different

alleles for a single gene are present, one is dominant and one is recessive.– A pea plant contains two discrete

hereditary factors, one from each parent – The two factors may be identical or

different – When the two factors of a single trait are

different • One is dominant and its effect can be seen • The other is recessive and is not expressed

Dominancesome traits mask expression of

others

• Dominant & Recessive allele traits– Dominant  -  trait that does

not disappear in the F1– Recessive -  trait that

disappears in the F1 generation   

each individual possess [only] 2 alleles for a specific trait         

    RR  -   homozygous dominant Rr   -   heterozygous

rr   -   homozygous recessive

T T

T

t t

t

t

Tall plantShort

plant

All tall plants

F1

T

The Law of Segregation

• Each pair of genes segregate or separate during Meiosis and the sex cells carry only one copy of each gene

• We know that now because we know about meiosis, but Mendel “discovered” this through analyzing his results…

Tt Tt cross

F1

Tall plant Tall plant

TTT

T t T

t T t tt

Tall Tall ShortTall

F2

t

The data suggested a theory of inheritance

– Results for all seven traits studied:• The F1 generation showed only one of the

two parental traits • The F2 generation showed an ~ 3:1 ratio of

the two parental traits

– These results contradicted a blending mechanism of heredity

Conclusions

– Gene: a section of chromosome that codes for a trait

– Most organisms have 2 copies of every gene • one from each

parent

– Alleles: various forms of a trait or gene

Probability

Probability is used to show all possible combinations of gametes and the likelihood that each will occur

•Like flipping a coin

In reality, you don’t get the exact ratio of results•Rules are up to chance

PUNNETT SQUARES CAN BE USED TO DETERMINE THE PROBABILITY OF AN ALLELE OCCURING IN THE OFFSPRING

Punnett Squares

• a grid that enables one to predict the outcome of simple genetic crosses

• by the English geneticist, Reginald Punnett

T T

t

t

Tt

Tt

Tt

Tt

R r

R

r

RR Rr

Rr rr

HOMOZYGOUS

• organisms have 2 identical alleles for a particular trait and are called true-breeding

• Homo = samo• HH or hh• AA or aa• GG or gg• QQ or qq

HETEROZYGOUS

• organisms have 2 different alleles for the same trait and are called hybrids

• He-te-ro = dif-fer-ent• Hh• Aa• Gg• Qq

Homologous Chromosome 4

a A

Terminal Axial

Inflated

D Constrictedd

Tall

T

Short

t

Results of Monohybrid Cross   

PHENOTYPE 

(PHYsical appearance)  

3 round    to   1 wrinkled          

GENOTYPE    

(allele make up)     

1(RR)  to  2(Rr)  to  1(rr)  

Dihybrid Crosses

• Mendel also performed a– Crossing individual plants that differ in

two traits

• For example – Trait 1 =  Seed texture (round vs.

wrinkled) – Trait 2 = Seed color (yellow vs. green)

Update…

• Mendelian factors are now called genes • Alleles are different versions of the same

gene • An individual with two identical alleles is

termed homozygous • An individual with two different alleles, is

termed heterozygous • Phenotype refers to the outward

appearance of an individual • Genotype refers to the specific allelic

composition of an individual

Meiosis backs Mendel

• The segregation of chromosomes in anaphase I of meiosis explains Mendel’s observation that each parent gives one allele for each trait at random to each offspring, regardless of whether the allele is expressed.

• The segregation of chromosomes at random during anaphase I also explains how factors, or genes, for different traits are inherited independently of each other.

Beyond Mendel

Incomplete, Codominance, Multiple Alleles, and Sex-

linked Traits

Incomplete DominanceBlended Inheritance    

                 • neither gene is dominant  • i.e., both contribute to expression of the trait • there is an intermediate phenotype…• Similar to mixing paint

R

R’

R

R’

RR’ RR’

RR’ RR’

R’

R

R R’

RR’

RR’

RR

R’R’

Red x White = Pink

Codominance

• Co—together, at the same time

• Equal expression

• Neither recessive

Moo!Methan

e

Morgan’s Experiment• The chromosome theory of inheritance

was confirmed through studies carried out by Thomas Hunt Morgan

• Morgan tried to induce mutations in the fruit fly Drosophila melanogaster

– Treatments included • Rearing in the dark • X-rays • Radium

• After 2 years, Morgan finally obtained an interesting result

• A male fruit fly with white eyes rather than the normal red eyes

• Morgan reasoned that this white eyed male must have arisen from a new mutation that converted a red-eyed allele into a white-eyed allele

•   • Morgan followed Mendel’s approach in

studying the inheritance of this white-eyed trait

• He made crosses then analyzed their outcome quantitatively

• Colorblindness...

• Frequency of crossover exchange...          exchange of chromatids pieces of a homologous pair during synapsis at a chiasma...      is GREATER the FARTHER apart 2 genes are

   is proportional to relative distance                                       between 2 linked genes

   Relative distance is measured as...        1% crossover frequency =                                   1 map unit of map distance              above CO Freq  =   8.5% + 8.5% = 17%

       1%   CrossOver  Freq   =    1   centiMorgan

Polygenic (Quantitative Trait) Inheritance        

Two or more genes affect 1 phenotype usually with quantitative characters,  with a continuous scale of measurement...  - height,  weight,  color* (and a  polygenic*model), etc...

Environmental Influences on Genes