Download 9BCC Bio 103 Mendelian Patterns of Inheritance CONCEPTS ONLY

Mendelian Patterns of
Inheritance
Chapter 9
Introduction
• Gazelle always produce baby gazelles, not bluebirds
• Poppy seeds always produce poppies, not dandelions
Introduction
• Everyone who observes this phenomenon
reasons that the parents must pass this
hereditary information to their offspring
Introduction
• It also occurs that offspring can appear markedly different than
either parent, however
• The laws of heredity must be able to explain not only the stability,
but the variation that is observed between generations of offspring
Gregor Mendel: “Mendelian
Genetics”
• Gregor Mendel, an Austrian monk in the 1860s,
formulated two fundamental laws of heredity
• He is known as the Father of Genetics
Gregor Mendel
• Mendel experimented with the garden pea, Pisum
sativum, in the gardens of the monestary, to test and
formulate his hypotheses about inheritance
Mendel and Inheritance: “blending
concept”
• Before Mendel, it was thought that both
sexes contribute equally to an individual,
and that parents of contrasting
appearance should always produce
offspring of intermediate appearance--the
“blending concept” of inheritance
Blending concept: not always true!
• If this were the case, then crossing red
and white flowers should always produce
pink flowers
• We know that this is not always the case
• This discrepancy—when white and red
flowers would show up in further
generations—was explained by some
instability in the breeding system
Mendel’s Experimental Procedure
• Mendel chose to work with the garden pea
• They were easy to cultivate and had a short
generation time
• They could easily be pollinated by hand
• Many varieties were available
Mendel’s experimental procedure
• Mendel chose 22 varieties for his
experiments
• When these varieties self-pollinated, they
were referred to as “true breeding”—
meaning that no offspring were like the
parents and like each other
Some characteristics of the pea
plants
Mendel’s experimental procedure
• Mendel studied simple and discrete traits
of the peas—seed shape, seed color, and
flower color
• He observed no intermediate
characteristics among the offspring
One trait inheritance
• For his first experiment, Mendel crossed the tall
and the short plant through cross pollination
One trait inheritance
• Mendel called the original parents the P
generation
• He called the first-generation offspring the
F1 generation
One trait inheritance: the first test
cross
• If the blending theory were correct, the
offspring should have the intermediate
trait: all medium height plants
One trait inheritance
• His result of crossing the Tall and the
Short plants: ALL TALL PLANTS! Not
medium sized plants
• So, therefore, the F1 generation were all
tall plants—resembling only one parent
• Did the characteristic for shortness
disappear?
One trait inheritance
• Mendel then allowed these F1generation
plants to self-pollinate with each other
• This next generation is referred to as the
F2 generation
• The result?
One trait inheritance
• In the F2 generation, ¾ of the plants were
tall, while ¼ of them were short, a 3:1 ratio
• So, the F1 plants were able to pass on the
factor for shortness and it just didn’t
disappear
• Perhaps the F1 plants were tall because
tallness was dominant to shortness?
