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 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
○ But there may be many alleles of a single gene in a population

Question 5

Dominant allele

Answer 5

Always expressed (shown in the phenotype)

Question 6

Recessive allele

Answer 6

Only expressed when 2 copies present (homozygous recessive)

Question 7

Codominant alleles

Answer 7

Both alleles expressed in the phenotype

Question 8

Homozygous

Answer 8

Alleles at a specific locus are the same

Question 9

Heterozygous

Answer 9

Alleles at a specific locus are different

Question 10

What do monohybrid crosses show?

Answer 10

inheritance of one phenotypic characteristic coded by a single gene

Question 11

What do dihybrid crosses show?

Answer 11

inheritance of two phenotypic characteristics coded by two different gene

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

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

Question 14

Explain how autosomal linkage affects inheritance of alleles

Answer 14

● 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 15

What is epistasis?

Answer 15

Interaction of non-linked genes where one masks and suppresses the expression of the other

Question 16

Describe when a chi-squared (X2) test can be used

Answer 16

● When determining if observed results are significantly different from expected results (frequencies)

● Data is categorical (can be divided into groups eg. phenotypes)

Question 17

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

Answer 17

● 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 18

Describe how a chi-squared value can be analysed

Answer 18

1. Number of degrees of freedom = number of categories - 1
2. Determine critical value at p = 0.05 (5% probability) from a table
3. If X2 value is / less
   than or equal to critical value at p < 0.05
   ● Difference is significant so we reject null hypothesis
   ● So there is less than 5% probability that difference is due to chance