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Biology A-Level > Genetics (Chapter 16) > Flashcards

Genetics (Chapter 16) Flashcards

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1
Q

What is a dominant allele?

A

One whose effect on the phenotype of a heterozygote is identical to its effect on a homozygote

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2
Q

What is a recessive allele?

A

One that is only expressed (in the phenotype) when no dominant allele is present

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3
Q

What are codominant alleles?

A

These both have an effect on the phenotype of a heterozygous organism

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4
Q

What is linkage?

A

The presence of two genes on the same chromosome so that they tend to be inherited together and do not assort independently

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5
Q

What is a test cross?

A

A genetic cross in which an organism showing a characteristic caused by a dominant allele is crossed with an organism that is homozygous recessive
- the phenotype of the offspring can be a guide to whether the first organism is homozygous or heterozygous

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6
Q

What is the F1 generation?

A

The offspring resulting from a cross between an organism with a homozygous dominant genotype, and one with a homozygous recessive genotype

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7
Q

What is the F2 generation?

A

The offspring resulting from a cross between two F1 (heterozygous) organisms

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8
Q

What is the phenotype?

A

The characteristics of an organism, often resulting from an interaction between its genotype and its environment

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9
Q

What is the genotype?

A

The alleles possessed by an organism

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10
Q

What does homozygous mean?

A

Having two identical alleles of a gene

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11
Q

What does heterozygous mean?

A

Having two different alleles of a gene

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12
Q

What is a mutation?

A

An unpredictable change in the genetic material of an organism

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13
Q

What is a gene mutation?

A

A change in the structure of a DNA molecule, producing a different allele of a gene

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14
Q

What is a chromosome mutation?

A

Mutations that cause changes in the structure or number of whole chromosomes in a cell

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15
Q

How do mutations occur?

A

Completely randomly (copying errors) or environmental factors e.g. ionising radiation or mutagens

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16
Q

What is a mutagen?

A

A substance that increases the chances of mutation occurring

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17
Q

What are the 3 different ways in which the sequence of bases in a gene may be altered during a gene mutation?

A

1) Base substitution
2) Base deletion
3) Base addition (insertion)

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18
Q

What happens during base substitution?

A

One base takes the place of another

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19
Q

What happens during base deletion?

A

One or more bases are lost from a sequence

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20
Q

What happens during base addition?

A

One or more extra bases are added to the sequence

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21
Q

Why do base additions/deletions usually have a very significant effect on the structure and therefore the function of the polypeptide that the allele codes for?

A
  • Because they alter every set of 3 bases that follows them in the DNA molecule - they cause ‘frame shifts’ in the code
  • Often the effects are so large that the protein made is totally useless
  • Or, the addition/deletion may introduce a ‘stop’ triplet part way through a gene, so that a complete protein is never made
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22
Q

What is a silent mutation?

A

A mutation that has no apparent effect on an organism

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23
Q

Why do base substitutions often have no effect at all?

A
  • They are often silent mutations because many amino acids have more than one triplet code, so even if one base is changed, the amino acid is still coded for
  • They also have no effect on any of the triplet codes that follow, only the one that is directly affected
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24
Q

How could base substitutions have a very large effect?

A

If the substituted base makes the triplet a ‘stop’ triplet

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25
Q

What is one example of a base substitution that has a significant effect on the phenotype?

A

Sickle cell anaemia

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26
Q

Describe the substitution in sickle cell anaemia

A
  • In people with the Hbs (sickle cell) allele, the base sequence CTT is replaced by CAT, changing one amino acid in the sequence which changes the beta-globin polypeptide
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27
Q

What is the effect of the small difference in the amino acid sequence in people with sickle cell anaemia?

A
  • It makes little difference to the Hb molecule when combined with oxygen
  • But when not combined with oxygen, the unusual beta-globin polypeptides make the Hb molecule much less soluble
  • The molecules tend to stick to each other, forming long fibres inside the RBCs
  • The RBCs are pulled out of shape, into a half-moon/sickle shape
  • The distorted cell cannot carry oxygen and also gets stuck in capillaries, stopping any unaffected cells from getting through
  • A person with this unusual beta-globin can suffer severe anaemia and may die
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28
Q

What is anaemia?

A

A lack of oxygen transported to the cells

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29
Q

What disease is an example of a relationship between a gene, enzyme and human phenotype?

