Genetic inheritance — AQA GCSE Combined Science

A gamete is a sex cell produced by meiosis. In humans, the gametes are sperm and egg cells; in flowering plants, they are pollen and egg cells. Each gamete is haploid, meaning it contains one chromosome from each homologous pair, so it carries one allele of each gene. At fertilisation, two gametes fuse to form a diploid zygote, restoring the full chromosome number and creating new allele combinations. For example, a human sperm carries 23 chromosomes and an egg carries 23; their fusion gives a zygote with 46 chromosomes. This explains why offspring inherit one allele from each parent and why gametes are central to genetic variation.

A chromosome is a thread-like structure of DNA found in the nucleus of eukaryotic cells. Most human body cells are diploid, containing 23 pairs of chromosomes, so 46 in total. One chromosome of each pair is inherited from each parent. Chromosomes carry genes in a fixed linear order, so each gene has a specific locus. In cells preparing to divide, chromosomes condense and become visible under a light microscope; they can be stained and photographed to produce a karyotype. Sex is determined by the sex chromosomes: XX in typical females and XY in typical males. Gametes are haploid, carrying 23 single chromosomes, so fertilisation restores the diploid number. Errors in chromosome number, such as trisomy 21, show why accurate separation in meiosis matters.

A gene is a short section of DNA that codes for a particular protein, or for a functional RNA. Genes are found at fixed positions called loci on chromosomes. The sequence of bases in a gene determines the sequence of amino acids in a protein, which in turn affects the protein's shape and function. Because humans have two copies of most chromosomes, they usually have two copies of each gene, one inherited from each parent. Different versions of the same gene are called alleles, and alleles may be dominant or recessive. A change in the DNA base sequence is a mutation, which can produce a new allele and may alter the protein produced. Understanding genes explains inherited characteristics and genetic disorders.

An allele is one of the different versions of a gene that can occupy the same locus on a chromosome. Humans are diploid, so body cells carry two copies of most genes; the two copies may be the same allele (homozygous) or different alleles (heterozygous). Alleles differ in their DNA base sequence, so they may code for slightly different versions of the same protein, producing different phenotypes. For example, the gene for eye colour has several alleles, and the combination inherited from each parent helps determine eye colour. In genetic diagrams, alleles are represented by letters: a dominant allele by a capital letter and a recessive allele by the lower-case version of the same letter. A gamete contains only one allele from each pair because meiosis separates homologous chromosomes.

A dominant allele is an allele whose characteristic is expressed in the phenotype even when only one copy is present. In a heterozygous individual, the dominant allele determines the visible characteristic, while the recessive allele is not expressed. Dominant alleles are represented by a capital letter, such as A, while the recessive allele is represented by the same letter in lower case, a. For example, if A codes for brown eyes and a codes for blue eyes, a person with genotype Aa has brown eyes because A is dominant. Dominance does not mean an allele is better, stronger or more common; it describes how the allele affects the phenotype. A dominant phenotype can result from a homozygous dominant genotype (AA) or a heterozygous genotype (Aa).

A recessive allele is one whose characteristic is only expressed in the phenotype when two copies are present, because its effect is masked by a dominant allele in a heterozygote. In a monohybrid cross, a recessive phenotype appears only in offspring that inherit the recessive allele from both parents, so both parents must carry at least one copy even if neither shows the trait. For example, if B codes for brown eyes and b for blue eyes, a bb individual has blue eyes, while Bb has brown eyes because B is dominant. Recessive alleles can therefore remain hidden for generations in carriers, which explains why two brown-eyed parents can have a blue-eyed child.

Homozygous describes an individual carrying two identical alleles for a particular gene, for example BB or bb. Because both alleles are the same, every gamete the organism produces for that gene carries the same allele. A homozygous dominant individual (BB) shows the dominant phenotype; a homozygous recessive individual (bb) shows the recessive phenotype. In a monohybrid cross between two heterozygotes (Bb × Bb), the possible genotypes are BB, Bb and bb in a 1:2:1 ratio. The homozygous genotypes BB and bb therefore each make up one quarter of the expected offspring, while the heterozygous genotype Bb makes up one half. This gives the familiar 3:1 phenotypic ratio of dominant to recessive.

Heterozygous describes a genotype in which the two alleles of a gene carried by an organism are different. Alleles are alternative versions of a gene, and a diploid organism inherits one allele from each parent. If the alleles differ, the organism is heterozygous for that gene, for example Bb or Tt. The observable characteristic depends on dominance: a dominant allele is expressed even when only one copy is present, so a heterozygote usually shows the dominant phenotype, while the recessive allele remains hidden unless two recessive alleles are inherited. Students should write allele pairs with a capital letter for the dominant allele and the matching lower-case letter for the recessive allele, and use Punnett squares or family trees to predict outcomes.

Genotype is the combination of alleles an organism carries for a particular gene or set of genes. Alleles are alternative versions of a gene, and because body cells are diploid, each gene is represented by two alleles, one inherited from each parent. The genotype is written as a pair of letters, such as BB, Bb or bb, where a capital letter represents the dominant allele and the matching lower-case letter represents the recessive allele. The genotype determines the phenotype, which is the observable characteristic, although the phenotype also depends on the environment. Students use genotypes in Punnett squares and family trees to predict the proportions of offspring with each characteristic.

The phenotype is the observable characteristic an organism shows, produced by the interaction between its genotype (the alleles it carries) and its environment. For example, a mouse with genotype BB or Bb for fur colour may appear black, while bb gives brown fur; the black or brown coat is the phenotype. You work out phenotype by identifying the alleles present, deciding which are dominant or recessive, then stating the visible feature. Environmental factors such as diet or sunlight can also affect phenotype, so identical genotypes need not always look identical. In exams, you may be asked to state a phenotype from a genetic cross, or to explain why two organisms with the same genotype can differ.

