Chapter 16: Inheritance

Question 1

Explain what is meant by homologous pair of chromosomes.

Answer 1

2 chromosomes that carry the same genes at the same loci.

Same shape, length, banding pattern and centromere position.

Pairs up to form bivalent in prophase I.

Question 2

Describe meiosis I.

Answer 2

Prophase I:
Chromosomes will condense. Homologous chromosomes will pair up to form bivalents, and non-sister chromatids cross over at chiasmata.

Nuclear envelope and nucleolus disintegrates. Centrioles move to opposite poles. Spindle fibre attached to centromeres.

Metaphase I:
Bivalents line up on equator, and independent assortment occurs.

Anaphase I:
Spindle fibres contract, whole chromosomes move to opposite poles led by centromeres. So, bivalents pulled apart.

Question 3

Describe meiosis II.

Answer 3

Generally same like mitosis:
Single chromosomes pair and line up on equator at right angles to first equator.

Centromeres divide, chromatids pulled to opposite poles. Chromosome number halved.

Question 4

Explain how independent assortment produces genetically different gametes.

Answer 4

Homologous chromosomes line up randomly and independent of each other on equator during metaphase 1.

Chromosomes also undergo independent assortment at metaphase 2.

There are 2^n possible combinations, so many different chromosome combinations in daughter cells.

This leads to new combinations of maternal and paternal alleles.

Question 5

Explain how crossing over produces genetically different gametes.

Answer 5

Crossing over occurs between non-sister chromatids of homologous chromosomes at chiasmata during prophase 1.

Exchange of DNA occurs, so linkage groups are broken.

New combination of alleles on chromatids.

Question 6

Explain how random fusion of gametes produces genetically different individuals.

Answer 6

Random mating and random fusion of gametes.

There could also be chromosome mutation.

Question 7

Define gene.

Answer 7

Length of DNA coding for a single polypeptide.

Question 8

Define locus.

Answer 8

Position of gene on chromosome.

Question 9

Define allele.

Answer 9

Different variations of the same gene due to different DNA base sequence.

Question 10

Define dominant and recessive.

Answer 10

Dominant: Allele always has effect on phenotype of heterozygote and homozygous dominant.

Recessive: Allele only expressed in phenotype when dominant allele not present.

Question 11

Describe sex linkage.

Answer 11

Some genes are found on 1 sex chromosome, and not present on the other. Usually, sex-linked genes found on X chromosome.

Inheritance of these genes depend on the individual’s sex.

Sex-linked genes are represented by writing alleles as superscript next to sex chromosome.

Question 12

Describe autosomal linkage.

Answer 12

Occurs on autosomes which are non-sex chromosomes.

2 or more genes do not assort independently during meiosis.

Genes are linked, and inherited together in original parental combination.

Question 13

Define F1 and F2.

Answer 13

F1:
First generation of offspring from parents that are homozygous dominant and recessive.
F1 generation is 100% heterozygous.

F2:
Offspring from 2 F1 individuals.

Question 14

Define phenotype and genotype.

Answer 14

Phenotype: Observable features of organisms.

Genotype: Combination of alleles in organism.

Question 15

State the Chi-squared test null hypothesis.

Answer 15

There is no significant difference between observed and expected results.

Question 16

When is chi-squared suitable to be used?

Answer 16

Data is discrete, and discontinuous distribution.

Question 17

Outline and explain effects of mutant alleles can result in albinism.

Answer 17

There is a mutant recessive allele of the TYR gene on chromosome 11 that causes lack of enzyme tyrosinase.

Tyrosinase would have converted tyrosine into DOPA, then dopaquinone. Then, dopaquinone is converted to melanin.

So, when homozygous recessive, no tyrosinase means no melanin. It is only expressed in homozygous recessive people.

Leads to pale hair, pale eyes, and susceptibility to skin cancer.

Question 18

Outline and explain the effects of mutant alleles can result in sickle cell anaemia.

Answer 18

There is an abnormal allele for the gene HBB on chromosome 11 which results in altered B-globin polypeptide in haemoglobin.

Haemoglobin is less soluble in low oxygen concentration, forming long fibres and RBCs form crescent shape.

So, RBCs carry less oxygen. They can get stuck in capillaries, blocks blood flow, and can cause pain. RBCs also break down faster.

Homozygous individual has 2 abnormal alleles for the HBB gene produces only sickle cell haemoglobin. Heterozygous will produce some normal and some sickle, but they are carrier and may have no symptoms.

Question 19

Outline and explain the effects of mutant alleles can result in haemophilia.

Answer 19

Haemophilia can be caused by a recessive allele of F8 gene on the X chromosome, so this is sex-linked.

