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.

More Terms






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





Incomplete Dominance:

One allele in a pair is not fully dominant over the other.

ex. snapdragons



A pair of nonidentical alleles specify two phenotypes, both expressed at the same time in the heterozygote.

ex. blood type


Multiple Alleles:

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


Polygenic Inheritance:

Continuous variation in a trait, the additive effects of two or more genes on a single phenotypic trait.

ex. human height


Linked Genes:

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


Chromosomal Aberrations

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


Sex Chromosomes

       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

Double stranded


Sugar-phosphate backbone

Nitrogen bases, face inward

Hydrogen bonds

Ladder-like shape

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