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Patterns of Inheritance
Chapter 9
Gregor Mendel
• Deduced the
fundamental principles
of genetics
• Cross-fertilization
What’s with the Peas?
• Little spontaneous variation
between generations
• Can self-fertilize
• Easy to control pollination
• Possessed several easily
observable traits
–
–
–
–
–
Pea form
Pea color
Flower location
Flower color
Stem size
What do we get??
• Genetic cross
– Bb x Bb
• P generation
• F1 generation
• F2 generation
Monohybrid Cross
• Cross between parent plants that
differ in only one characteristic
– Mendel developed four hypotheses
from the monohybrid cross:
• There are alternative forms of genes
– Alleles
• For each characteristic, an organism
inherits two alleles
– One from each parent
• Alleles can be dominant or recessive
• Gametes carry only one allele for each
inherited characteristic
Mendel’s Laws
• Genes
– Set of instructions that
determine characteristics of an
organism
– Segments of nucleic acid that
specifies a trait
– Found at designated place on
chromosomes
• Locus
– Not all copies of a gene are
identical
Mendel’s Laws
• Alternative forms of a gene lead to
the alternative form of a trait
– Alleles
• way of identifying the two members
of a gene pair which produce
opposite contrasting phenotypes
• Chromosomes that are homologous
are members of a pair and carry
genes for the same traits in the same
order
Genes v. alleles
• Genes
–
–
–
–
Basic instruction
Sequence of DNA
General
Hair color
• Alleles
– Variations of that
instruction
– Specifics
– Brown hair
Alternative alleles of genes are located on
homologous chromosomes
Genotype verse Phenotype
• Genotype
– the alleles an individual receives at fertilization
• Homozygous
– an organism has two identical alleles at a gene locus
• Heterozygous
– an organism has two different alleles at a gene locus
• Phenotype
– the physical appearance of the individual
The journey from DNA to phenotype
Mendel Proposes a Theory
• genetic traits are assigned a letter symbol
referring to their more common form
– Dominant traits are capitalized while a lowercase letter is reserved for the recessive trait
• P signifies purple
• p signifies white
Describing Genotypes
Homozygous Dominant
when both alleles are dominant
BB
Homozygous Recessive
when both alleles are recessive
bb
Heterozygous
when one allele is dominant and one is recessive
Bb
Punnet Square…..
Genetic cross determines arrangement
Variations on Mendel’s Laws
1.
2.
3.
4.
5.
Incomplete dominance
Multiple allelism & codominance
Pleiotropy
Polygenic inheritance
Environmental factors
Why Some Traits Don’t Show
Mendelian Inheritance
• Incomplete dominance
– Alleles have combined
(equal) effect on
phenotype of
heterozygote
– Phenotype is
intermediate
Multiple Allelism: existence of
more than 2 alleles of gene
Example: Blood type (A, B, O)
Remember: Each person still only has 2 alleles for
that trait, but more than 2 exist
Multiple Allelism: Blood typing
ABO Blood Type in Humans exhibits multiple allelism
Phenotype
Genotype
O
OO
A
AA or AO
B
BB or BO
AB
AB *
How many ALLELES are there?
3 ( A, B, O)
How many Phenotypes are there?
4 (A, B, AB, O)
How many Genotypes are there?
6
Question: If a woman with blood
type O marries a man with blood
type B, can they have a child with
blood type A?
Phenotype
Genotype
O
OO
A
AA or AO
B
BB or BO
AB
AB*
No. The mother’s genotype must be OO and the
father’s either BB or BO. Their child will either be
type B (BO) or type O (OO)
Why Some Traits Don’t Show
Mendelian Inheritance
• Codominance
– A gene may have more
than two alleles in a
population
• in heterozygotes, there
is not a dominant
allele
– Both alleles are
expressed
• Relationship between
the A & B alleles in
blood typing
Why Some Traits Don’t Show
Mendelian Inheritance
• Pleiotropic effects
– Allele that has more than
one effect on a
phenotype
– these effects are
characteristic of many
inherited disorders
• Sickle-cell anemia
• Must be homozygous for
sickle cell allele
Why Some Traits Don’t Show
Mendelian Inheritance
• Continuous variation
– Characters can show a
range of small
differences when
multiple genes act jointly
to influence a character
• Polygenic
Phenotypes are not always a
direct translation of genotype
Phenotypes may also be
influenced by the environment
Examples?
•
skin color influenced by sun
•
height/weight influenced by nutrition
•
animal coat influenced by climate
Remember…..
P=G+E
Sex-linked Traits
Female XX
Male XY
Genes located on the X or Y chromosome are sex-linked
X and Y chromosomes are not homologous, they contain different genes
Sex-linked traits
• Sex chromosomes
– Are designated X and Y
– Determine an individual’s sex
– Influence the inheritance of certain
traits
• Sex-linked genes
– Are any genes located on a sex
chromosome
Sex-Linked Traits
Females (XX) have 2 copies of each gene on the
X chromosome
Males (XY) have only 1 copy of each gene on
the X chromosome
Females can show a dominant condition if present on 1 or both X
chromosomes
Females can only show a recessive condition if present on both X
chromosomes
Males ALWAYS show X-linked alleles, regardless of dominance
Sex-Linked Disorders in Humans
• number of human
conditions result from
sex-linked (X-linked)
genes
• Red-green color
blindness
– characterized by a
malfunction of lightsensitive cells in the
eyes
Question…..
• Will a mother that is colorblind automatically have
a son that is colorblind?
Mutations
• Changes to the nucleotide sequence
of the genetic material of an
organism
• Can be caused by:
–
copying errors in the genetic
material during cell division
– exposure to UV light or chemical
mutagens
– Viruses
– can be induced by the organism itself
• Create variety within gene pool
• Less favorable verse more favorable
Human Heredity
• To study human heredity, scientists examine crosses that have
already been made
– Pedigree
– Determine whether a trait is sex-linked or autosomal and whether
the trait’s phenotype is dominant or recessive
Huntington’s disease is a dominant
genetic disorder
Recessive Disorders
• Most human genetic
disorders are recessive
– Individuals can be
carriers of these
diseases