Patterns of Inheritance

Question 1

What is the difference between homo and heterozygous?

Answer 1

Homo - Two identical alleles for a characteristic
Hetero - Two different alleles for a characteristic

Question 2

What is the difference between continuous and discontinuous variation?

Answer 2

Continuous - Characteristic that can take any value within a range
Discontinuous - Characteristic that can only appear in specific/discrete values

Question 3

What is the cause of variation, genetic control and examples in continuous variation?

Answer 3

Cause - genetic and environmental
Genetic control - Polygenes - controlled by a number of genes
Examples - leaf surface area, mass, skin colour

Question 4

What is the cause of variation, genetic control and examples in discontinuous variation?

Answer 4

Cause - mostly genetic
Genetic control - one or two genes
Examples - blood group, albinism, round or wrinkled pea shape

Question 5

What is monogenic inheritance?

Answer 5

An inheritace pattern where a single gene codes for a certain attribute

Question 6

In what scenario would an offspring have a 100% change of being heterozygous?

Answer 6

When the parental genotypes are homozygous recessive and homozygous dominant

Question 7

Explain codominance and give an example

Answer 7

• Occurs when two different alleles occur for a gene, and are both dominant
• Both alleles of the gene are expressed in the phenotype if present
• eg pink snapdragon flowers when the red and white allele is crossed

Question 8

How is codominance written/represented?

Answer 8

With a big letter C that is the same for both characteristics, and the different characteristic represented as a smaller letter, like a to the power in maths

Question 9

Explain what is meant by multiple alleles and give an example

Answer 9

• One gene, but more than two alleles
• Codominance can come within this
• eg blood type, represented by a big I with A,B, O (antigens) to the powers

Question 10

Explain sex linkage and why males are more likely to be affected by alleles on the X chromosome

Answer 10

• Genes found on sex chromosomes (X an Y)
• Males have XY, whereas females have XX
• The Y chromosome is much smaller, so there is less likely to be the same gene with an allele to counteract it

Question 11

What is dihybrid inheritance?

Answer 11

• Shows the inheritance pattern of 2 genes
• Each gamete carries one allele of both genes
• 4 possible phenotypes

Question 12

What is the typical ratio when two heterozygous genotypes are crossed in dihybrid inheritance?

Answer 12

9:3:3:1

Question 13

Why may there be unexpected ratios when gametes are crossed?

Answer 13

• Random fertilisation (egg and which sperm cell fused)
• Autosomal linkage (genes found on the same chromosome that are inherited together, no crossing over occurs to separate these genes)

Question 14

What does the chi squared (X2) test test for?

Answer 14

• To find significant difference between expected and observed ratios of offspring
• To test if there is autosomal linkage or epistasis

Question 15

When is there a significant difference between observed and expected ratio?

Answer 15

• When the chi squared value is bigger than the critical value

Question 16

What is meant by recombinant offspring?

Answer 16

Different combinations of alleles from either parent due to crossing over

Question 17

What is epistasis?

Answer 17

• Interaction of genes at different loci (different locations on different chromosomes)
• For example, gene A codes for protein A which could be an activator protein which would bind to gene B and cause it to be expressed

Question 18

What is meant by dominant epistasis?

Answer 18

• A dominant allele results in a gene having an effect on another gene
• If an epistatic gene codes for an enzyme that modified one of the precursor molecules in the pathway
• The next enzyme in the pathway would therefore lack a suitable substrate molecule so pigment would not be produced
• All the genes in this sequence would be masked

Question 19

What would be recessive epistasis?

Answer 19

If the presence of two recessive alleles at a gene locus led to the lack of an enzyme

Question 20

What does the Hardy-Weinberg principle state, and what assumptions does it follow?

Answer 20

• In a stable, non-evolving population, allele frequencies stay constant
• Assumes that there is a large population size, random mating, no mutations and no selection pressure, so no evolution

Question 21

Interpret the 2 Hardy Weinberg formulas

Answer 21

p + q = 1
where p = dominant allele frequency, and q = recessive allele
can be used to find p^2+2pq+q^2=1
p^2/q^2 = frequency of homozygous dominant/recessive genotype in population
2pq = frequency of heterozygous

Question 22

What are the 4 factors affecting evolution?

Answer 22

Mutations
Changes in population size
Genetic drift
Selection of favourable alleles (sexual and natural selection)

Question 23

In what 2 ways do changes in population size affect evolution?

Answer 23

density dependent factors affect population due to its size eg predation. Density independent factors affect population regardless of size, eg natural disaster

Question 24

Explain genetic drift

Answer 24

• Occurs in small populations
• Change in allele frequency due to random mutation
• New allele will have large impact due to small population
• Can lead to founder effect, when a mutation from an existing individual gives rise to a very favourable gene/allele (rare), which outcompetes rest of population, forming a new colony over time

Question 25

Explain the 3 types of natural selection

Answer 25

Stabilising selection - average phenotype selected/rises, shift in distribution where there are less individuals with extreme phenotypes, spike in middle of normal distribution curve
Directional selection - one extreme phenotype selected
Disruptive selection - two extreme phenotypes selected, very rare

Question 26

Define a species

Answer 26

Organisms that can interbreed to make fertile offspring

Question 27

Describe allopatric speciation

Answer 27

• Physical barrier a population into two different areas such as a mountain
• Therefore two different selection pressures such wet on one side, dry on the other
• Other genotypes, for phenotypes more favoured so favourable allele and more likely to survive and reproduce
• Change in allele frequency in the gene pool
• Characteristics accumulated until a new species is formed

Question 28

Explain sympatric speciation

Answer 28

• Occurs within the same habitat
• Disruptive selection or mating preferences that have caused a new species to arrive in the same habitat
• Much more likely in animals than plants, but still rare in animals