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A-level Biology Oxford AQA > 3.3.6 Inheritance > Flashcards

3.3.6 Inheritance Flashcards

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1
Q

Define gene

A

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

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2
Q

Define Genotype

A

The genetic constitution of an organism

(i.e. the alleles an organism has)

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3
Q

Define Phenotype

A

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

(i.e. an organism’s characteristics)

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4
Q

Define Allele

A

A different version of a gene

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5
Q

Explain what is meant by a recessive allele

A

Only expressed in phenotype when 2 alleles are present

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6
Q

Explain what is meant by a dominant allele

A

Is always expressed in the phenotype

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7
Q

Explain what is meant by co-dominant alleles

A

Both alleles are expressed in the phenotype

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8
Q

Define Homozygous

A

An organism that carries 2 copies of the same allele

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9
Q

Define Heterozygous

A

An organism that carriers 2 different alleles

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10
Q

Define Carrier

A

Person carrying an allele which isn’t expressed in phenotype but can be passed on to offspring

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11
Q

Define Locus

A

Fixed position of gene on a chromosome

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

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12
Q

Define Autosome

A

Any chromosome that is not a sex chromosome

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13
Q

Define Allosome

A

Sex chromosome

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14
Q

What is monohybrid inheritance?

A

Inheritance of a characteristic controlled by a single gene

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15
Q

Draw a monohybrid cross showing how wing length is inherited in fruit flies when the parent’s genotypes are NN x nn. Include the phenotypes and phenotypic ratio.

N - normal wings allele

n - vestigial (little) wings allele

A
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16
Q

Draw genetic diagram when the paren’ts genotypes are HNHS x HNHS (i.e. crossing 2 parents with sickle-cell trait (heterozygous))

HN = normal haemoglobin

HS = sickle haemoglobin

A
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17
Q

In the ABO blood group system in humans there are 3 alleles for blood type:

  • IO is the allele for blood group O
  • IA is the allele for blood group A
  • IB is the allele for blood group B

Allele IO is recessive. Alleles IA and IB are codominant - people with genotype IAIB will have blood group AB.

Draw a genetic diagram that shows a cross between a heterozygous person with blood group A and a heterozygous person with blood group B.

A
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18
Q

What do dihybrid crosses show?

A

Show how 2 genes are inherited are once

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19
Q

Draw a dihybrid cross diagram for when the both parents’ gentotype is RrYy. Include the phenotype and the phenotypic ratio.

  • R - round seed
  • r - wrinkled seed
  • Y - yellow seed
  • y - green seed
A
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20
Q
A
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21
Q

When are characteristics said to be sex-linked?

A

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

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22
Q

Most genes on sex chromosomes are only carried on ___ chromosomes

A

X

(X-linked genes)

23
Q

Explain why males are more likely than females to show recessive phenotypes for genes that are sex-linked

A
  • Males only have 1 X = 1 allele for sex-linked genes
  • ∵ have only 1 copy, express characteristic of allele even if its recessive
24
Q

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

Given that the female parent is a carrier and the male is unaffected, draw monohybrid diagram (n = faulty allele for a colour vision)

A
25
Q

What are autosomal genes?

A

Genes located on the autosomes

26
Q

Genes on the same autosome are said to be ____

A

linked

27
Q

Why are autosomal genes said to be linked?

A
  • ∵ they’re on the same autosome, they’ll stay together in independent segregation of chromosomes in meiosis I
  • And their alleles will be passed on to offspring together

(Only doesn’t occur if crossing over splits them up 1st)

28
Q

The closer together 2 genes are on autosome, the more …

A

closely they’re linked

29
Q

Why is that the more closer together 2 genes are on autosome, the more closely they’re linked?

A

∵ reduces the chance of genes being separated during crossing over

30
Q

What happens to the phenotypic ratio expected in offspring when 2 gene are autosomally linked?

A
  • It changes - won’t get the phenotypic ratio you expect in offspring of cross
    • e.g. Dihybrid cross between 2 heterozygous parents = 9:3:3:1 ratio in offspring
    • Instead phenotypic ratio more likely to be like monohybrid cross between heterozygous parents (3:1)
31
Q

Why is the phenotypic ratio different when 2 genes are autosomally linked?

