Mendel and Heredity
Ch 11.1-3 - Genetics
Genetics
• Study of Heredity
• Passing of traits from
parents to offspring
• Trait: a specific
characteristic
Gregor Mendel
• Father of genetics
• Parents were farmers
• He became ordained as a priest
• Studied science & math at Univ
of Vienna
Mendel’s Experiments
• Chose Pea plants as exp. Subjects
– Easy to cross
– Grow quickly
– Produce high number offspring
– Showed variety of contrasting traits
• Purple vs white flowers
• Tall vs short stems
• Round vs wrinkled seeds
Mendel’s experiments cont.
1. Chose true breeding lines of each
plant/trait he studied.
a. True breeding always produced offspring
of the same type
2. Crossed a true breeding plant with a
plant of the opposite trait. (purple x
white)
a. Called this the Parental (P) generation
3. Recorded data on the offspring of
this cross. (hybrid)
3) First Filial generation (F1)
4. Self pollinated the F1 offspring
5. Recorded data on the offspring of
the 2nd generation.
5) Second Filial generation (F2)
Analysis: F1 Generation
• Always displayed one trait (he later
called this the dominant trait)
• Must have within it the trait from
the original parents – the white
trait
Analysis: F2 Generation
• Displayed the hidden trait, ¼ of the F2
gen. had it (he later called this hidden
trait the recessive trait)
• Each individual has 2 ”factors” that
determine what external appearance
the offspring will have
– These factors are now called genes or
alleles
Mendel established 3 principles or
laws from his research
The Principle of Dominance
and Recessiveness
One trait is masked or covered up by
another trait
Principle of Segregation
The 2 factors (alleles) for a trait separate
during gamete formation
Principle of Independent
Assortment
Factors of a trait separate independently of one
another during gamete formation.
Example:
• Whether a flower is purple has
nothing to do with the length of
the plants stems – each trait is
independently inherited.
Modern Genetics
• Mendel’s factors are now called
Alleles. (different versions of a
gene)
– For every trait a person has, 2
alleles determine how that
trait is expressed
• We use letters to denote alleles,
since every gene has 2 alleles, all
genes can be represented by a pair
of letters
PP = purple,
Pp = purple
pp = white
Terms to know:
• Homozygous:
When the alleles are the same.
(true breeding)
Ex. AA, bb, EE, dd
Terms to know:
• Heterozygous:
When the alleles are different.
(dominant allele is
expressed)
Ex. Pp, Aa, Bb
Terms to know:
• Genotype:
Letters used to denote alleles
(BB, Pp, aa)
Terms to know:
• Phenotype:
What an organism looks like.
(brown, purple, curly hair….)
Terms to know:
• Monohybrid cross:
a cross involving one pair
of contrasting traits.
Ex. Pp x Pp
Terms to know:
• Punnett Square:
Used to determine the
Probability of having a certain
type of offspring given the
alleles of the parents.
How to solve a Punnett Square:
1. Determine the genotypes (letters) of
the parents.
1. (AA x aa ) or (Rr x Rr)
2. Set up the punnett square w/one
parent on each side.
3. Fill out the punnett square middle.
4. Analyze the number of offspring of
each type.