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Biology A-level > 7A Genetics > Flashcards

7A Genetics Flashcards

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1
Q

What is a gene?

A

A sequence of bases on a DNA molecule that codes for a protein (polypeptide), which results in a characteristic

e.g. a gene for eye colour

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2
Q

What is an allele?

A

A different version of a gene - There can be multiple alleles of a single gene, but most plants and animals carry two alleles of each gene –> one from each parent

The order of bases in each allele is slightly different

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3
Q

What is a genotype?

A

The genetic constitution of an organism - the alleles an organism has

e.g. BB, Bb or bb for eye colour

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4
Q

What is a phenotype?

A

The expression of the genetic constitution & its interaction with the environment

An organism’s characteristics e.g. brown eyes

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5
Q

What is a dominant allele?

A

An allele whose characteristic appears in the phenotype even when there’s only one copy

e.g. the allele for brown eyes (B) is dominant - if a person’s genotype is Bb or BB, they’ll have brown eyes

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6
Q

What is a recessive allele?

A

An allele whose characteristic only appears in the phenotype if two copies aren’t present.

e.g. the allele for blue eyes (b) is recessive - if a person’s genotype is bb, they’ll have blue eyes

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7
Q

What are codominant alleles?

A

Alleles that are both expressed in the phenotype

Neither one is recessive, e.g. the alleles for haemoglobin

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8
Q

What is a locus?

A

The fixed position of a gene on a chromosome

Alleles of a gene are found at the same locus on each chromosome in a pair

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9
Q

What is a homozygote?

A

An organism that carries two copies of the same allele e.g. BB or bb

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10
Q

What is a heterozygote?

A

An organism that carries two different alleles e.g. Bb

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11
Q

What is a carrier?

A

A person carrying an allele that is not expressed in the phenotype but that can be passed onto offspring

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12
Q

What does it mean that humans are diploid organisms?

A

We have two sets of chromosomes - so we have two alleles for each gene

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13
Q

How many alleles do gametes have?

A

Gametes contain only one allele for each gene

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14
Q

Where do the alleles in an offspring’s genotype come from?

A

When gametes from two parents fuse together, the alleles that they contain form the genotype of the offspring produced

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15
Q

Can the genotype be homozygous or heterozygous at each locus on the gene?

A

Yes

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16
Q

What can genetic diagrams be used for?

A

To predict the genotypes & phenotypes of the offspring produced if two parents are crossed

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17
Q

What is monohybrid inheritance?

A

The inheritance of a characteristic controlled by a single gene

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18
Q

What do monohybrid crosses show?

A

They show the likelihood of the different alleles of that gene (different versions of that characteristic) being inherited by offspring of certain parents

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19
Q

What is a punnett square?

A

Another way of showing a genetic diagram

They’re used to predict the genotypes & phenotypes of offspring

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20
Q

What is codominance?

A

Both alleles are expressed in the phenotype, neither one is recessive

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21
Q

Give an example of a codominant allele in humans

A

The allele for sickle-cell anaemia

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22
Q

Draw a genetic cross of two parents with the genotypes including their gametes and the phenotype:

H^N H^S X H^N H^S (heterozygotes)

A

Parent’s genotype: H^N H^S X H^N H^S

Gametes alleles: H^N H^S H^N H^S

Possible genotypes of offspring:

H^N H^N (unaffected)
H^N H^S (sickle-cell trait) - heterozygous
H^N H^S (sickle-cell trait) - heterozygous
H^S H^S (sickle-cell anemia)

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23
Q

What is the genotype for someone who doesn’t have sickle-cell anemia?

A

H^N H^N

They are homozygous for normal haemoglobin

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24
Q

What is the genotype for someone who does have sickle-cell anemia?

A

H^S H^S

They are homozygous for sickle cell anemia

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25
Q

What happens to people’s blood cells if they have sickle-cell anemia?

A

They are sickle-shaped (crescent-shaped)

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26
Q

What happens when someone is heterozygous for sickle-cell anemia (H^N H^S)?

A

They have an in-between phenotype - called sickle-cell trait

They have some normal haemoglobin and some sickle hemoglobin

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27
Q

Why do people with heterozygous alleles for sickle cell anemia have both normal haemoglobin and some sickle haemoglobin?

A

The alleles for sickle-cell anemia are codominant, they are both expressed

28
Q

Give an example of a gene that has more than one allele?

A

Blood group

29
Q

What does it mean that some genes have multiple alleles?

A

It is more complex, there aren’t just two alleles for the gene there could be 3 or 4

30
Q

How many alleles are there for the ABO blood group system?

A

There are 3 alleles for blood type

31
Q

What are the alleles for each blood group?

A
  • I^O is for blood group O
  • I^A is for blood group A
  • I^B is for blood group B
32
Q

What are each of the alleles for the blood groups like (dominance)?

A

I^O is recessive

I^A and I^B are codominant

People with genotype I^A I^B will have blood group AB

33
Q

Complete a genetic cross diagram for parents with the alleles:

I^A I^O X I^B I^O

A

Parent’s genotypes: I^A I^O X I^B I^O

Gametes alleles: I^A I^O I^B I^O

Possible offspring: 
I^A I^B (Group AB)
I^A I^O (Group A)
I^B I^O (Group B)
I^O I^O (Group O)
34
Q

How common is blood group O?

