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Patterns of Inheritance
Learning Outcomes
Explain the Laws of Segregation and of
Independent Assortment and what features of
meiosis they correspond to.
Explain the terms gene, allele, dominant, recessive,
genotype, phenotype, heterozygous, homozygous,
Solve genetics problems involving monohybrid and
dihybrid crosses.
Explain incomplete dominance and co-dominance.
Mendelian Genetics
“father of genetics”
Proposed that characteristics
were passed from generation to
generation by “heritable factors”
Gregor Mendel
1822-1884
Mendelian Genetics
Mendel investigated many
characteristics of garden peas
1 Stamen removal
Parents
(P)
2 Pollen transfer
Stamens
Carpel
3 Carpel matures into pod
4 Seeds from
pod planted
Offspring
(F1)
Monohybrid
Cross
True Breeding
Parents
F1 generation
Monohybrid
Cross
True Breeding
Parents
F1 generation
all purple
Monohybrid
Cross
F1 generation
F2 generation
3 purple : 1 white
Mendel studied 7 traits.
From his studies Mendel
developed several ideas
about the inheritance of traits.
Mendel’s Conclusions
A plant has two heritable factors for each
character (e.g., flower color).
When a plant makes gametes, only one heritable
factor for each trait is given to each gamete.
When gametes unite, the new plant will have
a heritable factor from each parent.
Some heritable factors dominate over others.
Mendel’s Law of Segregation
The two copies of a particular gene separate
when an individual makes gametes.
Each gamete receives only one copy.
Dihybrid Cross
True Breeding
Parents
F1 generation
all TALL with Purple flowers
Dihybrid Cross
F1 generation
F2 generation
9:3:3:1
PpTt
x
PpTt
Mendel’s Law of Independent Assortment
The law of independent assortment.
Alleles of different genes assort
independently during gamete formation.
This law applies to genes on different
chromosomes.
Terminology
Allele – one version of the gene for a particular character
many genes have multiple alleles
Genotype – the two specific alleles that an individual has
--the genetic makeup of the individual
Homozygous – the two alleles are the same
Heterozygous – the two alleles are different
Phenotype – the appearance of the individual
Terminology
Dominant – presence of the allele will confer the phenotype
SYMBOL USES CAPITAL LETTER
Recessive – the phenotype will only be seen when
both alleles are recessive
SYMBOL USES lowercase LETTER
Co-dominant – both alleles contribute to phenotype
How do Mendel’s Laws
apply to Meiosis?
DNA replication
meiosis I
2
1
meiosis II
3
gametes (P)
gametes (p)
zygote (Pp)
or
meiosis I
meiosis I
meiosis II
pt
meiosis II
PT
pT
Pt
Mendel’s Laws
Law of Segregation
each gamete contains only one heritable factor for each trait
Homologous chromosomes separate at meiosis I
Law of Independent Assortment
heritable factors for one trait segregate independently
of the heritable factors for other traits
The orientation of each tetrad at the metaphase plate
in Meiosis I is random
(i.e., independent from orientation of other tetrads.)
Alleles occupy a physical
position on a chromosome
The Law of Independent Assortment
applies to genes
on different chromosomes.
Genes on the same chromosome
tend to be inherited together
Incomplete Dominance
F1
F2
Incomplete Dominance
Genotypes
HH
Homozygous
for ability to make
LDL receptors
Hh
Heterozygous
hh
Homozygous
for inability to make
LDL receptors
Phenotypes
LDL
LDL
receptor
Cell
Normal
Mild disease
Severe disease
Codominance
Two alleles of a gene produce
phenotypes that are both present in
the heterozygote.
Example:
ABO blood group system has three
alleles of the gene: IA, IB, and IO.
Codominance
The A, B, O sugars are covalently linked to
phospholipid headgroups on the RBC plasma membrane.
O
A
B
Why do we care?
Blood
Carbohydrates Present
Group
Genotypes
on Red Blood Cells
(Phenotype)
A
I AI A
or
I Ai
Carbohydrate A
Carbohydrate B
B
I BI B
or
I Bi
AB
I AI B
O
ii
Antibodies
Present
in Blood
Reaction When Blood from Groups Below Is
Mixed with Antibodies from Groups at Left
O
AB
A
B
Anti-B
Anti-A
Carbohydrate A
and
Carbohydrate B
None
Anti-A
Neither
Anti-B
No reaction
Clumping reaction
Blood Group Effects on Health
Type O compared to Type A, AB, and B individuals
have a 14% reduced risk of squamous cell carcinoma
and 4% reduced risk of basal cell carcinoma.
a reduced risk of pancreatic cancer.
Type B correlated with increased risk of ovarian cancer,
Gastric cancer reported to be most common in blood group AB
and least in group O.
More about these in Chapter 14
Pleiotropy – one gene having many effects
on phenotype.
Epistasis – the protein product of one gene
controls the expression of a second gene.