Meiosis, Inheritance, and Evolution: Understanding Genetics and Sexual Reproduction, Slides of Geology

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Lecture 3 : Evolution Etc.

Overview

MEIOSIS AND INHERITANCE

• Meiosis– Sexual Reproduction– Dominant and Recessive Genes– Mutations

EVOLUTIONINFECTIOUS DISEASES

• Parasitism– Pathogenic Factors

Meiosis

•^

Meiosis

is a complex process involving two cell divisions

of a germ cell to produce 4 sex cells (or gametes) fromeach original cell.

-^

Each

gamete

contains a half set of chromosomes (23 in the

case of humans).

-^

DNA is exchanged between chromosomes during meiosis,in a process known as

crossing over

, resulting in greater

genetic diversity.

Sexual Reproduction

•^

Fertilisation occurs when a gamete produced by a male(sperm) fuses with a gamete produced by a female (ovum)to create a

zygote

with a complete set of chromosomes –

half from each parent.

-^

The embryo develops from the zygote through mitosis.

-^

Following meiosis, each of the four egg cells from thefemale always contains an X chromosome, whereas two ofthe

sperm

cells

from

the

male

will

contain

an

X

chromosome and two will contain a Y chromosome. Thesex of the offspring is therefore determined by the type ofchromosome passed on by the father via the fertilisingsperm cell.

Dominant And Recessive Genes

•^

Each person has two sets of chromosomes – 23 from theirmother and 23 from their father – and therefore 2 sets ofgenes.

-^

Each person may therefore have conflicting genes codingfor a particular function.

-^

One gene may become

dominant

, and the other is

recessive

Evolution

•^

Jean-Baptiste Lamarck (1744-1829) was an earlyproponent of evolution.

-^

Charles Darwin (1809-1892) introduced the notion of natural selection

in 1848.

•^

Diversity within a species is introduced by mutations;organisims which are best suited to their environment willsurvive in greater numbers to transmit their genes to thenext generation –

survival of the fittest

•^

The peppered moth (

Biston betularia

) provides an

excellent 19

th^

century example.

Biston betularia f. typica

Biston betularia f. carbonaria

Infectious Diseases

•^

Most infectious diseases are caused by a particular speciesof

microorganism

(or

microbe

), but some larger

organisms also cause disease.

-^

Microorganisms are ubiquitous. Only a small minority ofmicroorganisms are

pathogenic

(i.e. disease causing).

•^

Pathogens

do not have malicious intent. Jacques May

(1950): diseases are ‘merely the by-product of anaccidental collision between two or more forms of life,each pursuing its own destiny’.

Parasitism

•^

Only a minority of microorganisms are parasitic, but allpathogens are

parasitic

•^

A parasite is an organism that lives on or in anotherorganism (referred to as the

host

) and depends upon it for

nutrition, often to the detriment of the host.

-^

Parasitism is a form of

symbiosis

(living together).

•^

Other forms of symbiosis include

commensalism

and

mutualism

•^

Parasites tend to be particularly well adapted to specifichosts.

Disease Prevention

•^

All infectious diseases require at least two factors (host +agent).

-^

Many require additional factors (i.e. vector, intermediatehost, or reservoir).

-^

Each factor has its own ‘geography’. These geographiesmust overlap for the disease to be present.

-^

Diseases may be prevented by eliminating one of thefactors.

-^

This is not only difficult, but it may have unforeseenecological consequences.