A2 Booklet 1- Inheritance and Evolution

Question 1

What is the definition of:
Phenotype?
Genotype?
Alleles?
Dominant alleles?
Recessive alleles?
Codominant alleles?
Locus?
Homozygous?
Heterozygous/
Multiple alleles?
Sex-linked genes?
Autosomally-linked genes?
Homologous pairs of chromosomes?

Answer 1

Phenotype- Appearance of a characteristic due to expression of the genotype and interaction with the environment.

Genotype- Genetic constitution of an organism.

Alleles- Different forms of the same genes.

Dominant alleles- Always expressed in phenotype.

Recessive alleles- Only expressed when there’s a homozygous recessive genotype.

Codominant alleles- Both alleles expressed in genotype.

Locus- Position of gene on chromosome.

Homozygous- Both alleles of the genotype are the same.

Heterozygous- The two alleles of the genotype are different.

Multiple alleles- More than two alleles for a particular genes, but only two alleles can be present in a genotype.

Sex-linked genes- Gene is carried on only one type of sex chromosome, usually X chromosome.

Autosomally-linked genes- Genes linked because they’re on the same chromosomes, inherited together.

Homologous pairs of chromosomes- Have the same alleles of the same gene at the same locus.

Question 2

Explain Monohybrid Inheritance

Answer 2

Inheritance of one particular characteristic. T = tall, t = short:

Genotype: TT x tt
Gametes: T t
F1 Genotype: Tt

Question 3

What is a test cross?

Answer 3

A cross between a homozygous recessive individual and a corresponding suspected heterozygote to determine the genotype of the latter.

Question 4

What is codominance?

Answer 4

Both alleles are expressed in the phenotype:

Phenotype: Pink x Pink
Genotype: CrCw CrCw
Gametes: Cr + Cw Cr + Cw

F1 Genotype: CrCr CrCw CwCr CwCw
F1 Phenotype: Red Pink Pink White
F1 Ratio: 1 : 2 : 1

Question 5

What are multiple alleles?

Answer 5

More than two alleles for a particular gene but only two alleles can be present in a genotype.
ABO blood groups:
Phenotype          Genotype
       A                  |A|A or |A|O
       B                  |B|B or |B|O
      AB                      |A|B
       O                       |O|O

Question 6

How is sex determined?

Answer 6

1 pair of sex chromosomes in humans.
XX or XY.
Probability in humans is 50/50

Question 7

What are sex-linked genes?

Answer 7

Gene is carried on only one type of sex chromosome.
Colour blindness in humans is sex-linked. The alleles for normal sight and colour blindness are on the X chromosome at a locus which the Y chromosome does not have. This means that only 1 copy of the gene is needed for expression in a male. If the male has the allele for normal vision on the X chromosome, he will have normal vision. If the male has the allele for colour blindness on the X chromosome, he will have colour-blindness.
Females can be carriers (two X chromosomes, so heterozygous).

Haemophilia example:

XHXh x XHY
XH Xh XH Y

XHXH XHXh XHY XhY
2 : 1 : 1
Normal female, normal female (carrier), normal male, haemophilic male.

Question 8

What is a pedigree chart and how do you read it?

What are some of the rules?

Answer 8

Use the key to see which are male and which are female, and the colours will be unaffected, affected and carrier.

Pretend something IS sex-linked when trying to prove it’s not.
2 unaffected parents have an affected child = recessive.
2 unaffected parents have an affected DAUGHTER = not sex-linked when known to be recessive.
2 affected parents have an unaffected child = dominant, must be heterozygous parents that pass on recessive alleles.
2 affected parents have an unaffected DAUGHTER = not sex-linked when known to be dominant.

Question 9

Explain Dihybrid Inheritance

Answer 9

Inheritance of two different characteristics (genes are usually autosomally linked) controlled by different genes.

RRYY x rryy
RY ry
RrYy

Question 10

Why might observed ratios be different to expected ratios?

Answer 10

Small sample size = less likely to see expected ratios.
Fusion of gametes at fertilisation is random.
There is selection against a particular phenotype.

Question 11

How do you do the chi-squared test?

Answer 11

Test used to compare the goodness of fit of observed phenotypic ratios with expected ratios (is there a statistical difference between expected and observed ratios?)

Question 12

What are autosomally-linked genes?

Answer 12

Genes on the same chromosome that are inherited together. Reduced variety.

Question 13

What is Epistasis?

Answer 13

Occurs when two or more genes contribute to a phenotype. The allele of one gene affects or masks the expression of another gene at another locus. For example, colour in flowers. If AA and not Aa, enzyme Y is not produced so white doesn’t convert to pale blue etc.

