Inheritance

Question 1

What is a gene?

Answer 1

A length of DNA on a chromosome that codes for the production of one or more polypeptide chains and functional RNA

Question 2

Define genotype.

Answer 2

The genetic constitution of an organism.

Question 3

Define phenotype.

Answer 3

The expression of an organism’s genetic constitution, combined with its interaction
with the environment.

Question 4

What is a locus?

Answer 4

The fixed position on a DNA molecule occupied by a gene.

Question 5

What is an allele?

Answer 5

Different forms of a particular gene, found at the same locus on a chromosome. A single gene could have many alleles.

Question 6

Define homozygous

Answer 6

Having two identical alleles of a gene e.g. FF or ff

Question 7

Define heterozygous

Answer 7

Having two different alleles of a gene
e.g. Ff

Question 8

What is a dominant allele?

Answer 8

An allele that is always expressed
Represented with a capital letter e.g. F

Question 9

What is a recessive allele?

Answer 9

• An allele that is only expressed if 2 copies are present
• Represented with a small letter e.g. f

Question 10

What is meant by codominant alleles?

Answer 10

Two dominant alleles that both contribute
to the phenotype, either by showing a blend of both characteristics, or the characteristics appearing together.

Question 11

What is an autosome?

Answer 11

A chromosome that is not an X or Y
chromosome.

Question 12

What is a sex chromosome?

Answer 12

A chromosome that determines the sex of an organism, e.g. X and Y chromosomes in humans and other mammals.

Question 13

What is the F 1 generation?

Answer 13

The first generation of offspring resulting from the cross of two individuals in the
parental generation.

Question 14

What is the F 2 generation?

Answer 14

The second generation of offspring resulting from the cross of two individuals in the F 1 generation.

Question 15

Define monohybrid inheritance.

Answer 15

Where one phenotypic characteristic is controlled by a single gene.

Question 16

What is Mendel’s first law of inheritance?

Answer 16

Law of segregation
Alleles separate randomly into gametes
Each parent passes one allele to their offspring

Question 17

What is Mendel’s second law of inheritance?

Answer 17

Law of independent assortment
The alleles of genes assort independently of other genes during gamete formation

Question 18

What is the purpose of a test cross?

Answer 18

Used to determine whether an individual
with a dominant trait is heterozygous or homozygous dominant.

Question 19

Describe how a test cross is carried out.

Answer 19

An individual of unknown genotype is bred with a homozygous recessive individual
Offspring phenotypes are observed to determine
the genotype of the unknown individual

Question 20

Define dihybrid inheritance.

Answer 20

The determination of a trait by the inheritance of two genes.

Question 21

What does Mendelian inheritance assume?

Answer 21

It assumes that the genes involved are not linked.

Question 22

What is meant by sex-linkage?

Answer 22

• An allele is located on one of the sex chromosomes
• Its expression depends on the sex of the individual

Question 23

Give an example of a phenotype which shows sex-linkage.

Answer 23

• Haemophilia
• Duchenne muscular dystrophy

Question 24

Describe haemophilia.

Answer 24

• X-linked recessive condition
• Results in excessive bleeding and blood that is slow to clot

Question 25

Describe Duchenne muscular dystrophy.

Answer 25

* X-linked recessive condition * Characterised by muscle degeneration and weakness

Question 26

Why are haemophilia and Duchenne muscular dystrophy more common in males?

Answer 26

They are X-linked recessive conditions. Males only inherit one X chromosome so are more likely to express the gene in their phenotype.

Question 27

What is linkage?

Answer 27

* Two or more genes positioned on the same autosome * Unlikely to be separated by crossing over during meiosis so often inherited together

Question 28

What is the chi-squared test?

Answer 28

A statistical test used to determine whether the difference between observed and expected data is due to chance or a real effect. Can be used to compare expected phenotypic ratios with observed ratios.

Question 29

How is a chi-squared test performed?

Answer 29

Make a null hypothesis Used Mendelian ratios to calculate the expected numbers Calculate chi-squared value using chi-squared equation Calculate the degrees of freedom Select an appropriate significant level (normally 0.05) Find the critical value Compare the chi-squared value with the critical value Accept or reject the null hypothesis

Question 30

How are degrees of freedom calculated?

Answer 30

The number of categories minus 1.

Question 31

If the chi-squared value is greater than or equal to the critical value, is the null hypothesis accepted or rejected?

Answer 31

Rejected.

Question 32

What does it mean if the null hypothesis is rejected?

Answer 32

There is a significant difference between the observed and expected results.

Question 33

If the chi-squared value is less than the critical value, is the null hypothesis accepted or rejected?

Answer 33

Accepted

Question 34

What does it mean if the null hypothesis is accepted?

Answer 34

The difference between the observed and expected results is not significant. The results occured due to chance.

Question 35

What is a mutation?

Answer 35

* An alteration to the volume, arrangement or structure of DNA * May affect a single gene or a whole chromosome

Question 36

When do most mutations occur?

Answer 36

* Crossing over in prophase ! * Non-disjunction in anaphase I and II

Question 37

What is a gene mutation?

Answer 37

* A change to at least one nucleotide base in DNA or the arrangement of bases * May occur spontaneously during DNA replication and can be beneficial, damaging, or neutral

Question 38

Give an example of condition caused by a gene mutation.

Answer 38

Sickle cell anaemia

Question 39

Describe sickle cell anaemia.

Answer 39

* Recessive genetic disorder caused by a substitution mutation on chromosome 11 * Results in abnormal haemoglobin which distorts red blood cells

Question 40

What is a chromosome mutation?

Answer 40

A change in the structure or number of chromosomes, affecting many genes.

Question 41

Give an example of condition caused by a chromosome mutation.

Answer 41

Down's syndrome

Question 42

Describe Down's syndrome.

Answer 42

Genetic disorder characterised by delayed development and learning disabilities * Due to non-disjunction, an affected individual possesses three copies of chromosome 21

Question 43

What is chromosome nondisjunction?

Answer 43

Failure of homologous chromosomes to separate in meiosis I or sister chromatids to separate in meiosis II. Results in gametes with one extra or less chromosome than normal.

Question 44

What is a mutagen?

Answer 44

A chemical, biological or physical agent that increases the rate of gene mutations above normal level.

Question 45

Give some examples of mutagens.

Answer 45

lonising radiation (gamma radiation, UV, X-rays) Chemicals (e.g. polycyclic hydrocarbons)

Question 46

What is a carcinogen?

Answer 46

A type of mutagen that causes cancer.

Question 47

What is an oncogene?

Answer 47

Mutations of proto-oncogenes that are activated continuously, resulting in uncontrolled cell division.

Question 48

Define epigenetics,

Answer 48

The study of changes in gene expression that are not due to alterations in the nucleotide base sequence of DNA

Question 49

How can histone modification affect gene expression?

Answer 49

* Alteration of histones by the addition of methyl, acetyl or phosphate groups * Increases or decreases gene expression by causing the histone to coil more tightly or loosely

Question 50

How can DNA methylation affect gene expression?

Answer 50

* Addition of a methyl (CH3) group to cytosine bases * Prevents recognition of the bases, reducing gene expression