A

∵ higher proportion of offspring will their parents’ (heterozygous) genotype and phenotype

32
Q

Explain the difference in the phenotypic ratios

A
  • NR and nr linked (in NnRr parent)
  • NnRr parent produces mainly NR and nr gametes
  • So fewer Nnrr and nnRr
  • Crossing over produces some Nr and nR gametes
33
Q

What is epistasis?

A

When an allele of 1 gene masks (blocks) the expression of the alleles of other genes

34
Q

Name the 3 types of epistasis

A
  • Recessive epistasis
  • Dominant epistasis
  • Complementary epistasis
35
Q

Describe dominant epistasis

A

Having at least one copy of the dominant epistatic allele masks (blocks) the expression of the other gene

36
Q

Dominant Epistasis

State the phenotypic ratio when you cross a homozygous recessive parent with a homozygous dominant parent

A

12 : 3 : 1

dominant epistatic : recessive epistatic dominant other : recessive both

37
Q

Squash colour is controlled by 2 genes - the colour espistatic gene (W/w) and the yellow gene (Y/y). The no-colour, white allele (W) is dominant over the coloured allele (w). The yellow gene has the dominant yellow allele (Y) and the recessive green allele (y).

Draw a genetic digram crossing WwYy with WwYy. Include the phenotypes and phenotypic ratio.

A
38
Q

Describe recessive epistasis

A

Having 2 copies of the recessive epistatic allele masks (blocks) the expression of the other gene

39
Q

Recessive Epistasis

State the phenotypic ratio when you cross a homozygous recessive parent with a homozygous dominant parent

A

9 : 3 : 4

dominant both : dominant epistatic recessive other : recessive epistatic

40
Q

Flower pigment in a plant is controlled by 2 genes. Gene 1 codes for a yellow pigment (Y = dominant yellow allele) and gene 2 codes for an enzyme that turns the yellow pigment orange (R = dominant yellow allele). Cross YyRr x YyRr. Include phenotypes and phenotypic ratios.

A
41
Q

When do you use the chi-squared (χ2) test?

A

Used to see if results of experiment support a theory

42
Q

State the chi-squared formula

A
  • O = Observed result
  • E = Expected result
43
Q

To find if there’s a significant difference between your observed and expected results, you need to compare χ2 value to ____ ____

A

critical value

44
Q

If χ2 ≥ critical value, there’s a

A
  • Significant difference
  • (Something other than chance causing difference)
  • Null hypothesis can be rejected
45
Q

If χ2 < critical value, there’s …

A
  • NO significant difference
  • Accept null hypothesis
46
Q

Describe how you work out the degrees of freedom

A

Number of classes (no. of phenotypes) - 1

e.g. 2 - 1 = 1

47
Q

How do multiple alleles of a gene arise? (2)

A
  • Due to mutations
  • Which occur in different positions in the gene
48
Q

At extremities (e.g. legs/arms) temperature is _____

A

lower

49
Q

Explain one piece of evidence from the diagram which proves that the allele for Tay-Sachs disease is recessive (2)

A
  • 3 & 4 produce 9/11 who are affected
  • Both 3 and 4 are heterozygous (carry recessive allele)
50
Q

Explain one piece of evidence from the diagram which proves that the allele for Tay-Sachs disease is not on the X chromosome (2)

A
  • 11 is affected, 3 is not
  • (If on X) 11 / affected female would not receive the recessive allele on X chromosome / Xt from 3 / father
51
Q

Explain why sex-linked conditions are more common in males than in females (3)

A
  • Males have XY chromosomes and females have XX chromosomes
  • Recessive allele on X chromosome has no (equivalent) allele on Y chromosome
  • Male needs to inherit one recessive allele whereas female needs to inherit two
52
Q

Suggest one reason why observed ratios are often not the same as expected ratios (1)

A
  • Due to chance
  • Fertilisation is random
53
Q

How is variation genetically controlled? (1)

A

Polygenes / several genes

54
Q

Extension: State the genotype of the heterozygous parent using the correct notation

A

A-level Biology Oxford AQA (41 decks)

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