inheritance

DNA is located in the nucleus of a cell.

The genome is the entire set of genetic material of an organism. This includes all the alleles of all the genes in their DNA.

False. A gene provides the instructions that code for the amino acid sequence required to build a protein.

True. The nuclei of body cells contain a copy of the entire human genome, even though they don't use all of the information.

A gene is a section of DNA which codes for a specific protein.

Chromosomes are structures formed by the coiling of DNA double helices in the nucleus of a cell, visible during cell division, and containing genetic information in the form of genes.

True. Each chromosome carries many genes which code for many different types of protein.

Human body cells contain 23 pairs of chromosomes, totalling 46 chromosomes in diploid cells (often represented as 2n).

Homologous chromosome pairs are two chromosomes (one from each parent) that are similar in size, shape, and gene content. They carry corresponding genes, although they may have different alleles for each gene.

Gametes (sperm and egg cells) contain the haploid number of chromosomes (n), which is half the diploid number found in body cells.

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:

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

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:

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 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:

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:

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:

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:

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:

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

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

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:

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:

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: