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Biology · Edexcel Paper 1

Inheritance

How characteristics are passed on, from alleles, genotype and phenotype to monohybrid crosses, Punnett squares, family pedigrees and the inheritance of sex, and then, for separate Biology, Mendel's work, the ABO blood groups and sex-linked disorders. Matches Edexcel 3.11B to 3.18B. The first four lessons are for both Combined Science and separate Biology at Foundation and Higher. Lessons 5 and 6 are for separate Biology only, and lesson 7 is for separate Biology at Higher tier only.

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Key terms in this chapter

All 64, gathered from every lesson.

Allele
A different version of the same gene.
Dominant
An allele that shows its effect even when only one copy is present.
Recessive
An allele that only shows its effect when 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 for a gene.
Phenotype
The characteristics that an organism shows.
Gamete
A sex cell, with one allele of each gene.
Zygote
The cell formed when two gametes fuse.
Carrier
A person with one copy of a recessive allele who does not show the characteristic but can pass the allele on.
Monohybrid cross
A genetic cross that looks at the inheritance of a single gene.
Punnett square
A grid used to show all the possible combinations of gametes in a cross.
Genetic diagram
A diagram that shows the genotypes of parents, their gametes and their possible offspring.
Ratio
A way of comparing amounts, such as 3 : 1.
Probability
The chance that something will happen, written as a fraction, a decimal or a percentage.
Test cross
A cross between an organism of unknown genotype and a homozygous recessive one.
Offspring
The young produced by parents.
Parental generation
The parents at the start of a cross.
Expected ratio
The ratio of offspring predicted from a genetic diagram.
Pedigree
A family tree that shows how a characteristic is inherited through the generations.
Carrier
A person with one copy of a recessive allele who does not have the disorder.
Affected
Showing the characteristic or the disorder.
Generation
A set of individuals at the same stage in a family, such as the parents or the children.
Inherited disorder
A condition caused by faulty alleles passed from parents to children.
Cystic fibrosis
A recessive disorder that causes thick, sticky mucus in the lungs and gut.
Huntington's disease
A dominant disorder that causes the gradual death of brain cells.
Polydactyly
A dominant condition in which a person has extra fingers or toes.
Genetic counselling
Advice to people about the risk of passing on an inherited disorder.
Sex chromosomes
The pair of chromosomes that decides the sex of a person: XX in females and XY in males.
X chromosome
The larger sex chromosome, found in both males and females.
Y chromosome
The smaller sex chromosome, found only in males, which carries a gene for male development.
Sex determination
The way in which the sex of an offspring is decided.
SRY gene
The gene on the Y chromosome that makes an embryo develop as a male.
Independent event
An event whose outcome is not affected by the earlier ones.
Testes
The male reproductive organs that make sperm.
Ovaries
The female reproductive organs that make eggs.
Ratio
A comparison of two amounts, such as 1 : 1.
Mendel
The Austrian monk who discovered the basic rules of inheritance by breeding pea plants.
Pure-breeding
Always producing offspring with the same characteristic as the parents, because they are homozygous.
Cross-pollination
The transfer of pollen from one plant to the flower of another plant.
Self-pollination
The transfer of pollen to a flower on the same plant.
P generation
The parental generation, the plants that are crossed at the start.
F1 generation
The first generation of offspring from a cross.
F2 generation
The offspring of the F1 generation.
Blending inheritance
The old idea that the features of two parents mix in their offspring.
Hereditary factor
Mendel's name for what we now call a gene.
Codominance
When both alleles of a gene show their effect in a heterozygous person.
Multiple alleles
When a gene has more than two alleles in a population.
Blood group
The type of blood a person has, decided by antigens on the red blood cells.
Antigen
A molecule on the surface of a cell that the immune system can recognise.
ABO system
The system of blood groups A, B, AB and O.
Universal donor
A person with blood group O, whose blood can usually be given to any group.
Roan
An animal with a coat of mixed red and white hairs, an example of codominance.
Genotype
The combination of alleles that an organism has.
Transfusion
The transfer of blood from one person to another.
Sex-linked
Describes a gene that is found on one of the sex chromosomes, usually the X.
X-linked
Describes a gene on the X chromosome.
Haemophilia
A sex-linked recessive disorder in which blood does not clot properly.
Colour blindness
A sex-linked recessive condition in which a person cannot tell some colours apart.
Carrier
A female with one recessive allele on one X chromosome, who does not have the disorder.
Autosome
A chromosome that is not a sex chromosome.
Duchenne muscular dystrophy
A sex-linked recessive disorder that causes progressive muscle weakness.
Sex chromosomes
The X and Y chromosomes, which decide the sex of a person.
Clotting
The process by which blood forms a solid plug to stop bleeding.