                    Gene A                    Gene B
                   /            \                  /            \
                  A            a                B             b
                  \             /                  \              /
               active   inactive     active     inactive
                  \             /                \                 /
                  Enzyme Y                Enzyme Z
                          |                                 |
           White------------> pale blue----------> purple
         substrate              pigment              pigment

Question 14

What is the definition of:
Species?
Populations?
Gene pool?
Allelic frequency?

Answer 14

Species- a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding.
Populations- A group of organisms of same species occupying a particular space at a particular time that can potentially interbreed.
Gene pool- All of the alleles, expressed or not expressed, of all the genes of all the individuals of a population at any one time.
Allelic frequency- the number of time an allele of a particular gene occurs within the gene pool (individual, not genotype).

Question 15

What is the Hardy-Weinberg Principle?

Answer 15

A mathematical model which predicts that allele frequencies in a population will not change from generation, as long as certain conditions are upheld.

p = frequency of dominant allele.
q = frequency of recessive allele.
p2 = frequency of homozygous dominant genotype.
q2 = frequency of homozygous recessive genotype.
2pq = frequency of heterozygous genotype.
p + q = 1.0
p2 + 2pq + q2 = 1

Question 16

Which factors effect Variation in populations?

Answer 16

Gene mutations- changes in base sequence of DNA resulting in production of different polypeptides/enzymes.

Crossing over- during meiosis the exchange of alleles of same genes between chromosomes of a homologous pair, results in new combinations of alleles.

Independent segregation of chromosomes- first meiotic division separates homologous pairs of chromosomes.

Random fertilisation- random fusion of gametes occurs during sexual reproduction.

Question 17

Explain Natural Selection

Answer 17

• Organisms produce large number of offspring. Selection due to environmental conditions favouring particular phenotypes.
• Change in environment results in organisms with particular phenotype being better adapted to environment, so survive and reproduce and pass on these genes.
• Survival of only some organisms and them reproducing is known as differential survival and differential reproductive success.
• Organisms with alleles for selected phenotypes have greater chance of surviving and reproducing. They will pass on these alleles to next generation.
• Frequency of selected phenotype increases as does frequency of favourable allele(s) in gene pool.
• Continues from generation to generation as selection for phenotype continues. This is natural selection.

Question 18

What are the different types of Natural Selection?

Answer 18

Stabilising selection: results in organisms all having a phenotype closer to the middle of the range (higher peak and narrower range on a graph).

Directional selection: Environment changes so organisms with an advantageous allele coding for a phenotype closer to one extreme makes them more likely to survive and go on to breed and pass this allele on, results in shift of peak to one extreme.

Disruptive selection: selection against the mean.

Question 19

Describe how and why evolution occurs

Answer 19

Occurs as a result of change in allele frequencies in population.

Question 20

What are the two types of speciation?

Answer 20

Allopatric speciation and sympatric isolation.

Question 21

Describe allopatric speciation

Answer 21

• A population with variation already present due to mutations is split into two or more groups by geographical isolation (can be on a small scale or wide scale e.g. mountains/islands or smaller lakes from big lakes).
• No gene flow between the gene pools of separated populations.
• Different selection pressures in different environment means selection of different phenotypes.
• Organisms with selected phenotypes survive to breed and pass on advantageous alleles to next generations.
• Frequency of different selected phenotypes and alleles in each population increases.
• Over time, gene pools of separated populations become so different that the groups can no longer interbreed, meaning they are now different species.

Question 22

What conditions must you assume in the Hardy-Weinberg Principle?

Answer 22

• Population is large and isolated so no flow of alleles in or out of population.
• Mating within populations is random.
• No mutations of the gene occur.
• No natural selections so all alleles are equally likely to be passed on to next generation.

Question 23

Describe sympatric speciation

Answer 23

• Population becomes reproductively isolated (DOES NOT NEED TO BE GEOGRAPHICAL).
• Random mutations within population could lead to group of organisms not reproducing with organisms of same species in same habitat.
• May be due to different flowering/mating seasons or courtship behaviours as a result of mutation.
• Often involves disruptive selection.
• Living in same habitat, populations are reproductively isolated with no gene flow between gene pools.
• Allelic frequencies also change as mutations may occur independently in each populations.
• Over time, gene pools of separated populations become so different that the groups can no longer interbreed, meaning they are now different species.

Question 24

What is genetic drift?

Answer 24

• Occurs by chance, not natural selection.
• By chance, allele of particular gene is passed on to offspring more often than other alleles of same gene.
• Frequency of allele increases in successive generations.
• Much greater effect in small populations as there’s likely to be a smaller variety of alleles in gene pool (genetic diversity is less).
• Can lead to speciation occurring more rapidly in large population where frequency of mutant allele is likely to be much less and thus has less effect.