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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
– P Generation F1 Generation
purple white
X
All purple
Second Cross:Cross 2 F1 plants• Cross fertilized two of the offspring
of the F1 generation
purple
X
purple
white
purple purple
purple
Genes and AllelesAll 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
Dimples
Tongue roller
From Mom From Dad
No Freckles
No dimples
Non-roller
These are
alleles
Alleles are:
• Represented by a letter of the alphabet
Capital = dominant allele A lowercase = recessive allele 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: F2Mendel’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 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
You can determine phenotypes using a Punnet square.
Tt Tt
Tt Tt
If two parents are crossed (TT X tt), what are the resulting offspring?
T T
t
t
parent
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.
Tt
Tt
T
T
t
t
TT
tt
Genotypes: 1TT: 2Tt : 1ttPhenotypes: 3 Tall: 1 Short.
