(6) Patterns of Inheritance

Question 1

Types of variation

Answer 1

Interspecific - between 2 different species.
Intraspecific - between members of the same species.

Cause by environment or genetics

Continuous - polygenic, varies within a range, with no distinct categories. Could also be determined by environment.

Discontinuous - caused by one gene, each individual classed into one group. No environmental influence.

Question 2

Environmental factors affecting variation

Answer 2

Diet - continuous

Etiolation - elongation of stem in between nodes to reach sunlight, continuous

Chlorosis - not making chlorophyll because there is no light to catch

Question 3

Cause of variation - sexual reproduction

Answer 3

Random fusion of gametes at fertilisation - combining 2 random haploid gametes created from 2 genetically different individuals.
Crossing over and independent assortment.

Question 4

Monogenetic inheritance

Answer 4

Inheritance of a single pair of alleles of a single gene.

RR rr

Question 5

Dihybrid inheritance

Answer 5

Inheritance of 2 genes located on different pairs of homologous chromosomes (unlinked).

AaBb
Expected ratio - 9:3:3:1

Question 6

Sex linkage

Answer 6

Gene that codes for a characteristic is found on one of the sex chromosomes (X and Y in mammals).
Haemophilia, muscular dystrophy.

Question 7

Codominance

Answer 7

Inheritance of alleles that both contribute to the phenotype if they occur together.

Question 8

Autosomal linkage

Answer 8

Carried on non sex chromosomes.

Linkage - genes for different characteristics that are present on the same chromosome are linked and inherited together.

Crossing over results in some offspring having different combinations of alleles than either parent - recombinant.
When genes are on the same chromosome but very far apart, crossing over is more likely to occur, so each type of gamete is reproduced frequency (25% each).
When genes are on the same but close together, crossing over is less likely to separate the genes during meiosis = unequal frequency of each type of gamete. (Linked).

Question 9

Epistasis

Answer 9

Interaction of different green loci so that one gene locus masks or suppressed the expression of another.
Recessive - recessive allele of one gene prevents the expression of alleles of a second gene
9:3:4
Dominant - dominant allele of one gene prevents the expression of a second gene.
12:3:1

Question 10

Factors affecting evolution

Answer 10

Random mutation - randomly forms different alleles.
Sexual selection - frequency of alleles for improved mating success.
Natural selection - improved chance of survival on environment.
Population size.
Genetic drift - by chance not selection - better affect in smaller populations.
Gene flow - migration results in changes in allele frequency.

Question 11

Environment affecting common characteristics

Answer 11

Environment not changing much, individuals with alleles for characteristics towards the middle of the range are more likely to survive - stabilising selection.

When there is a change in the environment, individuals with the alleles for characteristics of extreme types are more likely to survive - directional selection.

Question 12

Genetic drift

Answer 12

Instead of environmental factors dictating which individuals survive and reproduce, chance dictates which alleles are passed on.
By chance the allele for one genotype is passed on more often than others, so the number of individuals with this allele increases.
If this occurs over and over, it can cause evolution as the allele becomes more common in the population.

Usually has a greater affect on smaller populations as chance has a greater influence.

Question 13

Genetic bottleneck

Answer 13

Causes genetic drift to have greater effect.
It is an event that causes a big reduction in a populations size for at least one generation, leading to reduction in gene pool.
Genetic diversity is also greatly reduced.

Question 14

Founder effect

Answer 14

Few organisms in a population start a new population and there are only a small number of different alleles in the initial gene pool.
Without any further gene flow the new population will grow with reduced genetic variation. As the population is small it’s more heavily influenced by genetic drift than a larger population.

Question 15

Hardy Weinberg principle

Answer 15

Calculate allele frequency for dominant and recessive alleles of any phenotypes controlled by 2 alleles of a single gene.

p + q = 1 (p is frequency of dominant allele, q is recessive).

p2 + 2pq + q2 = 1
(p2 = homozygous dominant individuals, 2pq = heterozygous, q2 = homozygous recessive).

Question 16

Isolating mechanisms in the evolution of new species.

Answer 16

Geographical mechanisms - allopathic speciation - difference between countries, results from 2 populations being geographically isolated. Favoured characteristics are selected resulting in differential reproductive success. Over time this results in new species.

Reproductive mechanisms - sympatric speciation - same country, formation of species that results within a population in the same area chasing them to become reproductively isolated.

Question 17

Artificial selection

Answer 17

When humans select individuals in a population to breed together to get desirable traits, can be done in animals and plants.

Animals - Morden dairy cattle, farmers select a female with a very high milk tiles and a male whose mother had a very high milk yield, the breed them together. Then select offspring with highest milk yield and breed them together. Continues over several generations.

Plants - bread wheat - same.

Question 18

Problems with artificial selection

Answer 18

Reduced gene pool - organism with similar traits and similar alleles are bred together , so less likely to be resistance to a new disease. Also means that potentially useful alleles may be lost.

Can exaggerate certain traits leading to health problems - pedigree dogs (pugs).

Increased incidence of genetic disease - hereditary deafness in Dalmatians.

Ethical issues.