Inheritance (Edexcel IGCSE Biology)

False.

The number of chromosomes differs between species.

True.

Chromosomes condense and become visible during cell division (like mitosis or meiosis) when the DNA coils tightly to form distinct structures.

The four nucleotide bases in DNA are:

• Thymine

• Adenine

• Guanine

• Cytosine

Complementary base pairing refers to the specific pairings of nucleotide bases in DNA.

• Adenine pairs with thymine

• Guanine pairs with cytosine

False.

Adenine pairs with thymine.

Complementary base pairing allows for the DNA template to be copied and a new strand to be synthesised. This ensures accurate DNA replication during cell division.

DNA is a polymer made up of two strands coiled around each other to form a double helix. The strands consist of a sugar-phosphate backbone with nitrogenous bases (adenine, thymine, guanine, cytosine) attached to the backbone via complementary base pairing.

The backbone of a DNA strand is formed from the deoxyribose sugar of one nucleotide base and the phosphate molecule of the adjacent nucleotide base.

A polymer is a large molecule made up of many repeating subunits.

The nucleotide bases found in RNA are adenine, guanine, cytosine and uracil.

A polynucleotide is a polymer, or large molecule, made up of many nucleotide monomers linked together in a chain.

Differences between DNA and RNA include:

• RNA is single-stranded and DNA is double-stranded

• RNA contains the base uracil instead of thymine (as in DNA)

• RNA contains ribose sugar and DNA contains deoxyribose sugar

True.

DNA stands for deoxyribonucleic acid and RNA stands for ribonucleic acid.

Protein synthesis is the process which uses mRNA as a messenger to carry the code from the DNA to the ribosome in order to make proteins.

tRNA is a molecule in the cytoplasm of cells. It plays a key role in the process of protein synthesis. tRNA binds to the mRNA using complementary base pairing and delivers the specific amino acid coded for by the mRNA.

False.

RNA relies on complementary base pairing during the formation of mRNA in transcription and pairing with tRNA for polypeptide synthesis in translation.

Uracil is present in RNA but not in DNA.

RNA contains ribose sugar in its backbone, which differs from the deoxyribose sugar found in the backbone of DNA molecules.

The main types of RNA involved in protein synthesis are messenger RNA (mRNA), transfer RNA (tRNA).

Protein synthesis is the process by which proteins are made in a cell. It is split into two parts:

• Transcription which copies the DNA code into mRNA

• Translation which uses the mRNA to produce a protein

Transcription occurs in the nucleus of the cell.

Transcription is the process of producing an mRNA copy of a gene's DNA sequence; it occurs in the nucleus of a cell.

mRNA is messenger RNA. It is a single-stranded RNA molecule that carries genetic information from DNA to ribosomes for protein synthesis.

Translation is the process during which mRNA is decoded to produce a specific sequence of amino acids; this produces a protein. Translation occurs on ribosomes.

tRNA is transfer RNA. It has a region called an anticodon and carries specific amino acids to the ribosome. Here the anticodon pairs with an mRNA codon during translation.

An anticodon is a triplet of unpaired bases on tRNA that is complementary to a specific mRNA codon during translation.

Translation occurs in the cytoplasm of the cell, specifically at ribosomes.

True.

DNA is transcribed and translated one gene at a time. Each gene will produce a different protein.

An allele is a version of a gene. For example the gene for eye colour has different alleles that give rise to different eye colours.

Individuals have two alleles of each gene; one on each chromosome of a homologous pair. One allele is inherited from the mother and one from the father.

True.

The genetic code of each allele will determine the protein made during protein synthesis. The proteins determine the phenotype of the individual.

Offspring tend to show similar characteristics to their parents because they inherit half their alleles from each parent.

A gamete is a sex cell, e.g. a sperm cell or an egg cell. Gametes are haploid cells.

A chromosome is a thread-like molecule of DNA present in the nucleus of a cell.

True.

Dominant alleles are always expressed in an organism's phenotype when present.

True.

Recessive alleles need to be inherited from both parents for their associated characteristic to be expressed in the phenotype. If either of the alleles inherited is dominant then it will mask the recessive form.

A homozygous individual has two identical alleles of a specific gene, e.g. DD or dd.

A heterozygous individual has two different alleles of a specific gene, e.g. Dd

Genotype refers to the combination of alleles present in an organism. A genotype can be represented by letters, e.g. EE, Ee or ee

Phenotype refers to the observable physical characteristics of an organism. Phenotype is influenced by genotype and the environment.

The genotype of a homozygous dominant individual in this example would be GG.

In a species of mammal where allele B = black fur and allele b = brown fur, the phenotype of an individual with the genotype Bb is black fur. This is because the dominant allele B masks expression of the recessive allele b.

Codominance occurs when both alleles of a genotype are expressed in the phenotype of an individual.

False.

Codominance results in both alleles being expressed equally in the phenotype.

