1. Traits and Inheritance
Chapter 3, section 2

2. Explaining Mendel
Mendel knew there were two sets of instructions for a trait -- genes
Each instruction is an allelle
Offspring get one instruction (allelle) from each parent
Allelles can be dominant or recessive

3. Allelle Symbols Dominant alleles - capital letters
Brown eyes are dominant -- B
Recessive alleles - lower case letters
Blue eyes are recessive -- b

4. Phenotype
Combinations of allelles and genes affect an organism’s traits or characteristics
Phenotype is the expression of the genes -- which allele is seen
Phenotype is the physical appearance

5. Genotype
Combinations of allelles (letters) that you inherit from your parents
Genotypes are made of two letters
Homozygous -- both allelles (letters) the same
Heterozygous -- two different allelles

6. Examples of Genotypes and Phenotypes
Homozygous
Genotype HH - dominant phenotype is seen
Genotype hh - recessive phenotype is seen
Heterozygous
Genotype Hh - dominant phenotype is seen

7. Predicting Offspring - Punnett Squares
Mendel’s 1st Experiment
Purebred Purple-flowering plant (PP) is crossed with a Purebred White-flowering plant (pp)
P P p P
p p P
Purple flowers
Purple flowers
p p
Ratio is 1:0
Purple flowers
Purple flowers

8. Predicting Offspring - Punnett Squares
Mendel’s 2nd Experiment
Offspring (Pp) are allowed to self pollinate
P p P p P
P P p
Purple flowers
Purple flowers
p p
Ratio is 3:1
Purple flowers
white flowers

9. Probability Mathematical chance something will happen
Written as a fraction or a percentage
Punnett squares are used to make probability predictions
End for today

10. Other Types of Inheritance - Exceptions to Mendelian Genetics
Mendel chose traits that had only two dominant or recessive alleles
Inheritance can be more complex than that

11. Incomplete Dominance or Co dominance
Sometimes the alleles don’t hide, they mix
Incomplete Dominance (Co dominance) is when the two variations of a trait blend together
Example - white flowers and red flowers blend to make pink flowers

12. Incomplete Dominance Allelles
Both alleles are shown as capitals of the same letter (R and R or R and R’)
The heterozygous genotype gives a blended phenotype
RR = red flowers
R’R’ = white flowers
RR’ = pink flowers

13. Incomplete Dominance Punnett Squares
R R R’
RR’
A red 4 o’clock (RR) plant and a white 4 o’clock (R’R’) plant are cross pollinated
Pink flowers
Ratio of 1:0

15. Multiple Alleles
Sometimes there are more than 2 variations or alleles for a trait
Human blood types have 3 alleles
One person has 2 of the 3 alleles
Two A, B, or O alleles combine to give the phenotype (bloodtype)

16. Polygenic Traits Traits can be controlled by more than one gene
Example - shades of eye color, shades of skin color