F8 not produced, so reduced blood clotting and excessive bleeding.

If males have an abnormal allele, they’ll have the condition because they only have 1 copy of the gene. Females can be heterozygous for F8 gene, and can only be a carrier.

Question 20

Explain why a man with haemophilia cannot pass the condition to his son.

Answer 20

Male’s sex chromosomes are XY. Male only passes Y chromosome to son, and Y chromosome don’t have the F8 gene.

Males don’t pass the X chromosome to son, but to daughter. It’s the female that pass the X chromosome to the son.

Question 21

Outline and explain the effects of mutant alleles can result in Huntington’s disease.

Answer 21

There is a mutation on the HTT gene on chromosome 4, which result in abnormal alleles.

The recessive allele has 10-35 repeats of CAG, but the dominant allele would have extra repeats of CAG.

The more repeats, the earlier onset is. Onset can occur in babies if numerous repeats. However, the usual onset is middle age.

This results in a misfolded protein, therefore causes a neurological condition where neurones are destroyed and causes involuntary movements and cognitive changes.

Question 22

Why are some plants tall and some short?

Answer 22

Some plant species height is partially controlled by genes.

Tall plants have dominant allele which codes for enzyme producing active gibberelin.

Dwarf plants have 2 copies of recessive allele, so no active gibberellin produced.

The recessive allele is only 1 nucleotide different to dominant allele, an amino acid substitution in primary structure of enzyme. So, primary structure changes, enzyme is non-functional.

Question 23

Describe the differences between repressible enzymes and inducible enzymes.

Answer 23

Repressible enzymes are produced continuously, and synthesis is prevented by binding of repressor protein to operator.

Inducible enzymes are only produced when inducer is present, and is constitutive.

Gene is switched on only when needed, transcription of this gene only occurs when inducer binds to TF, triggered by a stimulus. Concentration increases when substrate increases, for example galactose.

Question 24

Why are structural genes transcribed together?

Answer 24

They share 1 promoter, and the products work together.

Question 25

Why is it beneficial that structural genes are not expressed when lactose is not present?

Answer 25

Prevent wastage of amino acids.

Question 26

Describe lac operon when lactose is absent.

Answer 26

Lactose not bound to repressor protein, so repressor stays binding to operator.

RNA polymerase cannot bind to promoter and cannot move to operator.

mRNA not synthesised, so, this prevents transcription of structural genes.

Question 27

Describe lac operon’s genes and function.

Answer 27

Regulatory gene, gene I codes for repressor protein. Lac repressor protein has 2 binding sites, 1 for operator and 1 for lactose.

Lac Z codes for lactase, hydrolyses lactose into galactose and glucose.

Lac Y codes for lactose permease, increases lactose uptake.

Lac A codes for transacetylase.

Question 28

Describe lac operon when lactose is present.

Answer 28

When lactose is present, lactose binds to repressor protein.

Repressor protein changes shape, and no longer bind to operator.

RNA polymerase binds to promoter, genes lac Z, lac Y, lac A are transcribed. mRNA synthesis occurs, enzymes are synthesised.

Question 29

Describe transcription factors.

Answer 29

TF are proteins that controls transcription of genes by binding to specific region of DNA, some on promoter. So RNA polymerase too can bind to promoter, so transcription occurs.

TF controls gene expressions in eukaryotes by decreasing or increasing rate of transcription.

Question 30

Explain importance of transcription factors.

Answer 30

Genes are expressed at the correct time, and at the correct order and correct amount which leads to correct pattern of development.

This allows responses to environmental stimuli. For example, correct genes are expressed in response to very high temperatures.

TFs also regulates cell cycle and helps with cell signalling.

Question 31

Explain how gibberellin activates genes, resulting in amylase production.

Answer 31

DELLA proteins inhibit PIF, a TF.

Gibberellin binds to receptor in aleurone layer, so enzyme to break down DELLA is produced, so DELLA proteins are broken down by enzyme.

DELLA no longer inhibits PIF, PIF binds to promoter region of DNA, switching genes on.

Transcription of gene coding for amylase occurs, then translation, leading to amylase production.

Question 32

State the function of DELLA proteins.

Answer 32

DELLA proteins normally prevent activation of gene coding for amylase.

They are broken down when amylase is needed.

Question 33

Outline how a gene mutation may occur.

Answer 33

Spontaneous change in nucleotide, can look like substitution, deletion and addition. This may cause frame shift mutation.

Mutation can be due to mutagens.

Question 34

Explain why results of breeding could be different from expected ratios.

Answer 34

Autosomal linkage, some genes are on the same chromosome, therefore linked.

No independent assortment.