Transcript Dominant or Recessive trait?

Genes & Traits
Gene – section of DNA that encodes a
protein, resulting in/affecting a trait
 Genetics – study of heredity, or how
organisms inherit characteristics from
parents
 Trait – a characteristic of an organism;
*genetic traits are inherited*
Ex.: hair color, enzymes, size (potential),
etc.

Genetics (ch. 9 & 12)
There once was an Austrian
monk who liked peas and was
very good at math…
Gregor Mendel’s (1860s)
Pea Plant Experiments
Mendel crossed pea plants with
different traits and observed the
results in the offspring.
Mendel’s Pea Plant Experiments
When two purebreds were crossed, recessive traits disappeared in the F1,
reappeared in the F2.
Mendel hypothesized that each trait was controlled by a “factor” and
that there must be at least two forms of each “factor.”
Dominant
Recessive
Why pea plants?
•short life cycle
•traits easily observed, no blending
•self-fertilize easily, easy to manipulate/fertilize
Most genetic traits in nature are not so easy to distinguish.
Genes & Traits

Allele – one form of a gene


Dominant allele – expressed whenever
present, represented by a capital letter; ex.: B
for brown hair color
Recessive allele – only expressed when two
copies are present; represented by a lower
case letter; ex.: b for blonde hair color
A diploid organism inherits two alleles
(one from each parent) for every genetic
trait
 Genotype - the set of alleles for a trait
or all traits of an organism

Genotypes
Purebred/Homozygous
Hybrid/Heterozygous
Organism that has only one
form of a gene/trait
Organism that has two forms of
a gene/trait
(same allele received from
both parents)
(different alleles received from
each parent)
BB – homozygous dominant Bb – heterozygous genotype
genotype
bb – homozygous recessive
genotype
Phenotypes



Phenotype - the trait the organism actually
expresses; ex.: having brown hair, or having blonde hair
The phenotype may show on the appearance of the
organisms or not; ex.: being able to digest lactose is a
phenotype that does not “show”
A phenotype results from the expression of a gene  a
protein is made that affects the phenotype
Phenotype
Possible Genotypes
Dominant
Ex.: brown hair
BB or
Bb
Recessive
Ex.: blonde hair
bb
When we look at the genotypes in each
generation:
(not in notes)
Pedigrees
Pedigrees & Genetic Counseling
People with PKU (phenylketonuria) lack an enzyme needed to
break down the amino acid phenylalanine, which is found in milk
and other foods. Accumulation of the amino acid in the body
leads to developmental disabilities and death if not treated.
Pedigree -- Dominant or Recessive trait?
Not always obvious…
* Notice that
the hybrid
individuals are
not identified. *
Trait appears in every generation 
More likely to be a dominant trait, but cannot be sure.
Pedigree -- Dominant or Recessive trait?
Not always obvious…
* Notice that
the hybrid
individuals are
not identified. *
Trait skips a generation or more 
More likely to be a recessive trait, but cannot be sure.
Autosomal Recessive Traits in Humans
(know all traits in bold print)




Click for
Albinism – skin, hair, eyes lack melanin (pigment), sensitive to
Albino sunlight
pictures
Tay-Sachs – fat deposits prevent normal development of brain in
children
Cystic Fibrosis – lungs damaged from excessive mucous, respiratory
problems
Sickle Cell Anemia – red blood cells misshaped, clog capillaries 
circulation problems; many phenotypic effects, depending on
where in the body the blockage occurs
Inheritance Pattern - Autosomal Recessive Trait
Autosomal Dominant Traits in Humans
(know all traits in bold print)


Dimples, freckles, widow’s peak, farsightedness, broad lips, polydactyly
(extra fingers/toes)
Dominant disorders are less common than recessive disorders because
carriers (heterozygotes) are affected and may die before reproducing
Ex.: Huntington disease – deterioration of nervous system, especially
brain
Inheritance Pattern - Autosomal DominantTrait
Inheritantce Pattern
Autosomal Dominant (not in notes)

Variable
Expression:
Some people have
milder / more
severe simptoms
than others -Age of onset and
organs affected
may vary

Reduced
Penetrance:
Some people have
the dominant gene
but not have any
simptoms
Ex.: autosomal
dominant cancer
susceptibility
Predicting the Phenotype of Offspring:
Using a Punnett Square for One Trait
*Monohybrid cross
*
Genotypic Ratio:
YY : Yy : yy
Phenotypic Ratio:
Yellow : Green
1:2 :1
3:1
Predicting the Phenotypes of Offspring:
Using a Punnett Square for Two Traits
*Dihybrid cross
*
Phenotypic Ratio:
9
:
3
:
3
:
1
Round & Yellow : Round & Green : Wrinkled & Yellow : Wrinkled & Green
Walter Sutton’s (1903)
Chromosome Theory of Heredity
Sutton (American) noticed that
chromosomes behaved like Mendel’s
factors
 Sutton’s Chromosome Theory of Heredity
states that the material of inheritance
is carried by the genes in
chromosomes
 Theodor Boveri (German) reached the
same conclusion independently

Mendel’s Laws (modern form)

Law of Segregation - Gene pairs separate when
gametes form:
Half of an organisms gametes contain one gene from a
homologous pair, half contain the other gene

Law of Independent Assortment - Different
traits are inherited independently:
Genes for different traits segregate into gametes randomly
and independently from each other  new combinations
You can inherit your father’s eyes and your mother’s smile!

Law of Dominance - Dominant alleles are
expressed, recessive alleles can be hidden:
Hybrid pea plant: Py  P (purple flowers, dominant) is
expressed, p (white flowers, recessive) is hidden