Genetic Variation - Explanation

Describe the difference between autosomal and sex chromosome

Autosomal chromosomes are all the chromosomes except the sex chromosomes. Sex chromosomes determine an individual’s biological sex.

Explain diploid cell

A diploid cell has two sets of chromosomes (one from each parent), so it contains pairs of homologous chromosomes. In humans, diploid cells have 46 chromosomes (23 pairs) and are found in body (somatic) cells.

Explain haploid cell

A haploid cell has only one set of chromosomes, meaning it has no pairs. In humans, haploid cells have 23 chromosomes and are found in gametes (sperm and egg cells).

Distinguish between genes and alleles

A gene is the overall instruction for a trait, while alleles are the variations of that instruction.

Explain genotype and give an example

A genotype is the genetic makeup of an organism — the set of alleles that it carries for a particular trait

What is phenotype

physical expression of the genotype

What influences phenotype

genotype and environmental factors

Explain homologous chromosomes

pair of chromosomes that have the same size, shape, and locus

What is a locus

location of a gene on a chromosome

What do alleles for the same gene differ in

slight difference in the DNA base sequence which may cause a change in the protein that DNA codes for, which might affect the organism’s phenotype

Explain dominant allele

allele that is always expressed if present

Explain recessive allele

only expressed if no dominat allele is present

Describe “TT'“

homozygous dominant

Describe “Tt”

heterozygous

Describe “tt”

homozygous recessive

How does genetic variation impact evolution

Genetic variation allows evolution to take place by increasing the frequency of advantageous alleles and decreasing the frequency of disadvantageous alleles. This enables a species to adapt to changing environmental conditions and survive over time.

List sources of genetic variation (meiosis)

Crossing over, independent assortment, segregation

List sources of genetic variation (mutations)

Chromosomal mutations, DNA mutations

List sources of genetic variation (other)

mate selection, random fertilisation

Explain sexual reproduction

Sexual reproduction involves the fusion of two gametes (sperm and egg) from two parents, resulting in offspring with genetic variation. The offspring inherit a combination of genes from both parents, which increases diversity in the population.

Explain asexual reproduction

involves only one parent and produces offspring that are genetically identical (clones) to the parent. This process does not involve gametes or fertilisation.

Explain fertilisation

haploid gamete combines with another haploid gamete to produce a diploid zygote

What does a diploid zygote do

go through mitosis to create all of the diploid somatic cells in the body

Advantageous of sexual reproduction

• Creates genetic variation in offspring

• Increases adaptability to changing environments

• Helps eliminate disadvantageous alleles through natural selection

• Leads to evolution and survival of the species

Disadvantageous of Sexual reproduction

• Requires two parents, which can be time-consuming and energy-intensive

• Finding a mate may be difficult or risky

• Slower reproduction rate compared to asexual reproduction

Advantageous of asexual reproduction

• Only one parent needed

• Faster and more efficient than sexual reproduction

Disadvantageous of asexual reproduction

• No genetic variation in offspring

• Offspring are all identical — if the environment changes, they may not survive

• Greater risk of entire population being wiped out by disease or environmental change

Explain meiosis

process of nuclear reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically unique cells

Where does meiosis occur and why

sex organs to produce gametes (sex cells)

Why must the chromosome number be halved

when fertilisation occurs, the zygote has the correct diploid number. If gametes had the full number of chromosomes, the zygote would have double the normal number, which would be harmful to the organism.

Purpose of mitosis

produce genetically identical cells for growth, repair, and asexual reproduction

Number of cells produced in meiosis

4 haploid cells

List the key differences between meiosis and mitosis

Purpose, cells identity, number of cell divisions, number of chromosomes

Explain what happens before the 1st phase of meiosis 1

chromosomes duplicate

Explain the first phase of Meiosis 1

Prophase 1- Homologous chromosome pairs come together to form a tetrad (four chromatids), Crossing over occurs

Explain the second phase of Meiosis 1

Metaphase 1- Pairs of homologous chromosomes (tetrads) line up on equator of the cell. The orientation of each tetrad is independent from any other pair (independent assortment)

