Genetics
The scientific study of heredity.
Vocabulary:
Make sure to DEFINE these
words as you go through the notes
Heredity
Variation
Dominant
Recessive
Purebred
Hybrid
Homozygous
Heterozygous
Phenotype
Genotype
DNA
Allele
Meiosis
What is Heredity ?
• The passing of traits from parents to their
offspring
• Traits are passed to you through your
parent’s sperm and egg
– 50% of DNA from mom, 50% from dad
• Variation (differences in people’s traits)
are due to the combination you inherit
1860s: Gregor Mendel
• “Father of Genetics”
(he was an Austrian monk)
• He figured out how traits are passed on before we
knew about DNA, or chromosomes or meiosis
To begin, Mendel created pure
breeding plants
• Purple
First Cross:
pure purple with pure white
• Cross fertilized P (Parental) generation
– Pure purple with pure white
purple
white
X
All purple
– P Generation
F1 Generation
Second Cross:
Cross 2 F1 plants
• Cross fertilized two of the offspring of the F1
purple purple
generation
purple
purple
X
purple
white
Mendel’s Experiments led him to
3 Laws of Inheritance
• The Law of Dominance
• The Law of Segregation
• The Law of Independent Assortment
The Law of Dominance:
• Each individual has TWO factors for each trait
known as alleles
– Some alleles dominate over others – the
dominant allele is expressed in the offspring
Freckles
No Freckles
No dimples
Dimples
These are
alleles
Non-roller
Tongue
roller
From Mom
From Dad
– Homologous chromosomes-same traits -different forms
Alleles are:
• Represented by a letter of the alphabet
Capital = dominant allele
lowercase = recessive allele
A
a
• Each allele corresponds to a gene on a
chromosome
- the alleles represent different forms of
the gene
Law of Segregation
– The two alleles for each trait separate and
move into different gametes – meiosis
– Since only one egg or one sperm will
contribute to the new offspring, only one allele
for a trait is passed on
– The chance that any allele will be passed on is
50%
Law of Independent Assortment
• the inheritance of alleles for one trait
doesn’t affect the inheritance of alleles for
another trait
- occurs in meiosis during metaphase 1
Meiosis: division of the sex cells
Genotype:
• Genotype is the combination of alleles for a
trait expressed with letters
• Homozygous dominant (purebred)
– TT - two capital letters
• Heterozygous (hybrid)
– Tt - one of each letter (big and small)
• Homozygous recessive (purebred)
– tt - two lower case letters
Phenotype:
• Phenotype is physical expression of the
genotype. (how it looks)
Genotype
Phenotype
TT
tall plant
Tt
tall plant
*dominant allele always is expressed over the recessive allele
tt
short plant
*recessive trait is expressed only if there is no dominant gene
If R = red, and r = white
• What is the genotype of homozygous
dominant?
– RR
• What is the heterozygous genotype?
– Rr
• What is the homozygous recessive
genotype?
– rr
If R=red and r =white
• What is the phenotype of of RR?
– Red
• What is the phenotype of Rr?
– Red
• What is the phenotype of rr?
– White
If two parents are crossed (TT X tt), what
are the resulting offspring?
You can determine phenotypes using a Punnet
square.
t
t
T
T
Tt
Tt
Tt
parent
Tt
parent
Genotypes: 4 Tt
Phenotypes: 4 Tall
This is the first or parent generation
Now try on your paper a cross of two of the
offspring from the parent generation. This is called
the first generation or the F1.
T
t
T
t
TT
Tt
Tt
tt
Genotypes: 1TT: 2Tt : 1tt
Phenotypes: 3 Tall: 1 Short.