Download Biology 345 Organic Evolution

Module II
Mendelian Genetics &
Probability Theory
GENETICS
• Genetics is the study of heredity and variation in
organisms
• Genes are the hereditary units transmitted from
generation to the next
• Genes reside in the long molecules of
deoxyribonucleic acid (DNA) that exists within
the cell
• DNA, in conjunction with a protein matrix forms
nucleoprotein and becomes organized into
structures called chromosomes that are found in
the nucleus of the cell
ALLELES
• Mutation is a change in some part of the DNA
• The result of a mutation is a change a character or
trait of the individual
• Through a mutation, a gene may be changed into
two or more alternative forms called alleles
• Homologous chromosomes are genetically similar
MENDEL’S LAWS
• Law of segregation: If a parent contains 2
alleles of a trait, only 1 is transmitted
through a gamete to the offspring
• Law of independent assortment:
Segregation of one gene pair occurs
independently of any other gene pair
Overview/Review of Basic Genetics
Genetic material (DNA) is stored within structures called
chromosomes.
In sexually reproducing organisms (e.g. mammals and birds),
an individual has two copies of each type of chromosome.
Every species has a specific number of chromosomes (in
humans there are 23 pairs of chromosomes = 46 total).
Overview/Review of Basic Genetics
Genes (sequences of DNA which encode proteins) are found
on chromosomes.
Each chromosome
in a pair has the
same genes in the
same order.
However, specific
genes can be
variable and the
two chromsomes
may not have the
same gene variant.
Different
Genes
Overview/Review of Basic Genetics
Variants of a specific gene are known as alleles.
Example: There is a gene in humans that controls basic eye
color. There are two alleles: Blue (b) and Brown (B).
The genotype of an individual is the set of alleles the
individual has.
Question: What are the possible genotypes for eye color?
Answer:
BB or Bb or bb
Overview/Review of Basic Genetics
The phenotype of an individual is the physical characteristic
formed by their specific alleles.
Example: An individual with the brown alleles (BB) has a
brown eyed phenotype. An individual with two blue alleles
(bb) has a blue eyed phenotype.
Question: Does an individual with 1 brown and 1 blue allele
(Bb) have one brown eye and one blue eye?
Answer: No. The brown allele is dominant over the blue
allele (which is recessive). An individual with both alleles
(heterozygous) has the phenotype of the dominant allele. Thus
the Bb genotype = Brown phenotype.
Overview/Review of Basic Genetics
Another example: Many plants can have flowers of different
colors.
Bougainvillea
Snap Dragons
Bougainvillea on Campus
Overview/Review of Basic Genetics
In many plants, flower color is controlled by a single gene.
There are alleles for Red (R) and White (W) and sometimes
other colors (e.g., yellow).
A plant with an RR genotype has a red phenotype.
A plant with a WW genotype has a white phenotype.
A plant with an RW genotype has a pink phenotype.
Neither R nor W are dominant. The are called co-dominant
and the observed phenotype of a heterozygote is in between
the phenotype of either homozygote.
Overview/Review of Basic Genetics
Question: Can you predict phenotype from genotype?
Answer: Yes. For eye color we know that BB and Bb are
always brown and bb is always blue.
Question: Can you predict genotype from phenotype?
Answer: No. While we know a blue eyed person must have a
bb genotype, a brown eyed person could have either Bb or BB.
All we know for sure is they have at least 1 B.
What does any of this have to do with math?
An individual’s chromosomes are inherited from their parents.
One from the father and one from the mother.
When an individual has children, half of its chromosomes (one
from each chromosome pair) will go into the child.
The process by which a specific chromosome (and therefore
specific alleles) goes into a child is random.
What does any of this have to do with math?
Pretend you are going to breed a pair of plants with pink
flowers.
Question: What are the phenotypes of these plants? What are
the genotypes?
Answer: Phenotypes are both pink. Genotypes are both RW.
Questions: What are the possible genotypes and phenotypes of
the offspring? What is the probability of getting these
genotypes and phenotypes?
Answer: You may know the answer, but do you know the
mathematics behind it…