Module 6 Patterns of Inheritance

Question 1

Key Terms

Answer 1

• Chromosomes contain multiple genes that code for proteins. A different form of the same gene is an allele. A locus is the point on a chromosome a gene is found. Chromosomes found in homologous pairs so there are two copies of the same gene. Genotype is the combination of alleles found in the genome of an organism. Phenotype is the observable characteristics.
• A dominant allele is always expressed. A recessive allele is only expressed when the dominant allele is not present. If alleles are codominant then both alleles are expressed. If an organism has two of the same alleles, they are homozygous. If they have two different alleles, they are heterozygous.
• The F1 generation is the result of a cross between a homozygous dominant and a homozygous recessive individual. The F2 generation is the result of a cross between 2 F1 individuals. A test cross is used to determine the genotype of an unknown individual by crossing with a homozygous recessive individual. If any of the offspring display the recessive characteristic, the individual must be heterozygous.
• Sex linkage – some genes are found on the X chromosome but not the Y. The inheritance of genes depends on sex and can lead to sex linked diseases. Autosomal linkage – on autosomes genes on the same chromosome do not assort randomly. They are linked and stay together in the original parental combination.

Question 2

Phenotypic + genetic variation

Answer 2

• Phenotypic variation is the difference in phenotypes between organisms of the same species. Caused by genetic factors as with blood groups or environmental factors as with height. Genetic factors can be caused by a single gene or multiple genes.
• Diet in animals is an environmental form of variation. With fruit flies, they are generally grey. An uptake of silver salts as larvae will cause them to turn yellow.
• Chlorosis is the yellowing of plants caused by a decreased synthesis of chlorophyll in dark conditions or due to a lack of magnesium.
• Etiolation is caused by growing plants in the dark and leads to a long stem and small curled leaves.
• Sexual reproduction causes small differences in the DNA base sequences of organisms.
• Crossing over in prophase 1 – non sister chromatids exchange alleles when homologous chromosomes pair up and non-sister chromatids cross over at chiasmata. This entanglement places stress on DNA molecules and chromatid sections breaks off and re-join forming new combinations of alleles on the two chromosomes.
• Independent assortment in metaphase 1 + 11 causes a different combination of alleles in daughter cells. The random alignment of homologous chromosomes on the equator of the spindle with the orientation of each pair being independent. Number of possible combinations represented by 2^n (2^23).
• Random fusion of gametes to form a zygote leads to genetic variation between zygotes.

Question 3

Genetic diagrams + ratios

Answer 3

• Genetic diagrams show patterns of inheritance from Punnett squares. Parent phenotypes, parent genotypes, possible gametes, Punnett squares, offspring genotypes, offspring phenotypes.
• Monohybrid crosses- show how alleles for a single gene are passed from one generation to another. If multiple alleles, then gene represented by capital letter and alleles by superscript letters. Codominance – more possible phenotypes so ratio is changed.
• Dihybrid crosses- inheritance of two genes so more possible genotypes and phenotypes. Gametes have two genes.
• Autosomal/sex linkage – not all genes assort independently. Some on the same chromosome stay in original parental combination. Causing an unexpected phenotypic ratio. Sex linked genes- only present on one sex chromosome. Phenotypes not spread evenly as one sex is disproportionately affected. Males only have one X chromosome so are more likely to show recessive conditions.
• Epistasis – one gene affects the expression of another. Two genes on different chromosomes affect the same feature. Causes deviation from expected phenotypic ratios.

Question 4

Continuous/discontinuous variation

Answer 4

• Variation – the differences that exist between individuals of the same species. Can be qualitative or quantitative.
• Continuous – quantitative measurable differences represented by a range of values. Caused by interactions between genotype and environment. Different alleles have an additive effect. Large number of genes that have an effect are known as polygenes E.g., height or mass.
• Discontinuous – qualitative categoric differences that fit into discrete classes. Only influenced by genetic factors. Different alleles at a single gene locus have a large effect. Generally monogenic. E.g., blood groups.

Question 5

Chi squared

Answer 5

• Determines if there is a significant difference between expected and observed results. If they are statistically significant this suggests linkage/epistasis. If not, then any difference is due to chance. E= expected O= observed. Formula given in exam.
• Critical value found at probability value of 0.05. Degrees of freedom = no. of classes - 1.
• If x^2 > critical value, then difference is significant and H0 can be rejected. Suggests there is another factor involved.
• If x^2 < critical value, then the difference is not significant and H0 is accepted. Any difference is due to chance.

Question 6

Factors affecting evolution

Answer 6

• Selection pressures are environmental factors that affect survival chances and allele frequencies.
• Stabilising selection keeps allele frequencies constant, as this is favourable. Human birth weights are maintained as low and high birth weights are selected against.
• Directional selection causes a gradual change in allele frequencies. This may be due to a change in environment or a new allele which is advantageous. Selection pressures favour phenotypes caused by this new allele and those with it are more like to survive and pass it on. Therefore, the frequency of the allele increases.
• Genetic drift occurs when chance affects which individuals survive and pass on certain alleles. This is more likely to have an effect in small populations as alleles are lost from the gene pool. In large populations, it will have less effect.
• Genetic bottlenecks occur when there is a sudden fall in numbers in a large population, which reduces genetic diversity as alleles are lost and individuals must reproduce with close relatives.
• Founder effect is when a small number of individuals start a new population, so not all the gene pool of the parent population is present. Changes in allele frequency may occur in a different direction to the parent population.

Question 7

Hardy-Weinberg

Answer 7

• Used to calculate allele frequencies. p2 + 2pq + q2 = 1 p + q = 1
• The Hardy-Weinberg principle states that if the conditions are met then the allele frequency in a population will not change from one generation to the next.
• Assumptions are that the population is large, there is no overlap between generations, mating is random, there is no mutation, migration or selection, the allele frequencies are equal between the two sexes, organisms are diploid and only reproduce sexually.
• Using proportions p= frequency of the dominant allele q = frequency of the recessive allele p2= proportion of individuals who are homozygous dominant. q2 = proportion of individuals who are homozygous recessive, 2pq = proportion of individuals who are heterozygous.

Question 8

Isolating mechanisms

Answer 8

• When two populations are reproductively isolated and do not interchange genes, they become genetically isolated. They therefore evolve independently, and this leads to speciation, where genetic differences lead to an inability to interbreed and produce fertile offspring.
• Allopatric speciation is caused by geographical isolation, when two populations are separated by a geographical barrier. There is no genetic exchange between populations and selection pressures change the gene pools. Populations diverge and differ physiologically, behaviourally and morphologically.
• Sympatric speciation occurs when no geographical barrier is present and two populations inhabit the same area, but there is no gene flow between them. This could be due to ecological factors where the populations live in the same area but in different environments or behavioural factors such as differences between communication or courting.
• Speciation relies on mutations otherwise no new alleles would be produced.

Question 9

Artificial selection

Answer 9

• Artificial selection occurs when humans choose organisms with desirable characteristics to selectively breed. These organisms are selected for their phenotypes not their genotypes.
• The process of artificial selection begins as phenotypic variation occurs within a species. Humans select individuals that have desirable phenotypes and are not closely related and breed them together. Offspring with the most desirable phenotypes are selected and bred together. This continues for many generations until all offspring have the desirable phenotype.
• Animals are selected for higher milk yield, large eggs, quality wool and fast pace.
• Plants are selected for large flowers, better taste and disease resistance.
• It is important to maintain a resource of genetic material close to the wild species, to increase the gene pool.
• Ethical considerations include that artificial selection can lead to inbreeding depression and a reduction of the gene pool. This can lead to harmful genetic defects and vulnerability to new diseases as there is a lower chance of resistant alleles, which can be damaging to animals.