Fundamentals of
Genetics
Patterns of Inheritance

The History of Genetics
 Genetics – scientific study of heredity
 Trait – characteristic that can be passed from
parents to offspring
Father of Genetics
 Gregor
Mendel
(born 1822)
 Austrian
Monk
 Studied 7 different
pea plant traits that
are true-breeding
 Produce
offspring
identical to
themselves
Mendel’s
st
1
Conclusion
 Biological
inheritance is passed
from one generation to the next-
Genes
 Alleles- different forms of a gene
Parental Cross
 Cross-pollinated
flowers with
opposite traits (tall, short)
P
generation
 Plants
from the first cross were all
tall- hybrids.
 F1
generation
F1 Cross
 Allowed
plants from P cross to self-
fertilize
 The recessive trait had reappeared!!
 About 25% of the offspring (F2
generation) were short.
Mendel’s
 Principle
 Some
nd
2
Conclusion
of Dominance
alleles are dominant
Capital
Letter -R,D
This trait will be always be seen
 Some
alleles are recessive
Lower
case Letter –r,d
This trait will only be seen when a
dominant allele is NOT present
Mendel’s
 Principle
 Alleles
rd
3
Conclusion
of Segregation
separate from each other during
formation of sex cells, or gametes.

Purebred – organism receives the same genetic
traits form both of its parents
 Homozygous – AA or aa

Hybrid – organism receives different forms of
a genetic trait
 Heterozygous - Aa
Important Terms




Genes – sections of a chromosome that code
for a trait
Allele – distinct form of a gene
Dominant Allele – expressed when two different
alleles are present; represented with capital letter
Recessive Allele – form of gene that is not
expressed when paired with a dominant allele;
represented with lower case letter
Genes Represent Traits

Genotype – genes that make up an organism

Includes both genes in a homologous pair

Phenotype – outward expression of the trait

Homozygous – two alleles are identical (AA or aa)

Heterozygous – two alleles are different (Aa)

Also known as a hybrid organism
Genetics and
Prediction
Predictions for One Trait


Probability – predict likelihood of an event or
outcome
Punnett square – grid for organizing genetic
information
Can be used to make predictions about a cross
between two organsims
 Monohybrid Cross – cross between two parents and
one trait

Monohybrid Cross
Predictions for Two Traits

Dihybrid Cross – cross between two parents and
two traits
Use a 4 x 4 Punnett square
 Sixteen possible outcomes

Incomplete Dominance

Heterozygous offspring show a phenotype that
is in-between the phenotypes of the two
homozygous parents
Blending of traits
 Color in snapdragons;
instead of white or red,
color is pink

Codominance

Both alleles are expressed
 Example: Blood type
 IA, IB, i(O), or IAIB blood type
 Both A and B are dominant so they are both
expressed in the IAIB blood type
Polygenic Trait

Trait controlled by more than one gene
 Example: eye color
Pleiotropy

Single gene affects
more than one trait

Example: sickle cell
anemia
 Effects
include
blood cell shape,
anemia, weakness,
brain damage,
spleen damage,
and heart damage
Environmental Effects

Phenotype is a combination of genetic and
environmental influences
 Example: Himalayan rabbit – fur color
depends on body temperature
Human Genetics
Difficulties in Studying
Human Heredity
Controlled
experiments are not
possible
 Small numbers of
offspring per
generation
 Long periods
between generations

Sex Determination

Autosomes – body chromosomes – first 22 pair


Same in both male and female
Sex Chromosomes – 1 pair, last pair

XX – Female

XY - Male
X
X
X
XX
XX
XX
XX = 50%
XY = 50%
Y
XY
XY
You always have a 50% chance of having
a girl and a 50% chance of having a boy!
Sex Linked Traits


Traits controlled by recessive genes located on sex
chromosomes (normally associated with the X
chromosome)
Hemophilia – blood clotting enzyme
is absent
 Queen Victoria’s family affected
Sex Linked Traits

Red – Green Colorblindness – individuals cannot
distinguish between these two colors
Sex Linked Traits

Duchene Muscular
Dystrophy –
wasting away of
skeletal muscle
XR
Xr
XR
XR XR
XR Xr
Y
XR Y
Xr Y
XR
Xr
Xr
XR Xr
Xr Xr
Y
XR Y
Xr Y
Females
•XRXR = normal
•XRXr = carrier
•XrXr = disease
Males
•XRY = normal
•XrY = disease
Sex – Limited Traits
Controlled by genes located in the autosomes
 Only expressed in the presence of sex
hormone
 Only expressed in one gender
 First seen at puberty (hormones produced in
large enough quantities

Sex – Limited Traits

Examples:
 Male bird’s
colorful plumage
 Beard growth in
males
 Milk production in
females
Sex Influenced Traits




Traits that are caused by a gene whose expression
differs in males and females
Also located on autosomes
Expressed in the presence of male or female
hormones
Example: male pattern baldness



BB = normal
Bb = males bald, females normal
bb = males and females bald
Hereditary Disorders

Techniques for detecting genetic disorders
 Amniocentesis – technique used that removes and
studies amniotic fluid
during pregnancy;
 identifies only
chromosomal disorders
Hereditary Disorders

Karyotyping – an
enlarged photo of the
chromosome pairs
(map) to identify any
abnormalities in the
chromosomes
Chromosomal Disorders
 Chromosomes
fail to separate
during meiosis
 Nondisjuction-abnormal
numbers
of chromosomes enter gametes
Pedigree Charts


Chart which shows how a trait and the genes that
control it are inherited within a family
Identifies the presence or absence of particular trait
in members of each generation
Pedigree Charts







Males =
Females =
Generations = Roman Numerals
Individuals = Numbered sequentially
Trait Expressed = Filled in
Non Carriers = Empty
Carrier (not ill) = Half filled

Individual who carries a recessive allele that is not
expressed