Achondroplastic dwarfism is autosomal dominant condition caused by a gene on chromosome number 4. Color blindness is an X-linked recessive condition. An achondroplastic dwarf male with normal vision mates with a color-blind woman of average height. The man's father was average height. How many of their sons would be color-blind?

Given that colour blindness is X linked recessive condition which means that the gene is located on X chromosome and when present in recessive homozygous condition, it will be expressed. Hence if XC is dominant allele and Xc is recessive allele, XcY and XcXc will be colourblind.

The male has normal vision and males have only one X chromosome so his genotype is XCY.

The female is colourblind and females have two X chromosomes so her genotype is XcXc.

Their children:

XC Y

Xc XCXc XcY

All the sons have XcY genotype where gene for colourblindness is in recessive condition hence all of their sons would be colourblind.