6.2 - Patterns of inheritance

Question 1

Continuous vs discontinuous variation

Answer 1

Continuous

• no defined categories/distinct groups
• there is a range - any value is possible
• caused by more than one gene (polygenic) and often, the environment
• the greater the number of gene loci contributing to the characteristic, the greater the range in variation
• quantitative

Discontinuous

• discrete categories with no intermediates
• usually caused by one gene (mongenic)
• genes at different loci may interact to influence one characteristic and cause discontinuous variation (epistasis)
• no (very little) environmental effects cause it
• qualitative

Question 2

Environmental factors that influence variation

Answer 2

• Diet in animals can lead to changes in mass/malnutrition
• Language
• Scars
• Plants grown in too little light experience etiolation - rapidly growing  stems, weakening of cell walls, chlorosis (lack of chlorophyll  production). In chlorosis the environmental factors prevent the expression of genes for chlorophyll production

Question 3

How is genetic variation produced

Answer 3

• Mutations e.g. substitutions change the DNA base sequence
• Which leads to changes in primary structure of protein and therefore their shape and function
• Independent assortment of homologous chromosomes in metaphase
• Independent assortment of chromatids in metaphase II
• This produces large number of allele combinations
• Crossing over in prophase
• so chromatids will have new combination of alleles
• Non-disjunction means homologous chromosomes don’t separate in metaphase
• which can cause one more/less chromosome to be present in gamete and subsequent zygote (causes Down’s syndrome)
• Random fusion of non-genetically identical gametes at fertilisation produces a large number of allele combinations

Question 4

2 types of natural selection:

Answer 4

Stabilising selection

• occurs when organisms’ environment doesn’t change
• favours intermediate phenotypes (over extremes)
• reduces variation in a population
• e.g. animals with very short/long fur in constant temperatures will be selected against - those with mid length fur will survive - higher frequency of alleles for mid length fur

Directional selection

• occurs when environment changes
• favours a new (extreme) phenotype
• causes a change in population mean phenotype
• e.g. climate temperature decreases. Those with long fur survive, breed and pass on allele for long hair - over time this allele becomes more frequent in population

Question 5

2 special types of genetic drift

Answer 5

Genetic bottleneck

• an event e.g. flood rapidly reduces the numbers of a population
• some alleles lost from population at random
• genetic variation reduced - genetic drift

Founder effect

• a small number of individuals from an original larger population establish a new population
• some alleles lost from population at random (these could be advantageous)
• genetic variation reduced - genetic drift

Question 6

Why does genetic drift only happen in small populations?

Answer 6

• Each individual forms a large proportion of the gene pool
• therefore has a greater effect on the gene pool.
• It is also easier to ‘lose’ a gene from a small gene pool

Question 7

2 types of isolating mechanisms that cause speciation

Answer 7

Geographical isolation leads to allopatric speciation

• Populations are physically separated e.g. by water/mountains/fences
• Barrier prevents gene flow between populations
• Genetic changes occur in species
• caused by genetic drift, mutations or natural selection (different pressures in different areas)
• Ultimately the populations become genetically so different they can no longer interbreed to produce fertile offspring (reproductively isolated) - new species have been formed

Reproductive isolation leads to sympatric speciation

• Several things can lead to individuals in a population becoming reproductively isolated :
• Behavioural changes e.g. changes to sleep patterns, courtship behaviours
• Biological changes e.g. size differences, genetalia differences
• Genetic changes e.g. change in chromosome number prevents zygote viability
• Once populations can no longer interbreed to produce fertile offspring (reproductive isolation) - new species have been formed

Question 8

Stages of selective breeding (artificial selection):

Answer 8

1. male and female with desired characteristic chosen
2. male and female interbred
3. best offspring selected and interbred
4. this is repeated over many generations

Question 9

Principles behind selective breeding (artificial selection)

Answer 9

Humans chose parents with desired phenotypes and therefore the desired alleles and interbreed them to produce offspring with higher frequency of these phenotypes. Repeated over many generations

Question 10

The importance of maintaining a resource of genetic material for use in selective breeding including wild types (the original population you bred from):

Answer 10

• Selective breeding tends to reduce the gene pool
• This could mean if there was a rapid environmental change e.g. temperature/disease - genetically similar e.g. crops would not have the variation needed to survive
• We use gene banks (seed/sperm/egg banks/embryo, rare breed farms, botanic gardens/zoos) to maintain a source of alleles for future breeding
• This can counteract the loss in genetic variation, inbreeding, and extinction in the event of a disease etc
• It can also preserve currently unknown useful traits/alleles e.g. medicinal uses

