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3 Genetics > 14 Mendel and the Gene Idea > Flashcards

14 Mendel and the Gene Idea Flashcards

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1
Q

What terminology does Mendel use to describe the characteristic of an organism?

A

A heritable feature i.e. seed colour is a ‘character’

A specifici form of the ‘character’ is a ’trait’

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2
Q

In Mendelian genetics, what is a ‘character’?

A

A heritable feature i.e. flower colour

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3
Q

In Mendelian genetics, what is a ’trait’?

A

A specific character i.e. blue flowers.

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4
Q

How did Mendel control which plants pollinated each other?

A

He removed the stamens and brushed them against the stigma of specific flowers.

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5
Q

What word means true-breeding?

A

Pedigree

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6
Q

What does ‘pedigree’ mean?

A

It is ‘pure breeding’ and thus homozygous for a trait. This means that two pedigrees for one trait will have offspring that are homozygous for that trait.

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7
Q

If an organism is homozygous for a trait, what property does it have?

A

It is pure-breeding i.e. a ‘pedigree’

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8
Q

What is the crossing of two pure-breeding organisms?

A

‘Hybridisation’ (as a hybrid of the two pure traits has been formed)

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9
Q

What is ‘hybridisation’?

A

The crossing of two true-breeding varieties.

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10
Q

How are the generations labelled in crosses?

A

The true-breeding parents are the ‘P generation’.

Their hybridised descendants are the F1 generation, their offspring are the F2 and so on.

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11
Q

Technically speaking, what condition must the organisms of the P1 generation be?

A

They must be pure breeding.

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12
Q

What does ‘F1 generation’ stand for?

A

“First filial generation”

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13
Q

What are Medel’s basic laws?

A

The ‘Law of Segregation’ and the ‘Law of Independent Assortment’

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14
Q

What previous belief did Mendel dispute?

A

‘Blending’

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15
Q

What are some examples of evidence that blending does not occur?

A

-Primarily: A white flower and a purple flower will have white or purple flowers, not a mixture of each i.e. pale purple (except incomplete dominance)

  • The phenotypic ratios are 3:1 during a heterozygous cross
  • Organisms can be carriers which do not show that phenotype
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16
Q

What is the law of segregation?

A

“Two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes.”

Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of the organism making the gamete.

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17
Q

What historical concept does the ‘Law of Segregation’ directly refute?

A

‘Blending’ as the gamete gets only one allele per gene.

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18
Q

What are the main traits Mendel followed and what are there recessive and dominant forms?

A

Character : Dominant Trait : Recessive Trait

Flower Colour : Purple : White
Flower Position : Axial : Terminal
Seed Colour : Yellow : Green
Seed Shape : Round : Wrinkled
Pod Shape : Inflated : Constricted
Pod Colour : Green : Yellow
Stem Length : Tall : Dwarf
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19
Q

What does phenotype include?

A

All the ‘heritable traits’ of an organism including visible structural traits i.e. eye colour, physiological traits i.e. lipase sequence and behavioural.

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20
Q

How can the genotype of an organism be determined?

A
  • If it has the recessive trait it must be homozygous recessive.
  • Else a test cross can be performed.
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21
Q

How is a test cross performed?

A

The organsism with the unknown genotype is crossed with a homozygous recessive organisim.

If the offspring has the recessive trait the unknown organisim must be heterozygous.

Else with each cross the chance of it being heterozygous decrease.

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22
Q

What is Mendel’s “Law of Independent Assortment”?

A

“Each pair of alleles segregates independently of each other pair of alleles during gamete formation.”

Thus each characteristic is inherited independently of each other.

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23
Q

What is a cross involving two characteristics?

A

A dihybrid cross.

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24
Q

What is a bivalent’?

A

A tetrad

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25
Q

What is a ‘dihybrid cross’?

A

One involving two characteristics.

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26
Q

What conditions are necessary for the patterns of independent assortment to appear?

A

It must be a dihybrid or higher cross so that the independent sorting of the two chromosomes can be seen.

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27
Q

What rules of probability dictate the probability of genetic outcomes?

A

The ‘multiplication rule’ and the ‘addition rule’

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28
Q

What is the ‘multiplication rule’?

A

To find the probability of two events both occurring the probabilities of each occurring are multiplied.

For example: p (two heads) = 0.5 x 0.5

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29
Q

What is the ‘addition rule’?

A

To find the probabilities of two ‘mutually exclusive events’ occurring, add the probability of each occurring.

For example: p( roll a 6 OR a 5) = 1/6 + 1/6

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30
Q

What is the expected phenotype ratio of two heterozygous traits in a dihybrid cross?

