8. Inheritance And Selection

Question 1

What does genotype describe?

Answer 1

Describes all the alleles an organism contains

-Sets the limits within the characteristics of an organism may vary

Question 2

How can a change to the genotype be inherited by offspring?

Answer 2

If the mutation occurs in the formation of gametes

Question 3

What does phenotype describe?

Answer 3

The observable characteristics of an organism

Question 4

What is a modification? (In terms of inheritance and selection?)

Answer 4

A change to the phenotype that does not affect the genotype and is not inherited

Question 5

What is the position of a gene on a chromosome called?

Answer 5

Locus

Question 6

What is a homologous chromosome?

Answer 6

Homologous chromosomes are chromosome pairs, one from each parent, that are similar in length, gene position, and centromere location. The position of the genes on each homologous chromosome is the same, however the genes may contain different alleles.

Question 7

What does co-dominant/incompletely dominant mean?

Answer 7

When both alleles appear in the phenotype- there is a blend of both features

Question 8

What does multiple alleles mean?

Answer 8

When there are more than two alleles for a particular characteristic. However as there are only two chromosomes in a homologous pair, it follows that there can be only two of the multiple alleles present in any one organism

Question 9

What is an example of multiple alleles? Which are recessive/ dominant in your example?

Answer 9

Blood groups where groups A B ad O are possible. A and B are co-dominant and O is recessive to both.

Question 10

How are genetic crosses represented? What must you not forget to include?

Answer 10

Represented using punnet squares with genes represented as different letters (eg G for green seeds and Y for yellow seeds) and uppercase represents a dominant allele and a lower case represents a recessive allele

You must not forget to include:

• Phenotype of the parents
• Genotype of the parents
• What alleles their sperm and egg would include (during meiosis)

Question 11

What is sex linkage of a gene?

Answer 11

A gene is said to be sex linked when carried on either the X or the Y chromosome

Question 12

Why could some characteristics found on some parts of the X chromosome appear more frequently in males than in females?

Answer 12

• The X chromosome is much longer than the Y chromosome which means there is a part on the X chromosome which has no homologous portion on the Y chromosome
• Therefore if there is a recessive allele on the portion of the X chromosome with no homologous Y portion then there will be no dominant allele to mask the expression of the recessive allele

Question 13

Why does haemophilia occur almost exclusively in males?

Answer 13

Haemophilia is caused by a recessive allele in which the nucleotides do not code for teh required protein for blood clotting. It occurs almost exclusively in males because it occurs on the portion of the X chromosome with no homologous Y section therefore if present in a male, there is no dominant allele to mask the effect. However as females have two X chromosomes, it is more likely that the other X will have a dominant allele so the recessive allele for the faulty protein would not be expressed.

Question 14

What is a carrier? (In the context of inheritance and selection)

Answer 14

An organism that is heterozygous for a certain gene, perhaps for a genetic disease which is caused by a recessive allele- individuals do not express any characteristics/ symptoms of the disease/ disorder but may pass on the allele for the genetic disorder onto their offspring.

Question 15

What is a pedigree chart and how are different individuals represented? (Using the example of tracking a genetic disease)

Answer 15

A chart similar to a family tree. Males represented as square, females represented as circles. Affected individuals have their circle/ square shaded in.

Question 16

How are co-dominant alleles represented (eg in a punnet square)

Answer 16

They cannot be represented using upper and lowercase because this would imply that one is recessive to another. Therefore they are both represented as capital letters.
Eg colour of flowers, red and white:
C^W or C^R

Question 17

What is an allele hierarchy?

Answer 17

A ranking of alleles to show which alleles are dominant/recessive to which other alleles

Question 18

What are the two Hardy-Weinburg equations to calculate allelic frequencies?

Answer 18

p + q = 1

p^2 + 2pq + q^2 = 1

Question 19

What are the assumptions that we make when using the Hardy Weinburg equation?

Answer 19

• There are no mutations
• Population of isolated (no flow of alleles into or out of the population)
• No selection- all alleles equally likely to be passed on to the next generation
• Population is large
• Mating within the population is random

Question 20

(2 marks)

Explain what is meant by the term phenotype

Answer 20

1. Expression / appearance /
characteristic due to genetic
constitution/genotype/allele(s);
2. (Expression / appearance /
characteristic) due to
environment;

Question 21

What evidence would you look for on a pedigree chart to show that the allele for a recessive genetic disorder is NOT on the X chromosome?

Answer 21

• An unaffected father has an affected female child.
• This shows that it cant be sex linked, on the X chromosome, because she has inherited one of her X chromosomes from her father so therefore the father would have had the disease too if it was on the X chromosome

Question 22

What evidence would you look for on a pedigree chart to show that an allele for a disease is recessive?

