Chapter 14
Mendel and the Gene Idea
Gregor Mendel
Augustinian Monk
1857 he began breeding pea plants to study inheritance
Pea plants had many distinguishable characteristics
Character = a heritable feature that varies among individuals
Traits = variant for a character
He could control which pea plant were crossed
Mendel only tracked characters that had two traits
Began with two true bred plants of different varieties (white and purple flowers)
 P-generation (parental generation)
True Bred plants have offspring that are all of the same variety (purple)
Cross-pollinated these two varieties together  Hybridization
First hybrid offspring  F1 generation (first filial generation)
F1 generation plants are self-pollinated
Second hybrid offspring F2 generation
The Law of Segregation
All F1 generation offspring had the same trait as one of the plants the P-generation (purple)
In the F2 generation the trait of the other parent reappeared (purple and white)
This led to the Dominant and Recessive Allele Idea
Allele = alternative versions of a gene
Dominant allele (trait that will always appear if present)  purple
Recessive trait (only appear if the Dominant allele is not present  white
In the F1 only the dominant allele affects the visible characteristics of the plant
In F2 some plants have two recessive alleles present so that trait will show
3:1 ratio of Dominant to Recessive characteristics
Mendel’s Model
1. Alternative versions of genes account for variations in inherited characters
2. For each character an organism inherits two alleles, one from each parent
3. If the two alleles at a locus differ, then one, the dominant allele, determines the
organism’s appearance; the other, the recessive allele, has no noticeable effect on the
organism’s appearance
4. Law of Segregation – the two alleles for a heritable character separate (segregate)
during gamete formation an end up in different gametes
Helpful Vocabulary
Homozygous – an organism that has two of the same allele (P-generation)
Heterozygous – an organism that has two different alleles (F1 generation)
Phenotype – the expression of genes; the visible traits
Genotype – the genetic makeup of an organism; which alleles are present:
expressed in upper (dominant) and lower case (recessive) letters
The Testcross
Crossing a known-genotype plant (usually homozygous recessive)
with an unknown-genotype plant to determine its genotype
The Law of Independent Assortment
Monohybrid – heterozygous for one character (color)
Cross between heterozygotes for one character is a monohybrid cross
Dihybrids – heterozygote for two characters (color and shape)
Cross between dihybrids is a dihybrid cross
The alleles for sort themselves into gametes during meiosis
The alleles are not sorted together but sort independently of each other
This means that gametes can have any combination of alleles
Law of Independent Assortment
Each pair of alleles segregates independently of other pairs of alleles during gamete
formation
Punnett Squares and Genotypic/Phenotypic Probability
Punnett square – diagrammatic device used to predict the
allelic composition of offspring
Example - Heterozygous cross
– genotypic ratio 1:2:1 All dominant: Heterozygous: all recessive
– Phenotypic ratio 3:1 Dominant: Recessive
Multiplication Rule (Monohybrid crosses)
The chance of two independent events occurring together – multiply the probabilities
together
(Example; coin toss – ½ heads and ½ tails ½ x ½ = ¼ for all combinations)
Inheritance patterns
Spectrum of Dominance (example of: homozygous red flowers
and homozygous white flowers)
Complete Dominance – one allele is dominant over another
= red flowers (like Mendel’s flowers)
Co dominance – both alleles affect the phenotype
= spotted red and white flowers
Incomplete Dominance – the phenotypes are mixed
= pink flowers
coin toss
Co Dominance
Multiple Alleles
Example; blood type
A, B, AB, O
All IA alleles or IA and i= type A
All IB alleles or IB and i = type B
IA and IB = type AB
All i = type O
Incomplete Dominance
Epistasis
Pleiotrophy
Each allele has many different affects on phenotype
Epistasis
One gene affects the expression of another gene
(bald and curly hair - can’t have curly hair if you’re bald!)
A continuum of possible phenotypes (quantitative characters)
(skin color)
Polygenic Inheritance
Nature vs. Nurture
Humans – nutrition influences height, exercise affects build
Plants – soil acidity affects expression, flower color – blue-violet to red-violet color
Pedigrees
Help us to predict the future for traits
Answer whether or not the trait is dominant, recessive, or sex-linked
Recessive Sex-linked
Recessively Inherited Disorders
Homozygous Dominant – Normal
Heterozygous –Carrier (could pass trait to offspring; does not show the trait)
Homozygous Recessive – Has Trait
Cystic Fibrosis
Sickle-Cell Disease
Dominantly Inherited Disorders
Homozygous Dominant – Has Trait
Heterozygous – has Trait
Homozygous Recessive – Normal
Acohondroplasia (dwarfism)
Huntington’s disease