8.1 Genetic Variation

What is meiosis? (3)

- Meiosis is a form of cell division that produces gametes, or sex cells, that are used in sexual reproduction.

- The process reduces the chromosome number of the parent cell by half.

- This leads to the production of haploid cells that have half the number of chromosomes as the original cell.

How do mutations act as a source of genetic variation? (2)

- Point mutations result in changes to the base sequence of a specific gene.

- This creates an alternative form of the gene, known as an allele.

How does crossing over during meiosis introduce genetic variation? (3)

- During prophase I of meiosis, homologous chromosomes align to form bivalents, resulting in the formation of chiasmata.

- At these chiasmata, parts of the non-sister chromatids are exchanged.

- This process introduces a combination of paternal and maternal genes, resulting in new combinations of alleles in the gametes.

How does random independent assortment contribute to genetic variation? (2)

- During metaphase I, the bivalents arrange themselves randomly along the metaphase plate.

- This random assortment results in new combinations of paternal and maternal chromosomes in the resulting daughter cells.

How does independent assortment create genetic variation? (3)

- During meiosis, each pair of homologous chromosomes is split up.

- The chromosome from each homologous pair goes to which daughter cell completely randomly.

- This means the inheritance of one chromosome does not affect the inheritance of another, creating genetically unique gametes.

How is the possible number of chromosome combinations in gametes calculated? (3)

- The number of possible chromosome combinations resulting from independent assortment can be calculated using the formula (n²).

- In this formula, 'n' represents the number of homologous chromosome pairs in the organism.

- This calculation only accounts for the variation from independent assortment and not the additional variation created by crossing over.

Why is meiosis important for producing variation? (3)

- Meiosis is important for producing variation because it shuffles the genetic material.

- This shuffling leads to the formation of new combinations of alleles, or different genetic makeups.

- This variation contributes to the diversity of traits found in a population and is an essential mechanism for evolution.

How does meiosis lead to the production of new combinations of alleles? (3)

- Meiosis creates new combinations of alleles through the process of genetic recombination.

- This occurs when homologous chromosomes pair up and exchange segments of DNA during the first meiotic division.

- The result is the production of daughter cells that each contain a unique combination of alleles.

What are the key differences between mitosis and meiosis? (3)

- Mitosis produces genetically identical daughter cells, while meiosis produces genetically unique daughter cells.

- Mitosis results in diploid daughter cells with the same chromosome number as the parent.

- Meiosis results in haploid daughter cells with the chromosome number halved.

What are the two main stages of meiosis and what occurs in each? (3)

- Meiosis is divided into two main stages: meiosis I and meiosis II.

- Meiosis I is characterised by the separation of homologous chromosomes.

- Meiosis II is characterised by the separation of sister chromatids.

How does meiosis contribute to evolution? (3)

- Meiosis contributes to evolution by producing the genetic variation that can be subjected to natural selection.

- This variation allows species to adapt to changing environments.

- This helps to drive the process of evolution over time.

What happens if meiosis goes wrong? (2)

- If meiosis goes wrong, it can lead to a variety of genetic disorders, such as Down syndrome, Turner syndrome, and Klinefelter syndrome.

- These disorders are typically caused by the presence of an extra or missing chromosome.

Why is genetic variation important for a species? (3)

- Genetic variation ensures that the gene pool of a species contains a plethora of different alleles.

- This results in the expression of genetic diversity within the population.

- This diversity allows a species to adapt to environmental changes over time.

What is the difference between a gene, an allele, and a locus? (3)

- A gene is a section of DNA that codes for a particular polypeptide.

- An allele is an alternative form of a gene that occupies a specific position on a chromosome.

- A locus is the specific position on a chromosome where a gene is located.

What is the difference between a dominant and a recessive allele? (3)

- A dominant allele is the allele that is expressed in the phenotype when at least one copy is present in the genotype.

- A recessive allele is only expressed in the phenotype when an individual is homozygous for that allele.

- In a heterozygote, the dominant allele masks the expression of the recessive allele.

What is codominance? (1)

Codominance occurs when two different alleles of a heterozygote are both fully expressed in the phenotype.

What is the difference between autosomal and sex-linked genes? (2)

- Autosomal genes are located on chromosomes that are not sex chromosomes.

- Sex-linked genes are located on the sex chromosomes, such as the X or Y chromosome.

What is autosomal linkage? (3)

- Autosomal linkage occurs when the alleles for different traits are present on the same non-sex chromosome.

- These linked genes tend to be inherited together as if they were a single gene.

