Biology A-Level > Inheritance > Flashcards

Inheritance Flashcards

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1
Q

Define a gene.

A

A gene is a base sequence of DNA that codes for a polypeptide or functional RNA.

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2
Q

What is meant by the term allele?

A

An allele is one of two or more alternative forms of a gene found at the same locus on homologous chromosomes.

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3
Q

Define genotype.

A

The genetic composition of an organism — the combination of alleles it possesses.

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4
Q

Define phenotype.

A

The observable characteristics of an organism, resulting from the interaction between the genotype and the environment.

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5
Q

What is meant by the term locus?

A

The fixed position of a gene on a chromosome.

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6
Q

Define dominant allele.

A

An allele that is always expressed in the phenotype when present.

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7
Q

Define recessive allele.

A

An allele that is only expressed in the phenotype when no dominant allele is present.

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8
Q

Define codominance.

A

When both alleles in a heterozygote are expressed in the phenotype.

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9
Q

Define multiple alleles.

A

When a gene has more than two possible alleles at the same locus, although only two are present in any one individual.

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10
Q

What is meant by a dihybrid cross?

A

A genetic cross considering the inheritance of two different genes.

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11
Q

What does Mendel’s law of segregation state?

A

Each gamete receives only one allele for each gene, as alleles separate during meiosis.

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12
Q

What does Mendel’s law of independent assortment state?

A

Each pair of alleles separates independently of other pairs during gamete formation (if on different chromosomes).

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13
Q

What is meant by linkage?

A

When two or more genes are located on the same chromosome and are inherited together.

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14
Q

What is autosomal linkage?

A

Genes that are located on the same autosome (non-sex chromosome).

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15
Q

What is sex linkage?

A

When a gene is located on a sex chromosome (usually the X chromosome), causing differences in inheritance patterns between males and females.

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16
Q

Why are males more likely to express X-linked recessive traits?

A

Because males have only one X chromosome, so any allele present on it will be expressed, even if recessive.

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17
Q

Define epistasis.

A

When one gene affects or masks the expression of another gene at a different locus.

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18
Q

What is the phenotypic ratio for a typical dihybrid cross (when both genes show complete dominance)?

A

9:3:3:1.

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19
Q

Why might observed ratios differ from expected ratios in genetic crosses?

A

Due to random fertilisation, small sample size, or linkage between genes.

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20
Q

What is a test cross and why is it used?

A

A cross between an individual with a dominant phenotype and one that is homozygous recessive, used to determine the genotype of the dominant individual.

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21
Q

What is a monohybrid cross?

A

A genetic cross involving the inheritance of a single gene.

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22
Q

What is the expected phenotypic ratio for a monohybrid cross between two heterozygotes (complete dominance)?

A

3 dominant phenotype : 1 recessive phenotype.

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23
Q

What is the phenotypic ratio for codominant alleles in a monohybrid cross (e.g., red × white flowers)?

A

1 red : 2 pink : 1 white.

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24
Q

In a dihybrid cross between two heterozygotes (AaBb × AaBb), what is the expected phenotypic ratio?

A

9:3:3:1 — where both genes show complete dominance and assort independently.

