Unit 5C Genetic Inheritance
The Work of Gregor
Mendel & Applying
Mendel’s Principles
Heredity and Genetics
• Heredity- the delivery of
characteristics from parent to
offspring
• Genetics- the scientific study of
heredity
• The Modern study of genetics was
founded by a monk named Gregor
Mendel in the mid- 1800’s
• Mendel studied genetics in pea
plants
Traits
• Specific characteristics of an individual
– Example: in pea plants- tall or short,
round or wrinkled
– In humans, brown eyes, blue eyes
Crossing Organisms
• To study genetic inheritance, Mendel
crossed pea plants and looked at how
certain traits were passed from generation
to generation
• To cross means to cause one organism to
reproduce with another (like breeding)
Mendel Arrived at Two Conclusions
1. An individual’s characteristics are determined
by factors that are passed from one parental
generation to the next.
-
Today, these “factors” are called genes
Each gene has (at least) two contrasting varieties
The different forms of the gene are called alleles
Example: the gene for height in pea plants comes
in alleles for tall and short.
2. The Principle of Dominance- some alleles are
dominant and some are recessive
- Organisms carry two alleles for each trait
- Only one dominant allele needs to be present for
that trait to be expressed
(it “dominates” over the recessive allele)
- Two copies of the recessive allele must be present
for that trait to be expressed.
TT
Two dominant
alleles
Tt
One dominant
and one
recessive allele
tt
Two
recessive
alleles
Why do organisms have two alleles for
each gene?
• Because we have homologous pairs of
chromosomes, one from our mom and one
from our dad, we have two copies of every
gene.
If an allele is dominant does that mean
it codes for the more common trait in
a population?
• Not necessarily! There are many recessive
traits that are more common than dominant
ones.
• If more people in an area have a recessive
trait, their offspring will have it too, continuing
the trend.
• Example: Almost 80% of Northern Europeans
have light eyes even though light eyes are
recessive to brown eyes.
Allele Combinations
• Homozygous- organisms with
two identical alleles for a
particular gene (Ex: TT is
homozygous dominant, and tt is
homozygous recessive)
• Heterozygous- organisms that
have two different alleles for the
same gene (Tt)
Genotype and Phenotype
• Genotype- the genetic makeup of an organism
(Ex: TT)
• Phenotype- the observable characteristic or
trait (Ex: Tall)
• A plant with the genotype _________ has a
phenotype of short.
• A plant with the genotype of Tt has a
phenotype of ____________.
Using Punnett Squares
• Probability can be used to predict the
outcome of genetic crosses.
• Example:
– Parent 1 is TT
– Parent 2 is tt
T
t
Both
possible
gametes
from
Parent 2
t
T
Both
possible
gametes
from
Parent 1
Genotypes:
• What percent of the offspring are homozygous
dominant?
• What percent of the offspring are heterozygous?
• What percent of the offspring are homozygous
recessive?
Phenotypes:
• What percent are tall?
• What percent are short?
• When organisms are crossed to determine the
inheritance of one gene it is called a
monohybrid cross.
• The parents used are called the “P” generation
and the offspring are called the “F1” (first
filial) generation.
• Ex: what percent of F1’s will be tall (T) if both parents
are heterozygous tall?
What is the phenotype ratio
of tall (dominant) to short
(recessive)?
Test Cross
• Used to determine if an individual with the
dominant trait is homozygous or heterozygous
• Example: A pea plant is tall. Is its genotype TT or Tt?
– Do an experiment: cross the tall pea plant with a short
pea plant (tt) to see the phenotypes of the offspring.
– We can see why this works by setting up both potential
situations using punnett squares
If all offspring
have the
dominant
phenotype
then the
parent was TT
If 50% of the
offspring have
the dominant
phenotype
and the other
50% have
recessive,
then the
parent was Tt
Two Factor Crosses
• Experiments testing how two genes are
passed down are called two-factor or dihybrid
crosses.
