Download Mendelian Genetics - Byron High School

Mendelian
Genetics
Gregor Johann Mendel
• Monk, Czech Republic
• Founder of modern
genetics
• Studied traits in garden
peas in 1854
• Published his theory of
inheritance in 1865
• Mendel was
“rediscovered” in 1902
Gregor Mendel 1822-1884
Mendel’s Experiments
•
•
Mendel cross-pollinated pea plants
Focused his work on 7 different traits
seed color:
seed shape:
pod color:
pod shape:
stem height:
flower position:
flower/seed coat color:
•
yellow vs. green
smooth vs. wrinkled
green vs. yellow
inflated vs. pinched
tall vs. short
axial vs. terminal
purple vs. white flowers
grey vs. white seed coats
Counted the offspring of each phenotype and
analyzed the results mathematically
Some basic terminology:
Generations:
P = parental generation
F1 = 1st filial generation, progeny of the P generation
F2 = 2nd filial generation, progeny of the F1 generation
(F3 and so on)
Crosses:
Monohybrid cross = cross of two different true-breeding
strains (homozygotes) that differ in a single trait.
Dihybrid cross = cross of two different true-breeding
strains (homozygotes) that differ in two traits.
*Genetics etiquette - female conventionally is written
first
Dominant & recessive alleles (Fig. 10.7):
Results of Mendel’s monohybrid parental cross:
Why?
Smooth seeds (allele S) are completely dominant to wrinkled seeds
(allele s).
Smooth and wrinkled parental seed
strains crossed.
1. Genotypes (alleles –letters)
4/4 Ss
2.
Phenotypes (physical characteristics)
4/4 smooth
Punnett square animation
Punnett square
Genotypes
1/4 SS
1/2 Ss
1/4 ss
Phenotypes
3/4 smooth
1/4 wrinkled
Confirming the Principle of Segregation with test-crosses:

SS x SS

true breeding (100% homozygous dominant)

ss x ss

true breeding (100% homozygous recessive)
How do you determine whether an individual with the dominant
phenotype is homozygous or heterozygous?
Cross it with homozygous recessive:
SS x ss

4/4 dominant trait
Ss x ss

1/2 dominant trait + 1/2 recessive trait
Test Crosses - Cross
with a homozygous
recessive individual
Dihybrid cross:
Possible Gametes
Dihybrid cross:
Ratio:
9:3:3:1
Mendel’s dihybrid crosses:
1.
Mendel also performed crosses involving two pairs of traits, e.g.,
seed shape (smooth vs. wrinkled) and color (yellow vs. green).
2.
If alleles sort independently, four possible phenotypes (2n) appear
in the F2 generation in a 9:3:3:1 ratio.
Cross the following:
SSgg x
ssGG

Mendel’s Principle of Segregation:
Two members of a gene pair segregate (separate) from each other during
the formation of gametes.
Animation of
Segregation
Mendel’s Principle of Independent Assortment:
Alleles for different traits assort independently of one another.
Genes on different chromosomes behave independently in gamete production.