Genetics
11.1 & 11.2
The scientific study of heredity.
What is Heredity ?
• The passing of traits from parents to their
offspring
1860s: Gregor Mendel
• “Father of Genetics”
• Mendel figured out how traits are passed on
– before we knew about DNA, or chromosomes or
meiosis
Mendelstudied heredity with pea
plants
• Why pea plants? (was it luck?)
–
–
–
–
–
–
–
1. Small
2. Easy to grow
3. Produces large numbers of offspring
4. Matures quickly
5. Plants can self or cross fertilize
6. Many varieties
7. Each trait has two forms
• Tall/short
purple/white flowers
• green/yellow seeds
colored/white seed coat
• green/yellow seed pods
inflated/narrowed seed pod
• wrinkled/smooth seed
The Role of Fertilization
Male part of each flower makes pollen, which contains
sperm—the plant’s male reproductive cells.
Female portion of each flower produces reproductive
cells called eggs.
To begin, Mendel self fertilized to
create 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
Genes and Alleles
All of Mendel’s crosses had the characteristics of only
one of their parents.
An individual’s characteristics are determined by
factors (genes) that are passed from parent to offspring
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
Non-roller
Tongue
roller
From Mom
From Dad
These are
alleles
Alleles are:
• Represented by a letter of the alphabet
Capital = dominant allele
lowercase = recessive allele
A
a
- Each allele 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
Ex: Eye color traits will not affect a person’s
hair color.
The Two-Factor Cross: F2
Mendel’s experimental
results were very close
to the 9:3:3:1 ratio
Genotype:
• Genotype is the combination of alleles for a
particular expressed with letters
• Homozygous dominant:
– TT - two capital letters
• Heterozygous
– Tt - one of each letter (big and small)
• Homozygous recessive
– tt - two lower case letters
Phenotype:
• Phenotype is physical expression of the
genotype
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
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.
•
http://www.youtube.com/watch?v=CBezq1fFUEA