The individual has a heterozygous genotype with one allele for blood type and A and one allele for blood type B. Both alleles are expressed in the phenotype.

Polygenic inheritance refers to characteristics which are determined by multiple genes.

False.

The majority of characteristics are polygenic and are therefore determined by multiple genes.

Many characteristics have a wide range of phenotypes because they are polygenic and are therefore controlled by many different genes, all of which contribute to the range of possible phenotypes.

A Punnett square is a diagram which shows the potential allele combinations that can result from a genetic cross between two individuals.

Dominant alleles are usually represented by a capital letter whereas recessive alleles are represented by a lower case letter.

Monohybrid inheritance refers to the inheritance of a trait that is determined by a single gene.

If a tall pea plant with genotype TT is crossed with a short pea plant with genotype tt, all offspring would be heterozygous with the genotype Tt. This is because a T allele is inherited from the tall parent and a t allele is inherited from the short parent.

If a tall pea plant with genotype Tt is crossed with another tall pea plant with genotype Tt, the phenotype ratio of the offspring would be 3: 1 (3 tall and 1 short). This is because the cross would result in 1 x TT (tall), 2 x Tt (tall) and 1 x tt (short).

False.

Punnet squares can be used to predict other types of cross, e.g. dihybrid crosses. In this case the diagram may become more complicated and will require more squares.

False.

Monohybrid inheritance involves characteristics controlled by a single gene.

When a homozygous dominant individual is crossed with a homozygous recessive individual there is 100 % probability that the offspring will display the dominant phenotype. This is because all offspring will be heterozygous.

If two heterozygous individuals are crossed there is a 1/4, 0.25 or 25 % probability of offspring having a homozygous recessive genotype.

The parent genotypes that produce the offspring genotypes DD, DD, Dd and DD are DD and Dd.

A family pedigree is a genetic diagram used to trace the inheritance of a characteristic through generations of families.

Features of an individual that can be shown on a family pedigree diagram include:

• Whether an individual is a biological male or female

• Whether an individual is affected by a genetic trait or not

• Vertical lines are used to represent parentage

• Horizontal lines are used to represent couples that have produced children

• Whether an individual is a carrier of a genetic trait (in some diagrams)

A carrier is a heterozygous individual who carries a single recessive allele that codes for a genetic condition. They do not have the condition because the recessive allele is masked by the healthy dominant allele, but they can pass the condition on to their offspring. Family pedigree diagrams do not always label carriers.

Sex chromosomes are the chromosomes that determine the biological sex of an individual; in humans they are the X and Y chromosomes.

In biological females the sex chromosomes are XX.

False.

Biological sex is determined by an entire chromosome pair (XX for females, XY for males), not by a single gene.

In biological males the sex chromosomes are XY.

True.

The father determines the biological sex of the child because he can pass on either an X or a Y chromosome.

The probability of a child inheriting two X chromosomes is 1/2 or 50 %.

Mitosis is nuclear division that results in the production of genetically identical cells. It is used for growth, repair, and asexual reproduction.

Diploid cells have two sets of chromosomes; one set comes from each parent. In humans the diploid number of chromosomes is 46 (23 pairs).

False.

Mitosis results in genetically identical cells.

Before mitosis each chromosome replicates, producing two identical copies of each chromosome; one for each of the two daughter cells.

A mitotic division produces two genetically identical daughter cells. The daughter cells are genetically identical to each other and to the parent cell.

The gametes (i.e. sperm, egg cells, pollen and ovules) are not produced by mitosis, instead they are produced by meiosis.

Human cells produced by mitosis contain 23 pairs of, or 46, chromosomes.

Mitosis is important in the body to produce new cells for growth, repair of damaged tissues, and the replacement of cells.

Meiosis is a type of nuclear division that produces genetically different cells. The cells produced by meiosis are gametes (sex cells).

Meiosis is essential for sexual reproduction as it produces gametes (sperm cells and egg cells) with half the normal chromosome number.

False.

Meiosis results in genetically different cells.

During meiosis the chromosome number is halved from diploid (46) to haploid (23). For this reason meiosis is known as reduction division.

The process of meiosis involves the following events:

• During the first division homologous chromosomes pair up and recombine, resulting in genetic variation

• The second division separates sister chromatids

Meiosis increases genetic variation by creating new combinations of maternal and paternal chromosomes in each gamete, leading to unique offspring.

The daughter cells produced by meiosis can be described as follows:

• Four daughter cells

• Cells are genetically different

• Cells are haploid

• The cells produced are gametes

Meiosis is crucial because it ensures that gametes (sperm and egg cells) have half the chromosome number, allowing for the correct chromosome number after fertilisation.

Meiosis produces four daughter cells.