Parent “P”
organisms
R= round
r= wrinkled
Y=yellow
y=green
All possible
gamete
combinations
If one parent is
homozygous
recessive for both
genes and the
other is
homozygous
dominant for both
genes, then all
offspring are
heterozygous for
both traits
Heterozygous Dihybrid
R= round
r= wrinkled
Y=yellow
y=green
1. Determine the
possible gametes
from each
parent and place
each
combination on
the lines outside
the box
2. Cross each
combination into
the boxes with
alleles for the
same gene
coming back
together
What is the ratio of offspring phenotypes? (Both dominant to one dominant one
recessive to one recessive one dominant to both recessive)
Predictable Phenotypic Ratios for
Offspring of Heterozygous Parents
• Monohybrid Cross:
– Tt X Tt (Two heterozygous parents)
– Offspring ratio of dominant phenotype to
recessive phenotype will be 3:1
• Dihybrid Cross:
– TtGg X TtGg (Two parents heterozygous for both
genes)
– Offspring phenotypic ratio of both dominant to
one dominant, one recessive to one recessive, one
dominant to both recessive will be 9:3:3:1
Beyond Dominant and Recessive
• Not all genes follow the principle of
dominance
• There are 4 exceptions to the principle of
dominance:
– Incomplete dominance
– Codominance
– Multiple alleles
– Polygenic traits
Incomplete Dominance
• Occur when one allele is not
completely dominant over
the other
• Example: four o’clock
flowers
– Red (RR) x White (WW) = Pink
(RW)
– Neither red nor white is
dominant
– Heterozygous phenotypes are
a blending of the two
homozygous phenotypes
Codominance
• The phenotypes of both alleles are
expressed
• Example: In some chickens, black
feathers are codominant with white
feathers
– Heterozygous chickens will have both
black and white feathers
– Black and white are NOT blended, they
appear separately
Multiple Alleles
•
•
•
•
A gene with more than two alleles has “multiple alleles”
Individuals only have two copies of each gene
But, many alleles for the gene can exist in a population
Example:
– There are multiple alleles (3) for human blood
type: IA, IB, and i.
– Alleles IA and IB are codominant
– Each person inherits 2 of these alleles,
one from mom and one from dad.
Two Different Patterns of Inheritance
(besides simple dominance)
Multiple Alleles and
Codominance
Sex-Linked Inheritance
Example: Colorblindness
Example: Human blood Types
Key
XX=
XY=
C=
c=
Polygenic Traits
• Traits that are produced by interactions between multiple
genes
• Examples: At least three genes work together to make the
reddish-brown pigment in the eyes of fruit flies
– The variety of skin color in humans is because
multiple genes interact to produce skin color.
Sex-Linked Inheritance
• Sex-linked genes- genes located on the sex
chromosomes (X and Y)
• Genes on the Y are only found in males and
are passed from father to son
• Genes on the X are found in both sexes, but
remember… males have just one X
The Consequence of only having one X
Chromosome
• Recessive disorders related to genes on the X
chromosome are more common in males
since they only have one X.
• Example: color blindness
– 3 genes work together to produce color vision and ALL are
located on the X chromosome
– If any of these alleles is defective in males, they will
experience problems seeing colors
– In females, if one of these genes is defective, they still may
have a good copy of the allele on their other X
chromosome!
Two Different Patterns of Inheritance
(besides simple dominance)
Multiple Alleles and
Codominance
Sex-Linked Inheritance
Example:
Example:
Key
XX=
female
XY=
male
C=
Normal color vision
c=
Color blind
Results:
Human Pedigrees
• A pedigree is a chart that shows patterns of
inheritance in a family.
How to read a pedigree
• Circles represent females
• Squares represent males
• Shaded shapes mean the individual expresses
the trait
• Not shaded shapes mean the individual DOES
NOT express the trait
• Vertical lines connect parents to offspring
• Horizontal lines represent a marriage
How to read a pedigree
•This pedigree is for the dominant “white forelock” trait
•The grandfather expresses the trait
•What is the genotype of the circled individual?
•What do you think the genotype of the grandfather must be?
• The trait represented by the pedigree in the
figure has two alleles: P (dominant) and p
(recessive). The black symbols show the
dominant phenotype, and the white symbols
show the recessive phenotype. What is the
genotype of individual number 1?