Genetics

Question 1

what is a gene

Answer 1

section of DNA on a chromosome coding for one or more polypeptides

Question 2

what is an allele

Answer 2

one of a number of alternative forms of a gene

Question 3

what are nucleotides

Answer 3

-complex chemicals made up of an organic base, a sugar and phosphate
-they are basic units of which the nucleic acids DNA and RNA are made

Question 4

what is a genotype

Answer 4

-genetic constitution of an organism
-determines the limits within which the characteristics of an individual may vary

Question 5

what is a phenotype

Answer 5

-observable/biochemical characteristics of an organism
-result of interaction between the expression of the genotype and the environment

Question 6

what is a dominant allele

Answer 6

allele that is always expressed in the phenotype of an organism

Question 7

what is a recessive allele

Answer 7

allele that isn’t expressed in the phenotype. Only expressed if homozygous recessive

Question 8

what is a codominant allele

Answer 8

two alleles both contribute to the phenotype

Question 9

what is a locus

Answer 9

position of a gene on a particular DNA molecule

Question 10

what is meant by homozygote

Answer 10

alleles of a particular gene are the same

Question 11

what is meant by heterozygote

Answer 11

alleles of a particular gene are different

Question 12

what is meant by the term carrier

Answer 12

individual carrying an allele not expressed in phenotype

Question 13

what is meant by the term multiple allele

Answer 13

-gene that has two or more alleles
-organism will have 2 alleles on each homologous pair of chromosomes

Question 14

define monohybrid inheritance

Answer 14

where one phenotypic characteristic is controlled by a single gene

Question 15

what is the expected phenotypic ratio from crossing two heterozygous parents (monohybrid)

Answer 15

3:1

Question 16

example of codominance

Answer 16

inheritance of sickle-cell anaemia

Question 17

what is sickle cell anaemia

Answer 17

→ a condition where a faulty haemoglobin allele
causes red blood cells to be sickle shaped
→ in the heterozygous genotype, some cells
are sickle shaped but some are normal

Question 18

what is the expected phenotypic ratio for crossing two heterozygous parents (codominance)

Answer 18

1:2:1

Question 19

what is an example of multiple alleles

Answer 19

human blood group has three alleles

Question 20

which blood groups are codominant and which are recessive

Answer 20

-group a + group b = codominant
-group 0= recessive

Question 21

define dihybrid inheritance

Answer 21

where two phenotypic characteristics are determined by two different genes present on two different chromosomes at the same time

Question 22

what is the expected phenotypic ratio from crossing two parents heterozygous for both genes (dihybrid)

Answer 22

9:3:3:1

Question 23

what can be an example of dihybrid crossing

Answer 23

inheritance of wing length and wing colour in an insect

Question 24

What is a sex-linked gene?

Answer 24

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

Question 25

Explain why males are more likely to express a recessive X-linked allele ( in humans)

Answer 25

● 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 26

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

Answer 26

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

Question 27

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

Answer 27

Only males tend to have [named recessive phenotype].

Question 28

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

Answer 28

● [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 29

Explain how autosomal linkage affects inheritance of alleles

Answer 29

● 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 30

what is epistasis

Answer 30

a gene interactions-> one gene masks the expression of another gene

Question 31

what is a n example of epistasis

Answer 31

flower colour

Question 32

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

Answer 32

● 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 33

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

Answer 33

● 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 34

Describe how a chi-squared value can be analysed

Answer 34

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 X2 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