One trait inheritance
• Mendel explained why the short plants
showed up in a 3:1 ratio in the F2
generation and not the F1 generation
• The F1 parents contained 2 separate
copies of each hereditary factor, one being
dominant and the other recessive
One trait inheritance
• These factors separated when gametes
were formed, and each gamete carried
only one gamete of each factor
• And random fusion of all possible gametes
occurred upon fertilization
The Law of Segregation
• Each individual has 2 factors for each trait
• The factors segregate (separate) during
the formation of gametes
• Each gamete contains only one factor
from each pair of factors
• Fertilization gives each individual two
factors for each trait
As viewed by modern genetics
• Each trait in a pea plant is controlled by
two alleles, alternate forms of the gene—in
this case, that control the length of the
stem for tallness and shortness
• The dominant allele is so named because
of its ability to mask the expression of the
other allele, called the recessive allele
Dominance and recessiveness
• The dominant allele is identified by an
uppercase (capital) letter
• The recessive allele is identified by a
lowercase (small) letter
• So, in this reference, the allele for tallness
(the dominant allele) is “T”, and the allele
for shortness (the recessive gene) is “t”
alleles
• These alleles occur on a homologous pair of
chromosomes at a particular location that is
called the gene locus
alleles
• One allele for each trait is located in each gamete
because of the division of chromosomes during gamete
formation in meiosis
Homozygous alleles
• When an organism has two identical
alleles, we say it is homozygous
• For instance, in Mendel’s P generation of
Tall plants, the parents were homozygous
for tallness: TT. The short plants were
homozygous for shortness: tt
Heterozygous alleles
• When an organism has two different
alleles at the same gene locus, we say
that it is heterozygous
• Therefore, these F1 plants all had the
alleles “Tt”
Genotype vs. Phenotype
• Two organisms with different allelic
combinations for a trait can give the same
outward appearance: for example, TT and
Tt plants are both tall
• We distinguish between the alleles present
in an organisms and the appearance of
that organism
Genotype vs. phenotype
• The word genotype refers to the alleles an
individual receives at fertilization
• Genotype may be referred to by letters or
by short descriptive phrases
• Genotype TT is called homozygous
dominant, and genotype tt is called
homozygous recessive
• Genotype Tt is called heterozygous
Genotype vs. phenotype
• Phenotype refers to the physical
appearance of the individual
• The homozygous dominant (TT) individual
and the heterozygous (Tt) individual both
show the dominant phenotype of being tall
• The homozygous recessive phenotype is
short
Genotype vs. phenotype
Exceptions to simple Mendelian
Inheritance
• 1) incomplete dominance: the offspring have an
intermediate phenotype compared to the parents
with two different phenotypes
• 2) multiple alleles: the offspring inherits 2 of
several possible alleles
• 3) codominance: two inherited alleles are
expressed equally
• 4) polygenic inheritance: This occurs when a
single physical trait is governed by two or more
sets of alleles
1. Incomplete Dominance
• Incomplete dominance is exhibited when
the heterozygote has an intermediate
phenotype between that of either
homozygote
• For example (next slide), a red and a white
flower will produce a pink flower
2) Multiple Allelic Traits
• When a trait is controlled by multiple
alleles, the gene exists in several alleleic
forms
• But, each person can only have two of the
possible alleles
• Blood types are an example of this
Multiple Allelic traits and codominance: ABO blood types
• Three alleles for the same gene control
the inheritance of ABO blood types
• These alleles determine the presence or
absence of antigens on red blood cells:
– IA = A antigen on red blood cells
– IB = B antigen on red blood cells
– i = neither A nor B antigen on red blood cells
3) Co dominance
• An example of co-dominance is:
• If a white flower and a red flower were
crossed, the resulting offspring would be
flowers with red and white stripes (not
pink)
4) Polygenic inheritance
• This occurs when a trait is governed by
two or more sets of alleles
• Each dominant allele has a quantitative
effect on the phenotype, and these effects
are additive
• The result is a continuous variation of
phenotypes, resulting in a distribution that
resembles a bell-shaped curve
• Skin color and height are examples of polygenic
inheritance
Polygenic inheritance and epistasis
• Epistasis: this occurs
when a gene at one locus
interferes with a gene at a
different locus
• Albinism is an example of
this: no matter what
genes for skin color are
inherited from the
parents, the gene for
albinism interferes with
the expression of alleles
for skin color
Environment and phenotype
• Nutrition plays a part in height determination
• Temperature can effect the color of primroses
and Himalayan rabbits
• Soil acidity effects the color of certain flowers
Ex. hydrangea
Sex-linked inheritance
• We have two types of chromosomes:
• 1. sex chromosomes (X and Y: XX for a
female, XY for a male)
• 2. autosomal chromosomes: all of our
other chromosomes not including sex
chromosomes
• Males produce 2 types of gametes: those
that have an X and those that have a Y
• Females produce only one type of gamete:
those that have an X
• Therefore, sex of the offspring is
determined by the FATHER
Sex chromosomes
• Not only are sex-specific traits carried on
sex chromosomes, but genes that have
nothing to do with sex of the individual are
carried here as well
• These are termed SEX-LINKED TRAITS
• i.e., X-linked traits are carried on the X
chromosome