Inheritance

Question 1

What is a gene?

Answer 1

A sequence of DNA bases that codes for a specific polypeptide

Question 2

What is a locus?

Answer 2

The particular region that the gene occupies on a chromosome

Question 3

What is an allele?

Answer 3

Alternative forms of a particular gene, coding for an altered phenotype

Question 4

Define genotype

Answer 4

The genetic make up of an individual - all of the alleles they possess

Question 5

Define phenotype

Answer 5

The observable characteristics of an organism determined by the genotype

Question 6

What is the gene pool?

Answer 6

All the alleles in a population at any one time

Question 7

What is monohybrid inheritance?

Answer 7

The inheritance of a single gene eg that controlling a plant’s height

Question 8

Why were pea plants a useful choice for Mendel’s inheritance experiments?

Answer 8

• They’re easy to grow
• They can self or cross fertilise
• They can produce flowers and fruits in the same year

Question 9

How would Mendel ensure that offspring produced were only from the parents being investigated?

Answer 9

Pollen transferred by hand - receiving flower could be in a bag to prevent further cross pollination
Remove anthers before they mature so no self pollination

Question 10

What is Mendel’s first ‘law of inheritance’?

Answer 10

The characteristics of an organism are determined by (factors) alleles, which occur in pairs. Only one allele of a pair is present in each gamete.

Question 11

What is continuous variation?

Answer 11

Characteristic often has a range of values (bell shaped curve). Controlled by a number of genes. Eg height Environmental factors have an effect

Question 12

What is discontinuous variation?

Answer 12

The characteristic has distinct categories controlled by a single gene. Environmental factors have no effect eg tongue rolling, blood type

Question 13

What is the ‘backcross’ or test cross?

Answer 13

A method used in genetics to determine whether a particular dominant characteristic observed in an organism is caused by 1 or 2 dominant alleles (homozygous dominant or heterozygous). The unknown is always crossed with an individual displaying the recessive phenotype.

Question 14

What is pure breeding?

Answer 14

Two organisms with the same phenotype that, when bred together, produce offspring with the same phenotype (homozygous)

Question 15

What is codominance?

Answer 15

Both alleles for a gene are expressed when present in a heterozygote. Both alleles are written using a capital letter. Eg chickens with black feathers and chickens with white feathers produce speckled offspring

Question 16

What is incomplete dominance?

Answer 16

The phenotype of the heterozygote is intermediate of the two homozygous parents eg red carnations crossed with white flowered carnations produce pink offspring

Question 17

What is dihybrid inheritance?

Answer 17

Inheritance of 2 unlinked genes. Independent assortment of these genes produces recombinants (different allele combinations in the gametes)

Question 18

What is Mendel’s second law (law of independent assortment)?

Answer 18

Either of a pair of alleles may combine randomly with either of another pair

Question 19

What are the headings of the columns when working out chi squared?

Answer 19

Observed. Expected. O-E. (O-E)^2. (O-E)^2/E - Then work out the sum of (O-E)^2/E

Question 20

What is the value for degrees of freedom?

Answer 20

One less than the number of categories

Question 21

When do we accept/reject the null hypothesis?

Answer 21

• If the chi squared value is less than the value in the table, we accept the null hypothesis, any difference is down to chance
• If the chi squared value is greater than the value in the table, we reject the null hypothesis, therefore other factors are influencing

Question 22

What 4 things to mention when concluding results of chi squared

Answer 22

1. Chi squared value is less than/more than critical value at 5%
2. Therefore there is/is no significant difference between observed and expected values
3. Any difference is down to chance/other factors
4. Null hypothesis is accepted/rejected

Question 23

What are the 2 types of chromosome?

Answer 23

• Autosomes: any chromosome that is not a sex chromosome
• Sex chromosomes: a chromosome concerned in determining the sex of an organism, typically one of 2 kinds

Question 24

What is sex linked inheritance?

Answer 24

Alleles that are carried on the X chromosome. Male Y chromosome is much smaller than the X so carries far fewer genes. For most genes carried on the X chromosome in the male there is only the one allele which must be expressed. Therefore in the male any recessive genes on the X chromosome will be expressed in the phenotype.

Question 25

If a male with a sex-linked disease had a son, what are the chances of him having the disease?

Answer 25

No chance as the father will only pass on his Y chromosome to his son. Sons can only inherit the recessive allele from the mother

Question 26

If a male with a sex-linked disease had a daughter, what are the chances of her having the disease?

Answer 26

She would become a carrier with no symptoms, unless the mother had also passed on a recessive allele and then she would have the disease.

Question 27

Why might women with a carrier mother and an unaffected father want genetic counselling before having children?

Answer 27

To find out if they have inherited the recessive allele as this would make them carriers. If they passed this recessive allele onto their son, the son would have the disorder. They may decide to screen the embryo when they become pregnant.

Question 28

Haemophilia is an example of a sex-linked condition in humans. The individual cannot produce enough of one particular blood clotting protein. What would be a symptom of this?

Answer 28

Blood doesn’t clot normally - slow, persistent bleeding

Question 29

Duchenne muscular dystrophy is caused by a sex linked recessive allele of the dystrophin which stabilises cell membranes of muscle fibres. What are symptoms for individuals without this working allele?

Answer 29

Progressive weakening and wasting of the muscles

Question 30

Why might there be a low number of children who have sex-linked diseases through affected parents?

Answer 30

Affected people might die before they can reproduce or decide they don’t wish to conceive so don’t pass the disease onto offspring

Question 31

When do Mendel’s ratios apply (3:1 and 9:3:3:1)?