A gene is a short section of DNA that codes for a characteristic. Some characteristics, such as fur colour in mice or red-green colour blindness in humans, are controlled by a single gene, so inheritance follows simple dominant and recessive patterns. Each gene can exist in different forms called alleles; for example, a mouse fur-colour gene may have a black allele and a brown allele. You represent alleles with letters: dominant as a capital, recessive as a lower case. A homozygous organism has two identical alleles, heterozygous has two different alleles. Red-green colour blindness is caused by a recessive allele on the X chromosome, so it is more common in males.

Alleles are versions of a gene, and the combination an organism carries is its genotype. At a molecular level, a gene is a length of DNA whose base sequence codes for a polypeptide; the polypeptide may be an enzyme or a structural protein. The allele present determines which polypeptide is made, and that protein produces the characteristic. For example, a dominant allele may code for a functional enzyme that makes a pigment, while a recessive allele may code for a non-functional enzyme, so no pigment is made. The observable feature, such as eye colour or flower colour, is the phenotype. Phenotype results from the proteins produced, and the environment can also influence it. So genotype determines the proteins made, and those proteins develop the phenotype.

Alleles are different versions of a gene. A dominant allele is expressed in the phenotype even when only one copy is inherited, so a heterozygous organism with one dominant and one recessive allele shows the dominant characteristic. A recessive allele is expressed only when two copies are present, meaning the organism is homozygous recessive and no dominant allele is present. For example, in pea plants, the allele for tall stems (T) is dominant over the allele for short stems (t). A plant with genotype Tt is tall because T is expressed; a plant with genotype tt is short because no dominant T allele is present. Genetic diagrams use these rules to predict the ratios of phenotypes in offspring, and the 3:1 ratio in a monohybrid cross between two heterozygotes follows from dominance.

Alleles are different versions of the same gene, and a diploid organism carries two alleles for each gene, one inherited from each parent. When both alleles are identical, the genotype is homozygous for that trait, for example AA or aa. When the two alleles differ, the genotype is heterozygous, for example Aa. The combination of alleles present is the genotype, while the visible or detectable characteristic is the phenotype. In a heterozygote, a dominant allele is expressed even though a recessive allele is also present, so Aa usually shows the dominant phenotype. Only when both alleles are recessive, aa, is the recessive phenotype seen. Writing genotypes with a capital letter for the dominant allele and the matching lower-case letter for the recessive allele makes homozygous and heterozygous states easy to identify and use in genetic crosses.

Although simple monohybrid examples such as pea colour or earlobe shape are useful for learning inheritance, most characteristics are polygenic, meaning they are controlled by several genes acting together. Each gene may have multiple alleles, and the combined effects produce a continuous range of phenotypes rather than a few distinct categories. Human height, skin colour, body mass and many crop yields are examples. Environmental factors also influence these characteristics, so the final phenotype depends on both genotype and environment. Because several genes contribute, inheritance patterns are more complex than a single dominant or recessive pair, and outcomes are often shown as a normal distribution. Understanding polygenic inheritance explains why most people do not fall neatly into two groups and why predictions for individuals are less certain than in simple monohybrid crosses.

A single gene cross predicts the chance of each offspring genotype, not the exact family outcome. If two heterozygous parents (Aa × Aa) are crossed, each child has a 1 in 4 chance of aa, so probability describes long-run proportions across many offspring. A Punnett square shows the four equally likely fertilisation outcomes, and ratios such as 3:1 express those probabilities. However, most phenotype features, including human height, skin colour and eye colour, are polygenic: several genes plus environmental factors contribute, giving continuous variation rather than two clear categories. So a single gene cross is a model that works for clear-cut traits such as cystic fibrosis or pea seed shape, but it does not explain most features.

The outcome of a genetic cross can be expressed as a ratio or as a proportion. In a monohybrid cross between two heterozygotes (Aa × Aa), the genotype ratio is 1 AA : 2 Aa : 1 aa, which simplifies to 1:2:1, and the phenotype ratio for a dominant trait is 3 dominant : 1 recessive. Direct proportion converts these ratios into fractions or percentages of the total: 1:2:1 means 1/4 AA, 1/2 Aa and 1/4 aa, while 3:1 means 3/4 dominant and 1/4 recessive. To use direct proportion, add the parts of the ratio to find the total, then divide each part by the total. For example, in a ratio of 3:1 the total is 4, so the proportions are 3/4 and 1/4. Ratios compare parts to parts, whereas proportions compare a part to the whole.

A Punnett square is a grid that predicts the possible allele combinations in offspring. Write the two alleles of one parent across the top and the two alleles of the other parent down the side, then fill each cell by combining the row and column alleles, keeping dominant alleles capitalised, for example Tt × Tt gives TT, Tt, Tt and tt. From the completed square you can state the ratio of genotypes and phenotypes, such as 3 tall to 1 short. Family trees show the same inheritance over generations: squares are males, circles are females, shaded symbols show the trait, and you deduce genotypes by working from affected offspring back to parents, remembering that a recessive trait can appear in a child only if both parents carry the allele.

Higher tier students construct a Punnett square from parental genotypes and use probability to predict outcomes. For a cross such as Bb × Bb, the square gives BB, Bb, Bb and bb. The probability of a dominant phenotype is 3/4, 0.75 or 75%, and of a recessive phenotype is 1/4, 0.25 or 25%. Probability means the chance for each offspring, not a guarantee about a particular family; a 75% chance does not mean that every litter of four will contain exactly three dominant offspring. Express predictions as fractions, decimals or percentages, and apply the same reasoning to sex determination, where XX × XY gives a 1/2 chance of each sex. Ratios describe relative frequencies but are distinct from probabilities.