Explain the third phase of Meiosis 1

Anaphase 1 - Homologous chromosomes move to opposite poles of the cell

Explain the 4th phase of Meiosis 1

Telophase 1 & Cytokinesis - Chromosomes gather at poles of the cells, and cytoplasm divides

Explain the first phase of Meiosis 2

Prophase 2 - A new spindle forms around the chromosomes

Explain the second phase of Meiosis 2

Metaphase 2 - Chromosomes line up at the equator

Explain the third phase of Meiosis 2

Anaphase 2 - Centromeres divide and chromatids move to opposite poles of the cell - this is called segregation

Explain the fourth phase of Meiosis 2

Telophase 2 & Cytokinesis - Nuclear envelope forms around each set of chromosomes. Cytoplasm divides and 4 daughter cells are formed

How are genetically different gametes produced

Crossing Over, Independent Assortment, Segregation

When does crossing over occur

When pairs of homologous chromosomes form tetrad during Prophase 1

What happens during Crossing Over

Corresponding segments of non-sister chromatids in a tetrad may cross over, exchanging DNA between the two different chromatids

What is the name of the point where non-sister chromatids exchange DNA

Chiasma

What happens at the end of meiosis

chromatids that were bound together by the centromere are separated

Where do Each of the four chromatids from each pair of homologous chromosomes go

to different gametes

Explain Independent Assortment

the orientation of each tetrad is independent from any other pair

How does independent assortment contribute to genetically different gametes

Homologous pairs of chromosomes line up in random orientation along the equator of the cell. Only one chromosome from each homologous pair ends up in each gamete. This leads to different combinations of existing chromosomes in gametes resulting in new combinations of existing alleles

How does segregation contribute to genetically different gametes

Two alleles of each gene are separated (segregated) when chromatids separate during anaphase II. Therefore, each gamete only receives one allele for each gene 

Explain spontaneous mutation

Mistake that occurs when DNA is replicated. The rate of this increases with age

Explain induced mutation

Result of exposure to mutagens

Name 3 mutagens

Cigarette, UV light, X-Rays

Describe chromosomal mutations and give an example

changes to the structure or number of whole chromosomes, which can affect many genes at once. Trisomy 21 (3 Copies for Chromosome 21).

Describe gene mutations. Give an example

changes to the DNA base sequence of a single gene. Sickle Cell Anaemia (substitution mutation)

Affect of Somatic (body cell) Mutations

They can cause changes in the cells where they occur, such as uncontrolled cell division leading to cancer. Because they affect only the individual, somatic mutations can cause diseases like skin cancer, but they do not contribute to genetic variation in a population.

Affect of Gametic Mutations

These mutations become part of the zygote’s DNA and therefore affect every cell of the offspring.. They can lead to inherited genetic disorders or contribute to genetic variation within a population, which is a driving force of evolution.

How to introduce NEW alleles into a population

Gametic mutations

Explain how mutations increase genetic diversity

Mutations create new alleles that did not exist before. These new alleles can potentially lead to new traits. When mutations occur in gametes, they can be passed on to offspring and become part of the population’s gene pool. Over generations, the accumulation of different mutations increases the genetic variation within a species.

Explain a test cross

performed to determine if an individual is homozygous dominant or heterozygous for a trait

How is a test cross performed

Individual with unknown genotype is crossed with individual that is homozygous recessive for same trait

Explain what it means if the recessive trait appears in offspring for Test Cross

The unknown individual must be heterozygous because it inherited the allele from both parents

Explain what it means if no recessive trait appears in offspring for Test Cross

it is likely that the unknown individual is homozygous dominant (purebred)

What is a holdback for test crosses

Test crosses only work well with organisms that can quickly produce large numbers of offspring

What is a pedigree chart

A diagram that shows how a trait is passed through generations.