Answer 31

When genes are on different chromosomes - not linked Genes on the same chromosome are linked and usually inherited together so Mendel’s ratios don’t apply

Question 32

An organism with the genotype AaBb has 2 genes (A and B). That are found on different chromosomes - not linked. What gametes could be produced by this organism?

Answer 32

AB, Ab, aB, ab

Question 33

In an organism, the genotype AaBb has 2 genes (A and B) that are found on the same chromosome - linkage. What gametes can be produced by this organism if crossing over doesn’t disrupt the linkage?

Answer 33

AB, ab

Question 34

Summarise linkage

Answer 34

- Genes are inherited together - Crossing over is very unlikely to separate them as the genes are too close together on the same chromosome

Question 35

Summarise incomplete linkage

Answer 35

- Genes are on the same chromosome but are far apart - Genes could be separated due to crossing over (at prophase I) and not be inherited together - Rare (only 5-10% of the time)

Question 36

What is a mutation?

Answer 36

An unpredictable change in the genetic material of an organism

Question 37

What are the 2 types of mutation?

Answer 37

- **Gene mutations:** affect single genes - **Chromosome mutations:** cause changes in the structure or number of whole chromosomes

Question 38

Why are mutations important?

Answer 38

They provide an important source of genetic variation.

Question 39

In which organisms are mutation rates more frequent?

Answer 39

Those with short life cycles and frequent cell division

Question 40

When do chromosomal mutations most often occur?

Answer 40

During crossing over in prophase I and non-disjunction during anaphase I and II

Question 41

What are mutagens? Give examples

Answer 41

Factors in the environment that increase the frequency of mutation in an organism Ionising radiation eg X-rays Mutagenic chemicals eg polycyclic hydrocarbons

Question 42

Why is it incorrect to say that mutagens cause mutations?

Answer 42

Mutagens do not cause mutations but increased exposure to mutagens increases the rate of mutations occurring

Question 43

What are carcinogens?

Answer 43

Mutagens that increase the rate of cancer

Question 44

What regulates cell division in humans? What happens if these genes become mutated?

Answer 44

**Proto-oncogenes** and **tumour-suppressor genes** regulate cell division They form oncogenes if mutated

Question 45

What are oncogenes involved in?

Answer 45

Uncontrollable cell division and can lead to cancer

Question 46

Tobacco smoke contains a number of harmful chemicals that affect human health. What do these include?

Answer 46

- Nicotine - Polycyclic aromatic hydrocarbons - Carbon monoxide

Question 47

How may tar cause lung cancer?

Answer 47

Tar contains carcinogens which target DNA in the cells of the alveoli. If mutation occurs in a proto-oncogene or tumour suppressor gene, lung cancer may develop

Question 48

What do gene mutations affect?

Answer 48

Single bases within a gene

Question 49

What could a change in a single base of a gene result in?

Answer 49

- Change in DNA base sequence, resulting in a change in triplets/codons for amino acids - Different amino acid sequence produced in translation - Bonds form in different places and result in different shaped protein (may not function)

Question 50

An enzyme is required for melanin to be produced. Explain how a mutation in the gene coding for this enzyme results in a lack of pigmentation in individuals with albinism

Answer 50

- Changes to the DNA base sequence results in a change in the sequence of amino acids during translation - A mutation in the gene coding for the enzyme could result in the active site changing shape - The enzyme cannot function and no melanin is produced

Question 51

Give an example of a substitute gene mutation

Answer 51

Sickle cell anaemia RBCs become sickle shaped, affecting the ability of the haemoglobin to bind to oxygen, resulting in anaemia Mutant allele is co-dominant

Question 52

Sickle cell trait confers some resistance to the malaria parasite. Why is this?

Answer 52

Plasmodium parasites cannot successfully reproduce within the RBCs because of their sickle shape. This is a selective advantage in countries where malaria is prevalent and so natural selection has meant this characteristic is not selected against

Question 53

What are 3 ways chromosome mutations can occur?

Answer 53

1. **Changes in chromosome structure** - errors occur when chromosomes exchange sections of DNA during crossing over at prophase I 2. **Changes in whole sets of chromosomes (polyploidy)** - if a gamete receives 2 sets of chromosomes during a failed meiotic division, the gametes will be diploid rather than haploid 3. **Changes in chromosome number (nondisjunction)** - process where faulty cell division results in one daughter cell getting two copies of a chromosome whilst one daughter cell gets none

Question 54

Describe why someone may be born with Down’s syndrome (trisomy 21)

Answer 54

During meiosis in the female ovary, an oocyte with 2 copies of chromosome 21 is produced and survives. If this oocyte is fertilised, the zygote would have 3 copies of chromosome 21 - 2 from the mother and one from the father

Question 55

State 3 features of Down’s syndrome

Answer 55

- Open facial features - Low muscle tone - Some degree of learning disability

Question 56

Explain how a trisomy such as trisomy 16 in mice can occur in a zygote

Answer 56

Homologous chromosomes fail to separate/nondisjunction at anaphase I in oogenesis. Secondary oocyte has 2 copies of chromosome 16. Third added from sperm at fertilisation.

Question 57

What is epigenetics?

Answer 57

The control of gene expression by modifying DNA or histone proteins, without affecting the DNA nucleotides sequence

Question 58

How can the addition of a methyl group to nitrogenous bases lead to epigenetic changes?

Answer 58

It prevents the bases being recognised and therefore reduces the ability of the whole gene to be transcribed (expressed). If transcription doesn’t occur, translation will not occur and the polypeptide and eventual protein will not be produced

Question 59

How does modification of histone proteins lead to epigenetic changes?

Answer 59

Histone proteins are used to organise the DNA in a chromosome. If the DNA becomes more tightly coiled around the histone proteins, this can prevent gene expression (transcription and translation). If coiled more loosely it can increase gene expression (transcription and translation)