Biology 101 Chapter 9
Patterns of Inheritance
Genetics = science of heredity.
Heredity: passing of characteristics from parents to
Wildtype vs. Variant
Roots of Genetics
Hippocrates and Aristotle
The Father of Modern Genetics
· Began in 1860s with Gregor Mendel
· Discovered fundamental principles of genetics
· Parents pass on to offspring discrete heritable factors (genes)
Terms to Know
Self-fertilize Hybrids P-generation
Cross-fertilize Monohybrid cross F1-generation
Cross Dihybrid cross F2-generation
Principle of Segregation
- Mendel's hypotheses
- Alleles = alternate forms of genes
- Dominance and recessiveness
Pairs of genes, on homologous chromosomes, separate during gamete formation, and are paired again at fertilization, with one half coming from each parent.
Genes and Chromosomes
Gene: a segment of DNA that codes fro a heritable trait
- located at gene loci or loci
- chromosomes are 1000's of genes
Principle of Independent Assortment
- Each trait's (gene's) alleles segregate, or split to opposite sides of the cell, independently from one another during gamete formation (shown with a dihybrid cross)
- Useful with a test cross: a cross of a unknown genotype with a homozygous recessive to identify the unknown genotype
Principle of Dominance
- Trait has two different alleles, one is dominant and one is recessive, called complete dominance
- The dominant allele masks the effects of the recessive
- Anytime you have the dominant allele, that phenotype is expressed
The Relationship of Genotype to Phenotype is Not a Simple One
NOW!! EXCEPTIONS TO MENDEL'S PRINCIPLES
One allele in a pair is not fully dominant over the other.
A pair of nonidentical alleles specify two phenotypes, both expressed at the same time in the heterozygote.
ex. blood type
A gene may have more than two alleles.
ex. blood type
A single gene affects many phenotypic characteristics.
ex. growth hormone
One set of alleles (a gene) may mask or inhibit the expression of another gene's alleles.
ex. coat color in Labradors
Continuous variation in a trait, the additive effects of two or more genes on a single phenotypic trait.
ex. human height
Genes located close together on the same chromosome tend to not separate during crossing over and to be inherited together.
Problems of Genes
Pedigree = family history of genetic traits
Mutation = a change in the base code of DNA for a gene
Genetic Abnormality vs. Genetic Disorder
(genetic "disease" not correct)
Autosomal recessive disorders + Autosomal dominant disorders
Carriers = heterozygotes for a recessive disorder
Kinds of Mutations
1) Base Substitutions = replacing a base with another
2) Base Insertions = adding extra bases to a sequence
3) Base Deletions = removing bases from a sequence
1) Deletions = loss of whole sections of chromosome
2) Duplications = section of chromosome copied
3) Inversions = piece of chromosome reversed
4) Translocation = part of one chromosome is put into another
Changes in Chromosome Number
Aneuploidy = loss of whole chromosome
Polyploidy = addition of whole chromosome or chromosomes
Sex Linked Genes
Autosomes = pairs 1-22
Sex Chromosomes = pair 23
· Contain genes that determine gender
· Two of them; X and Y
· XX is female, XY is male
· Sex-linked genes are those found on the sex chromosome but are unrelated to sex determination
· Most sex-linked genes are found on X chromosome (80%)
· Passed on maternally
! Most disorders occur in males! Why? There are no such things as male carriers for sex-linked traits.
ex. colorblindness and hemophilia
Some final notes on probability
Mendel's crosses and rules reflect chance, not certainty. Genetic crosses show only the odds of getting a particular genotype at any one time, not what must be.
Genes, Natural Selection and Adaptation
Some mutations are good. Mutations, genetic recombination and crossing over provide novel new gene combinations.
Genome = the total of all genes in an individual
Gene Pool = the total collection of genes in a population
The Hardy-Weinberg Equilibrium
Equilibrium law. The frequency of each allele in the gene pool will remain constant. If the frequency of one is known, the other can be determined.
P + q = 1
p2 + 2pq + q2 = 1
p and q frequency of alleles for a gene, p = A q = a
p2, 2pq, q2 are frequencies of genotypes in a population
Molecular Biology of the Gene: Revisited
Genes = short pieces of DNA, are encoded in DNA
· ~30,000 in humans
· Located on chromosomes (loci)
· Many on one chromosome, but lots of empty space
· Have a beginning and end
What are exons and intron?
What is Junk DNA? Junk DNA and aging
Many believe that the extra DNA has regulatory functions
Structure of DNA
Made up of nucleotides
Nitrogen bases, face inward
Complimentary base pairing
· 4 bases: adenine (A), guanine (G), cytosine (C), thymine (T)
· Base sequence determines nature of gene [ATTACGTATCG]
· What is a codon?
· The Genetic Code
· Restriction enzymes and gene swapping