Principles of Inheritance and Variation

An affected mother has three copies of chromosome 21 (trisomy 21).
During meiosis, the three chromosomes 21 are distributed so that half of her eggs receive two copies of chromosome 21 and half receive one copy.
The normal father gives one chromosome 21 in every sperm.
Egg with two copies + sperm with one = trisomy 21 (Down's syndrome); egg with one copy + sperm with one = normal.
So about 50% of the offspring would be affected.

H. J. Muller showed in 1927, working with Drosophila, that X-rays induce mutations, including sex-linked recessive lethal mutations, which he detected with his ClB method.
For this discovery of the production of mutations by X-ray irradiation, Muller received the Nobel Prize in Physiology or Medicine in 1946.
Cytoplasmic inheritance (Correns) and recombination of linked genes as such were not awarded Nobel Prizes; genetic engineering is a technology rather than a single discovery.
So the discovery that X-rays induce sex-linked recessive lethal mutations resulted in a Nobel Prize.

Cytoplasmic male sterility (CMS) is the inability of a plant to produce functional pollen, inherited through the female parent (maternally).
It is caused by changes (rearrangements forming new open reading frames) in the mitochondrial genome, which affect the energy supply needed for pollen development.
Nuclear restorer (Rf) genes can restore fertility.
CMS is widely used in producing hybrid seeds (e.g. maize, rice, sorghum), as it removes the need for manual emasculation.
So genes for cytoplasmic male sterility are generally located in the mitochondrial genome.

Sex-limited traits are controlled by autosomal genes, but they are expressed in only one sex, because their expression depends on the sex hormones.
The genes for beard and moustache growth are present in both men and women, but they are expressed only in males under the influence of testosterone.
Pattern baldness is also autosomal and expressed mainly in males because of the male hormones (strictly it is described as sex-influenced, since it is dominant in males and recessive in females).
Sex-linked traits, such as haemophilia and colour blindness, are due to genes on the X chromosome.
So these are examples of sex limited traits.

In Drosophila, sex is determined by the genic balance mechanism proposed by C. B. Bridges.
Sex depends on the ratio of the number of X chromosomes (X) to the number of sets of autosomes (A), i.e. the X/A ratio. The Y chromosome does not decide sex (it is needed only for male fertility).
X/A = 1 (e.g. XX + 2A): female. X/A = 0.5 (e.g. XY + 2A): male. X/A between 0.5 and 1: intersex. X/A greater than 1: metafemale; less than 0.5: metamale.
Determination by fertilised or unfertilised egg (haplodiploidy) occurs in honeybees.
So in Drosophila, sex is determined by the ratio of the number of X chromosomes to the sets of autosomes.