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
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
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
Pedigree Chart
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
Types of Phenotypic Traits
• Discrete traits
• Quantitative traits
Types of Phenotypic Traits
1) Discrete Traits: determined by the action of
a single gene
Only a few distinct categories exist for trait
Shape of human hairline is a discrete trait
There are 2 alleles and 2 varieties:
Widow’s Peak
= dominant
No Widow’s Peak
= recessive
Human earlobe type is a discrete trait
There are 2 alleles and 2 varieties:
Attached earlobe =
recessive
Free earlobe
= dominant
Types of Phenotypic Traits
2) Quantitative Traits: determined by 2 or more
genes
Has a range of phenotypes for that trait
Height is a quantitative trait
There’s a range of possible values
Others: weight, skin color
Past the Peas:
Types of Dominance
1) Complete Dominance: 1 or other
2) Incomplete Dominance: 3rd effect
3) Codominance: some of both
Complete Dominance
Dominant Allele is always expressed when present
Recessive Allele is only expressed as homozygote
Incomplete Dominance
Alleles have
combined (equal)
effect on phenotype
of heterozygote
Phenotype is
intermediate
Codominance
Both alleles are visible in the
phenotype of the heterozygote
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
What is the relationship between A and B alleles? codominance
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)
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?
Question…..
• Will a mother that is colorblind automatically have
a son that is colorblind?
Yes!!!!!
Beyond Simple Inheritance Patterns
• Polygenic Inheritance
– Occurs when a trait is
governed by two or more
sets of alleles
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
Recessive Disorders
• Most human genetic
disorders are recessive
– Individuals can be
carriers of these
diseases
Dominant Disorders
• Some human genetic
disorders are dominant
– Achondroplasia is a
form of dwarfism
– Huntington's disease