Brent Cornell

Sexually reproducing organisms inherit DNA from both parents, meaning they possess two copies of every chromosome (i.e. diploid)

• This means that these organisms will possess two alleles for every autosomal gene (one maternal allele and one paternal allele)
• One caveat is that males only have one allele for each gene located on a sex chromosome, as these chromosomes aren’t paired (XY) 

Types of Zygosity

For any given gene, the combination of alleles can be categorised as follows:

• If the maternal and paternal alleles are the same, the offspring is said to be homozygous for that gene
• If the maternal and paternal alleles are different, the offspring is said to be heterozygous for that gene
• Males only have one allele for each gene located on a sex chromosome and are said to be hemizygous for that gene

Genotype:

The combination of alleles for a specific trait is referred to as the genotype (e.g. homozygous or heterozygous)

Phenotype:

The observable characteristic of a specific trait is referred to as the phenotype (determined by both genotype and the environment) 

• Some phenotypes are determined by a single gene (monogenic), while other phenotypes are influenced by many genes (polygenic)

Modes of Inheritance

Two alleles (i.e. genotype) may interact in different ways to affect the physical expression of a characteristic (i.e. phenotype)

• The way in which the alleles are expressed in combination is called the mode of inheritance (e.g. dominance, codominance, etc.)

1. Complete Dominance

Most traits follow a classical dominant / recessive pattern of inheritance, whereby one allele is expressed over the other

• The dominant allele will mask the recessive allele when in a heterozygous state
• Homozygous dominant and heterozygous forms will be phenotypically indistinguishable
• The recessive allele will only be expressed in the phenotype when in a homozygous state

When representing alleles, the convention is to capitalise the dominant allele and use a lower case letter for the recessive allele

• An example of this mode of inheritance is mouse coat colour – black coats (BB or Bb) are dominant to brown coats (bb)

2. Codominance

Co-dominance occurs when pairs of alleles are both expressed equally in the phenotype of a heterozygous individual

• Heterozygotes therefore have an altered phenotype as the alleles are having a joint effect

When representing alleles, the convention is to use superscripts for the different co-dominant alleles (recessive still lower case)

• An example of co-dominance is blood groups – blood type A (I^A) and blood type B (I^B) can give rise to an AB blood group (I^AI^B)

Codominance can influence a phenotype in two different ways:

• Both traits may be seen equally within the heterozygous phenotype (i.e. forms a mosaic phenotype)
• The traits may mix to create a new outcome within the heterozygote (i.e. forms a blended phenotype)

Expression of both traits is categorised as classical codominance, while blending of traits is called incomplete dominance 

3. Sex Linkage

Sex linkage refers to when a gene controlling a characteristic is located on a sex chromosome (X or Y)

• Because males and females have different sex chromosome combinations, the patterns of inheritance will differ according to sex 
• Sex linked alleles are represented as superscripts attached to the relevant sex chromosome (example: X^A or X^a)

Y-Linked Traits:

• Only males possess a Y chromosome and so Y-linked traits are only ever present in men (not women)
• As males only have one Y chromosome, the trait is not dominant or recessive (it is always expressed)
• A Y-linked trait will always be inherited by the sons and will never be inherited by the daughters

X-Linked Dominant Traits:

• As females possess two X chromosomes, an X-linked dominant trait will be more common in women
• An affected father always has affected daughters (daughters must inherit the father’s X chromosome)
• Sons only inherit the condition if the mother is affected (sons inherit their X chromosome from mother)

X-Linked Recessive Traits:

• As males only possess one X chromosome, an X-linked recessive trait will be more common in men
• This is because males cannot be carriers for X-linked recessive traits (they cannot be heterozygous)
• Affected mothers always have affected sons, while unaffected fathers cannot have affected daughters