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A little background…
• Gregor Mendel- “Father of Genetics”– Austrian monk and biologist in the mid-1800s– Used pea plants to figure out how traits were
inherited (passed down)
• Dominant:
allele that can mask the other
(uppercase letters- A)
• Recessive:
allele that can be masked by the other (lowercase letters- a)
• Homozygous:
both alleles in the pair are the same
(AA or aa)
• Heterozygous:
both alleles in the pair are different
(Aa)
• Genotype:
genetic make-up (the genes) of an organism
Ex. Uu or UU
• Phenotype:
physical appearance expressed by the genes
Ex. Unibrow
G – green pea pods g – yellow pea pods
(Dominant) (Recessive)
Genotypes GG Gg gg
Terms
Phenotypes
Homozygous dominant
Homozygous recessive
Heterozygous
Green pea pods
Green pea pods
Yellow pea pods
Laws of Heredity
• Law of Independent Assortment:– Alleles from different genes separate
independently of one another during gamete formation
Punnett Squares
• Used to figure out the possible genotypes and phenotypes (ratios)
• Monohybrid cross:
one trait of an individual is crossed with the same trait of another individual
Monohybrid Cross Example
Let’s look at pea pod color:– Green is dominant (G) and yellow is recessive (g)– Example: Homozygous dominant (GG) crossed with
homozygous recessive (gg)
G G
g
g
Genotype ratio 4 Gg: 0 GG: 0 gg 100% Gg
Phenotype ratio 4 green: 0 yellow 100% green
Gg Gg
Gg Gg
Punnett Squares
• Dihybrid cross:
two traits of an individual are crossed with the same two traits of another individual
Dihybrid Cross ExampleMust find all possible allele combinations:
Traits - seed shape and seed color
Alleles: R = Round r = Wrinkled
Y = Yellow y = green
Female: RrYy Male: RrYy
both are round and yellow
All combinations for each:
Female: RY, Ry, rY, ry Male: RY, Ry, rY, ry
Dihybrid Cross Example
Cross these combinations to find possible genotypes/phenotypes of offspring:
RY
RY
Ry rY ry
Ry
rY
ry
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
Dihybrid Cross ExampleCross these combinations to find possible genotypes/phenotypes of offspring:
Genotype ratio: RRYY 1/16 RRYy 2/16 RrYY 2/16 RrYy 4/16
Rryy 1/16 Rryy 2/16
rrYY 1/16 rrYy 2/16
rryy 1/16
Round and Yellow 9/16
Round and Green 3/16
Wrinkled and Yellow 3/16
Wrinkled and Green 1/16
Phenotype ratio:
Types of Dominance:
• 1) Complete Dominance-
One allele can completely mask another
RR- Red Rr- Red rr- white
Types of Dominance
• Incomplete Dominance:
one allele is partially covered by another
RR- RED RR’- PINK R’R’- WHITE
Types of Dominance
• Co-dominance:
Both alleles are dominant, so they both show through
RR- RED RW- Red & White WW- White
Types of Dominance
• 4) Multiple Alleles- when a trait has more than two alleles
• Ex. Blood Type– IA- Type A– IB- Type B– i- Type O
• Both complete and co-dominance are shown in creating phenotypes
• Type A- IAIA, IAi• Type B- IBIB, IBi• Type AB- IAIB
• Type O- ii