Inheritance

Inheritance is the process by which genetic information is passed on from parent to child. This is why members of the same family tend to have similar characteristics.

How is genetic material inherited? #

• Most of our cells contain two sets of 23 chromosomes (they are diploid).

• An exception to this rule are the sex cells (egg and sperm), also known as gametes, which only have one set of chromosomes each (they are haploid).

• However, in sexual reproduction the sperm cell combines with the egg cell to form the first cell of the new organism in a process called fertilisation.

• This cell (the fertilised egg) has two sets of 23 chromosomes (diploid) and the complete set of instructions needed to make more cells, and eventually a whole person.

• Each of the cells in the new person contains genetic material from the two parents.

• This passing down of genetic material is evident if you examine the characteristics of members of the same family, from average height to hair and eye colour to nose and ear shape, as they are usually similar.

• If there is a mutation in the genetic material, this can also be passed on from parent to child

• This is why diseases can run in families.

How is sex determined? #

• The sex of an individual is determined by the sex chromosomes called the X chromosome and the Y chromosome.

• Females have two X chromosomes (XX).

• Males have an X chromosome and a Y chromosome (XY).

• Female gametes (eggs) therefore always carry an X chromosome.

• Male gametes (sperm) can carry either an X or a Y.

• When an egg joins with a sperm containing an X chromosome, the result is a girl.

• When an egg joins with a sperm containing a Y chromosome, the result is a boy.

What is Mendelian inheritance? #

• The simplest form of inheritance was uncovered from the work of an Austrian monk called Gregor Mendel in 1865.

• From years of experiments using the common pea plant, Gregor Mendel was able to describe the way in which genetic characteristics are passed down from generation to generation.

• Gregor used peas in his experiments primarily because he could easily control their fertilisation, by transferring pollen from plant to plant with a tiny paintbrush.

• Sometimes he transferred pollen to and from flowers on the same plant (self-fertilisation) or from another plant’s flowers (cross fertilisation).

Mendel’s law of inheritance #

• Law of Segregation – The alleles for each character segregate during gamete production so that each gamete will only have one of the two alleles for each gene.

• Law of Independent Assortment – Pairs of alleles for each characteristic/gene segregate independently of each other.

Genotype #

• The genotype is the genetic combination of two alleles.

• If, for example, a child has received one brown-eye allele represented by ‘B’ and one blue-eye allele represented by ‘b’ then their genotype would be ‘Bb’.

• If, however, the child received two brown-eye alleles their genotype would be ‘BB’, and a child with two blue-eye alleles ‘bb’.

• As previously mentioned, the brown-eye allele is dominant over the blue-eye allele so a child with the genotype ‘Bb’ would, in theory, have brown eyes, rather than blue or a mix between the two.

• Genotypes with two alleles that are the same, i.e. ‘BB’ and ‘bb’, are known as homozygous genotypes and genotypes with two different alleles are known as heterozygous genotypes.

Phenotype #

• The physical appearance of the genotype is called the phenotype.

• For example, children with the genotypes ‘BB’ and ‘Bb’ have brown-eye phenotypes, whereas a child with two blue-eye alleles and the genotype ‘bb’ has blue eyes and a blue-eye phenotype.

• The phenotype can also be influenced by the environment and sometimes certain alleles will be expressed in some environments but not in others.

• Therefore two individuals with the same genotype can sometimes have different phenotypes in they live in different environments.