- The closer together the genes are on the chromosome, the more closely linked they are.

What is sex-linkage? (3)

- In sex-linkage, the alleles for certain genes are located on one of the sex chromosomes, typically the X chromosome.

- The inheritance of sex-linked disorders is much more common in men.

- This is because men only need to inherit one copy of a recessive allele on their single X chromosome for the trait to be expressed.

What is the purpose of the Chi-squared test and when can it be used? (3)

- The chi-squared test is a statistical test used to establish whether the difference between observed and expected results is significant.

- It can be used to determine whether a null hypothesis should be accepted or rejected.

- The test can only be used if the sample size is sufficiently large, typically greater than 20.

How are the results of a Chi-squared test interpreted? (3)

- The calculated chi-squared value is compared to a critical value from a probability table.

- If the calculated value is less than the critical value, the null hypothesis is accepted as any difference is considered due to chance.

- If the calculated value is greater than the critical value, the null hypothesis is rejected as there is a significant difference between the results.

What is the formula for the Chi-squared test? (1)

The formula is X² = ∑(O − E)² / E.

What do the terms in the Chi-squared formula represent? (3)

- (X²) is the chi-squared value.

- ∑ represents the sum of all values.

- 'O' is the observed frequency and 'E' is the expected frequency.

What is polygenic inheritance? (2)

- Polygenic inheritance occurs when a single feature is determined by the interaction of multiple genes.

- A particular feature can also be significantly influenced by environmental factors.

What is the difference between genotype and phenotype? (2)

- The genotype is the genetic constitution of an individual at a particular locus.

- The phenotype is the observable set of characteristics or traits that is manifested by a gene.

What is the relationship between the Chi-squared test and gene mapping? (3)

- The Chi-squared test is used to evaluate whether observed inheritance patterns fit expected ratios.

- When results deviate significantly, it can suggest gene linkage.

- Gene mapping is a technique used to locate the specific position that a gene occupies on a chromosome.

What is Mendel's Law of Segregation? (3)

- An individual's characteristics are controlled by two sets of homologous chromosomes, each containing a set of alleles.

- The gamete cells produced by an individual carry only one set of these homologous chromosomes.

- The genes located on these chromosomes are responsible for expressing a particular characteristic.

What is a dominant and recessive character in Mendelian genetics? (2)

- A dominant character is the characteristic that is expressed in the heterozygous F1 generation.

- A recessive character is a characteristic that is obscured in the F1 generation but may reappear in the F2 generation.

How is the ABO blood group system an example of codominance? (2)

- Codominance occurs when the phenotypes of both homozygotes are expressed completely in the heterozygote.

- A key example of this pattern of inheritance is the ABO blood group system in humans.

What are the three alleles that determine the ABO blood group system? (3)

- There are three different allelic forms for the gene that determines the ABO blood group.

- These alleles are represented as Iᴬ, Iᴮ, and Iᴼ.

- (Iᴬ) and (Iᴮ) are codominant with each other, while the I allele is recessive to both.

What antigens and antibodies are present in type A and type B blood? (2)

- Type A blood has A antigens on its erythrocytes and anti-B antibodies in its plasma.

- Type B blood has B antigens on its erythrocytes and anti-A antibodies in its plasma.

What antigens and antibodies are present in type AB and type O blood? (2)

- Type AB blood has both A and B antigens on its erythrocytes and neither anti-A nor anti-B antibodies in its plasma.

- Type O blood has no A or B antigens on its erythrocytes but contains both anti-A and anti-B antibodies in its plasma.

What are multiple alleles? (2)

- It refers to a gene for which three or more alleles exist within a population.

- An individual organism can only carry two of these alleles at any one time, one on each homologous chromosome.

What is genetic recombination? (2)

- Genetic recombination is a process that occurs during meiosis where alleles are exchanged between homologous chromosomes.

- This process results in gametes with different combinations of alleles than the parent cells, which increases genetic variation in the offspring.

What is recombination frequency? (2)

- Recombination frequency is a measure of the likelihood that a crossing over event will occur between two specific genes on the same chromosome.

- It is dependent on the physical distance between the genes; the further apart they are, the higher the recombination frequency.

Why does crossing over result in a maximum of 50% recombinant gametes? (3)

- Crossing over occurs between only two of the four chromatids within a homologous chromosome pair.

- As a result, two of the four chromatids remain in their original parental combination, while the other two become recombinant.

- Therefore, when the chromatids separate during anaphase II, only half (50%) of the resulting gametes can be of the recombinant type.