Determining probability of inheriting disease gene

Presentation

Evolution & Heredity

In this unit you will explore the basics of heredity and the complex
process of cell copying. First you will take a look at RNA and DNA. Then
you will take a brief tour of the process known as Mitosis.
Additionally, the process of Meiosis, or sex-cell formation, will be
explained and presented. Finally, how organisms adapt to specific
environments will be examined as well as the repercussions for organism
reproduction and genetic selection.

All living organisms need to reproduce themselves and find ways to
survive given specific environmental constraints. How reproduction and
adaptation occur is based on genetic transference and evolutionary
change. Let's examine these concepts more closely.

If a cell is to operate as it should, it must be able to reproduce sex
genes and control chemical reactions. DNA replication creates an exact
duplicate of the genetic material and almost always passes on identical
strands of DNA to the next generation of cells. The primary control
molecules of the cell, however, are the nucleic acids that direct the
production of proteins.

Protein structure, which is determined by the structure of DNA and RNA,
will determine their role in the life cycle of the cell. They must be
synthesized through the decoding of the DNA. This is done at the
ribosome. If an error occurs, the cell will malfunction and die.

For more information on DNA replication click the following button and
complete Activity 9.2 of your Student CD-Rom.

Diversity within species is caused by alteration of the frequency of
genes within a given gene pool. Mutation, migration, and genetic
recombination, for example, all generate variety within the gene pool.
Subpopulations may have different gene frequencies from one another and
are called demes. Demes exist because local conditions may demand
certain characteristics, founding populations may have had
unrepresentative gene frequencies, and barriers may prevent free flow of
genes from one locality to another. Demes are often known as subspecies,
varieties, strains, breeds, or races.

All sexually reproducing organisms naturally exhibit genetic variety
among the individuals in the population as a result of mutations and the
genetic recombination resulting from meiosis and fertilization. These
genetic differences are important for the survival of the species
because natural selection must have genetic variety from which to
select. Natural selection by the environment results in better-suited
individual organisms that have greater numbers of offspring than those
that are less well off genetically. Some genes express themselves only
during specific periods in the life of an organism and may be recessive
genes that show themselves only when in the homozygous state.
Characteristics that are acquired during the life of the individual and
are not determined by genes cannot be raw material for natural
selection.

Let's examine these concepts further. Click on the following button and
complete Activity 15.1 of your Student CD-Rom.

Populations are usually genetically diverse. Mutations, meiosis, and
sexual reproduction tend to introduce genetic variety into a population.
Organisms with wide geographic distribution often show different gene
frequencies in different parts of their range. The process of speciation
usually involves the geographic separation of the species into two or
more isolated populations. While they are separated, natural selection
operates to adapt each population to its environment. If this generates
enough change, the two populations may become so different that they
cannot interbreed. Similar organisms that have recently evolved into
separate species normally have mechanisms to prevent interbreeding. Some
of these are habitat preference, seasonal isolation, and behavioral
isolation. Plants have a special way of generating a new species by
increasing their chromosomes numbers as a result of abnormal mitosis or
meiosis.



Questions and Answers



Question #1

What are the differences between DNA and RNA?

The DNA of a eukaryotic cell is located in the cell’s nucleus, but
protein synthesis occurs on ribosomes in the cytoplasm. DNA, therefore,
cannot directly guide protein synthesis necessary for reproduction.
There must be an intermediary, a molecule that carries the information
from the DNA in the nucleus to the ribosomes in the cytoplasm. That
intermediary is RNA. RNA is similar to DNA but differs structurally in
three respects: (1) RNA is normally single-stranded; (2) RNA has the
sugar ribose in place of the deoxyribose in the backbone of DNA; and (3)
RNA has the base uracil instead of the base thymine which is found in
DNA.

Question #2

What is the process of DNA replication?

When a chromosome is replicated, the two DNA strands of the double helix
unwind. DNA polymerase enzymes move along each strand, linking up free
nucleotides into new DNA strands. The sequence of nucleotides in each
newly formed strand is complementary to the sequence on the parent
strand. As a result, two double helices are synthesized, each consisting
of one parental DNA strand plus one newly synthesized complementary
strand that is an exact copy of the other parental strand. This type of
replication is called semi-conservative replication.

Question #3

How do meiosis and sexual reproduction produce genetic variability?

The random shuffling of homologous maternal and paternal chromosomes
creates new chromosome combinations. Crossing over creates chromosomes
with allele combinations that may never before have occurred on single
chromosomes. Because of crossing over, a parent probably never produces
two gametes that are completely identical. The fusion of tow such
genetically unique gametes adds further genetic variability to the
offspring.

Question #4

What is the difference between convergent evolution and adaptive
radiation?

Evolution by natural selection predicts that, given similar
environmental demands, unrelated species might independently evolve
superficially similar structures in response to environmental pressures.
Adaptive radiation, on the other hand, occurs when populations of a
single species invade new habitats and evolve in response to the changed
environment.

Question #5

What is a gene pool and how do you determine the allele frequencies
within a gene pool?

A gene pool is all the genes that are present in a population. Allelic
frequencies are determined by adding all the alleles for a trait in a
population and determining their relative proportions.

Question #6

The Hardy-Weinberg concept is theoretical. What factors restrict its
ability to operate in a natural gene pool?

Population geneticists call the Hardy-Weinberg idealized population an
equilibrium population. Population equilibrium will remain in genetic
equilibrium as long as several conditions are met:

There must be no mutation.

There must be no gene flow between populations; that is, there must be
no net migration of alleles into the population through immigration or
out of the population through emigration.

All mating must be random with no tendency of certain genotypes to mate
with specific other genotypes

There must be no natural selection. In other words, all genotypes must
be equally adaptive and reproduce with equal success.

As you might expect, few if any natural populations meet these criteria.
The Hardy-Weinberg principle, however, remains a useful starting point
for studying the mechanics of evolution and for understanding a
perfected, although, theoretical equilibrium baseline.

Question #7

How do new species form?

Speciation requires that two populations be isolated from gene flow
between them and develop significant genetic divergence. Allopatric
speciation occurs by geographical isolation and subsequent divergence of
the separated populations through genetic drift or natural selection.
Sympatric speciation occurs by ecological isolation and subsequent
divergence or by rapid chromosomal changes.