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25
What does it mean if genes are linked?
They are located on the same chromosome and are inherited together unless separated by crossing over.
26
How can linkage affect phenotypic ratios?
It reduces the number of recombinant offspring, causing observed ratios to differ from the expected 9:3:3:1.
27
How can crossing over affect linked genes?
Crossing over during meiosis can separate linked alleles, producing new combinations of alleles (recombinants).
28
What is a recombinant phenotype?
A phenotype that results from new combinations of alleles due to crossing over.
29
How is recombination frequency calculated?
\text{Recombination frequency} = \frac{\text{Number of recombinant offspring}}{\text{Total offspring}} \times 100
30
What does a recombination frequency of less than 50% indicate?
That the genes are linked and close together on the same chromosome.
31
What is meant by complete linkage?
When genes are so close together on a chromosome that crossing over does not separate them.
32
What is epistasis?
The interaction between genes at different loci where one gene masks or modifies the expression of another.
33
Give an example of recessive epistasis.
If gene A codes for pigment production and gene B codes for pigment deposition, individuals homozygous recessive for A (aa) will be albino regardless of B.
34
Give an example of dominant epistasis.
If a dominant allele at one locus masks the expression of alleles at another (e.g., a dominant inhibitor allele preventing pigment formation).
35
What are typical phenotypic ratios for epistasis?
• Recessive epistasis: 9:3:4 • Dominant epistasis: 12:3:1 or 13:3 (depending on interaction type)
36
How does the environment influence phenotype?
It can affect gene expression — e.g., temperature affecting fur colour in Himalayan rabbits.
37
What statistical test is used to compare observed and expected genetic ratios?
The chi-squared (χ²) test.
38
What is the purpose of the chi-squared test in genetics?
To test whether differences between observed and expected ratios are due to chance.
39
State the formula for the chi-squared test.
\chi^2 = \sum \frac{(O - E)^2}{E} Where O = observed, E = expected.
40
When do you accept the null hypothesis in a chi-squared test?
When the calculated χ² value is less than the critical value, meaning differences are due to chance.
41
What assumptions must be met for the chi-squared test to be valid?
• Large sample size • Data are categorical (discrete) • Expected frequencies are greater than 5 • Individuals are randomly sampled and independent
42
Why are sex-linked conditions more common in males?
Because males have only one X chromosome, so a single recessive allele on the X is expressed.
43
Give an example of a sex-linked disorder.
Haemophilia or red–green colour blindness.
44
What type of inheritance pattern does haemophilia show?
X-linked recessive.
45
How would you identify sex linkage from pedigree data?
The condition appears more frequently in males, and affected males cannot pass the trait to their sons (since they pass the Y chromosome).
46
What is the phenotypic ratio for codominant alleles in a monohybrid cross (e.g., red × white flowers)?
1 red : 2 pink : 1 white.
47
In a dihybrid cross between two heterozygotes (AaBb × AaBb), what is the expected phenotypic ratio?
9:3:3:1 — where both genes show complete dominance and assort independently.
48
What does it mean if genes are linked?
They are located on the same chromosome and are inherited together unless separated by crossing over.
49
How can linkage affect phenotypic ratios?
It reduces the number of recombinant offspring, causing observed ratios to differ from the expected 9:3:3:1.
50
How can crossing over affect linked genes?
Crossing over during meiosis can separate linked alleles, producing new combinations of alleles (recombinants).
51
What is a recombinant phenotype?
A phenotype that results from new combinations of alleles due to crossing over.
52
How is recombination frequency calculated?
\text{Recombination frequency} = \frac{\text{Number of recombinant offspring}}{\text{Total offspring}} \times 100
53
What does a recombination frequency of less than 50% indicate?
That the genes are linked and close together on the same chromosome.
54
What is meant by complete linkage?
When genes are so close together on a chromosome that crossing over does not separate them.
55
What is epistasis?
The interaction between genes at different loci where one gene masks or modifies the expression of another.
56
Give an example of recessive epistasis.
If gene A codes for pigment production and gene B codes for pigment deposition, individuals homozygous recessive for A (aa) will be albino regardless of B.
57
Give an example of dominant epistasis.
If a dominant allele at one locus masks the expression of alleles at another (e.g., a dominant inhibitor allele preventing pigment formation).
58
What are typical phenotypic ratios for epistasis?
• Recessive epistasis: 9:3:4 • Dominant epistasis: 12:3:1 or 13:3 (depending on interaction type)
59
How does the environment influence phenotype?
It can affect gene expression — e.g., temperature affecting fur colour in Himalayan rabbits.
60
What statistical test is used to compare observed and expected genetic ratios?
