Download Modeling Mendel*s Law

Mendelian Heredity
(Fundamentals of Genetics)
Chapter 9
Heredity

The transmission of characteristics from parent to
offspring.

Trait (allele)- variant of a characteristic

Mendel observed 7 characteristics of pea plants.
Characters vs. Traits
Mendel’s Methods

Mendel observed traits by controlling how pea plants
were pollinated.

Pollination occurs when pollen grains (male) are
transferred to the stigma (female.


Normally, self-pollination occurs

Mendel utilized cross-pollination, prevented selfpollination
The offspring of a “cross” will have characteristics from
both “parents”

Hybrid
Manual Cross-Pollination
Mendel’s Experiments

Used pea plants, easy to grow and quick to reproduce

Began by growing true-breeding (purebred) plants


Always produce offspring with that trait when they selfpollinate
Cross-pollinated pairs of plants that were true-breeding
for contrasting traits of a single characteristic

Parents were P generation

Offspring were F1 generation

F1 offspring then self-pollinated  F2 generation
Mendel’s Laws of Heredity


Each trait is controlled by a pair of factors (alleles)

Dominant (Capital letter)

Recessive (Lowercase letter)
Law of Dominance - One factor (trait) in a pair may mask the effect of
the other.
Mendel’s Laws of Heredity

Each reproductive cell gets one factor (allele) of each pair


When two gametes combine, the offspring has two factors for each
characteristic
Law of Segregation - The two traits for a characteristic separate during
the formation of eggs and sperm.
Mendel’s Laws of Heredity

Factors for individual characteristics are not connected (Anaphase I)


Ex. Flower color vs. seed color (Green pods on a white plant)
Law of Independent Assortment - The traits for different characteristics
are distributed to reproductive cells independently.
Genotype and Phenotype


Genotype- organism’s genetic makeup

Ex. Genotype of a white-flowering plant consists of two
recessive alleles for white color, pp.

Ex. Genotype of a purple-flowering plant can either be PP
or Pp.
Phenotype- organism’s appearance

Ex. Phenotype of a PP or Pp pea plant is purple flowers.

Ex. Phenotype of a pp pea plant is white flowers.
Genetic Terms


Homozygous- both alleles of a pair are alike

Dominant PP

Recessive pp
Heterozygous- two alleles in the pair are different

Heterozygous for flower color- Pp (Purple)
Using Punnett Squares

Punnett Square

Model that predicts the likely outcomes of a genetic cross
with the alleles taken from the parents

Shows all the genotypes that could result from a given
cross
Monohybrid Cross

A cross in which only one characteristic is tracked

Contrasting traits (purple vs white flower)

Homozygous Cross (PP or pp)

Heterozygous Cross (Pp)

Paternal alleles across the top

Maternal alleles down the left side
Using Probability

Likelihood that a specific event will occur

Probability = # of one kind of possible outcomes
total # of all possible outcomes

Probability of a specific allele in a gamete

Represented as a fraction or ratio
Punnett Square Practice

1. Homozygous x Homozygous


2. Homozygous x Herterozygous


Cross between a pea plant homozygous for purple flower
color (PP) and a pea plant homozygous for white flower
(pp)
Cross between a guinea pig that is homozygous dominant
for the trait of black coat color (BB) and a guinea pig that
is heterozygous for this trait (Bb)
3. Heterozygous x Heterozygous

Cross between two rabbits that are heterozygous for coat
color. In rabbits, the allele for black coat color (B) is
dominant over the allele for brown coat color (b).
Many Genes, Many Alleles


Polygenic Inheritance

Example: Eye Color

Several genes affect the characteristic

Relative greenness or brownness, blue eyes are recessive
Incomplete Dominance

Intermediate between traits

Example: Snapdragon (red and white = pink)
Many Genes, Many Alleles

Codominance

Both alleles for the same gene are fully expressed


Example: Roan Shorthorn Cattle
Multiple Alleles

3 or more possible alleles, only two alleles for a gene can
be present

Complex dominance

Example: Blood types and Labrador coat color
Heredity & The Environment

Genes can be affected by the environment

Nutrients and temperature

Personality and behavior
Uses for Heredity

Pedigrees

Several generations

Determine genetic disorders or diseases that can be
inherited

Can answer questions about the three factors of
inheritance

Sex-linked genes

Dominant or Recessive

Heterozygous or Homozygous