A

Albinism

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30
Q

Describe what happens in someone with albinism

A
  • The dark pigment melanin is totally or partially missing from the eyes, skin and hair
  • They have blue/pink irises, very pale skin and hair and poor vision
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31
Q

What kind of mutation causes albinism?

A

Mutations at several loci may be responsible, but in its classic form, the mutation is autosomal recessive and therefore homozygous recessive individuals show albinism

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32
Q

What is the effect of the mutation that causes albinism?

A
  • A mutation in the gene for the enzyme, tyrosinase, results in either the absence of tyrosinase or the presence of inactive tyrosinase in the cells responsible for melanin production
  • In these melanocytes, the first of 2 steps of the conversion of the amino acid, tyrosine into melanin cannot take place (tyrosine cannot be converted into DOPA and dopaquinone)
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33
Q

What is tyrosinase?

A
  • An oxidase with two Cu atoms in its active site which bind to an oxygen molecule
  • It is a transmembrane protein and found in the membrane of large organelles in the melanocytes called melanosomes
  • Most of the protein, including the active site, is inside the melanosomes
  • Occurs in plant tissues as well
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34
Q

What disease is an example of a mutation inherited by a dominant allele?

A

Huntington’s disease - therefore most people are heterozygous with a 1 in 2 chance of passing on HD

35
Q

What is Huntington’s Disease?

A
  • A neurological disorder resulting in involuntary movements (chorea) and progressive mental deterioration
  • Brain cells are lost and the ventricles of the brain become larger
36
Q

What is the effect of the age of onset of HD being variable but most commonly middle age?

A

Individuals may have children before they know they have HD and pass on the gene

37
Q

Describe the mutation that causes HD

A
  • The mutation is an unstable segment in a gene on chromosome 4 coding for a protein: huntigtin
  • In people without HD, the segment is made up of a small number of repeats of the triplet bases CAG
  • People with HD have a larger number of repeats of the CAG triplet - called a ‘stutter’
  • There is a rough inverse correlation: the more stutters, the earlier the onset of HD
38
Q

What is factor VIII?

A

A protein needed for blood clotting

39
Q

Where is the gene that codes for the production of factor VIII?

A

On the X chromosome

40
Q

Describe how haemophilia is caused

A
  • There are two alleles of the gene that codes for the production of factor VIII: dominant (H), producing normal factor VIII and the recessive one (h), resulting in a lack of factor VIII
  • The recessive allele causes haemophilia, in which the gene fails to clot properly
41
Q

Why is haemophilia more common in males?

A

Because the Factor VIII gene is sex linked (on the X chromosome)
- e.g in the children of a man without haemophilia and a female carrier, there is a 25% chance of a boy with haemophilia but 0% chance of a girl with haemophilia (only 25% chance of female carrier)

42
Q

What kind of alleles are the alleles of the haemoglobin gene?

A

Codominant

43
Q

What does the HbA allele code for?

A

Normal beta-globin polypeptide

44
Q

What does the HbS allele code for?

A

Sickle cell beta-globin polypeptide

45
Q

What does a HbAHbS genotype result in?

A

1) 1/2 of the person’s Hb will be normal and 1/2 is sickle cell Hb
2) these people have sickle cell trait and are carriers
3) they have no problems and appear to be healthy bc they have enough normal Hb to carry enough O2
4) 1/2 of the gametes will have HbA genotype and half will have HbS genotype

46
Q

What is the aim of a test cross?

A

To find out of the genotype of an organism is heterozygous or homozygous dominant

47
Q

How does a test cross work?

A

1) breed organism with homozygous recessive organism
2) if the genotype is BB, all offspring will be Bb (brown)
3) if the genotype is Bb, 1/2 offspring will be Bb (brown) and 1/2 will be bb (blue)

48
Q

How many alleles to most genes have?

A

More than 2 (multiple alleles)

49
Q

Give an example of a gene which has multiple alleles

A

Human blood groups (4 of them)

50
Q

How many alleles are there of the human blood group gene?

A

3

51
Q

Describe the alleles of the human blood group gene?

A
  • IA and IB are codominant

- IO is recessive to both IA and IB

52
Q

How are the sex chromosomes different from the autosomes?

