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A-Level Biology A2 > Genetics > Flashcards

Genetics Flashcards

1
Q

Definition of a gene

A

A sequence of bases on a DNA molecule that codes for a protein which results in a characteristic i.e. gene for eye colour.

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2
Q

Definition of an allele

A

A different version of a gene. Most plants and animals only carry two alleles of each gene, one from each parent. They code for different versions of the same characteristic.

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3
Q

Definition of a genotype

A

The genetic constitution of an organism, the alleles an organism has (BB, Bb, bb).

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4
Q

Definition of a phenotype

A

The expression of the genetic constitution and its interaction with the environment - an organisms characteristic.

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5
Q

Definition of a dominant allele

A

An allele whose characteristic appears in the phenotype even when there’s only one copy. (shown by a capital letter).

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6
Q

Definition of a recessive allele

A

An allele whose characteristic appears in the phenotype when two copies.

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7
Q

Definition of co dominant alleles

A

Alleles that are both expressed in the phenotype.

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8
Q

Definition of a locus

A

Fixed position of a gene on a chromosome.

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

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9
Q

Definition of a homozygous

A

An organism that carries two copies on the same allele.

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10
Q

Definition of a heterozygous

A

An organism that carries two different alleles.

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11
Q

Definition of a carrier

A

A person carrying an allele which isn’t expresed in the phenotype but can be passed onto offspring.

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12
Q

What do genetic diagrams show

A

possible genotypes of offsprings

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13
Q

are humans diploid organisms or haploid

A

diploid

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14
Q

what may there be many of of a single gene

A

There may be many alleles of a single gene

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15
Q

How many alleles do gametes contain

A

one allele for each gene.

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16
Q

What is formed when two parents gametes fuse

A

The alleles they contain form the genotype of the offspring

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17
Q

At each locus, the genotype can be either ____ or ____

A

homozygous

heterozygous

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18
Q

What is a monohybrid inheritance?

A

Inheritance of a characterisitc controlled by a single gene.

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19
Q

What are monohybrid crosses?

A

Shows likelihood of different alleles of that gene being inherited by offspring of certain parents.

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20
Q

When would you use a dihybrid cross?

A

To look at how two different genes are inherited at the same time.

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21
Q

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

A

due to:

  • sex linkage
  • autosomal linkage
  • epistasis
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22
Q

Where is the information for a gender carried on?

A

Two sex chromosomes

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23
Q

How many X chromosomes do females have?

A

2- XX

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24
Q

How many X chromosomes do males have?

A

1, they have a Y chromosome - XY

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25
Q

What is a sex linked characterisitic?

A

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

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26
Q

What’s the difference between a X and Y chromosome?

A

Y chromosome is smaller than the X chromosome and carried fewer genes so most are only carries, so most genes on the sex chromosomes are only carried on the X chromosome.

27
Q

Why do males express the characterisitc of the allele if it’s recessive?

A

Due to males only having one X chromosome, so they often only have one allele for sex-linked genes and so that one copy allows the expression.

28
Q

What causes genetic disorders?

A

Faulty alleles on the sex chromosome.

29
Q

Examples of genetic disorders?

A
  • Colourblindness
  • Haemophilia
30
Q

What are X-Linked disorders?

A

The faulty alleles for both of the disorders are carried on the X chromosome.

31
Q

Is it rarer for male or female to become colour blind?

A

For female, due to them needing two copies of the recessive allele, whilst males only need one.

32
Q

What is the ratio when it’s between a carrier female and unaffected male?

A

3:1

33
Q

What are autosomal genes?

A

A chromosome that isn’t a sex chromosome. They are the genes located on the autosomes.

34
Q

What are genes on the same autosome said to be?

A

Linked because they’re on the chromosomes in meisois I, and their alleles will be passed on to the offspring together. The only reason that won’t happen is if crossing over splits them up first.

35
Q

When are two genes more closely linked?

A

When they’re closer together on the autosome.

36
Q

What won’t you get if two genes are autosomally linked?

A

The expected phenotypic ratio

37
Q

What ratio would you expect in a dihybrid cross between two heterozygous parents for the offspring?