A

9:3:3:1

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31
Q

What are the basic patterns of inheritance based on dominance?

A

Co-dominance, complete dominance and incomplete dominance.

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32
Q

What is complete dominance?

A

The dominant allele is the sole allele expressed as the phenotype when present. Thus the phenotype is the same regardless of whether the organism is homozygous dominant or heterozygous.

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33
Q

What is ‘in-complete dominance’?

A

Neither allele is completely dominant over the other.

Therefore a heterozygous organisim has an intermediate phenotype.

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34
Q

What is ‘co-dominance’?

A

A situation in which both alleles are equally dominant and thus both expressed simultaneously.

This leads to both phenotypes begin present simultaneously.

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35
Q

What is ’Tay-sachs disease’?

A

A recessive disease in which the enzyme to break down a specific lipid is present in a non-functional mutated form.

This causes the lipid to accumulate in the brain causing seizures, blindness, degraded motor and mental function and death.

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36
Q

Is Tay-Sachs a recessive or dominant disease?

A

Recessive

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37
Q

Using ’Tay-sachs’ as an example, how can the distinction between co-dominance and incomplete dominance etc. be relative?

A

Only children who inherit two copies of the Tay-Sachs allele (homozygotes) have the disease. Thus, at the organismal level, the Tay-Sachs allele qualifies as recessive.

However, the activity level of the lipid-metabolizing enzyme in heterozygotes is intermediate between that in individuals homozygous for the normal allele and that in individuals with Tay-Sachs disease. The intermediate phenotype observed at the biochemical level is characteristic of incomplete dominance of either allele. The heterozygote condition does not lead to disease symptoms, apparently because half the normal enzyme activity is sufficient to prevent lipid accumulation in the brain.

Heterozygous individuals produce equal numbers of normal and dysfunctional enzyme molecules. Thus, at the molecular level, the normal allele and the Tay-Sachs allele are codominant.

38
Q

What allele will be more common: the dominant one or the recessive one?

A

Either the dominant or recessive can be most common.

39
Q

What is it called when more than two alleles are present for a gene?

A

‘multiple alleles’

40
Q

Besides the basics, what are the patterns of inheritance seen in mendelian genetics?

A

‘Pleiotropy’, ‘Epistasis’ and ‘Polygenic inheritance’

41
Q

What is ‘pleiotropy’?

A

When one gene has multiple phenotypic effects.

For example, pleiotropic alleles are responsible for the multiple symptoms associated with certain hereditary diseases such as cystic fibrosis.

42
Q

What is it called when one allele has multiple phenotypic effects?

A

‘Pleiotropy’

43
Q

What is ‘epistasis’?

A

When the phenotypic expression of a gene at one locus alters that of a gene at a second locus.

44
Q

What is an example of ‘epistasis’?

A

Albinisim: if a person has the albino gene at the albino locus it will prevent the expression of the hair colour allele at its locus

45
Q

What is it called when an allele at one locus affects the expression of the allele at another?

A

‘Epistasis’

46
Q

What is ‘polygenic inheritance’ refer to?

A

When multiple genes act together to form ‘quantitative’ characteristics

(note that they are not epistatic as they do not affect the expression of each other.)

47
Q

What is a ‘quantitative trait’?

A

One in which phenotype acts as a range i..e heights

48
Q

What is a ‘qualitative trait’?

A

On in which the phenotype is discrete i.e. blue, brown or green eyes.

49
Q

What is it called when the phenotype of a trait exists as a range?

A

A quantitative trait

50
Q

What is it called when the phenotype of a trait is discrete?

A

A qualitative trait.

51
Q

What is it called when multiple alleles act together in a non-epistatic fashion to affect on phenotype?

A

‘Polygenic inheritance’

52
Q

What is an example of ‘polygenic inheritance’?

A

Human skin colour:

In a simplified model imagine three loci. Each locus encodes either for a ‘melanin’ allele or no melanin produced.

If the individual has 3 ‘melanin’ alleles they will have very dark skin. If they have none they will have very light skin.

This is an example of a ‘quantitative trait’ as if the person has 2 ‘melanin alleles’ they will have an intermediate skin darkness and so on.

53
Q

What does the phenotype include?

A

The characteristics of an organism both from genetic and environmental characteristics.

54
Q

What is it called when a phenotype is affected by both genetic and environmental factors?

A

‘Multifactorial’

55
Q

What does ‘multifactorial’ refer to?

A

When a phenotype is affected by both genetic and environmental factors.

56
Q

What is the range of phenotypes a gene can lead to based on environmental factors called?

