It’s usually found in males, which only express one X chromosome (XY). X– linked recessive inheritance is where a mutation passes on a gene on the X chromosome. Morgan deduced that the inherited trait must travel on a sex-determined chromosome. He then crossed the white-eyed (mutant) male with red eye (wild-type) females.Īfter two years he found white-eyed males. He knew this had a mutated eye color gene that he could test in further experiments. His work discovered the model of X-linkage and sex inheritance.įirstly, it took two years for Morgan to find a white-eyed male. In 1908, Thomas Hunt Morgan performed a two-year long experiment on fruit flies. They also don’t explain why some traits show up more in boys than in girls. Simple Mendelian laws, for example, don’t explain why certain traits skip generations. The discovery of the chromosome helped to reveal that genetics and DNA had more and more layers. Duchenne Muscular Dystrophy – Muscular weakness, progressive deterioration of muscle tissue, and loss of coordination.Hemophilia – a condition lacking the enzyme for blood clotting.Red-green colorblindness – Inability differentiate between red and green.What are some examples of sex linked traits? Usually such linked genes are found on the X chromosome. If a gene is found only on the X chromosome and not the Y chromosome, it is said to be a sex-linked trait.Īs the gene that expresses the trait is located on the sex chromosome, sex linkage is linked to the gender of the offspring. These 2 chromosomes are known as the sex chromosomes. One pair of chromosomes (the 23rd pair) determines the gender of the individual. This means haemophilia is much more common in males than in females.In humans, there are 23 pairs or 46 chromosomes.
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Some genetic diseases, are caused by sex linked genes, for example haemophilia.This means females have two alleles for X-linked genes while males only have one.Females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY).These genes are inherited with the X chromosome (from the mother if it is a boy or from either mother or father if it is a girl).Some genes are found on the sex chromosome, X.Image credit: Genome Research Limited What are sex-linked genes? Illustration to show the inheritance of dominant and recessive alleles for eye colour. For example, the allele for blue eyes is recessive, therefore to have blue eyes you need to have two copies of the 'blue eye' allele. Recessive alleles only show their effect if the individual has two copies of the allele (also known as being homozygous).An example of this is the blood group AB which is the result of codominance of the A and B dominant alleles.
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The resulting characteristic is due to both alleles being expressed equally. If both alleles are dominant, it is called codominance.For example, the allele for brown eyes is dominant, therefore you only need one copy of the 'brown eye' allele to have brown eyes (although, with two copies you will still have brown eyes). Dominant alleles show their effect even if the individual only has one copy of the allele (also known as being heterozygous).Alleles can be either dominant or recessive.These different versions of a gene are called alleles. Since human cells carry two copies of each chromosome they have two versions of each gene.