Patterns of Inheritance

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Sexually reproducing animals are typically diploid, inheriting one allele from each parent to possess two alleles for each trait

The allele combination (genotype) may be the same (homozygous) or different (heterozygous)

There are a number of distinct patterns of inheritance which determines how the genotype may be expressed as a phenotype


Autosomal Dominance / Recessiveness

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

  • The dominant allele has the same effect on the phenotype whether it is present in the homozygous or heterozygous state
  • The recessive allele only has an effect on the phenotype when present in the homozygous state
  • When assigning alleles, the convention is to use a capital letter for the dominant allele and the lower case form for the recessive allele


In this pattern of inheritance, the homozygous dominant (e.g. BB) and heterozygous (e.g. Bb) forms will be phenotypically indistinguishable

It may be possible to determine the genotype of a phenotypically dominant individual by performing a test cross

  • A test cross involves testing a suspected heterozygote by crossing it with a known homozygous recessive
  • If the unknown parent is homozygous dominant, all offspring must express the dominant phenotype
  • If the unknown parent is heterozygous, half the offspring should express the recessive phenotype


Incomplete / Partial Dominance

Incomplete dominance is a form of intermediate inheritance, whereby neither of the two alleles are wholly dominant

As such, heterozygous individuals exhibit a blending of the two traits to produce an intermediate phenotype

  • An example in nature is the colouration of the snapdragon (flower) - homozygotes may be red or white, heterozygotes are pink
  • In humans, male voice pitch is typically high or low in homozygotes, with an intermediate range of pitches in heterozygotes


Codominance

Codominance occurs when pairs of alleles are both expressed equally in the phenotype of a heterozygous individual 

When assigning alleles for codominance, the convention is to use a common letter to represent dominant and recessive and use superscripts to represent the different codominant alleles 

An example of codominance is the ABO blood grouping in humans

  • I stands for immunoglobulin (antigenic protein on blood cells)
  • A and B stand for the codominant variants


The ABO gene has three alleles: IA, IB and i

  • IA and IB are codominant, wherease i is recessive (no antigenic protein is produced)
  • Codominance means that both IA and IB alleles will be expressed within a given phenotype


The genotypes and phenotypes of the ABO blood groups are:


The ABO Blood Group System


Multiple Alleles

Some genes have more than two alleles for a single given trait (e.g. the ABO blood group system)

The alleles which are not recessive may either: 

  • Share codominance (be expressed equally in the phenotype) 
  • Share incomplete dominance (neither is fully expressed in the phenotype, resulting in blending) 
  • Demonstrate a dominance order (e.g. allele A > allele B > allele C)


Sex Determination

Humans have 23 pairs of chromosomes for a total of 46 (excluding instances of aneuploidy)

The first 22 pairs are autosomes - each chromosome pair possesses the same genes and structural features

The 23rd pair of chromosomes are heterosomes (or sex chromosomes) and determine gender

  • Females are XX - they possess two X chromosomes
  • Males are XY - they posses one X chromosome and a much shorter Y chromosome


The Y chromosome contains the genes for developing male sex characteristic - hence the father is always responsible for determining gender

  • If the male sperm contains the X chromosome the growing embryo will develop into a girl
  • If the male sperm contains a Y chromosome the growing embryo will develop into a boy
  • In all cases the female egg will contain an X chromosome (as the mother is XX)


The Y chromosome is much shorter than the X chromosome and contains only a few genes 

  • Includes the SRY sex-determination gene and a few others (e.g. hairy ears gene)


The X chromosome is much longer and contains several genes not present on the Y chromosome

  • Includes the genes for haemophilia and red-green colour blindness


In human females, only one of the X chromosomes remains active throughout life 

  • The other is packaged as heterochromatin to form a condensed Barr body
  • This inactivation is random and individual to each cell, so heterozygous women will be a mosaic - expressing both    alleles via different cells


Sex Linkage

Sex linkage refers to when a gene controlling a characteristic is found on a sex chromosome (so the trait is associated with a predominant gender)

  • Sex-linked conditions are usually X-linked, as very few genes exist on the shorter Y chromosome
  • As human females have two X chromosomes (and therefore two alleles for any X-linked gene), they can be either homozygous or heterozygous
  • Males only have one X chromosome (and therefore only one allele) and are hemizygous


Consequently, X-linked dominant traits are more common in females (as either allele may be dominant for the trait)

X-linked recessive traits are more common in males, as females may be carriers for the recessive trait

  • A carrier is an individual who is heterozygous for a recessive disease condition
  • As males only have one allele for this gene they cannot be a carrier for the condition, meaning they have a higher frequency of being recessive 
  • Males will always inherit an X-linked recessive condition from their mother
  • Females will only inherit an X-linked recessive condition if they receive a recessive allele from both parents
  • Examples of X-linked recessive conditions include haemophilia and red-green colour blindness


When assigning alleles for sex-linked traits the convention is to write the allele as a superscript to the sex chomosome (usually X)

  • Haemophilia:  XH = unaffected ; Xh = affected
  • Colour Blindness:  XA = unaffected ; Xa = affected


Male and Female Genotypes for a Sex-Linked Condition