3b Inheritance¶

Part of 3 Reproduction and Inheritance.

Inheritance explains how information stored in DNA influences phenotype and how that information passes between generations. The topic links molecular ideas to genetic crosses, cell division, variation, mutation and natural selection.

Learning Objectives¶

ID	Route	Official specification wording	Main teaching sections
`3b-lo-1`	All students + Biology-only detail	3.14 understand that the genome is the entire DNA of an organism and that a gene is a section of a molecule of DNA that codes for a specific protein 3.15 understand that the nucleus of a cell contains chromosomes on which genes are located 3.16B describe a DNA molecule as two strands coiled to form a double helix, the strands being linked by a series of paired bases: adenine (A) with thymine (T), and cytosine (C) with guanine (G) 3.17B understand that an RNA molecule is single stranded and contains uracil (U) instead of thymine (T) 3.18B describe the stages of protein synthesis including transcription and translation, including the role of mRNA, ribosomes, tRNA, codons and anticodons	DNA, Genes and Protein Synthesis
`3b-lo-2`	All students	3.19 understand how genes exist in alternative forms called alleles which give rise to differences in inherited characteristics 3.20 understand the meaning of the terms: dominant, recessive, homozygous, heterozygous, phenotype, and genotype 3.21B understand the meaning of the term codominance 3.22 understand that most phenotypic features are the result of polygenic inheritance rather than single genes 3.23 describe patterns of monohybrid inheritance using a genetic diagram 3.24 understand how to interpret family pedigrees 3.25 predict probabilities of outcomes from monohybrid crosses 3.26 understand how the sex of a person is controlled by one pair of chromosomes, XX in a female and XY in a male 3.27 describe the determination of the sex of offspring at fertilisation, using a genetic diagram	Alleles and Inheritance Patterns
`3b-lo-3`	All students	3.28 understand how division of a diploid cell by mitosis produces two cells that contain identical sets of chromosomes 3.29 understand that mitosis occurs during growth, repair, cloning and asexual reproduction 3.30 understand how division of a cell by meiosis produces four cells, each with half the number of chromosomes, and that this results in the formation of genetically different haploid gametes 3.31 understand how random fertilisation produces genetic variation of offspring 3.32 know that in human cells the diploid number of chromosomes is 46 and the haploid number is 23 3.33 understand that variation within a species can be genetic, environmental, or a combination of both	Mitosis, Meiosis and Variation
`3b-lo-4`	All students	3.34 understand that mutation is a rare, random change in genetic material that can be inherited 3.35B understand how a change in DNA can affect the phenotype by altering the sequence of amino acids in a protein 3.36B understand how most genetic mutations have no effect on the phenotype, some have a small effect and rarely do they have a significant effect 3.37B understand that the incidence of mutations can be increased by exposure to ionising radiation (for example, gamma rays, x-rays and ultraviolet rays) and some chemical mutagens (for example, chemicals in tobacco) 3.38 explain Darwin’s theory of evolution by natural selection 3.39 understand how resistance to antibiotics can increase in bacterial populations, and appreciate how such an increase can lead to infections being difficult to control	Mutation and Natural Selection

Biology-Only Content: DNA, Genes and Protein Synthesis¶

This content is required for Biology-only students and is not required for Combined Science students.

DNA structure: - DNA (deoxyribonucleic acid) is a double helix — two strands wound around each other. - It is a polymer made of repeating units called nucleotides. Each nucleotide contains a sugar molecule, a phosphate group and one of four organic bases: adenine (A), thymine (T), cytosine (C) or guanine (G). - The two strands are joined by complementary base pairing: A pairs with T; C pairs with G. - A sequence of three bases (a triplet) codes for one amino acid.

From gene to protein — protein synthesis: 1. The DNA double helix is unwound and unzipped in the nucleus. 2. Free mRNA (messenger RNA) nucleotides bind to the complementary bases on one strand. The bases in mRNA are A, U (uracil, replacing thymine), C and G. 3. The mRNA nucleotides are joined together to form an mRNA strand — a complementary copy of the gene. This is called transcription. 4. The mRNA strand leaves the nucleus and attaches to a ribosome in the cytoplasm. 5. At the ribosome, bases on the mRNA are read in groups of three (codons). Each codon specifies one amino acid. 6. Carrier molecules bring the corresponding amino acids to the ribosome. 7. The amino acids are joined together in the correct sequence to form a polypeptide chain, which folds into a specific three-dimensional protein. This is called translation.

The order of bases in DNA therefore determines the order of amino acids in a protein, which in turn determines the protein's shape and function.

DNA, Genes and Protein Synthesis¶

The genome is the entire DNA of an organism. Chromosomes are structures in the nucleus made of long DNA molecules. Genes are sections of a chromosome that code for a specific protein. Humans have 23 pairs of chromosomes — 22 pairs control characteristics, and the 23rd pair determines sex.

Alleles and Inheritance Patterns¶

An allele is an alternative form of a gene. Most body cells are diploid, containing two alleles for each gene (one inherited from each parent).

Key terms: - Dominant allele: expressed (visible in phenotype) when only one copy is present. Represented by a capital letter (e.g. A). - Recessive allele: only expressed when two copies are present. Represented by a lowercase letter (e.g. a). - Homozygous: both alleles are the same (AA or aa). - Heterozygous: one dominant and one recessive allele (Aa). - Genotype: the combination of alleles an individual has (e.g. Aa). - Phenotype: the observable characteristic produced (e.g. brown eyes). - Codominance: when neither allele is dominant over the other, so both contribute to the phenotype (e.g. a speckled chicken from crossing a white and a black chicken).

