GCSE Biology: Inheritance

Justina: Genetic variation and inheritance is what makes each of us unique and determines our characteristics.

Georgina: And it’s also why we each look similar, but not identical, to our parents and other family members.

Justina: It starts in the very first formation of the very first cell that we developed from.Shall we take a closer look?

Georgina: There are two ways that organisms can reproduce: asexual and sexual reproduction.Asexual reproduction requires a single parent.This process does not occur in humans.It uses the process of mitosis to create identical clones of the parent cell.Sexual reproduction, requiring two parents (a male and a female) involves the fusion of two gametes or reproductive cells into a zygote.In humans, the two gametes are the sperm cell from the male parent and the ovum, or egg, from the female parent.Their fusion or fertilisation, produces variation in the offspring.

Justina: Our genetic material is stored in our cells.The nucleus of each cell contains 46 chromosomes arranged in Georgina:3 pairs.Chromosomes are polymer strands made of DNA.This DNA provides the code for sequences of amino acids that produce a specific protein.These genetic codes are the units of heredity that may be copied and passed on to the next generation of offspring.

Georgina: Gametes (the sperm cell and the ovum) are known as haploid cells.They each contain Georgina:3 DNA chromosomes.When they fuse during fertilisation, the zygote that is formed now has the Georgina:3 chromosomes from each gamete; a full set of 46 chromosomes required in the new cell.

Justina: The chromosomes from the male parent and the chromosomes from the female parent each carry the same genes, within each strand of DNA.For example, the gene that defines eye colour or the sex of the offspring.

Georgina: When the two chromosomes join into pairs, the new cell has pairs of the same type of genes within the DNA.

Justina: Alleles are different versions of the same gene.For example, the eye colour genes have alleles for different eye colour: blue, brown, green.

Georgina: In the newly formed chromosome pairs, one of the alleles may be dominant and one may be recessive.

Justina: The combination of alleles determines one of our characteristics.

Georgina: We may inherit a blue-eyed allele from one parent and a brown-eyed allele from the other.As the brown-eyed genes are dominant and the blue-eyed genes are recessive, the offspring would have brown eyes.However, most characteristics are a result of multiple genes interacting, rather than a single gene.

Justina: We inherit alleles for all of our characteristics.Sometimes, the alleles we inherit from each parent will be identical.These are known as homozygous alleles, but when they are different, they are known as heterozygous.

Georgina: The characteristic expressed by each allele is called a phenotype.For example, blue eyes is a phenotype.

Justina: The combination of alleles we inherit from our parents is known as our genotype.By studying genotypes, we can predict the probably of inheritance using genetic cross diagrams.These diagrams show the probability of different combinations of alleles being inherited and which are recessive and dominant.

Georgina: This can be used to predict the likelihood of inherited genetic diseases that may be more likely due to particular alleles.

Justina: DNA and the genes within really are the building blocks of life on a microscopic level.

Georgina: And our parents do provide us with every single gene, but the endless combinations of alleles is what makes each and every one of us truly unique.