Linkage
- Created by: beth-marie2511
- Created on: 21-06-16 13:31
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- Linkage
- Alleles of different genes cannot assort independently if they are on the same chromosome
- During meiosis linked genes are inherited together because they pass into the gamete and offspring together
- During meiosis at least one chiasma forms between two homologous chromosomes but it may not form between a particular pair of genes
- The further apart the genes are the more likely it is that crossing over will result in the formations of recombinants
- Sex-Linked Inheritance
- Genes that are located on one or the other of the sex chromosomes are 'sex-linked'
- These genes determine body characters and have nothing to do with sex
- Sex-linked genes may be on the X chromosome (X-linkage) or on the Y chromosome (Y-linkage)
- X chromosomes carry many sex-linked genes whereas Y chromosomes carry very few
- In humans features of Y chromosomes will only arise in males but features of X chromosomes arise in either sex
- X-Linkage
- Most sex-linked genes are located on the X chromosome
- X chromosomes carry genes for characteristics such as the ability to see certain colours and to clot blood properly
- Recessive alleles express themselves more often in the male phenotype
- They are unpaired so there is no dominant allele present (Y chromosome)
- Females have 2 X chromosomes and therefore 2 alleles for the gene
- Females with a recessive allele on only one X chromosome won't be affected because of the dominant allele on the other X chromosome
- 'Carriers'
- Females with a recessive allele on only one X chromosome won't be affected because of the dominant allele on the other X chromosome
- Males cannot pass on X-linked characteristics to their sons because they must receive the Y chromosome to become male
- However all his daughters must receive the recessive gene from him
- Haemophilia
- Sex-linked trait in humans
- Blood doesn't clot normally
- Causes intensive internal and external bleeding
- Haemophilia A is a sex-linked characteristic caused by a recessive allele carried on the X chromosome
- If a male inherits one allele for haemophilia A he has the disease because he doesn't have another dominant allele to mask it
- A female only inherits the disease with 2 recessive alleles
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