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
Mendel Changed Biology Forever by
Working in the Monastery Garden!!
• Worked with garden
peas- a “Model System”
– Convenient to study
– Easy to grow
– Produce many offspring
quickly
– Info from peas can tell
us about other
organisms
Some other model systems are fruit
flies, bacteria, mice, monkeys, etc.
A Little Bit About Peas
• Pea plants have
flowers
• Pea flower
Anatomy:
– Anther- creates
pollen which
contains sperm
Ovary- Contains
eggs
– Stigma- receives
pollen to bring to
the egg cells
Fertilization in Peas
• Normally the pollen of the pea plant will fertilize
the eggs of the SAME pea plant (self pollination)
– This will always produce offspring identical to the
parent plant- “true-breeding” plants
• Mendel decided to “cross” pea plants
– To cross means to cause one organism to reproduce
with another
– Mendel used a paint brush to take pollen from one
plant and place it on the stigma of a different pea
plant
Crossing Pea Plants- Cross Pollination
Setting Up A Cross
• P Generation (parental)–
the original pair of plants
• F1 Generation (“filial”
meaning son or
daughter)- the offspring
of the P generation
• F2 Generation- the
offspring of a selfpollinated F1.
Traits
• Specific characteristics of an individual
– Example: in pea plants- tall or short,
round or wrinkled
– In humans, brown eyes, blue eyes
The Traits Mendel Studied
• Mendel crossed parents with opposite traits
to study the following 7 traits in the offspring:
A- flower on
side
T- flower at
the end
• The offspring produced by crossing parents with different traits are called hybrids
Setting Up A Cross
• P Generation (parental)– the original pair of
plants
• F1 Generation (“filial” meaning son or
daughter)- the offspring of the P generation
Mendel’s F1 Results
• All F1 offspring had the trait of only one parent
• The trait of the other parent seemed to have
disappeared
Two Conclusions From These Data
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 Trait Mendel studied was controlled by a
single gene
- Each gene has two contrasting varieties
- The different forms of the gene are called alleles
- Example: the gene for height comes in alleles for
tall and short.
Two Conclusions From These Data
(cont.)
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
Segregation
• After seeing the trait of only
one parent show up in the
F1, Mendel wanted to know
if the other parent’s trait
had disappeared.
• He allowed the F1 hybrid
plants to self-pollinate.
• The offspring of the F1 cross
are called the F2 generation
(second filial)
Cross
Pollination
Self
Pollination
Segregation (cont.)
• The recessive traits
reappeared in the F2
generation
• 25% of the F2 had the
recessive trait
• He then asked, why did
these recessive traits
disappear in the F1
generation only to reappear
in the F2 generation?
Cross
Pollination
Self
Pollination
Segregation
• During gamete
formation, the pair of
alleles for each trait will
segregate, so that each
gamete carries only 1
allele
Two Gametes Come Together During
Fertilization
• When the gametes of the
F1 come together in the
F2, new combinations of
alleles are created
• The recessive trait will
show up when two
recessive alleles come
together
Two recessive alleles
leads to the recessive
trait
Allele Combinations
• Homozygous- organisms with two identical
alleles for a particular gene (Ex: TT, tt)
• 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?
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?
– Cross the tall pea plant with a short pea plant (tt)
Ifto
all offspring
see the phenotypes of the offspring.
have the
dominant
phenotype
then the
parent was TT
Independent Assortment
• Mendel wanted to know if the segregation of
one pair of alleles affects the segregation of
another pair
– Ex: if a parent pea plant is round and yellow does
its off have to have both round and yellow also, or
can they have round and green, or wrinkles and
yellow?
• Experiments testing how two genes are
passed down are called two-factor or dihybrid
crosses.
Dihybrid Cross: creating the F1
• Mendel crossed plants with round, yellow
peas (RRYY) with plants with wrinkled, green
peas (rryy)
All offspring are
heterozygous for
both traits
Parent
genotype
All possible gamete
combinations
Dihybrid Cross: creating the F2
• F1 plants reproduced with each other (or
self pollinate)
Do all of the F2 have the
same combinations of
genes as the parents?
Thus, genes that
segregate independently
do not influence each
other’s inheritance
Predictable Phenotypic Ratios for
Offspring of Heterozygous Parents
• Monohybrid Cross:
– Tt X Tt
– Offspring ratio of dominant phenotype to
recessive phenotype will be 3:1
• Dihybrid Cross:
– TtGg X TtGg
– Offspring ratio of both dominant to one dominant,
one recessive to both recessive will be 9:3:3:1
Independent Assortment Defined:
• This principle states that genes for different
traits can segregate independently during the
formation of gametes.
• This helps to explain why organisms with the
same parents are genetically varied!
Summary of Mendel’s Contributions:
• Inheritance is determined by genes which are
passed from parents to offspring
• Some forms of a gene (alleles) are dominant,
some are recessive
• Adult organisms have two copies of each gene
(one from each parent)
– These genes segregate during gamete formation
• Alleles for different genes segregate
independently.
• Mendel’s Principles apply to all organisms, not
just plants!
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:
Example:
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
• 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=
XY=
C=
c=
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 circles individual?
•What do you think the genotype of the grandfather must be?