Some key differences between meiosis and mitosis might include:

• Mitosis involves one division WHILE meiosis involves two

• Mitosis produces diploid daughter cells WHILE meiosis produces haploid daughter cells

• Mitosis produces two daughter cells WHILE meiosis produces four

• Daughter cells in mitosis are genetically identical to each other and the parent cell WHILE in meiosis they are genetically different

• Mitosis is used for growth and repair WHILE meiosis produces sex cells for sexual reproduction

Genetic variation is the differences between the genetic information of different organisms.

Random fertilisation describes the nature of the fertilisation process; any one of millions of sperm cells may fertilise whichever egg cell happens to have been released during ovulation.

Random fertilisation contributes to genetic variation because there are trillions of different possible combinations of sperm cell and egg cell that can fuse at fertilisation; this means that the new embryo will be genetically unique.

Variation refers to the differences between individuals.

Variation can be caused by genetic factors and/or environmental factors.

True.

Meiosis increases genetic variation by creating gametes with unique combinations of alleles.

Examples of variation in humans that is caused by genetic factors alone include:

• Blood group

• Eye colour

• Biological sex

• Ability to roll the tongue

Environmental variation refers to differences in traits caused by factors in the environment, e.g.

• Events that cause physical trauma

• Stress

• Culture

• Lifestyle

Examples of variation in humans that is caused by environmental factors alone include:

• Body scarring

• Spoken language

• Regional accent

• Tattoos

Note that there are likely to be very few factors that are caused by environmental factors alone; most traits have a genetic element even if they seem to be environmental, e.g. body mass or sporting ability

True.

Genetic and environmental factors interact with each other to produce observable traits in individuals, e.g. body mass is affected by diet and lifestyle, but there is also a large genetic component that affects the brain and metabolism.

Mutations are rare and random change to the genetic material that can be inherited.

False.

It is rare for cells to contain mutations in their genetic material.

True.

Mutations are changes in the genetic material, so contribute to genetic variation between individuals.

Inherited traits are those that can be passed on to the next generation. DNA containing mutations can be passed on to offspring, so mutations can be inherited.

Mutations can affect amino acid sequence as follows:

• Mutations result in changes to the DNA base sequence

• The changed base sequence results in a different amino acid sequence

An altered amino acid sequence can affect the phenotype of an organism because the changed amino acid sequence might result in the formation of altered proteins.

False.

Most mutations have no effect, or just a small effect, on the phenotype. This may be the result of an unaltered proteins structure, or a minor change to protein structure that does not affect protein function.

True.

It is rare for mutations to have a significant effect on phenotype. When this occurs it can lead to harmful effects, e.g. genetic diseases.

True.

The incidence of mutations can be increased by exposure to ionising radiation (gamma rays, x-rays, ultraviolet rays) and certain chemicals, e.g. tar in tobacco.

A mutagen is an agent, such as radiation or a chemical, that can increase the frequency of mutations.

Gamma rays, x-rays, and ultraviolet rays are types of ionising radiation that can damage DNA and cause changes in DNA base sequences, leading to mutations.

The chemicals found in tobacco tar can increase mutation rates.

Darwin's theory of natural selection states that:

• Individuals within a species exhibit variation due to genetic differences

• Those with advantageous traits are more likely to survive and reproduce

• The alleles that code for advantageous traits are passed on to offspring

• Over time the advantageous alleles become more common in the population

Evolution is the change that occurs in populations over many generations. Natural selection in the mechanism by which this change occurs.

"Survival of the fittest" refers to the idea that individuals with traits better suited to their environment are more likely to survive. These individuals will reproduce and pass on their traits.

False.

Natural selection can only occur when variation is present. This is why populations with no, or limited, genetic variation are unable to adapt to changes in their environment.

True.

Advantageous characteristics promote survival in organisms. These organisms can then reproduce and pass on their alleles to the next generation.

Antibiotic resistance is the ability of bacteria to withstand the effects of an antibiotic; bacteria with resistance to a particular antibiotic are more likely to survive when they are exposed to the drug.

True.

Random mutations in bacterial DNA can lead to antibiotic resistance, enabling some bacteria to survive antibiotic treatment.

Antibiotic resistant strains of bacteria can arise by the following process:

• Mutation gives rise to an allele that codes for antibiotic resistance in one individual

• The resistant individual survives when treated with the antibiotic

• The individual reproduces and passes on the resistance allele

• This occurs over many generations until only resistant individuals remain

Antibiotic use can increase the probability that a resistant population of bacteria will arise because the antibiotics kill any non-resistant individuals, meaning that any randomly occurring alleles for resistance become highly advantageous. The resistance alleles are therefore more likely to be passed on.

True.

Infections caused by antibiotic resistant strains of bacteria can be hard to treat, and several different types of antibiotic may be needed to kill all the bacteria

The development of antibiotic resistance can be prevented by measures such as:

• Using antibiotics only when necessary, e.g. not for minor illness or viral infections

• Prescribing specific types of antibiotic for specific types of infection

• Limiting antibiotic use in agriculture

• Improve hygiene measures in hospitals so that resistant strains do not spread