The chi-squared (χ²) test.
61
What is the purpose of the chi-squared test in genetics?
To test whether differences between observed and expected ratios are due to chance.
62
State the formula for the chi-squared test.
\chi^2 = \sum \frac{(O - E)^2}{E} Where O = observed, E = expected.
63
When do you accept the null hypothesis in a chi-squared test?
When the calculated χ² value is less than the critical value, meaning differences are due to chance.
64
What assumptions must be met for the chi-squared test to be valid?
• Large sample size • Data are categorical (discrete) • Expected frequencies are greater than 5 • Individuals are randomly sampled and independent
65
Why are sex-linked conditions more common in males?
Because males have only one X chromosome, so a single recessive allele on the X is expressed.
66
Give an example of a sex-linked disorder.
Haemophilia or red–green colour blindness.
67
What type of inheritance pattern does haemophilia show?
X-linked recessive.
68
How would you identify sex linkage from pedigree data?
The condition appears more frequently in males, and affected males cannot pass the trait to their sons (since they pass the Y chromosome).
69
What does a higher number of recombinant offspring suggest?
Crossing over occurred more frequently — the genes are further apart on the chromosome.
70
What does a chi-squared result that rejects the null hypothesis indicate in a linkage test?
That the observed ratios differ significantly from expected ratios, suggesting gene linkage.
71
Why might a test cross be used in linkage analysis?
To determine whether genes are linked by examining phenotypic ratios in offspring when a heterozygote is crossed with a homozygous recessive.
72
How does independent assortment affect genetic variation?
It produces new combinations of maternal and paternal chromosomes in gametes, increasing variation.
73
How does crossing over contribute to genetic variation?
It exchanges segments of DNA between homologous chromosomes, creating new allele combinations.
74
Why does linkage reduce genetic variation in gametes?
Linked genes are inherited together, so fewer allele combinations are produced unless crossing over occurs.
75
Why is the Y chromosome shorter than the X?
It carries fewer genes — many traits are only found on the X chromosome.
76
Why can’t a male be a carrier of an X-linked condition?
He only has one X chromosome, so if he has the allele, he expresses the condition.
77
What does the term “carrier” mean in genetics?
An individual who has one copy of a recessive allele but does not express the phenotype.
78
How does random fertilisation increase variation?
Any sperm can fuse with any egg, combining alleles in new ways.
79
State the Hardy–Weinberg equation for genotype frequencies.
p^2 + 2pq + q^2 = 1
80
State the Hardy–Weinberg equation for allele frequencies.
p + q = 1
81
What does each symbol represent in the Hardy–Weinberg equation?
• p = frequency of the dominant allele • q = frequency of the recessive allele • p^2 = frequency of homozygous dominant genotype • 2pq = frequency of heterozygous genotype • q^2 = frequency of homozygous recessive genotype
82
What is the Hardy–Weinberg principle?
It predicts that allele frequencies in a population will remain constant from generation to generation if certain conditions are met.
83
What are the assumptions of the Hardy–Weinberg principle?
1. Large population size 2. No mutations 3. No migration (no gene flow) 4. Random mating 5. No selection (all genotypes equally likely to reproduce)
84
What does it mean if observed genotype frequencies differ from Hardy–Weinberg predictions?
That one or more of the conditions (e.g., selection, mutation, migration) are not being met.
85
How can you calculate q when given the frequency of a recessive phenotype?
If the recessive phenotype is expressed only in homozygotes, q = \sqrt{\text{frequency of recessive phenotype}}
86
Once q is known, how do you find p?
p = 1 - q
87
How do you find the heterozygote frequency using Hardy–Weinberg?
2pq
88
What does Hardy–Weinberg equilibrium imply about evolution?
That no evolution is occurring — allele frequencies remain stable.
89
How can selection pressure affect Hardy–Weinberg equilibrium?
Selection changes reproductive success of certain genotypes, altering allele frequencies over time.
90
How can mutation affect Hardy–Weinberg equilibrium?
Mutations introduce new alleles or alter existing ones, changing allele frequencies.
91
How does migration affect Hardy–Weinberg equilibrium?
Migration introduces or removes alleles from a population, changing allele frequencies (gene flow).
92
Why is random mating important in Hardy–Weinberg equilibrium?
Because if mating is non-random (e.g. assortative mating), certain alleles may become more common.
93
Example: If 9% of a population shows a recessive trait, calculate the percentage of heterozygotes.
Step 1: q^2 = 0.09 \Rightarrow q = 0.3 Step 2: p = 1 - 0.3 = 0.7 Step 3: 2pq = 2(0.7)(0.3) = 0.42 Answer: 42% are heterozygotes.
94
Why might Hardy–Weinberg predictions not match real data in natural populations?
Because conditions such as selection, non-random mating, migration, and mutation often occur in nature.
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