A2C2 - Chapter 9 - 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 (genotype). 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. Eg: FF or ff.

Question 7

Define heterozygous.

Answer 7

Having two different alleles of a gene. Eg: Ff.

Question 8

What is a dominant allele?

Answer 8

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

Question 9

What is a recessive allele?

Answer 9

An allele that is only expressed in the absence of a dominant allele. Only expressed if 2 copies are present.
Represented with a small letter eg: 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, eg: X and Y chromosomes in humans and other mammals.

Question 13

What is the F1 generation?

Answer 13

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

Question 14

What is the F2 generation?

Answer 14

The second generation of offspring resulting from the cross of two individuals in the F1 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 independant assortment.
The alleles of genes assort independantly of other genes during gamete formation.

Question 18

Draw an example of a genetic cross diagram.

Answer 18

Parental phenotypes
Parental genotypes
Gametes
Offspring genotypes (work out via punnett square)
Offspring phenotypes

Question 19

A female who is homozygous recessive for cystic fibrosis (ff) has a child with heterozygous male (Ff). Draw punnett square to illustrate this monohybrid inheritance.

Answer 19

(punnet square drawn)

Question 20

PKU is a recessive condition. Two heterozygous parents have offspring. Predict the proportion of offspring that will have PKU.

Answer 20

0.25 or 25%

Question 21

What is the purpose of a test cross? (aka punnett square)

Answer 21

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

Question 22

Describe how a test cross is carried out.

Answer 22

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

Question 23

Define dihybrid inheritance.

Answer 23

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

Question 24

What does Mendelian inheritance assume?

Answer 24

It assumes that the genes involved are not linked.

Question 25

What is meant by sex linkage?

Answer 25

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

Question 26

Give an example of a phenotype which shows sex linkage.

Answer 26

Haemophilia Duchenne muscular dystrophy.

Question 27

Describe haemophilia.

Answer 27

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

Question 28

Describe Duchenne Muscular Dystrophy.

Answer 28

X-linked recessive gene. Characterised by muscle weakness and degeneration.

Question 29

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

Answer 29

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

Question 30

What is linkage?

Answer 30

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

Question 31

What is a chi-squared test?

Answer 31

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

Question 32

How is chi-squared test performed?

Answer 32

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

Question 33

How are degrees of freedom calculated?

Answer 33

The number of categories minus 1.

Question 34

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

Answer 34

Rejected

Question 35

What does it mean if the null hypothesis is rejected?

Answer 35

There is significant difference between the observed and expected results.

Question 36

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

Answer 36

Accepted

Question 37

What does it mean if the null hypothesis is accepted?

Answer 37

No significant difference between the observed and expected results. The results occured due to chance.

Question 38

What is a mutation?

Answer 38

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

Question 39

When do most mutations occur?

Answer 39

Crossing over during Prophase I Non-disjunction in Anaphase I and II.

Question 40

What is a gene mutation?

Answer 40

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

Question 41

Give an example of a condition caused by a gene mutation.

Answer 41

Sickle cell anaemia.

Question 42

Describe sickle cell anaemia.

Answer 42

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

Question 43

What is a chromosome mutation?

Answer 43

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

Question 44

Give an example of a condition caused by chromosome mutation.

Answer 44

Down Syndrome

Question 45

Describe Down Syndrome.

Answer 45

Genetic disorder characterised by delayed development and learning disabilities. Due to non-dysjunction, an affected individual possesses 3 copies of chromosome 21 (Trisomy 21)

Question 46

What is chromosome nondisjunction?

Answer 46

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 47

What is a mutagen?

Answer 47

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

Question 48

Give some examples of mutagens.

Answer 48

Ionising radiation (gamma radiation, uv, xrays) Chemicals (polycyclic hydrocarbons)

Question 49

What is a carcinogen?

Answer 49

A type of mutagen that causes cancer.

Question 50

What is an oncogene?

Answer 50

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

Question 51

Define epigenetics.

Answer 51

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

Question 52

How can histone modification affect gene expression?

Answer 52

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 53

How can DNA methylation affect gene expression?

Answer 53

Addition of methyl group to cytosine bases. Prevents the recognition of the bases, reducing gene expression.