8. Inheritance And Selection Flashcards
What does genotype describe?
Describes all the alleles an organism contains
-Sets the limits within the characteristics of an organism may vary
How can a change to the genotype be inherited by offspring?
If the mutation occurs in the formation of gametes
What does phenotype describe?
The observable characteristics of an organism
What is a modification? (In terms of inheritance and selection?)
A change to the phenotype that does not affect the genotype and is not inherited
What is the position of a gene on a chromosome called?
Locus
What is a homologous chromosome?
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.
What does co-dominant/incompletely dominant mean?
When both alleles appear in the phenotype- there is a blend of both features
What does multiple alleles mean?
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
What is an example of multiple alleles? Which are recessive/ dominant in your example?
Blood groups where groups A B ad O are possible. A and B are co-dominant and O is recessive to both.
How are genetic crosses represented? What must you not forget to include?
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)
What is sex linkage of a gene?
A gene is said to be sex linked when carried on either the X or the Y chromosome
Why could some characteristics found on some parts of the X chromosome appear more frequently in males than in females?
- 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
Why does haemophilia occur almost exclusively in males?
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.
What is a carrier? (In the context of inheritance and selection)
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.
What is a pedigree chart and how are different individuals represented? (Using the example of tracking a genetic disease)
A chart similar to a family tree. Males represented as square, females represented as circles. Affected individuals have their circle/ square shaded in.
How are co-dominant alleles represented (eg in a punnet square)
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
What is an allele hierarchy?
A ranking of alleles to show which alleles are dominant/recessive to which other alleles
What are the two Hardy-Weinburg equations to calculate allelic frequencies?
p + q = 1
p^2 + 2pq + q^2 = 1
What are the assumptions that we make when using the Hardy Weinburg equation?
- 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
(2 marks)
Explain what is meant by the term phenotype
1. Expression / appearance / characteristic due to genetic constitution/genotype/allele(s); 2. (Expression / appearance / characteristic) due to environment;
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?
- 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
What evidence would you look for on a pedigree chart to show that an allele for a disease is recessive?
- Two unaffected parents have affected offspring
- They must be carriers/ heterozygous
(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.
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;
(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.
Fertilisation is random / fusion of gametes is random / small/not large population/sample / selection advantage/disadvantage / lethal alleles;
(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
- Males have one allele;
- Females need two recessive
alleles / must be homozygous
recessive / could have dominant
and recessive alleles / could be
heterozygous/carriers;
(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.
- Offspring ratios are a probability/not fixed/arise by
chance/ - gametes may not be produced in equal numbers/
- fertilisation/fusion of gametes is random/
- small sample;
(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.
- Possible if parents homozygous/ bb;
- Don’t know genotype of chocolate cat / chocolate cat
could be homo- or heterozygous / chocolate cat could be
bb or bbi
; - Two chocolate cats could give cinnamon kittens;
(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.
- Directional;
- The recessive allele confers disadvantage/ the dominant
allele confers advantage/more likely to survive / reproduce;
(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.
- Breeding less successful;
- Feathers in poor condition;
- Less energy for breeding/reproduction/ stated aspect of
reproduction;
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.
- Avoids bias;
- Data representative/choice of nest not influencing results;
- Allows use of statistical tests/named statistical test;
(1 mark)
What does a negative correlation coefficient indicate about the relationship between number of feather mites on birds and their breeding success?
Breeding success decreases as feather mites increases/
negative correlation between feather mites and breeding
success ;
(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.
No mark for effect on reliability, marks are for explanation.
- Oil gland size/number of mites could vary;
- At different times of the day/due to preening;