Determine a phenotypic ratio for a dihybrid punnett square (state holdback)

9:3:3:1, if genes are unlinked

Cause of unexpected phenotypic ratio from a monohybrid punnett square,

Lethal Allele

Explain why lethal alleles remain in the population

Recessive allele, phenotype expresses itself later in life (after reproduction), spontaneous mutation

Explain linked genes during independent assortment and segregation

Linked genes cannot be separated by independent assortment or segregation during meiosis, so they are likely to be inherited together. However, during meiosis, linked genes will sometimes be separated because of crossing over.

How do linked genes affect genetic variation?

Linked genes affect genetic variation if a parent is heterozygous. We get phenotypic ratios that are different to what we would normally expect from a dihybrid cross with unlinked genes. We get a lower number of recombinant genotypes and phenotypes than we would expect.

Formula for allele frequency

Number of ‘X’ alleles/Total number of alleles (gene pool) = frequency of ‘X’ allele

Explain how alleles relate to genetic diversity and importance

The greater the range of different alleles in a population, the greater the genetic diversity. This is important, because if the environment changes, populations with greater genetic diversity are more likely to survive. This is because there is a greater chance that there will be individuals in the population who possess alleles to help them survive in new environmental conditions

Explain Selection Pressure

abiotic or biotic factor that affects the survival and reproductive success of a population. It increases the chance of some alleles being passed onto the next generation and decreases the chance of others

Explain how Natural Selection leads to evolution

1. Individuals in population show variation

2. Individuals with less favourable characteristics (less fit) do not survive/reproduce due to selection pressure

3. Individuals with more favourable characteristics (fitter) survive and produce offspring

4. Survivors pass their alleles onto the next generation

5. Over time, successful alleles become more common and unsuccessful alleles are removed, characteristics of the species changes

Explain stabilising selection

Selective pressure selects against two extremes of a phenotype and for ‘average’ phenotypes. As a result, the population’s genetic variation decreases; however, allele frequencies remain relatively constant over many generations

Explain directional selection

Selective pressure selects against one extreme of a phenotype. As a result, the population’s phenotype distribution shifts towards one extreme and allele frequencies change over a number of generations - can ultimately lead to evolution

Explain disruptive selection

Selective pressure selects for two different sets of alleles; individuals with intermediate features and allele sets are not selected for. As a result, two different phenotypes are maintained in the population

Explain how gametic mutations contribute to evolution of a species

Gametic mutations are the ONLY way that new alleles can enter a population’s gene pool. These increase the genetic variation within a species

Explain beneificial mutations

mutations that produce traits that make organisms better suited for their environment, therefore increasing their likelihood of survival and reproduction. These are passed down to next generation more frequently, and become more common in gene pool

Explain Sexual Selection

a type of natural selection where the preference of one sex (usually females) for certain traits (usually in males) acts as selection pressure

Who drives it?

Natural: environment, Sexual: mate choice, Artificial: Humans

Purpose?

Natural Selection: Survival and reproduction, Sexual: Reproductive Success, Artifical:Desired Traits

Traits Favoured?

Natural: adapted to environment, Sexual: Attractiveness or mating success, Artificial: Traits useful or appealing to humans

Explain holdback of artificial selection

Closely related individuals are often bred together. These increases the chance that harmful alleles are expressed in the individual’s phenotype - so this might shorten their lifespan

Migration impact on gene frequency

Migration has a more noticeable impact on the allele frequency of small populations

Founder Effect & Bottleneck Effect on Allele Frequency

Alleles present in gene pool of the new small population are unlikely to be representative of the whole population. Some alleles are lost from the new population. Other alleles may have increased in frequency (or even become fixed as there are no other options)

Founder Effect > New Species

Founder effect populations may be isolated for a long time with no gene flow (migration). May face new selection pressures in new environment, so different alleles selected for than in original population. Can lead to relatively rapid evolution, formation of new specie

Explain how genetic drift differs from natural selection

Genetic drift is when allele (gene) frequencies change by random chance, especially in small populations. Natural selection is when allele frequencies change because some traits help organisms survive or have more babies. These helpful traits become more common over time.

Effect of genetic drift on alleles

 Random process that usually results in loss of alleles from gene pool (even beneficial alleles). Mainly affects allele frequencies in small populations