MENDELIAN GENETICS
Chapter 10.2
Gregor Mendel
• Made the first discoveries in
genetics
• Studied basic traits of
Pea pod plants
• Pea pods are true breeders – always
produced offspring with the same trait.
• Developed Mendel’s laws
Inheritance of traits
• When Mendel crossed plants with
different traits he expected them to
blend.
• Instead all offspring had the trait of
one parent.
F1 and F2 Generations
• P – Cross green
And yellow peas
• F1 – all yellow.
• Self fertilization Cross F1 with F1
• F2 – mostly yellow
Some green
Seven traits
• Mendel studied
Seven characters
• For all traits he
found That one trait
was Dominant over
the Other trait.
Mendel’s Conclusions
1)
Inheritance – factors are passed on
from generation to generation
These factors are called genes
Allele – Different forms of a gene
Mendel’s Conclusions
2) The Principle of Dominance: Some
alleles are dominant and some are
recessive
• Dominant covers recessive
• Recessive only shows if there is no
Dominant allele
Mendel’s Conclusions
3) There are two alleles for each gene
• Genotype - the two alleles (Yy)
• Phenotype – the observable trait
(yellow)
• Dominant alleles are written as a
Capital letter (Y)
• Recessive alleles are written as a
lower case letter (y)
Genotypes and phenotypes
• Homozygous Dominant (YY) – shows
dominant trait (yellow)
• Homozygous Recessive (yy) –shows
recessive trait (green)
• Heterozygous (Yy) – shows dominant
trait (yellow)
Mendel’s conclusions
4) Law of segregation – the two alleles
for each character separate during
meiosis
Monohybrid cross
• Hybrid = heterozygote (Yy)
• Monohybrid cross – cross between
two individuals that have different
alleles for one gene.
Example: Yy X Yy
Punnett squares
• Predict the possible offspring of a
cross between two known genotypes.
Punnett Square rules
A. Divide the parent genotype in half
(meiosis)
B. Put the genotype of each HAPLOID
cell along top and left edge of square
C. Combine sperm and egg in each
box of square
D. Capital Letters first, same letters go
together
Monohybrid cross Example
2 parents - heterozygous genotypes
Purple flower is the dominant trait
X
Genotype of
each
haploid
MOM
DAD
B= dominant
(purple)
b= recessive
(white)
Phenotype
3:1 ratio
Punnett Square practice.
Use R = red and r = white flowers
1)Homozygous dominant mom and
heterozygous dad
2)Heterozygous mom and homozygous
recessive dad
3) True breeding red mom and white
dad
Can you cross more than one allele
in a punnett square?
• Dihybrid cross - Cross two organisms
that differ in two traits.
Dihybrid cross
• R- round
• r – wrinkled
• Y – Yellow
• y- green
Law of independent assortment
• There is a random assortment of
alleles that occurs during gamete
formation.
• 4 possible combinations of gametes
• Genotype – YyRr
• Gametes:
YR, Yr, yR, yr
• 4 different
phenotypes
• Phenotypic ratio
9:3:3:1
• 9 different
genotypes
Practice Dihybrid Cross
• S – short tail
• s – long tail
• B – Brown
• b – white
• Cross SsBb X SsBb