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Biology · AQA Paper 2

Genetic Inheritance

How characteristics are passed from parents to offspring: the language of genetics (alleles, dominant and recessive, genotype and phenotype), how to predict the results of a single gene cross with a Punnett square, how inherited disorders such as polydactyly and cystic fibrosis are passed on and followed in family trees, and how sex is determined. Matches AQA topics 4.6.1.6 (genetic inheritance), 4.6.1.7 (inherited disorders) and 4.6.1.8 (sex determination). Every lesson is for both Combined Science and separate Biology; the Higher tier extras are marked.

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Key terms in this chapter

All 34, gathered from every lesson.

Gamete
A sex cell, such as a sperm cell or egg cell, that carries one copy of each gene.
Chromosome
A long molecule of DNA in the nucleus that carries many genes.
Gene
A small section of DNA on a chromosome that codes for a particular protein.
Allele
A different form of the same gene.
Dominant
An allele that is always expressed, even if only one copy is present.
Recessive
An allele that is only expressed if two copies are present.
Homozygous
Having two identical alleles of a gene.
Heterozygous
Having two different alleles of a gene.
Genotype
The combination of alleles that an organism has.
Phenotype
The characteristic that is expressed, which results from the genotype and the environment.
Punnett square
A diagram that shows all the possible combinations of gametes in a genetic cross.
Genetic cross
A mating between two parents, used to predict the genotypes and phenotypes of the offspring.
Gamete
A sex cell that carries one allele of each gene.
Fertilisation
The joining of a male gamete and a female gamete.
Offspring
The young produced in a cross.
Ratio
A comparison of two numbers, such as 3 : 1.
Probability
The chance of something happening, written from 0 to 1, or as a fraction or percentage.
Carrier
A person who has one copy of a recessive allele and does not have the disorder.
Inherited disorder
A disorder caused by alleles that are passed from parents to offspring.
Polydactyly
An inherited disorder in which a person has extra fingers or toes, caused by a dominant allele.
Cystic fibrosis
An inherited disorder of cell membranes, caused by a recessive allele, that produces thick sticky mucus.
Carrier
A person who has one copy of a recessive allele and does not have the disorder.
Family tree
A diagram that shows how a characteristic is inherited through several generations of a family.
Embryo screening
Testing an embryo for a faulty allele before it is placed in the womb.
Gene therapy
Treating an inherited disorder by changing or replacing the faulty gene in a person's cells.
IVF
In vitro fertilisation, in which eggs are fertilised outside the body.
Sex chromosomes
The pair of chromosomes that determines the sex of a person: XX in females and XY in males.
X chromosome
A sex chromosome carried by all eggs and by half of sperm.
Y chromosome
The shorter sex chromosome, carried by half of sperm, which makes an embryo develop as male.
Autosomes
The 22 pairs of chromosomes that control characteristics but not sex.
Gamete
A sex cell with one copy of each chromosome.
Fertilisation
The joining of an egg and a sperm.
Punnett square
A diagram showing all possible combinations of gametes in a cross.
Ratio
A comparison of two numbers, such as 1 : 1.