Transcript Mendels Laws of Heredity

10.1
Martin aka Tha’ Boss
Heredity: the passing on of
characteristics from parents to
offspring
◦ Traits: the characteristics that are
inherited
 Genetics: the branch of biology that
studies heredity


A. Mendel Chose his subject carefully
◦ He chose to use garden peas to
experiment with
 Because they reproduce asexually and
have both male and female reproductive
organs
 Gametes: male and female sex cells
 Fertilization: the union of male and
female gametes to form a zygote
 Zygote develops into a “seed”

A. Mendel Chose his subject carefully
◦ Pollination: when plants transfer pollen
grains from the male reproductive organ
to the female reproductive organ
◦ Self-pollination: plants usually do this
because they have both male and female
organs
◦ Let’s take a look at what Mendel did…
1. Mendel cut away male
pollen organs from
purple
2. Mendel got pollen from
the white plant and
transferred it to the
female part of the
purple plant
3. Allowed purple plant to
produce “seeds” from
this pollination and
planted seeds to see
what color offspring
would be produced.
Hybrid: the offspring of parents that
have different forms of a trait
 Monohybrid: where two parents only
differ by “one” trait


A. The First Generation
◦ Cross Pollination of 2 plants
 1 short, 1 tall
◦ All offspring grew as tall as the
tallest parent
◦ The short plant characteristic has
been “masked” in this generation
◦ Parent 1 x Parent 2 = F1 Generation

B. The Second Generation
◦ Mendel allowed the F1 offspring to “self
pollinate”, then planted the seeds
◦ He noticed now that:
 3/4ths of the plants were tall
 1/4th of the plants were short
◦ 3:1 ratio in the F2 offspring
◦ F1 x F1 = F2
C. The Rule of Unit Factors
 Alleles: the different forms that genes
can exist in

◦ in other words, each gene is represented
by two “letters”. Letters can be capital or
lowercase. Each letter is an allele and
each gene is represented by 2
letters/alleles
 TT, Tt, tt
D. The Rule of Dominance
 Dominant: the observed trait in the
F1
 Recessive: the trait that disappears in
F1, only to return in the F2


E. The Law of Segregation
◦ States that every individual has two alleles
of each gene and when gametes are
produced, each gamete receives one of
these alleles
◦ During fertilization, gametes randomly
pair to produce combinations of alleles

Phenotype: the way an organism
looks or behaves
◦ Its physical appearance
 Tall, or short

Genotype: the allele combination an
organism contains
 TT, Tt, tt

Genotypes can be described two ways
◦ Homozygous: TT, or tt
 Because the alleles are the “same” (homo)
 TT is “homozygous dominant”
 tt is “homozygous recessive”
◦ Heterozygous: Tt
 Because the alleles are different (hetero)
A. The First Generation:
 Mendel took true breeding round
yellow (RRYY) seeds and crossed with
true breeding green/wrinkled (rryy)
seeds

◦ This produced an F1 that had all
round/yellow seeds
 This proved that yellow/round were the
dominant traits
B. The Second Generation:
 Mendel allowed a single F1 offspring
to self-pollinate

◦ This produced an F2 that had the
following phenotypes




9
3
3
1
–
–
–
–
Round/Yellow
Round/Green
Wrinkled/Yellow
Wrinkled Green
B. The Second Generation:
 The results from Mendel’s F2
experiment led him to his 2nd law
regarding genetics

◦ The Law of Independent Assortment

C. The Law of Independent
Assortment:
◦ States that genes for different traits are
inherited independently of each other




We should all be able to complete
monohybrid and dihybrid crosses using
Punnett Squares at this time
We should be able to describe genotypes and
phenotypes of offspring
We should be able to describe ratios among
offspring and describe probabilities of
phenotypes among offspring
But can we?