A

They are not always homologous as they do not always have the same genes in the same position because there are two types (X and Y)

53
Q

How is the Y chromosome different from the X?

A

It is shorter and carries fewer genes

54
Q

Where is the factor VIII gene located?

A

It is located on the X chromosome

55
Q

What does the factor VIII gene code for?

A

The production of a protein called factor VIII, needed for blood clotting

56
Q

What are the 2 alleles of the factor VIII gene?

A

1) H = normal factor VIII

2) h = lack of factor VIII (haemophilia)

57
Q

What is the effect of the factor VIII gene only being on the X chromosome?

A

1) females have two copies of the gene and males only have one
2) haemophilia is much more common in males
3) it is sex-linked

58
Q

Describe the genotypes of the factor VIII gene (5)

A

1) XHXH - normal
2) XHXh - normal (carrier)
3) XhXh - lethal (zygote does not normally develop)
4) XHY - normal
5) XhY - haemophilia

59
Q

What is a sex-linked gene?

A

A gene that is found on a part of the X chromosome and not matched by (not found on) the Y chromosome

60
Q

What do dihybrid crosses look at?

A

The inheritance of 2 genes at once, instead of just one (monohybrid cross)

61
Q

When does independent assortment occur?

A

Metaphase of meiosis 1

62
Q

What is independent assortment?

A

When the pairs of homologous chromosomes line up on the equator independently of each other

63
Q

What is the effect of independent assortment?

A

In many cells undergoing meiosis, roughly 1/2 will line up one way and the other 1/2 the other way

64
Q

How many types of gamete are there at the end of meiosis II for a dihybrid cross?

A

4 (AB, Ab, aB, ab) because each orientation of chromosome gives 2 types of gamete

65
Q

How many types of genotype/phenotype are there are there at the end of a dihybrid cross between heterozygous and homozygous recessive and what is the ratio?

A

4 in a 1:1:1:1 ratio

66
Q

How many types of genotype are there are there at the end of a dihybrid cross between heterozygous and heterozygous?

A

16

67
Q

How many types of phenotype are there are there at the end of a dihybrid cross between heterozygous and heterozygous and what is the ratio?

A

4 in a 9:3:3:1 ratio

68
Q

What is the effect of interactions between loci?

A

Different loci interact to affect one phenotypic character

69
Q

Give an example of an interaction between loci affecting one phenotypic character

A
  • Feather colour: W/w and C/c
  • W dominant over C
  • wwcc also white
70
Q

What does it mean when genes are linked?

A

1) When two or more genes loci are on the same chromosome, they do not assort independently as they would if they were on different chromosomes
2) Their genotype is written like (BT)(bt) and the brackets do not split in a cross

71
Q

What are genes on a chromosome other than the sex chromosomes described as?

A

Autosomally linked

72
Q

What is another word for a homologous pair of chromosomes?

A

A bivalent

73
Q

What are homologous chromosomes joined by?

A

Chiasmata

74
Q

What happens during crossing over?

A

1) the chromatids of a bivalent may break and reconnect to another, non-sister chromatid
2) this results in an exchange of gene loci between a maternal and paternal chromatid

75
Q

In autosomal linkage, how many recombinant types of phenotype are produced from crossing over and in what ratio?

A

A small number but in the ratio of 1:1

76
Q

What is the cross over value?

A

The percentage of offspring that belong to the recombinant classes e.g. 6% + 6% = 12%

77
Q

What is the cross over value a measure of?

A

The distance apart of the two gene loci on their chromosomes - the smaller the cross over value, the closer the loci are

78
Q

What is the change of a cross over directly related to?

A

The distance apart of the two gene loci

79
Q

What does the chi squared test allow us to do?

A

Compare our observed results with the expected results and decide whether or not there is a significant difference between them

80
Q

What is the formula for the chi squared test?

A

sum of (observed-expected)squared/expected

81
Q

How do you calculate the degrees of freedom?

A

number of classes (n) (options)-1

82
Q

How do you calculate degrees of freedom for two sets of data?

A

(n-1) + (n-1)

83
Q

What do the degrees of freedom take into account?

A

The number of comparisons made

84
Q

What does it mean if the chi squared value is LESS than the critical value at P=0.05?

A

Any difference between the observed and expected results is due to chance and there is no significant difference between the observed and expected results

Biology A-Level (44 decks)

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