Genetics
Heredity passing down of traits or
characteristics from parents to offspring
Genetics  study of heredity
Gregor Mendel (1822-1884)
Worked with Pea Plants. 2 major traits for each plant
Plants were either tall (T) or short (t)
Plants were either Purple (P) or white (p)
Mendel’s Experiment
Male and Female sex organs in pea plants are on the same
flower (stamen have pollen and stigma contains the egg)
Pure Traits  Traits established through self-fertilization
Stamen
Stigma
Genetics
•Pure white individuals x pure purple individuals
•F1 All purple offspring
•F1 x F1  3:1 ratio of purple to white pea plants
•Out of 1000 plants 750 were purple and 250 were white
•Results of the Parental cross showed that one trait covered up the other
trait
X
=
Genetics Vocabulary
Dominant Trait  Trait that masks the recessive
trait
Recessive Trait  Trait that is being covered up
In this cross PURPLE color was dominant
Mendel’s Hypothesis
Gene  sequence of DNA on a chromosome
Each pea plant trait is a combination of 2 genes,
one from mom and one from dad
Each alternative gene is called an allele
Dominant Gene  shown by a capital letter (P)
Recessive Gene  shown by a lower case letter (p)
Pure Purple plants  PP
Pure white plants pp
PP x pp  all Pp
Pp x Pp PP or Pp or pP or pp
Mendel’s Terms
Homozygous  2 of the same allele
(PP or pp)
Heterozygous  2 different alleles
(Pp)
Punnett Square  Results of a
cross can be predicted
Incomplete Dominance
Some traits do not have dominant alleles
Heterozygotes appear as a mix between 2 parents
1 trait = 3 phenotypes
4 O’clock Flowers
Rr x Rr
Ratio of Offspring
1 Red
2 Pink
1 white
Codominance
Neither allele is dominant. Multiple alleles are
expressed
Presence of heterozygote means both alleles are
expressed equally
Blood type is an example
Sex Determination
Thomas Morgan discovered that chromosomes differed in males and
females
Females- XX
Males- XY
Called Sex Chromosomes
All other chromosomes are called autosomes
Sex-linked Inheritance
Morgan crossed white-eyed male with red-eyed
female
F1Ratio: All Red Eyes
F2 Ratio: 3:1 ratio, but never any white females
Reasoned that there are genes on sex
chromosomes
Allele for eye color is on X chromosome
No gene for eye color on Y chromosome
Presence of homozygous recessive, or ONLY a
recessive allele, yields recessive trait
Morgan’s Crosses
Sex Influenced Traits (Not on test!)
• Sex-influenced traits are autosomal traits
that are influenced by sex. If a male has
one recessive allele, he will show that trait,
but it will take two recessive for the female
to show that same trait. One such gene is
baldness.
• BB normal male & female
• Bb  bald male; normal female
• bb  bald male; bald female
Dihybrid Crosses
Dihybrid Cross: Cross: Cross of 2 individuals for 2 different traits
Example:
In Pea Plants
Tall(T) is dominant to short
Purple(P) is dominant to white
Crossing homozygous tall, purple plant with a homozygous short, white
plant
TTPP x ttpp
Crossing heterozygous tall, purple plant with a heterozygous tall, purple
plant
TtPp x TtPp
Genetic Diseases
Autosomal dominant disease  Presence of dominant allele means
that individual will have the disease
Autosomal recessive disease  Disease only present when BOTH
recessive alleles are present
Sex-linked Dominant  Disease present when dominant allele is
present on either sex chromosome
Sex-linked Recessive  Disease present when recessive allele is
present on either sex chromosome
Common Genetic Diseases
Huntington’s Disease Autosomal dominant ;
Neuromuscular disease; degeneration of muscle tissue; onset in early 30’s.
Folk Singer Woody Guthrie had the disease
Cystic Fibrosis Autosomal Recessive; Defective Protein is made that
creates excess mucus; clogs lungs.
Color Blindness Sex-linked recessive
Inablity to distinguish colors (8% of male population)
Some Genetics Diseases
Hemophilia Sex-linked recessive
Inablity of blood to clot
Nicholas, Czar of Russia (Mid 1800’s) child was
hemophiliac
Muscular Dystrophy Sex-linked recessive
Tay-Sach’s Disease  Autosomal Recessive
Degeneration of Central Nervous System; infant mortality
Changes in Chromosome Number
Nondisjunction – occurs when:
In meiosis I, homologous pair both go into the same
daughter cell or
In meiosis II, the sister chromatids both go into the
same gamete.
The result:
Trisomy (3 copies of a single chromosome) or
Monosomy (1 copy of a single chromosome)
Nondisjunction in meiosis I
Changes in Chromosome Structure
• Mutation - a permanent genetic change.
• Chromosome mutation - a change in chromosome
structure
• Radiation, organic chemicals, or even viruses may
cause chromosomes to break, leading to mutations.
• Types of chromosomal mutations: inversion,
translocation, deletion, and duplication.
Deletions
Deletions occur when a single break causes a lost end
piece, or two breaks result in a loss in the interior.
An individual inherits a normal chromosome from one
parent and a chromosome with a deletion from the
other parent
No longer has a pair of alleles for each trait
A syndrome can result – type depends on
chromosome(s) affected.
Williams Syndrome
• Chromosome 7 loses an end piece
• Children have a pixie look (turned-up noses, wide
mouth, small chin, large ears)
• Poor academic skills, good verbal and musical
abilities
• Skin ages prematurely from lack of the gene that
controls the production of elastin (also affects
cardiovascular health).
Duplication
Duplication results in a chromosome segment being
repeated in the same chromosome
Produces extra alleles for a trait.
Ex: An inverted duplication in chromosome 15 causes
inv dup 15 syndrome
Poor muscle tone, mental retardation, seizures,
curved spine, and autistic characteristics
Duplication
• Translocation
Translocation is exchange of chromosomal segments
between two, non-homologous chromosomes.
Ex: Alagille syndrome results from a deletion of
chromosome 20 or a translocation that disrupts an
allele on chromosome 20.
Distinctive face, abnormalities of eyes & internal
organs, and severe itching.
Translocation
Inversion
• Inversion involves a segment of a chromosome
being turned 180 degrees
• The reverse sequence of alleles can alter gene
activity.
• Crossing-over between inverted and normal
chromosomes can cause duplications and
deletions in resulting chromosomes.
Inversion