Topic 7.1 Inheritance

Question 1

What is meant by the term genotype?

Answer 1

Genetic constitution of an organism

Question 2

What is meant by the term phenotype?

Answer 2

The expression of this genetic constitution (genotype)
and its interaction with the environment

Question 3

What are alleles and how do they arise?

Answer 3

Variations of a particular gene (same locus) → arise by mutation (changes in DNA base sequence)

Question 4

How many alleles of a gene can be found in diploid organisms?

Answer 4

● 2 as diploid organisms have 2 sets of chromosomes (chromosomes are found in homologous pairs)
○ But there may be many (more than 2) alleles of a single gene in a population

Question 5

Describe the different types of alleles

Answer 5

Dominant allele:

• Always expressed (shown in the phenotype)

Recessive allele : Only expressed when 2 copies present (homozygous recessive)
/ NOT expressed when dominant allele present (heterozygous)

Codominant alleles:
Both alleles expressed / contribute to phenotype (if inherited together)

Question 6

What is meant by the terms homozygous and heterozygous?

Answer 6

Homozygous : Alleles at a specific locus (on each homologous chromosome) are the same

Heterozygous : Alleles at a specific locus (on each homologous chromosome) are different

Question 7

What do monohybrid and dihybrid crosses show?

Answer 7

● Monohybrid cross - inheritance of one phenotypic characteristic coded for by a single gene
● Dihybrid cross - inheritance of two phenotypic characteristics coded for by two different genes

Question 8

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Answer 8

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Question 9

Explain the evidence from a pedigree diagram which would show that the
allele for [named phenotype] is dominant

Answer 9

● [Named phenotype] parents [n & n] have child [n] WITHOUT [named phenotype]
● So both parents [n & n] must be heterozygous / carriers of recessive allele
○ If it were recessive, all offspring would have [named phenotype]

Question 10

Explain the evidence from a pedigree diagram which would show that the
allele for [named phenotype] is recessive

Answer 10

● Parents [n & n] WITHOUT [named phenotype] have child [n] WITH [named phenotype]
● So both parents [n & n] must be heterozygous / carriers of recessive allele

Question 11

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Answer 11

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Question 12

What is a sex-linked gene?

Answer 12

A gene with a locus on a sex-chromosome (normally X)

Question 13

Explain why males are more likely to express a recessive X-linked allele

Answer 13

This assumes males are XY and females are XX, as in humans. In some organisms, it is swapped. In these cases,
females (XY) would be more likely to express a recessive X-linked allele.

● Females (XX) have 2 alleles → only express recessive allele if homozygous recessive / can be carriers
● Males (XY) have 1 allele (inherited from mother) → recessive allele always expressed

Question 14

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Answer 14

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Question 15

Explain the evidence from a pedigree diagram which would show that the
allele for [named phenotype] on the X-chromosome is recessive

Answer 15

● Mother [n] WITHOUT [named phenotype] has child [n] WITH [named phenotype]
● So mother [n] must be heterozygous / carrier of recessive allele

Question 16

Explain the evidence from a pedigree diagram which would suggest that
[named recessive phenotype] is caused by a gene on the X chromosome

Answer 16

Only males tend to have [named recessive phenotype]

Question 17

Explain the evidence from a pedigree diagram which would show that the
gene for [named phenotype] is not on the X chromosome

Answer 17

● [Named phenotype] father [n] has daughter [n] WITHOUT [named phenotype]
● Father [n] would pass on allele for [named phenotype] on X chromosome so
daughter [n] would have [named phenotype]
OR
● [Named phenotype] mother [n] has son [n] WITHOUT [named phenotype]
● Mother [n] would pass on allele for [named phenotype] on X chromosome so
son [n] would have [named phenotype]

Question 18

Explain how autosomal linkage affects inheritance of alleles

Answer 18

● Two genes located on same autosome (non-sex chromosome)
● So alleles on same chromosome inherited together
○ Stay together during independent segregation of homologous chromosomes during meiosis
● But crossing over between homologous chromosomes can create new combinations of alleles
○ If the genes are closer together on an autosome, they are less likely to be split by crossing over

Question 19

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Answer 19

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Question 20

What is epistasis?

Answer 20

Interaction of (products of) non-linked genes where one masks / suppresses the expression of the other

Question 21

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Answer 21

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Question 22

Describe when a chi-squared (X
2
) test can be used

Answer 22

● When determining if observed results are significantly different from expected results (frequencies)
○ Eg. comparing the goodness of fit of observed phenotypic ratios with expected ratios
● Data is categorical (can be divided into groups eg. phenotypes)

Question 23

Suggest why in genetic crosses, the observed phenotypic ratios obtained in
the offspring are often not the same as the expected ratios

Answer 23

● Fusion / fertilisation of gametes is random
● Autosomal linkage / epistasis / sex-linkage
● Small sample size → not representative of whole population
● Some genotypes may be lethal (cause death)

Question 24

Describe how a chi-squared value can be analysed

Answer 24

1. Number of degrees of freedom = number of categories - 1 (eg. 4 phenotypes = 3 degrees of freedom)
2. Determine critical value at p = 0.05 (5% probability) from a table
3. If X
   2 value is [greater / less] than critical value at p < 0.05
   ○ Difference [is / is not] significant so [reject / accept] null hypothesis
   ○ So there is [less / more] than 5% probability that difference is due to chance

Question 24

Describe how a chi-squared value can be calculated

Answer 24

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