Chapter 16: Variation and Evolution

Definitions

Species
Specific group of organisms that are capable of mating and producing fertile offspring.

Gene
Section of chromosome that controls a specific trait.

Allele
Different form of the same gene.

Locus
Location of a gene on a chromosome

Genotype
Set if genes and individual possesses.

Genome
All the genes in an organism.

Heterozygous
One dominant and one recessive gene.
Homozygous
Two dominant or two recessive genes.

Phenotype
The physical expression of the genotype.

Dominant Genes
Always expressed when present.

Recessive Genes
Genes that are only expressed in the absence of dominant genes.

Mutation
Any permanent change in the structure of the gene.
(additory, deletion, exchange)

Homologous Chromosomes
Pairs of chromosomes with identical genes and loci on them

F1 Cross
Cross between two pure breeding parents.

Haploid
Single set of chromosomes

Diploid

Paired set of chromosomes.

Gregor Mendel

Father of Genetics

Studied inherited characteristics in pea plants

Mendel’s First Law (Law of Segregation)

Inherited characteristics are controlled by pairs of genes.

  • Alleles of a gene exist in pairs but when gametes are formed the members of each pair segregate into different gametes.

  • This means that a gamete has only one allele of each gene.

  • At fertilisation, the offspring will have two alleles again, one from each parent.

Mendel’s Second Law (Law of Independent Assortment)

At gamete formation, the separation of one pair of alleles is completely independent to the separation of all other alleles.

Monohybrid Crosses

Involve the study of a single characteristic.

Cross between heterozygous parents has ratio of 3:1

Dihybrid Crosses

Involves the study of two characteristics.

Cross between heterozygous parents has ratio of 9:3:3:1

Incomplete Dominance

No allele is dominant and the offspring produce traits that are inbetween.

Sex Determination

In humans, all cells (except gametes) contain 23 pairs of chromosomes.

44 of these are called autosomes (found in males and females)

The other 2 are sex chromosomes, X and Y chromosomes.

Females are XX, males are XY.

Meiosis

Half sperm have X chromosome, half have Y chromosome.

All eggs have X chromosomes.

Y chromosome has SRY gene (sex determining region of Y)

Y chromosome is missing genes on X chromosome.

e.g. colour vision, blood clotting, muscle development.

These traits are X linked/sex linked traits.

Genetic Study using Fruit Flies

Easy to grow

Reproduce offspring in just two weeks

Have large chromosomes

Reproduce in large numbers

Mutations

Spontaneous and permanent change in the DNA sequence of a gene.
e.g. white eyes in fruit flies

Lethal Gene
Mutation that leads to death of an organism
e.g. sickle cell anaemia

Causes

  • Radiation

  • UV Light

  • Radon

  • X-Rays

  • Perservatives

  • Pesticides

Point Mutation

Change in one base pair

e.g. Sickle Cell Anaemia

  • Inherited blood disorder

  • RBCs have irregular shape

  • 2 copies of recessive allele

  • Cellular lifespan is only a few days

  • Symptoms include short of breath, dizzy, headaches

Chromosome Mutation

Large change in structure of a chromosome

e.g. Down Syndrome

  • Presence of an extra chromosome, chromosome 21

  • Spectrum of physical and mental features

Linkage

Genes that are linked mean that they are on the same chromosome, so if you get one, you get the other.

Sex Linkage

When a characteristic is controlled by a gene on an X chromosome.

There are very few genes on the Y chromomes, so all genes on the X chromosome are expressed in males.

Variation

Differences between members of the same species.

Acquired Variation

Not genetically controlled but learned throughout life.

Inherited Variation

Genetically controlled variation.

Causes of Inherited Variation

  • Independent assortment of chromosomes when gametes are formed.

  • Crossing over during meiosis

  • Fertilisation

  • Mutations

Evolution

Inheritable change in a population whereby there is a genetic change to produce new form of life, occur over time or by natural selection.

Credited to Charles Darwin and Alfred Wallace

Darwin proposed a mechanism for evolution by natural selection.

Darwin’s Observations

  • Overbreeding
    Organisms produce offspring in large numbers

  • Population remains constant
    Environment can only support a certain amount of life

  • Inherited variation occurs in populations
    Independant assortment, mutations, sexual reproduction, genetic engineering.

Darwin’s Conclusions

There is a struggle for existence, so competition will occur.

Only the fittest will survive.

Survival of The Fittest

Organisms with genetically controlled characteristics can adapt to their environment, these will survive and reproduce, therefore passing on genes to the next generation.

Speciation

When a new species is formed as a result of evolution.

  • Certain characteristics are selected over time.

  • These characteristics accumulate over time.

  • Eventually differences are so vast the species cannot interbreed.

  • A new species is formed.

Evidence of Evolution

  • The fossil record

  • Paleontology - Study of Fossils

  • Fossil is the remains of an organism.

  • Can be dated using radioactive carbon (carbon-14 dating).

  • Fossils show changes compared to modern organisms (modern fossils are more complex).

  • Fossils can be aged.

  • Some fossil organisms are extinct.

  • Fossil evidence can be linked to environmental change.