Occurs when alternative alleles are present in the genotype and fully observed in the phenotype
E.g. ABO blood grouping system, where a single gene locus features multiple alleles- IA, IB, and i. Individuals carrying alleles for both A and B express both in the phenotype AB.
Genes are carried on the sex chromosomes (X or Y)
XBXB normal female
XBXb carrier female
XbXb affected female
XBY normal male
XbY affected male
Dominant gene on X chromosome
Affected males pass to all daughters and none of their sons
If the mother has an X- linked dominant trait and is homozygous (XAXA) all children will be affected
If Mother heterozygous (XAXa) 50% chance of each child being affected
E.g. dwarfism, rickets, brown teeth enamel.
Sex linked dominant disorders
Sex linked dominant problems
The barred pattern of chicken feathers is inherited by a pair of sex linked genes, B for barred, b for no bars. If a non-barred female is mated to a barred male,
a) What will the proportion and appearance of the offspring?
What will be the appearance and proportion of the progeny produced by mating an Fl male with an Fl female?
Gene located on the X chromosome
More males than females affected (males inherit X from mother)
Females can only inherit if the father is affected and mother is a carrier (hetero) or affected (homo)
An affected female will pass the trait to all her sons
Daughters will be carriers if father is not affected
Males cannot be carriers (only have 1 X so either affected or not)
Can skip generations
E.g. colour blindness, haemophilia, Duchene muscular dystrophy
Red-green colour blindness in men is caused by the presence of a sex-linked recessive gene c, whose normal allele is C.
a) Can two colour blind parents produce a normal son?
b) Can they produce a normal daughter?
c) Can two normal parents produce a colourblind son or daughter?
d) Can a normal daughter have a colourblind father or mother?
e) Can a colourblind daughter have a normal father or mother?