Fundamentals of
Genetics
Chapter 9
Trait Description
Tongue rolling
Dimples
Widow’s peak
Hitchhiker’s Thumb
Hand clasping, left over right thumb
Arm folding fold your arms across your chest/stomach…which one
is on top?
Free earlobe
Chin cleft
Hair on middle joints of fingers
Short big toe
Ear points
Round face
Curly hair
Blue Eyes
Brown Hair
Class numbers
Genetics is…..
• The study of heredity, how
characteristics are transmitted
from parents to offspring.
Gregor Mendel
“Father of Genetics”
• Mendel studied garden peas and
constructed the basis of genetics still
used today.
• He observed 7 traits in the plants (ex.
plant height )
• He noticed that traits in the offspring
varied from the parent (ex. short
plants came from tall plants).
7 traits- pg 167 (DO NOT COPY)
1.
2.
3.
4.
5.
6.
7.
Position of flower along stem
Height of plant
Pod appearance
Pod color
Seed texture
Seed color
Flower color
Mendel’s Experiments
• Mendel grew pure plants for each of the
7 traits (pure meaning the offspring were
just like the parent plants)
• He called this the P1 generation
• The pure plants were then crosspollinated (tall plant crossed with a short
plant)
• These plants were the F1 generation
• The F1 generation was allowed to selfpollinate
• These plants were called the F2
generation
Test
• P1 generation – Tall plant x short plant
• F1 generation – All tall plants (where are
the short plants?)
• F2 generation – Tall and short plants
present
Mendel’s Conclusions
• There are dominant and recessive traits.
A dominant trait masks a recessive trait.
• During the formation of sex cells, alleles
are separated (one allele for a trait per
sex cell)- Law of segregation
• Alleles for different characteristics are
distributed to sex cells independently –
Law of independent assortment.
Genes
• A gene is a segment of DNA on a
chromosome that controls a particular trait
(ex. height of a plant, eye color)
• Genes occur in pairs, alleles make up the pair
• Letters are used to represent alleles. Capital
letter represents a dominant allele and a
lower case represents a recessive allele.
• T= tall plant; t = short plant
• When both alleles of a pair are alike it is
said to be homozygous (TT or tt)
• When the alleles of a pair are different it
is said to be heterozygous (Tt)
Genotype vs. Phenotype
• Genotype – genetic makeup of an
individual ; ex. Bb or TT
• Phenotype – physical expression of
the genotype; ex. brown eyes or tall
plant
Punnett Square
A punnett square…..
• A diagram used by biologists to aid them in
predicting the probability that certain traits
will be inherited by offspring
• Types of crosses
– Monohybrid – cross b/t one pair of traits
Ex. Tt x TT
– Dihybrid – cross b/t two pair of traits
Ex. TtGG x TTgg
3 possibilities….
• Homozygous x Homozygous
• Homozygous x Heterozygous
• Heterozygous x Heterozygous
Do you know the genotype of a guinea
pig just by looking at its appearance?
• Black is the dominant coat color
• BB or Bb would produce a black coat
• To tell which, do a test cross and
compare to offspring of the pig
• Cross with a homozygous recessive
Incomplete Dominance
• Two or more alleles influence the
phenotype and cause a blending of traits.
• Flower color, mouse coat, sickle cell
anemia
Codominance (co= exist together)
• Two or more alleles influence the
phenotype and cause both phenotypes
to appear.
• Cattle coat color (spots) or Blood types
(AB)
• Do the Punnett square using different
letters because neither traits are
dominant
The “real” story behind eye
color......
There are many genes involved and eye color can range from brown
to hazel to green to blue to...
How does eye color work? Eye color comes from a combination of
black and yellow pigments called melanin in the iris of your eye. If you
have no melanin in the front part of your iris, you have blue eyes. An
increasing proportion of the yellow melanin, in combination with the
black melanin, results in shades of colors between brown and blue,
including green and hazel.
What we are taught in high school biology is generally true, brown eye
genes are dominant over green eye genes which are both dominant
over blue eye genes. However, because many genes are required
and certain combinations of pigments , there many possibilities in
shades and color of ones eyes.