BIOL 22000 Lecture Notes - Lecture 12: Convergent Evolution, Disruptive Selection, Parallel Evolution
8 Jun 2018
School
Department
Course
Professor
Chapter 12 Genetics
12.1Fundamental Concepts of Genetics
• Each human being possesses two copies of each chromosome, called homologues
o Except for male sex chromosomes, one Y and one X
• Each gene has a locus, or location on a specific chromosome
• If only one copy of an allele is needed to express a given phenotype, the allele is said to
be dominant
o Represented with a capital letter
• If two copies are needed to express a given phenotype, the allele is said to be recessive
o Represented with a lowercase letter
• If both alleles are the same for a given gene, the individual is said to have a homozygous
genotype
• If the alleles are different, the individual has a heterozygous genotype
• A hemizygous genotype describes a situation in which only one allele is present for a
given gene
o X chromosome in males
PATTERNS OF DOMINANCE
• When only one dominant and one recessive allele exist for a given gene, there is said to
be complete dominance
o The presence of one dominant allele will mask the recessive allele
• When more than one dominant allele exists for a given gene, there is codominance
o For example, a person with one allele for the A blood antigen and one allele for
the B blood antigen will express both antigens simultaneously
• Incomplete dominance occurs when a heterozygote expresses a phenotype that is
intermediate between the two homozygous genotypes
o For example, mating of certain flowers, red crossing white produce pink
PENETRANCE AND EXPRESSIVITY
• Penetrance is a population measure defined as the proportion of individuals in the
population carrying the allele who actually express the phenotype
o It is the probability that, given a particular genotype, a person will express the
phenotype
o Alleles can be classified by their degree of penetrance
o Huntington’s disease, caused by an expansion of a repetitive sequence in the
huntingtin gene
▪ Individuals with over 40 sequence repeats have full penetrance—100
percent of individuals with this allele show symptoms of Huntington’s
disease
▪ Individuals with fewer sequence repeats show high penetrance, wherein
most of those with the allele show symptoms of the disease
find more resources at oneclass.com
find more resources at oneclass.com
▪ With fewer sequence repeats, the gene comes to have reduced
penetrance, low penetrance, or even nonpenetrance
• Expressivity is defined as varying phenotypes despite identical genotypes
o If expressivity is constant, then all individuals with a given genotype express the
same phenotype
o If expressivity is variable, then individuals with the same genotype may have
different phenotypes
• KEY CONCEPT
o Penetrance—the proportion of the population with a given genotype who
actually express the phenotype
o Expressivity—the different manifestations of the same genotype across the
population
MENDELIAN CONCEPTS
Mendel’s First Law: Law of Segregation
• There are four basic tenets of the modern interpretation of Mendel’s first law (of
segregation)
o Genes exist in alternative forms (alleles)
o An organism has two alleles for each gene, one inherited from each parent
o The two alleles segregate during meiosis, resulting in gametes that carry only one
allele for any inherited trait
o If two alleles of an organism are different, only one will be fully expressed and
the other will be silent
Mendel’s Second Law: Law of Independent Assortment
• Mendel’s second law (of independent assortment) states that the inheritance of one
gene does not affect the inheritance of another gene
• Segregation and independent assortment
allow for greater genetic diversity in the
offspring
DNA AS GENETIC MATERIAL
• It is theorized that the live, nonvirulent
bacteria must have acquired the ability to
form smooth capsules from the dead virulent
bacteria
• This was known as the transforming principle
find more resources at oneclass.com
find more resources at oneclass.com
• To determine the exact material underlying the transformation principle
o When this substance was treated with enzymes known to degrade DNA, the
bacteria were not transformed, and the mice lived
o When the substance was treated with enzymes known to degrade proteins, the
bacteria were still transformed, and the mice died
o Thus, the group concluded that the transforming substance must be DNA
• DNA could independently carry genetic information
o One group of bacteriophages contained radiolabeled sulfur, which is found in
protein but not in DNA
o Another group contained radiolabeled phosphorus, which is found in DNA but not
in protein
o The bacterium injects its content into the cell, and it is incubated
o It was determined that no radiolabeled protein entered the cells, but that
radiolabeled DNA had
o It was known that viruses must enter a cell to cause disease and replicate, so this
experiment once again confirmed that DNA was the heritable genetic material
12.2 Changes in the Gene Pool
• All the alleles that exist within a species are known as the gene pool
MUTATIONS
• A mutation is a change in DNA sequence
• Substances that can cause mutations are called mutagens
• Elements known as transposons can insert and remove themselves from the genome
o If a transposon inserts in the middle of a coding sequence, the mutation will
disrupt the gene
Nucleotide-Level Mutations
• Point mutations occur when one
nucleotide in DNA (A, C, T, or
G) is swapped for another
• Silent mutations occur when the
change in nucleotide has no effect
on the final protein synthesized
from the gene
o This most commonly
occurs when the changed
nucleotide is transcribed
to be the third nucleotide
in a codon because there
is degeneracy (wobble) in the genetic code
• Missense mutations occur when the change in nucleotide results in substituting one
amino acid for another in the final protein
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
12. 1fundamental concepts of genetics: each human being possesses two copies of each chromosome, called homologues, except for male sex chromosomes, one y and one x, each gene has a locus, or location on a specific chromosome. If only one copy of an allele is needed to express a given phenotype, the allele is said to be dominant: represented with a capital letter. If two copies are needed to express a given phenotype, the allele is said to be recessive: represented with a lowercase letter. If both alleles are the same for a given gene, the individual is said to have a homozygous genotype. If the alleles are different, the individual has a heterozygous genotype: a hemizygous genotype describes a situation in which only one allele is present for a given gene, x chromosome in males. Individuals with over 40 sequence repeats have full penetrance 100 percent of individuals with this allele show symptoms of huntington"s disease.
