Patterns of Inheritance
Gregor Mendel
Pea plants
Complete dominance
Law of Segregation
Law of Independent Assortment
Law of Independent Assortment
Practice
Two organisms, with genotypes BbDD and BBDd are crossed.
Assuming independent assortment occurs for both genes, what
genotype ratios do you expect?
Practice
Y = yellow seeds and y = green seeds / T = tall plant and t = short plant
An F1 generation yields 12 yellow, tall plants and 4 yellow, short plants.
What are the genotypes of the P generation plants?
Law of Multiplicatiom
Used to determine whether two or more independent events will occur
together in some specific combination.
e.g. What is the probability that an Rr x Rr cross will yield the rr
genotype? 1/2 x 1/2 = 1/4
Law of Additon
Used to determine the probability that any one of two or more mutually
exclusive events will occur.
e.g. In the Rr x Rr cross, the Rr genotype can result from two mutually
exclusive events. The sperm can donate the R and the egg r or vice
versa. So, 1/4 + 1/4 = 1/2 chance at Rr.
Practice
What is the probability that YyRr will result from a YyRr x YyRr cross?
What is the probability that ppYyrr will result from a PpYyRr x Ppyyrr
cross?
Mendelian Inheritiance
Aka. complete dominance
Each character is determined by a single gene, for which there are two
alleles, one completely dominant and the other recessive.
A note on dominance...
Dominance does not mean the allele is most common in a population.
It means that in a complete dominance situation, the heterozygote
expresses only the dominant trait.
Pedigrees
Practice
Draw the following pedigree. Identify
the genotypes and whether the trait is
dominant or recessive.
Types of Characters
Discrete (e.g. Tongue rolling, ear-lobe attachment)
Quantitative (e.g. Height, skin color)
Multifactorial (e.g. Height, build, skin, intelligence)
Non-Mendelian Patterns
Incomplete Dominance
Neither gene is completely dominant.
e.g. flower color in some plants,
Codominance
Each allele affects the phenotype in
separate, distinguishable ways.
e.g. Human mn blood group, flower
color in some plants
Multiple Alleles
More than two alleles for a character exist in a population of organisms.
e.g. ABO blood groups
Practice
Cross an individual heterozygous for type A blood with one who is
heterozygous for type B blood. What are the expected genotype and
phenotype ratios?
Polygenic Inheritance
When two or more genes have an additive affect on a single
phenotypic character.
e.g. Skin pigmentation
Plieotropy
One gene has multiple phenotypic effects.
e.g. cystic fibrosis, sickle-cell disease
Epistasis
The phenomenon when the
phenotypic expression of one
gene alters the phenotypic
expression of another gene.
e.g. Fur color in mice and dogs
Practice
B = black fur, b = brown fur and E = pigment deposition, e = no pigment
deposition
These genes have an epistatic relationship.
What are the expected fur color phenotype ratios (black : brown :
yellow) when two dogs heterozygous for both traits are crossed?
X-linked
When a gene is located on the X
chromosome.
e.g. Color blindness, hemophilia
Practice
Neither Tim nor Rhoda have Duchenne muscular dystrophy, but their
first born son has it. What is the probability that a second child of this
couple will have the disease?
Practice
A man with hemophilia (a recessive, x-linked condition) has a daughter
of normal phenotype. She marries a man normal for the trait. What is
the probability that a daughter of this mating will be a hemophiliac?
Patterns of Inheritance Overview
Complete dominance
Incomplete dominance
Codominance
Multiple Alleles
Polygenic Inheritance
Plieotropy
Epistasis
Sex-linked, X-linked
The relationship between dominance
and phenotype
For any character, the observed dominant/recessive relationship of
alleles depends on the level (organism all, molecular, biochemical) at
which we examine the phenotype.
e.g. See Tay-Sachs description on p. 272
Remember an allele is a nucleotide sequence.