Answer 22

• Two unaffected parents have affected offspring

- They must be carriers/ heterozygous

Question 23

(2 marks)
The fruit fly is a useful organism for studying genetic crosses. Female fruit flies are
approximately 2.5 mm long. Males are smaller and possess a distinct black patch on
their bodies. Females lay up to 400 eggs which develop into adults in 7 to 14 days.
Fruit flies will survive and breed in small flasks containing a simple nutrient medium
consisting mainly of sugars.

Use this information to explain two reasons why the fruit fly is a useful organism for
studying genetic crosses.

Answer 23

1. Large number of
eggs/offspring/flies (therefore)
improves reliability / can use
statistical tests/ are
representative / large sample
(size) / reduces sampling error;
2. Small size / (breed) in small
flasks / simple nutrient medium
(therefore) reduces costs/easily
kept/stored;
3. Size / markings / phenotypes
(therefore) males/females easy
to identify;
4. Short generation time / 7-14
days / develop quickly /
reproduce quickly (therefore)
results obtained quickly / saves
times / many generations;

Question 24

(1 mark)
The number of red-eyed females and red-eyed males in the offspring was counted.
The observed ratio of red-eyed females to red-eyed males was similar to, but not the
same as, the expected ratio. Suggest one reason why observed ratios are often not
the same as expected ratios.

Answer 24

Fertilisation is random / fusion of
gametes is random / small/not large
population/sample / selection
advantage/disadvantage / lethal
alleles;

Question 25

(2 marks)
Male fruit flies are more likely than female fruit flies to show a phenotype produced by
a recessive allele carried on the X chromosome. Explain why

Answer 25

1. Males have one allele;
2. Females need two recessive
   alleles / must be homozygous
   recessive / could have dominant
   and recessive alleles / could be
   heterozygous/carriers;

Question 26

(1 mark)
The breeder had expected equal numbers of chocolate and cinnamon kittens from the
cross between the chocolate male and black female. Explain why the actual numbers
were different from those expected.

Answer 26

1. Offspring ratios are a probability/not fixed/arise by
   chance/
2. gametes may not be produced in equal numbers/
3. fertilisation/fusion of gametes is random/
4. small sample;

Question 27

(2 marks)
The gene controlling coat colour has three alleles. The allele B gives black fur,
the allele b gives chocolate fur and the allele bi gives cinnamon fur.
● Allele B is dominant to both allele b and bi
.
● Allele b is dominant to allele bi
.
The breeder wanted to produce a population of cats that would all have chocolate fur.
Is this possible? Explain your answer.

Answer 27

1. Possible if parents homozygous/ bb;
2. Don’t know genotype of chocolate cat / chocolate cat
   could be homo- or heterozygous / chocolate cat could be
   bb or bbi
   ;
3. Two chocolate cats could give cinnamon kittens;

Question 28

(2 marks)
Scientists studied the frequencies of two alleles of a gene in one population of sea otters.
The dominant allele, T, codes for an enzyme. The other allele, t, is recessive and
does not produce a functional enzyme. After some years, the frequency of the recessive allele had decreased.

What type of natural selection appears to have occurred in this population of sea
otters? Explain how this type of selection led to a decrease in the frequency of the
recessive allele.

Answer 28

1. Directional;
2. The recessive allele confers disadvantage/ the dominant
   allele confers advantage/more likely to survive / reproduce;

Question 29

(2 marks)
Parasites are organisms that live on or in host organisms. The populations of many
organisms may be reduced by the effects of parasites.
Feather mites are small parasites found on the wing feathers of many birds. The mites
feed on the oil that the birds produce. This oil keeps the feathers in good condition.
Birds unable to oil their feathers properly use more energy in maintaining their body
temperature. This results in less energy being available for other processes.
Scientists investigated the relationship between the numbers of feather mites and the
breeding success of one species of bird, the great tit.

Use the information above to suggest how feather mites could affect breeding in
great tits.

Answer 29

1. Breeding less successful;
2. Feathers in poor condition;
3. Less energy for breeding/reproduction/ stated aspect of
   reproduction;

Question 30

Study on feather mites and how they affect the breeding success of a certain species of bird. Explain why the scientists sampled the nests at random.

Answer 30

1. Avoids bias;
2. Data representative/choice of nest not influencing results;
3. Allows use of statistical tests/named statistical test;

Question 31

(1 mark)
What does a negative correlation coefficient indicate about the relationship between number of feather mites on birds and their breeding success?

Answer 31

Breeding success decreases as feather mites increases/
negative correlation between feather mites and breeding
success ;

Question 32

(2 marks)
Explain how measuring the oil gland at the same time as counting the feather mites
may have affected the reliability of the data.

Answer 32

No mark for effect on reliability, marks are for explanation.

1. Oil gland size/number of mites could vary;
2. At different times of the day/due to preening;