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630 Chapter 19 NEL

Morgan explained the results of his experiments by concluding that the Y chromo-

some does not carry the gene to determine eye colour. We now know that the gene for

eye colour in Drosophilais located on the part of the X chromosome that does not

match the Y chromosome. Therefore, Morgan’s conclusion was correct. The Y chro-

mosome does not carry an allele for the eye-colour gene. Traits determined by genes

located on sex chromosomes are called sex-linked traits.

The initial problem can now be re-examined. The pure-breeding, red-eyed female can

be indicated by the genotype XRXRand the white-eyed male by the genotype XrY.The

symbol XRindicates that the allele for red eye is dominant and is located on the X chro-

mosome. There is no symbol for eye colour on the Y chromosome because it does not

contain an allele for the trait. A Punnett square, as shown in Figure 4, can be used to

sex-linked traittrait that is
determined by genes located on
the sex chromosomes

Figure 3

Drosophilacontain three pairs of autosomes and a single pair of sex chromosomes.

male

autosomes

female

autosomes

XY

XX

red-eyed

male , XRY

females

(red-eyed)

male

(white-eyed)

male (red-eyed)

Y

Xr

XR

XR

XRXR XRXr

XRY XrY

red-eyed female , XRXR

females

(red-eyed)

males

(red-eyed)

Y

XR

Xr

XR

XRXr XRXr

XRYXRY

red-eyed female , XRXr

white-eyed

male , XrY

F 1 generation F 2 generation

X chromosome paired with a small, hook-shaped Y chromosome. Females have two

paired X chromosomes (Figure 3). Since the X and Y chromosomes are not completely

homologous (although they act as homologous pairs during meiosis), it was concluded

that they contain different genes.

CAREER CONNECTION

Entomologist

Entomologists study the life cycle

of insects and conduct research

into evolution and biodiversity. The

science of entomology has made a

significant contribution to

understanding genetics and gene

mapping. Would you like to work

with fruit flies or arthropods, such

as spiders and mites? Explore this

field of study.

http://www.science.nelson.com GO

Figure 4

Punnett squares showing F 1 and F 2 generations for a cross between a homozygous red-eyed

female and a white-eyed male.