A

9:3:3:1

38
Q

What masks the expression of another gene?

A

An epistatic gene

39
Q

How do many different genes control the same characterisitic?

A

Due to the allele of one gene masking the expression of the alleles of other genes - epistasis

40
Q

Definition of epistasis?

A

Allele of one gene masking the expression of the alleles of other genes

41
Q

Having two copies of the recessive epistasic allele ________________.

A

masks the expression of the other gene

42
Q

What phenotypic ratio would you get if you cross a homozygous recessive parent with a heterozygous dominant parent?

A

9:3:4

43
Q

Two genes are autosomally linked. What does this mean?

A

When genes are on the same autosome they’ll stay together during the independent segregation of chromosomes in meiosis I, and their alles will be passed onto the offspring together.

44
Q

A dihybrid cross produces the phenotypic 9:3:4 in the F2 generation. What does this indicate about the genes involved?

A

Homozygous parents - one dominant, one recessive

45
Q

What can the chi-squared test be used to check?

A

The results of the genetic crosses

46
Q

What is the statistical test used to see if results of an experiment support a theory

A

the chi-squared test

47
Q

what are the predicted results known as

A

expected result

48
Q

what are the actual results known as

A

observed results

49
Q

what is the null hypothesis

A

there’s no significant difference between the observed and expected results.

50
Q

What phenotypic ratio would you expect for a monohybrid inheritance if you cross a homo dominant with a homo recessive

A

3:1

51
Q

What can the results of the X2 test tell you?

A
  • if the value is larger than or equal to the critical value then there is a significant difference between the observed and expected results - and the null hypothesis can be rejected.
  • if the value is smaller than critical value then there is no significant difference between the observed and expected result and the null can’t be rejected.
52
Q

How would you find out the significant difference between the observed and expected value?

A

Compare the X2 value to the critical value.

53
Q

What is the critical value?

A

The value of X2 that corresponds to a 0.05 (5%) level of probability that the difference between the observed and expected results is due to chance.

54
Q

Explain what is meant by codominant alleles. (1)

A

Both alleles are expressed / shown (in the phenotype).

55
Q

Male cats with a tortoiseshell phenotype do not usually occur. Explain why. (1)

A

Only possess one allele / Y chromosome does not carry allele / gene / can’t be heterozygous.

56
Q

Explain why the scientists used the same restriction endonuclease enzymes on each DNA sample. (2)

A
  1. Cut DNA at same base sequence / place.
  2. So you get fragments of the required gene.
57
Q

Explain why primer A3 and primer A4 only bind to specific DNA fragments. (2)

A
  1. Each has / they have a specific base sequence;
  2. That is complementary
58
Q

Explain why they used cells that were in mitosis. (2)

A
  1. Chromosomes visible in mitosis;
  2. So you can see which chromosome DNA probe attached to.
59
Q

Explain why the scientists carried out the control experiment with the non-resistant flies (2).

A
  1. For comparison with resistant flies.
  2. To see death rate (in non-resistant) / to see effect of insecticide in non-resistant / normal flies.
60
Q

In fruit flies, the genes for body colour and wing length are linked. Explain what this means. (1)

A
  1. (Genes / loci) on same chromosome.
61
Q

If these genes were not linked, what ratio of phenotypes would the scientist have expected to obtain in the offspring? (1)

A

1:1:1:1

62
Q

Which statistical test could the scientist use to determine whether his observed results were significantly different from the expected results?

(1)

Give the reason for your choice of statistical test.

A
  1. Chi squared test;
  2. Categorical data.
63
Q

In birds, males are XX and females are XY.

Use this information to explain why recessive, sex-linked characteristics are more common in female birds than in male birds.

A
  1. (Recessive) allele is always expressed in females/
  2. females have one (recessive) allele /
  3. males need two recessive alleles /
  4. males need to be homozygous recessive /
  5. males could have dominant and recessive alleles /
  6. be heterozygous / carriers;
64
Q

Why is a chi test used to compare what?

A

compare the goodness of fit of observed phenotypic ratios with expected ratios.

A-Level Biology A2 (11 decks)

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