Sex Linkage And The Chromosome Theory Flashcards
Proof for chromosome theory
Thomas Hunt Morgan who made reciprocal crosses in Drosophila melanogaster.
Recessive mutant turned up in one stock: changed eye colour from red to white.
Reciprocal crosses gave different result, depending on whether the male of the female parent carried the mutant allele (mated F1 flies together to get F2)
Cross F1 F2
1) red female, All red All females red,
White male 1/2 males Red, 1/2 white
2) red male x All females Red 1/2 each for
White female All males white Male and fem
He knew that male and female Drosophilia different in one chromosome pair, the sex chromosomes
Homogametic sex
Two large X chromosomes
Heterogametic sex (males)
A large X and a much smaller Y
Hypothesis- the locus for eye colour is carried on the X
The pattern of inheritance of the white allele follows that of the X chromosome and strong evidence suggests that they are actually on those chromosomes
Definite proof for sex linkage
In some crosses, above pattern did not apply (check course booklet). Instead, the opposite results:
white females mated with red males gave all daughters WHITE, all sons RED! The stock’s
chromosomes were abnormal. Female parents are ANEUPLOIDS – because of an error during cell
division they produce eggs which are XX or have no sex chromosomes at all. (Check course booklet)
a chromosomal abnormality has led to a matching genetic abnormality. (Note – sex-
determination mechanism is different in Drosophila and humans: X0 and XXX humans are viable and
female, XXY are viable and male).
Sex linkage inheritance
Drosophilia white eye - first example of sex linkage - the gene is carried on X chromosome
In general X linked recessive alleles are carried through females but only express themselves mainly in males
Sex linkage diseases
Haemophilia, factor VIII deficiency, blood cannot clot properly
It was carried by Queen Victoria and passed onto the Royal Families of Europe
Note for exam - this is not an example of inbreeding. The mutation was most likely a new mutation which first occurred in the gametes of Victoria’s father, Prince Edward
Another example is Lesch-Nyhan syndrome - fatal error in purine metabolism to self mutilation and death. Colour blindness is due to a fault in one of the ops in proteins which are expressed in our retina
X- chromosome inactivation
Problem: females have 2 X chromosomes which means that females produce twice as much proteins as males
X-chromosome inactivation (XCI), equilises he dose of X linked alleles in males and females
In animals, the females inactivate one X in each cell randomly, during early development. The inactivated X chromosome can be visualised under a microscope
Example of X chromosome inactivation (cats)
Tortoiseshell cats - patches of black and orange fur: in females half the cells have the orange gene switched on and the other half have the black gene switched on
Example of X chromosome inactivation (humans)
Humans - females who are heterozygous for red-green colour blindness have patches of normal and colour-blind cells in retina.
Duchenne muscular dystrophy: caused by a mutation in the dystrophin coding gene which causes muscles to break down. Example of pleiotropy.
A remarkable case of heterozygous female twins, one with and one without the disease; had the normal X-chromosome inactivated very early on in development.
