topic 10 ib biology

what are Homologous Pairs?

The chromosomes of sexually reproducing organisms are homologous (i.e. they exist in pairs)

what copies are in Homologous Pairs?

There is a paternal copy and a maternal copy
(sex chromosomes may not be homologous)

when do Chromosomes replicate? and to form what?

during interphase to form genetically identical sister chromatids

when are these chromatids separated?

during meiotic division to become autonomous chromosomes

what are the two nuclear divisions that produce haploid gametes?

Homologous pairs are separated during meiosis I • Sister chromatids are separated during meiosis II

what is The final outcome of meiosis?

four genetically distinct haploid daughter cells (i.e. gametes)

what is Random Assortment?

The random organisation of chromosomes in the middle of the cell during the first stage of meiosis

what happens During Metaphase I?

homologous pairs of chromosomes line up in a random orientation along the equator

what is the orientation of homologous pairs?

The orientation of each homologous pair is independent to the orientation of any other homologous pair

The number of potential chromosome combinations can be determined by what formula?

2n (where n \= haploid number)

how many haploid number/ geminates combinations can a humans have?

haploid number of 23

what occours during During Prophase I?

homologous pairs of chromosomes form points of connection between non-sister chromatids

how do chromosome connection points form?

via a process known as synapsis and the resulting complex is called a bivalent (or tetrad)

what happens to non-sister chromatids While in synapsis?

they may break and recombine with their homologous partner (crossing over)
Crossing over may result in the exchange of alleles

what dose the chiasmata do?

The non-sister chromatids remain physically connected at points of exchange (chiasmata) until separated by anaphase

What are recombinants?

The non-sister chromatids that have had genetic material exchanged are called recombinants
Recombination may result in novel allele combinations

what is Mendel's Law of Independent Assortment?

Separation of alleles for one gene occurs independently of the separation of alleles for another gene

unlinked genes

The inheritance of two different genes / traits will occur independently provided the genes are on separate chromosomes
This is because unlinked genes segregate independently during meiosis (random assortment of homologous pairs)

what is A linkage group?

A linkage group describes a group of genes whose loci are on the same chromosome (i.e. linked genes)
Linked genes will function as a single inheritable unit and will not follow the law of independent assortment

do linked genes follow law of independent assortment?

no
Linked genes will function as a single inheritable unit and will not follow the law of independent assortment

how can Linked genes be separated (unlinked)?

by crossing over

what are The novel allele combinations called?

recombinants

Recombinant phenotypes will only be evident if ...

crossing over has occurred and thus occur at lower frequencies
Frequency increases the further apart the two genes are

what do Dihybrid crosses determine?

allele combinations of offspring for two genes that are unlinked (on different chromosomes)

2 genes × 2 alleles \= ....

\= 4 potential gamete combinations

Gamete combinations are calculated via what method?

FOIL method
First (AaBb \= AB)
Outside (AaBb \= Ab)
Inside (AaBb \= aB)
Last (AaBb \= ab)

What is a Punnett square used for?

used to calculate genotype and phenotype frequencies

who determined the concept of gene linkage and how?

Thomas Morgan
via breeding experiments involving fruit flies (Drosophila)

a number of different traits that did not conform to Mendelian ratios
what did Thomas Morgan summaries from his experiment?

• These traits represented linked genes (on same chromosome) • Linked genes can be uncoupled via recombination

how can Variation can be discrete or continuous?

discrete (finite patterns)
continuous (normal distribution)

Monogenic traits are characteristics that are controlled by what?

a single gene locus
They therefore have a finite pattern of expression (discrete variation)

Polygenic traits are characteristics controlled by what?

more than two gene loci
They exhibit a bell-shaped distribution pattern (continuous variation)

Polygenic traits (e.g. human height) may also be influenced by what?

the environment

where can A chi-squared test be applied?

to phenotypic ratios to determine if there is a statistically significant likelihood that two particular genes are linked or unlinked

what are the steps to conducting a chi-squared test?