A

The ‘norm of reaction’

57
Q

What does ’norm of reaction’ refer to?

A

The range of phenotypes a gene can lead to based on environmental factors called

58
Q

What are some examples of traits with ’norm of reaction’ ranges that are large and small?

A

The gene for red-blood cell type has no range because only genetics determines the if the person is Type A blood etc. Therefore it is not multifactorial’

The ’norm of reaction’ has a wide range in the alleles for height has this is a ‘multifactorial trait’ with diet etc. playing a major role.

59
Q

What is the genetic family history called?

A

A ‘pedigree’

60
Q

What does ’pedigree’ refer to? (not ‘pure breeding’)

A

A chart which shows the genetic history of the family i.e. which had a disease.

61
Q

What is the chart which shows the family’s genetic history called?

A

A ‘pedigree chart’

62
Q

How can the chance of offspring having a genetic diseases be quantified?

A

By using a pedigree chart to track the family history of the child and thus determine if its parents are heterozygous or homozygous.

Then a punnet square can be used to calculate the chance.

63
Q

What are the basic symbols of a pedigree chart?

A

Squares represent males, circles represent females.

64
Q

What does a square represent in a pedigree chart?

A

A male

65
Q

What does a circle represent in a pedigree chart?

A

A female.

66
Q

What is it called when tow closely related offspring have children?

A

A ‘consanguineous’ mating

67
Q

What does ’consanguineous mating’ refer to?

A

When two closely related individuals have children together.

68
Q

How is a ‘consanguineous mating’ represented on a pedigree chart?

A

By a double line between the parents, not a single one as is normally seen.

69
Q

What does a double line on a pedigree chart represent?

A

A ‘consanguineous mating’

70
Q

What does ’carrier’ refer to?

A

A person who has the allele for a gene but does not express it.

71
Q

What does the presence of a carrier indicate about the genotype of the individual and the nature of the allele?

A

This typically indicates that the individual is heterozygous and thus that the trait is recessive.

The presence of a carrier could theoretically be due to epistasis however.

72
Q

What are the basic recessive genetic disorders?

A

Cystic fibrosis, albinism and sickle-cell disease

73
Q

What is cystic fibrosis?

A

A recessive disease in which the chloride ions transport channel are not present.

This causes high concentrations of extracellular chloride, which causes the mucus that coats certain cells to become thick and sticky.

The mucus builds up in the pancreas, lungs, digestive tract, and other organs, leading to poor absorption of nutrients from the intestines, chronic bronchitis, and recurrent bacterial infections.

74
Q

What is the most common lethal genetic disease?

A

Cystic fibrosis.

75
Q

What disease causes a buildup of mucus in the lung etc.?

A

Cystic fibrosis

76
Q

What is the dominance of cystic fibrosis.

A

It is recessive

77
Q

What is albinism?

A

A recessive genetic disease in which an enzyme needed to make the skin pigment melanin is not present.

78
Q

What is the dominance of albinism?

A

Recessive.

79
Q

What is the dominance of sickle cell disease?

A

Recessive.

80
Q

How can an individual be a carrier and not realise?

A

1) There are heterozygous for a recessive allele.
2) It is a late onset dominant allele
3) It is controlled by epistatic interactions.

81
Q

What is are some examples of dominant genetic disease?

A

‘Huntingtons’s disease’ and achondroplasia

82
Q

What is ‘Huntington’s disease’?

A

A degenerative disease of the nervous system, is caused by a lethal dominant allele.

It has no obvious phenotypic effect until the individual is about 35 to 45 years old.

83
Q

What is ‘anchondroplasia’?

A

A dominant trait leading to ‘dwarfism’

84
Q

What is Huntington’s disease an example of?

A

A ‘late onset disease’

85
Q

What does ’late onset disease’ refer to?

A

One in which symptoms only develop during adulthood, even through it was present in the genotype since birth.

86
Q

What is the dominance of polydactyl?

A

It is a dominant trait.

87
Q

What can genetic diseases be grouped as, besides recessive and dominant?

A

‘Multifactorial’

88
Q

What is a ‘multifactorial disease’ and what are some example?

A

A disease in which genetic factors make an individual more susceptible to a disease but environmental factors influence whether the disease actually develops.

For example some individuals are more likely to get atherosclerosis but environmental factors i.e. diet also has a huge affect.

89
Q

Where is the gene for Huntington’s located?

A

At the tip of chromosome 4.

90
Q

What are the basic ways in which genetic disorders are detected?

A

With ‘fetal testing’ and ’newborn screening’

3 Genetics (9 decks)

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