A2 Biology 9700: Dihybrid Cross for Body Colour and Wing Length

In a species of fruit fly, body colour is controlled by gene G, with allele G (grey body, dominant) and allele g (ebony body, recessive). Wing length is controlled by a separate gene, gene W, located on a different chromosome, with allele W (normal wings, dominant) and allele w (vestigial wings, recessive).

A fly that is heterozygous for both genes (genotype GgWw, grey body with normal wings) is crossed with another fly that is also heterozygous for both genes (genotype GgWw).

(a) State the gametes that each parent fly can produce with respect to these two genes. [2] (b) Construct a genetic diagram for this cross and use it to determine the expected phenotypic ratio of the offspring with respect to body colour and wing length. [4] (c) Explain, with reference to meiosis, why the alleles of gene G and the alleles of gene W assort independently of each other in this cross. [2]

Part (a): Gametes produced by each parent

Each parent has genotype GgWw. During meiosis, the G/g alleles and the W/w alleles assort independently of one another (they are on different chromosomes), so each parent produces four types of gamete in equal proportions: GW, Gw, gW and gw.

Part (b): Genetic diagram and phenotypic ratio

Parents: GgWw (grey, normal wings) x GgWw (grey, normal wings)

Gametes from each parent: GW, Gw, gW, gw

	GW	Gw	gW	gw
GW	GGWW	GGWw	GgWW	GgWw
Gw	GGWw	GGww	GgWw	Ggww
gW	GgWW	GgWw	ggWW	ggWw
gw	GgWw	Ggww	ggWw	ggww

Reading off phenotypes (G_ = grey, gg = ebony; W_ = normal wings, ww = vestigial wings):

Genotypes with at least one G and at least one W (G_W_): 9 out of 16, grey body, normal wings.
Genotypes with at least one G but ww (G_ww): 3 out of 16, grey body, vestigial wings.
Genotypes with gg but at least one W (ggW_): 3 out of 16, ebony body, normal wings.
Genotype ggww: 1 out of 16, ebony body, vestigial wings.

So the expected phenotypic ratio is 9 grey/normal : 3 grey/vestigial : 3 ebony/normal : 1 ebony/vestigial.

Part (c): Why the two genes assort independently

Gene G and gene W are carried on different, non-homologous chromosomes. During metaphase I of meiosis, every pair of homologous chromosomes lines up at the equator independently of every other pair, so the chromosome carrying the G/g alleles is distributed to a gamete completely independently of the chromosome carrying the W/w alleles. This is why all four gamete combinations (GW, Gw, gW, gw) are produced in equal numbers, and why the dihybrid cross gives the standard 9:3:3:1 ratio rather than the alleles of the two genes staying grouped together as they were in the parents.

Final answers

(a) Each GgWw parent produces gametes GW, Gw, gW and gw in equal proportions.
(b) The cross GgWw x GgWw gives offspring in the phenotypic ratio 9 grey/normal wings : 3 grey/vestigial wings : 3 ebony/normal wings : 1 ebony/vestigial wings.
(c) Genes G and W are on different chromosomes, so independent assortment at metaphase I distributes them to gametes independently of each other.