Question 11

Problems with and how to avoid inbreeding in selective breeding/artificial selection

Answer 11

• As the genetic diversity decreases with each generation, individuals become more and more related - inbred
• The likelihood of unintentionally selecting 2 copies of a harmful recessive allele can increase in a small gene pool.
• This can lead to increased susceptibility to disease
• To avoid this, breeders can ‘outcross’ individuals with their wild types (or individuals from gene banks) to prevent the gene pool becoming too small

Question 12

Ethical considerations of selective breeding/artificial selection

Answer 12

• Often doesn’t take into account animal welfare
• Inbreeding can increase susceptibility to disease e.g. some breeds are highly susceptible to cancers
• Many domesticated animals e.g. pigs would not survive if released into the wild - easy prey, wrong fat:muscle ratio to survive in cold

Question 13

Explain how selectively breeding for one trait may result in many differences between selectively bred and wild animals

Answer 13

• selective breeding involves whole genomes
• hence other traits follow selected trait(s)
• for example, selecting for one trait may inadvertently select for another linked gene

Question 14

(Generally) how to do any Hardy Weinberg Q

Answer 14

1. Work out the proportion of individuals with the recessive phenotype (=q2)
2. Work out q (square root of q2)
3. Work out p (1-q=p)
4. Use p and q to work out the proportions asked for by the question (e.g. to work out the frequency of heterozygotes: 2pq = 2 x p x q

Question 15

Monohybrid inheritance

Answer 15

• Inheritance of 1 gene (one trait e.g. eye colour)
• 2 alleles – 1 dominant and 1 recessive
• 3:1 (AA,AB,BB)
• Look out for: 1 trait, only 2 phenotypes

Question 16

Multiple alleles

Answer 16

• Inheritance of 1 gene (one trait)
• 3 or more alleles – there may be a dominance hierarchy
• Look out for: 1 trait, multiple phenotypes
• e.g. ABO blood groups (also show codominance)

Question 17

Sex linkage

Answer 17

• Inheritance of 1 gene (one trait)
• Look out for: a clear difference between male and female proportions
• Written e.g. XAXa for females, XAY for males (no allele on Y)

Question 18

Codominance

Answer 18

• Inheritance of 1 gene (one trait)
• 2 (or more) alleles – neither dominant over the other – both alleles contribute to the phenotype of the heterozygote
• 1:2:1 (AA, AB, BB)
• Look out for: 1 trait, 3 (or more) phenotypes (heterozygous phenotype often appears to be 2 other phenotypes ‘combined’)
• Written e.g CACA (A’s phenotype), CACB (AB’s phenotype), CBCB B’s phenotype)

Question 19

Dihybrid

Answer 19

• Inheritance of 2 genes (two traits)
• Genes are NOT linked
• Genes do NOT have an effect on each other
• 2 (or more) alleles for each gene – 1 dominant and 1 recessive for each gene
• 9:3:3:1
• Look out for: 2 traits expressed in each of 4 phenotypes (e.g. long and yellow)
• Written: AABB, AaBb (i.e. 2 alleles for each gene)

Question 20

Autosomal linkage

Answer 20

• Inheritance of 2 genes (two traits) on the same chromosome – inherited together
• Genes do NOT have an effect on each other
• 2 (or more) alleles for each gene – 1 dominant and 1 recessive for each gene

Autosomal linkage when crossing over has not occurred – what we expect:

• Look out for: 2 traits expressed in each of 2 phenotypes (e.g. long and yellow)
• 3:1 (same as a monogenic cross because genes are linked and inherited together)

Autosomal linkage when crossing over has occurred:

• Crossing over produces recombinant gametes/phenotypes
• Look out for: 2 traits expressed in each of 4 phenotypes (e.g. long and yellow)
• A NON 9:3:3:1
• The further apart the gene loci for the linked genes, the more likely crossing over is and the higher the number of recombinant phenotypes

Question 21

Epistasis

Answer 21

• Inheritance of 2 genes but 1 trait
• Genes are NOT linked
• Allele from one gene masks the expression of alleles on another gene
• 2 (or more) alleles for each gene – 1 dominant and 1 recessive for each gene
• Modified 9:3:3:1
• 9:3:4 recessive epistasis (homozygous recessive on one gene is epistatic to alleles on other gene)
• 12:3:1, 13:3 dominant epistasis
• 9:7 or 9:3:4 epistasis by complementary gene action
• Look out for: 1 trait expressed in each of 2/3 phenotypes
• Written: AABB, AaBb (i.e. 2 alleles for each gene)