A

Recessive blood groups are really rare - however lots of people in Britain are descended from people who were I^O I^O so O is common

35
Q

What is a dihybrid cross used to look at?

A

How two different genes are inherited at the same time

36
Q

Complete a dihybrid cross between two pea plants with the genotypes:

RrYy and RrYy

A

Parents genotypes: RrYy RrYy

Gametes alleles: RY Ry rY ry RY Ry rY ry
(Draw grid out on paper and complete cross)

Possible offspring:
RRYY, RRYy, RrYY, RrYy, RRYy, RRyy, RrYy, Rryy, RrYY, RrYy, rrYY, rrYy, RrYy, Rryy, rrYy, rryy

37
Q

What are the alleles for genes in pea plants?

A 
R = round seed
r = wrinkled seed 
Y = yellow seed
y = green seed
38
Q

What would a monohybrid cross with two homozygous (one dominant, one recessive) parents result in?

A

Phenitypic ration in F1 = All heterozygous offspring

Phenotypic ratio in F2 = 3:1 - dominant:recessive

39
Q

What is the phenotypic ratio?

A

The ratio of different phenotypes in offspring

40
Q

What would a dihybrid cross with two homozygous (one dominant, one recessive) parents result in?

A

Phenotypic ratio in F1 = All heterozygous offspring

Phenotypic ratio in F2 = 9:3:3:1
dominant both : dominant 1st recessive 2nd : recessive first dominant 2nd : recessive both

41
Q

What would a codominant cross with two homozygous (one dominant, one recessive) parents result in?

A

Phenotypic ratio in F1 = All heterozygous offspring

Phenotypic ratio in F2 = 1:2:2
homozygous for one allele : heterozygous for another

42
Q

Why won’t you always get the expected phenotypic ratio from a cross?

A

This can be cuased by sex linkage or epistasis

43
Q

Where is the genetic info for biological sex carried?

A

On two sex chromosomes

44
Q

In mammals, what chromosomes do males/females have?

A

Males: One X and one Y chromosome - (XY)

Females: Two X chromosomes - (XX)

45
Q

When is a characteristic said to be “sex linked”?

A

When the allele that codes for it is located on a sex chromosome

46
Q

Why does the Y chromosome carry fewer genes than the X?

A

The Y chromosome is smaller than the X so it carries fewer genes

47
Q

Why are most genes of the sex chromosomes only carried on the X?

A

Because the Y chromosome is smaller and carries fewer genes

48
Q

Why do males often only have one allele for sex linked genes?

A

They only have one X chromosome and most genes are carries on the X, not the Y

49
Q

How does only having one X chromosome affect men?

A

They often only have one allele for sex-linked genes (only carried on the X)

Due to only having one copy they express the characteristic of the allele, even if its recessive

Makes them more likely (than females) to show recessive phenotypes for genes that are sex-linked

50
Q

Give two examples of genetic diorders caused by faulty alleles on sex chromosomes

A
  • Colour blindness

- Haemophilia

51
Q

Where are the faulty alleles for colour blindness and haemophillian carried and what name do they have?

A

They are both carried on the X linked chromosome

This makes them X-linked disorders

52
Q

What is colour blindness and what is it caused by?

A

It is a sex-linked disorder caused by a faulty allele carried on the X chromosome

53
Q

Why is it less common for females to be colourblind?

A

The gene for colour blindness is on the X chromosome and females have 2

Therefore for them to be colourblind the gene would have to be on both chromosomes

54
Q

Show a genetic cross between two parents with the genotypes (N-not colourblind, n-colourblind):

X^N X^n and X^N Y

What is the ratio of non-colourblind:colourblind

A

Gametes:
X^N X^n X^N Y

Possible genotypes of offspring:

  • X^N X^N
  • X^N Y
  • X^n N^N
  • X^n Y

3:1

55
Q

What is an autosome?

A

Any chromosome that isn’t a sex chromosome

56
Q

Where are autosomal genes located?

A

On an autosome

57
Q

What are genes on the same autosome like?

A

They are said to be linked

58
Q

Why are genes on the same autosome said to be linked?

A

Because they’re on the same autosome they’ll stay together during independent segregation of chromosomes in meiosis 1

Their alleles will be passed onto the the offspring together

59
Q

Why would genes on the same autosome not be passed down together?

A

If crossing over splits them up

60
Q

What does it mean if two genes are very close together on an autosome and why?

A

The closer two genes are on an autosome the more closely they’re said to be linked

This is because crossing over is less likely to split them up

61
Q

What are the phenotypic ratios like for a cross between two autosomally linked genes?

A

You won’t get the phenotypic ratio that youd expect in an offspring cross

62
Q

What sort of phenotypic ratio are autosomal genes most likely to have and why?

A

One similar to what you would expect from a monohybrid cross between two heterozygous parents (3:1)

Because the autosomally-linked alleles are inherited toether

63
Q

If genes are autosomally linked what does this mean for the offspring’s geotype and phenotype?

A

A higher proportion of the offspring will have their parent’s (heterozygous) genotype and phenotype

64
Q

What do epistatic genes do?

A

They mask the expression of other genes

65
Q

How do genes form phenotypes?

A

Many different genes can control the same characteristic

They all interact to form the phenotype

66
Q

What is epistasis?

A

The allele of one gene masks (blocks) the expression of other genes

Biology A-level (20 decks)

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