Step 1: Identify Expected Frequencies
Step 2: Apply the Chi-Squared Formula
Step 3: Determine Significance

what are the two possibilities regarding the phenotypic ratios of two genes?

Null Hypothesis (H0) - There is no association (i.e. genes are unlinked)
Alternative Hypothesis (H1) - There is an association (i.e. genes are linked)

A table is constructed to identify expected frequencies of distribution (unlinked)
This data will be compared against what?

the observed values previously identified

The expected ratios are calculated using what?

a dihybrid cross (ratios \= 9 : 3 : 3 : 1)
The ratios are applied to total population to determine expected frequencies

The chi-squared (X2) formula calculates what?

a value based on a comparison of the observed frequencies (O) and the expected frequencies (E)

what is the chi-squared formula ?

sum of (observed-expected)^2/expected

what is the degree of freedom?

df \= (number of rows - 1) × (number of columns - 1)

The chi-squared value is used to determine what?

statistical significance (p value)
p

what is Evolution?

the change in the allele frequency within a gene pool over several successive generations

what is a gene pool?

the sum total of all the genes (and the alleles) that are present within an interbreeding population

what is the allele frequency?

the relative proportion of a particular allele within a population

what is Genetic drift?

A change in allele frequencies caused by random events

within genetic drift where is there a higher or lower drift?

There is higher drift in smaller populations (faster change) There is lower drift in larger populations (greater stability)

What dose natural selection change?

the composition of a gene pool due to environmental selection pressures

what is Population Bottlenecks?

Population bottlenecks occur when an event reduces the population size by an order of magnitude
Surviving population has less genetic variability (⇧ drift)

what is the Founder Effect?

The founder effect describes the establishment of a new population by a fraction of a larger existing population
The new population has less genetic variability (⇧ drift)

what is Stabilising Selection? give example

When an intermediate phenotype is favored at the expense of extremes and Operates when conditions are stable
Example: Human birth weights ⇨ Too large \= birth complications ⇨ Too small \= high infant mortality

what is directional Selection? give example

When one phenotypic extreme is selected at the cost of the other and Operates when conditions change
Example: Antibiotic resistance ⇨ Antibiotic \= ⇧ resistance ⇨ No antibiotic \= ⇧ susceptibility

what is disruptive Selection? give example

When both extremes are favored at the expense of the intermediate and Operates when conditions fluctuate
Example: Moth pigmentation ⇨ Pigmentation \= camouflage ⇨ Benefit depends of conditions

What is speciation?

Formation of new species

What is reproductive isolation?

occurs when barriers prevent two populations from interbreeding (gene pools kept separate) and can lead to speciation

what are the two categories of reproductive isolation barriers?

Pre-zygotic barriers (no offspring are produced)
Post-zygotic barriers (offspring are not viable or infertile)

what are Isolation Barriers?

Something which causes a population to split. They include Temporal Isolation

What is geographic isolation?

Populations occupy different habitats / niches in an area
E.g. Lions and tigers don't often interact within a region

what is Behavioural Isolation?

Populations exhibit or respond to specific courtships
E.g. Different cricket species have distinct mating calls

what is temporal isolation?

Populations have distinct / separate reproductive cycles
E.g. Leopard and wood frogs mate at different times

what is Allopatric Speciation?

Occurs when geographic barriers isolate populations
The physically separated populations are exposed to different environmental conditions and begin to diverge

What is sympatric speciation?

Occurs when populations diverge within a shared location (i.e. the populations are not physically separated)
Reproductive isolation leads to genetic divergence

Sympatric speciation may be caused by what?

polyploidy - condition in which the cells of an organism have more than one pair of chromosomes.

Polyploidy is more common in what?

plant species that can self-pollinate or reproduce asexually

What is phyletic gradualism?

Speciation occurs at a constant pace over a period of time Involves a continuous / gradual accumulation of mutations Theory is supported by the presence of intermediate fossils (e.g. evolution of the modern horse hoof)

what is Punctuated Equilibrium?

Speciation occurs in rapid bursts with periods of stability
If conditions are stable