Mendelian Genetics
Gregor Mendel ~ Father of
modern genetics. Mendel was an
Austrian monk and mathematician.
One of his assignments in the
monastery was to tend the garden.
•Pea Plants
•Traits
•Self-pollination ~ purebred
•Cross-pollination ~ hybrid
Genes and Dominance
•Gene
•Allele
•Dominant
•Recessive
•Phenotype
Marfan’s
syndrome
•Genotype
•Homozygous
•Heterozygous
Holt-Oram
Syndrome
Segregation
• The separation of alleles on different chromosomes
during meiosis. Each allele ends up in a different
egg or sperm cell.
Tt
T
T
Diploid mother cell
t
t
Haploid
daughter
cells
Fertilization
• Fertilization: The
union of an ovum
and a sperm cell.
• Zygote: The first
new diploid cell of
an individual made
by fertilization.
Mendel’s Laws of Heredity
• Law of Segregation (1st Law) ~ Alleles for the same
gene are separated during meiosis when the
chromosomes separate.
• Law of Independent Assortment (2nd Law) ~ Genes
on different, nonhomologous chromosomes assort
independently during meiosis. (They are “shuffled”
like cards.) Assortment occurs during meiosis.
• Principle of Dominance ~ Some traits are
dominant, and some are recessive. Dominant traits
will always be expressed when they are present.
Punnett Square ~ A device that is used to predict the
possible gene combinations for a trait that two
parents can pass to their offspring.
Monohybrid cross ~ A cross of two parents to
evaluate what happens with one set of alleles.
In
other words, seeing how one gene will be passed
to offspring.
Phenotypic Ratios:
Show the ratio of the
different possible
phenotypes of offspring.
Tall vs. Short Pea Plants
Purebred Parents:
TT x TT
Genotypic Ratios:
Show the ratio of the
different possible
genotypes of offspring.
P1 Cross
Probabilities: Show the
percentages of chance
for each type of
phenotype or genotype
F1 Cross
TT x tt
Tt x Tt
Actual vs. Hypothetical
• A couple has a 50% chance of having a girl
every time they have a baby. They have
four children, and all are girls.
• What is the hypothetical number of girls
and boys?
• What is the actual number of girls and
boys?
Dihybrid Cross ~ Crossing the genes for two different
traits at the same time.
9:3:3:1 ~ The phenotype ratio if both parents are
heterozygous for both traits.
TtYy x TtYy
Build-a-Baby
• Eyes
– Cyclops dominant (E)
– Binocular recessive (e)
• Appendages
– Flippers dominant (A)
– Legs recessive (a)
• Color
– Green dominant (C)
– Blue recessive (c)
• Body
– Round dominant (B)
– Pear-shaped recessive (b)
• Hair
– Curly dominant (Z)
– Bald recessive (z)
• Heads
– One head dominant (H)
– Two heads recessive (h)
• Use the penny to decide
the genotype of your baby
for each trait. Heads –
dominant, Tails –
recessive.
• Make a text box showing
the different genotypes for
your baby, then draw the
baby according to what
phenotypes he or she
would show.
Dihybrid Cross ~ Crossing the genes for two different
traits at the same time.
9:3:3:1 ~ The phenotype ratio if both parents are
heterozygous for both traits.
Tall, green father:
Ttyy
Short, yellow
mother:
ttYY
Linked Genes ~ Genes for different traits that
are on the same chromosome, so they are
passed on to offspring together.
Multiple Alleles
• There are more than two possible genes that
can be inherited for a trait.
• Examples
– Flower color
– Blood type
Incomplete Dominance ~ Describes genes that are
neither dominant or recessive. The traits show as a “mixed”
phenotype.
r ~ red allele
y ~ yellow allele
w ~ white allele
rr ~ red flower
yy ~ yellow flower
ww ~ white flower
ry ~ orange flower
rw ~ pink flower
Co-dominance ~ Traits that have
more than one dominant allele.
Some traits have more than one possible
dominant or recessive gene. For instance, blood
type!
Blood type is determined by proteins on the
surface of your red blood cells.
Type A
Dominant
Type B
Dominant
Type AB Co-Dominant
Type O
Recessive
Co-dominance
Cows
T = tall pea plant
Dihybrid Cross
t = short pea plant
Y= yellow pea pods
TY
y = green pea pods
•
Fill in the missing
offspring
genotypes.
•
What is the
phenotype ratio
for the offspring?
•
What must the
genotypes of the
two parents be?
Ty
tY
TY TTYY TTYy
Ty TTYy
TTyy
tY TtYY
ty
Ttyy
ty
TtYy
TtYy
Ttyy
ttYY
ttYy
Bellwork:
Do a dihybrid cross for the following, then show the
genotypic and phenotypic ratios for the offspring.
Pansy color is incompletely dominant. Broad leaves are
dominant over narrow leaves.
An orange pansy with broad leaves ryBb
A red pansy with broad leaves rrBb
Do a dyhbrid cross for a mother who is AB+ and her
husband who is B-. Here are their genotypes:
Mother: IAIBRr
Father: IBirr
What are the phenotype and genotype ratios for their
potential children?