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Genetics
Chapter 11
History of Genetics
 Gregor Mendel 1822-1884
 “Father of genetics”
 a monk who studied inheritance traits in
pea plans
 worked with pea plants in the monastery
garden
What Mendel noticed about his pea
plants:
 Pea plants reproduced sexually- with
male and female gametes
 Plants could self-pollinate and short
plants bred short plants and tall
plants bred tall plants.
 He studied the following traits: seed
color, pod shape, plant height, etc.
Mendel’s Crosses
 Mendel bred plants with different
traits and studied the offspring.
 original parents are the P generation.
 offspring were the F1 (daughter/son)
generation
Mendel’s conclusions
 Law of Inheritance
 Traits are controlled by pairs of geneswith one member of each pair coming
from each parent
 Law of dominance
 some alleles are dominant and other
are recessive.
 Dominant – expressed
 Recessive – present but not expressed
Mendel’s Laws Continued
 Law of Segregation and
Recombination
 During gamete formation two
chromosomes separate
 Each gamete contains one allele for each
trait
 Law of Independent Assortment
 Traits are inherited independently of
each other
Key Terms
 Gene
 Sections of giant DNA molecules found in
chromosomes
 Are the units of heredity
 Allele
 Alternative genes for trait
 Example: Height: Tall or Short; Eyes:
Brown or Blue; Cheeks: Dimple or No
Dimple; Hair Line: Widows or Straight
D/R
 Each gene has two possible alleles
 Dominant- always expressed
 Recessive- always hidden by a dominant
allele.
 Example: Dimpled chin (cleft chin)
 D= dimpled
d= non-dimpled
More terms:
 Homozygous / Pure
 having 2 of the same alleles
 example: DD – homozygous dominant
dd – homozygous recessive
 Heterozygous / Hybrid
 having 2 different alleles ex: Dd
 Phenotype
 physical characteristic ex: dimpled chin
 Genotype
 genetic make up ex: DD
Punnett Square
 Used to predict the possible offspring
of a couple.
 Gives the probability of the matingnot the actual outcome
Punnett Square Rules
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
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Define traits, assign symbols
Determine the parental genotypes
Set up Punnett square
Work it out
List the genotype probability
List the phenotype probability
Practice
 In humans a widow’s peak is dominant over a
straight hairline. A man who is heterozygous
for widow’s peak married a woman without a
widow’s peak. Predict the genotype and
phenotype of the offspring:
 In humans, the ability to tongue roll is
dominant over not being able to tongue roll.
Cross a heterozygous man with a non-rolling
woman. List the possible genotypes and
phenotypes of the offspring.
 In humans, brown eyes are dominant over blue
eyes. Cross a homozygous dominant man with
a blue eyed woman. List the possible
genotypes and phenotypes of the offspring.
 In guinea pigs, black hair is dominant over
brown hair. Cross two guinea pigs that are
heterozygous for black hair. What are the
chances their offspring would have brown hair?
Intermediate Inheritance
 Incomplete Dominance
 a type of heredity in which the hybrid is
an intermediate between the pure
dominant and recessive parents.
(Blending of trait) Example
 Co-dominance
 expression of two dominant alleles
(spots, strips, etc) Example
Example of Incomplete Dominance
 A Japanese 4 o’clock flower can be red, white,
or pink. Pink is the result of the mixture of red
and white. Cross two pink flowers. List their
genotypes and phenotypes.
RR = Red WW = White RW = Pink
Example of Co-dominance
 Shorthorn cattle can have one of three color coats. Their
coats can be red, white or roan. Roan is patches of red
and white hairs. Cross a roan bull with a red cow.
CRCR = Red Hair
CWCW = White Hair
CRCW = Roan
Sex Linked Traits
 Sex Determination
 Female: XX
 Male: XY
 The sperm cell determines the sex of the child
 Sex Linked Trait
 The gene found only on the X or Y chromosome
 Males tend to be more vulnerable to sex-linked
genetic disorders because most disorders occur
on the X chromosome
Practice: Sex Linked Traits
 In humans, hemophilia is a sex linked
trait. Females can be normal, carriers, or
have the disease. Males will either have the
disease or not (but they won’t ever be
carriers). Cross a male with hemophilia with a
carrier female. List genotypes and phenotypes.
Multiple Alleles
 some traits have more than one
allele, but a single individual cannot
have more than two genes for a each
trait
 Example: Human Blood Type
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Type
Type
Type
Type
A
B
AB
O
Can you Write out the Key?
 Type A and B are co-dominant
 Type O is recessive
Blood Type Key
Phenotype
Type A
Genotype
IAIA
IAiO
Type B
IBIB
IBiO
Type AB
IAIB
Type O
iOiO
Practice: Multiple Alleles
 The ABO blood group system in humans is an
example of multiple alleles. Cross a
heterozygous type A male with a heterozygous
type B female. Record the possible genotypes
and phenotypes.
 Cross a person with type AB blood and a
person who is heterozygous for type B blood.
What are the chances the child will have type A
blood?
 What must the genotypes of a parent with type
A blood and a parent with type B blood be if
they have a child with type O blood?