Monohybrid crosses use Punnett square diagrams to predict the probability of offspring having particular genotypes and phenotypes. Dominant alleles are represented by capital letters; recessive by lowercase. Most phenotypic features result from multiple genes interacting rather than a single gene.

Sex determination: Humans have 23 pairs of chromosomes. The 23rd pair consists of the sex chromosomes, X and Y. Females have two X chromosomes (XX); males have one X and one Y (XY). During meiosis, eggs always contain an X chromosome, while sperm contain either X or Y. The sex of offspring depends on which sperm fertilises the egg, giving a 50% chance of each sex.

Mitosis, Meiosis and Variation¶

Mitosis produces two genetically identical daughter cells, each with the same diploid (46) chromosome number as the parent. It occurs during growth, repair, cloning and asexual reproduction.

Stages of the cell cycle leading to mitosis: 1. Interphase: the cell grows, organelles replicate, proteins are synthesised and all chromosomes are copied (producing the characteristic X shape). 2. Chromosomes line up at the cell's equator and spindle fibres pull each chromatid to opposite poles. 3. The cytoplasm and cell membrane divide, producing two identical daughter cells.

Meiosis produces four non-identical haploid cells (gametes), each with 23 chromosomes. It occurs in the reproductive organs.

The cell copies all its chromosomes.
The cell divides once to produce two cells, each with the normal 46 chromosomes (diploid).
Each cell divides again to produce four cells, each with 23 chromosomes (haploid).

Chromosomes are shuffled randomly during meiosis, so all four gametes are genetically different from one another and from the parent cell. Random fertilisation (the fusion of any egg with any sperm) further increases genetic variation in offspring.

Sources of variation: - Genetic variation: differences in DNA sequences between individuals. Examples include eye colour and blood group, which are determined by inherited alleles. - Environmental variation: differences caused by factors such as diet, climate or lifestyle. For example, a plant that could grow tall genetically may remain stunted if it lacks nutrients. - Both: many characteristics — including height — are influenced by a combination of genetics and environment.

Mutation and Natural Selection¶

A mutation is a rare, random change in DNA sequence that can be inherited. All alleles originally arose through mutation. Most mutations occur in non-coding DNA and have no effect on phenotype; some affect phenotype slightly; and a very few (occurring in coding regions) can have a major effect.

Mutations can occur more frequently when organisms are exposed to: - Ionising radiation (e.g. gamma rays, X-rays, ultraviolet light). - Chemical mutagens (e.g. chemicals in tobacco smoke).

Some mutations cause uncontrolled cell division, resulting in tumours. Mutagens that cause cancer are called carcinogens.

Natural selection and evolution: - Evolution is a change in inherited characteristics of a population over time as a result of natural selection, which may lead to new species forming. - Mutations create variation in a population. - If a mutation gives a survival advantage, the organism is more likely to survive to breeding age ('survival of the fittest') and pass the mutation to offspring. - Over many generations, the frequency of the advantageous mutation increases in the population.

Antibiotic resistance in bacteria: Bacteria reproduce very rapidly by binary fission, so advantageous mutations spread quickly. When bacteria are exposed to antibiotics, susceptible bacteria die while any that carry a resistance mutation survive and reproduce. Over time, the proportion of resistant bacteria in the population increases. This is an example of natural selection. MRSA ('superbug') is resistant to many antibiotics and spreads in hospitals when healthcare workers move between patients.

Speciation occurs when a population of a species becomes so genetically different from others that they can no longer interbreed to produce fertile offspring.

Common Confusions¶

Gene vs allele: A gene is the section of DNA coding for a specific protein. An allele is one particular version of that gene.
Mitosis vs meiosis: Mitosis makes two identical diploid cells. Meiosis makes four non-identical haploid gametes.
Dominant vs common: A dominant allele is expressed over a recessive one, but it does not necessarily become more common in a population over time.

Key Terms¶

Genome: the entire DNA of an organism.
Gene: a section of DNA that codes for the production of a specific protein.
Allele: an alternative form of a gene.
Genotype: the combination of alleles an organism possesses.
Phenotype: the observable characteristics of an organism.
Codominance: when two different alleles are both expressed in the phenotype.
Homozygous: having two identical alleles of a gene.
Heterozygous: having two different alleles of a gene.
Dominant: describing an allele expressed in the phenotype when only one copy is present.
Recessive: describing an allele expressed only when two copies are present.
Diploid: having two sets of chromosomes (46 in humans).
Haploid: having one set of chromosomes (23 in humans; found in gametes).
Mitosis: cell division producing two genetically identical diploid cells.
Meiosis: cell division producing four genetically non-identical haploid cells.
Mutation: a random change in DNA sequence that may be inherited.
Natural selection: the process by which organisms with advantageous traits reproduce more successfully, increasing the frequency of those traits in the population.
Evolution: change in inherited characteristics of a population over time.
Speciation: the formation of a new species when populations become so different they can no longer interbreed to produce fertile offspring.
Carcinogen: a substance or agent that causes mutations leading to cancer.
Transcription: the production of an mRNA molecule from a DNA template.
Translation: the assembly of a protein at a ribosome using the sequence of codons on mRNA.