20- patterns of inheritance and variation

Question 1

What is chlorosis and how does it occur?

Answer 1

• The leaves look pale or yellow. It occurs because the cells are not producing normal amounts of chlorophyll. This reduces the ability of the plant to make food via photosynthesis.

Question 2

What environmental factors can cause chlorosis?

Answer 2

• Lack of light, plants will turn off chlorophyll production to conserve resources.
• Mineral deficiencies- e.g. lack of iron or magnesium. Iron is needed as a cofactor by some enzymes that make chlorophyll.
• Virus infections- when viruses infect plants, they interfere with the metabolism of cells. A common symptom is yellowing of the infected tissues as they can no longer support the synthesis of chlorophyll.

Question 3

What factors are animal body mass influenced by?

Answer 3

Combination of both genetic and environmental.
Environmental e.g. amount and quality of foods eaten, quantity of exercise, or the presence of disease.
Genetic e.g. mutation of chromosome 7 in a mouse. It causes the pattern of fat deposition in the body to be altered. Studies showed this gene acts in conjunction with other genes that regulate the energy balance of the body.

Question 4

What is a genotype?

Answer 4

The combination of alleles an organism inherits for a characteristic.
- It is the genetic make-up of an organism in respect of that gene.

Question 5

What is a phenotype?

Answer 5

• The observable characteristics of an organism.
• The actual characteristics that an organism displays are also often influenced by the environment.

Question 6

What does homozygous mean?

Answer 6

They have 2 identical alleles for a characteristic. The organism could be be homozygous dominant (contains 2 alleles for the dominant phenotype) or homozygous recessive (contains 2 alleles for the recessive phenotype)

Question 7

What does heterozygous mean?

Answer 7

They have 2 different alleles for a characteristic. The allele for the dominant characteristic will be expressed.

Question 8

What is continuous variation?

Answer 8

• a characteristic that can take any value within a range
• The causes of continuous variation are genetic and environmental
• controlled by multiple genes
• e.g. leaf SA, animal mass, skin colour
• Shown on line graphs

Question 9

What is discontinuous variation?

Answer 9

• A characteristic that can only appear in specific values.
• Its mostly genetic causes
• It is controlled by one or two genes
• e.g. blood group, albinism, round and wrinkled pea shape.
• Shown on bar charts or histograms

Question 10

What is codominance?

Answer 10

It occurs when two different alleles occur for a gene both of which are equally dominant. As a result, both alleles of the gene are expressed in the phenotype of the organism if present.

Question 11

What is sex linkage?

Answer 11

Some characteristics are carried on the sex chromosomes- these genes are called sex linked.
- As the Y chromosome is much smaller than the X chromosome, there are a number of genes in the X chromosome that males have only one copy of.
- This means that any characteristic caused by a a recessive allele on the section of the X chromosome, which is missing in the Y chromosome, occurs more frequently in males. This is because many females will also have a dominant allele present in their cells.

Question 12

What is haemophilia?

Answer 12

Its a sex linked genetic disorder. Patients with haemophilia have blood which clots extremely slowly due to absence of a protein clotting factor. As a result, injury can result in prolonged bleeding which, of left untreated, is potentially fatal.
If a male inherits the recessive allele that codes for haemophilia on their X chromosome, they cannot have a corresponding dominant allele on their Y chromosome, and so develop the condition.
- Females that are heterozygous for the haemophilia coding gene are known as carriers.

Question 13

Why do actual ratios of offspring produced can differ from expected when using a dihybrid cross?

Answer 13

• Fertilisation of gametes is a random process so in a small sample a few chance events can lead to a skewed ratio
• The genes being studied are both on the same chromosome. These are known as linked genes. If no crossing over occurs the alleles for the two characteristics will always be inherited together.

Question 14

What is linkage?

Answer 14

the genes are located on the same chromosome. The effects of sex linkage can be seen in conditions such as haemophilia and red green colour blindness.
- When the genes that are linked are found on one of the other pairs of chromosomes it is called autosomal linkage.
- Linked genes are inherited as one unit- there is no independent assortment during meiosis unless the alleles are separated by chiasmata. They tend to be inherited together.

Question 15

What are recombinant offspring?

Answer 15

• they have different combinations of alleles than either parent.
• The closer the genes are on a chromosome, the less likely they are to be separated during crossing over and the fewer recombinant offspring produced.

Question 16

What is recombinant frequency? How is it measured?

Answer 16

• the amount of crossing over that has happened in meiosis
• Recombinant frequency = number of recombinant offspring/ total number of offspring
• A recombinant frequency of 50% indicates that there is no linkage and the genes are on separate chromosomes.
• Less than 50% indicates that there is gene linkage and the random process of independent assortment has been hindered.
• As the degree of crossing over reduces, the recombination frequency also gets smaller.
• The degree of crossing over is determined by how close the genes are on the chromosome.
• The closer they are, the less likely they will be separated during crossing over.

Question 17

What is the Chi-squared test?

Answer 17

It is a stats test that measures the size of the difference between the results you actually get and those you expect to get. It helps to determine whether differences in the expected and observed results are significant or not, by comparing the sizes of the differences and the numbers of observations.

Question 18

What do different chi-squared values mean?

Answer 18

• A large chi-squared value means there is a statistically significant difference between the observed and expected results and the probability that these differences are due to chance is low. There must be a reason, other than chance, for the unexpected results.

Question 19

What is epistasis?

Answer 19

It is the interaction of genes at different loci. Gene regulation is a form of epistasis with regulatory genes controlling the activity of structural genes e.g. the lac operon. Gene interaction also occurs in biochemical pathways involving only structural genes.

Question 20

What is dominant epistasis?

Answer 20

• It occurs if a dominant allele results in a gene having an effort on another gene. This would happen if an epistatic gene coded for an enzyme that modified one of the precursor molecules in the pathway.
• The next enzyme in the pathway would then lack a suitable substrate molecule and so the pigment would again not be produced. All of the genes in the sequence would be effectively ‘masked’.

Question 21

What factors can lead to changes in the frequency of alleles within a population and eventually affect the rate of evolution?

Answer 21

• Mutation is necessary for the existence of different alleles in the first place, and the formation of new alleles lead to genetic variation.
• Sexual selection leads in an increase in frequency of alleles which code for characteristics that improve mating success.
• Gene flow is the movement of alleles between populations. Immigration and emigration results in changes of allele frequency within a population.
• Genetic drift occurs in small populations. This is a change in allele frequency due to the random nature of mutation. The appearance of a new allele will have a greater impact in a smaller population than in a much larger population where there is a greater number of alleles present in the gene pool.
• Natural selection; leads to an increase in the number of individuals that have characteristics that improve their chance of survival.