Which of the following best describes how this gene is inherited from parents to offspring?

• Since human cells carry two copies of each chromosome they have two versions of each gene. These different versions of a gene are called alleles.
• Alleles can be either dominant or recessive.
• Dominant alleles show their effect even if the individual only has one copy of the allele (also known as being heterozygous). For example, the allele for brown eyes is dominant, therefore you only need one copy of the ‘brown eye’ allele to have brown eyes (although, with two copies you will still have brown eyes).
• If both alleles are dominant, it is called codominance. The resulting characteristic is due to both alleles being expressed equally. An example of this is the blood group AB which is the result of codominance of the A and B dominant alleles.
• Recessive alleles only show their effect if the individual has two copies of the allele (also known as being homozygous). For example, the allele for blue eyes is recessive, therefore to have blue eyes you need to have two copies of the ‘blue eye’ allele.

Illustration to show the inheritance of dominant and recessive alleles for eye colour.
Image credit: Genome Research Limited

What are sex-linked genes?

• Some genes are found on the sex chromosome, X.
• These genes are inherited with the X chromosome (from the mother if it is a boy or from either mother or father if it is a girl).
• Females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY).
• This means females have two alleles for X-linked genes while males only have one.
• Some genetic diseases, are caused by sex linked genes, for example haemophilia.
• The allele for haemophilia is recessive so two copies are needed for a female to have the disease
• However, because males only have one X chromosome, they only need one copy of the haemophilia allele to have the disease.
• This means haemophilia is much more common in males than in females.

For example:

Functioning allele = H

Haemophilia allele = h

XH XH = healthy female

XH Xh = carrier female

Xh Xh = haemophilia female

XH Y = healthy male

Xh Y = haemophilia male

This page was last updated on 2021-07-21

• We actually have two genomes each
• We get one copy of our genome from each of our parents
• Inheritance describes how genetic material is passed on from parent to child.

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 a genotype?

• The genotype is a description of the unique genetic makeup of an individual. It can be used to describe an entire genome or just an individual gene and its alleles.
• The genotype of an individual influences their phenotype.
• For example, if we are talking about the genotype for eye colour we may say an individual has one brown eye allele (B) and one blue eye allele (b).
• As a result, the individuals phenotype will be brown eyes.
• This is because the allele for brown eyes is dominant, while the allele for blue eyes is recessive (see image below).

Illustration to show the inheritance of dominant and recessive alleles for eye colour.
Image credit: Genome Research Limited

What is a phenotype?

• The phenotype is a description of the physical characteristics of an organism. For example, if we are talking about eye colour the phenotype of an individual may mean blue, brown or green eyes.
• Most phenotypes are influenced by an individual’s genotype, although environment can also play a role (nature versus nurture).

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).
• In one experiment he cross fertilised smooth, yellow pea plants with wrinkly, green peas:
  • Every single pea resulting from this first cross, the first generation (F1), was smooth and yellow.
  • However, when two smooth, yellow peas from this first generation were crossed to produce a second generation (F2), the result was 75 percent smooth, yellow peas and 25 percent wrinkly, green peas (3:1).
  • This outcome shows that the genes for smooth, yellow peas are dominant while the genes for wrinkly, green peas are recessive.
• The results from this and further experiments led Gregor Mendel to come up with three key principles of inheritance:

1. The inheritance of each trait is determined by ‘factors’ (now known as genes) that are passed onto descendants.
2. Individuals inherit one ‘factor’ from each parent for each trait.
3. A trait may not show up in an individual but can still be passed onto the next generation.

• Genetic traits that follow these principles of inheritance are called Mendelian.

Gregor Mendel
Image credit: Mendel’s Principles of Heredity: A Defence by William Bateson

This page was last updated on 2021-07-21

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