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Sex Linkage

Sex Linkage . By: Taylor and Sav. Sex Linkage. 4.3.5 Sex Determination : Sex chromosomes X and Y. In humans, the chromosomes for females are homologous (XX) and non-homologous in males (XY) Shows that there will always be a 1:1 ratio of males to females.

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Sex Linkage

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  1. Sex Linkage By: Taylor and Sav

  2. Sex Linkage • 4.3.5 • Sex Determination: • Sex chromosomes X and Y. In humans, the chromosomes for females are homologous (XX) and non-homologous in males (XY) • Shows that there will always be a 1:1 ratio of males to females. • Sex is therefore solely determined by sperm because it carries the Y chromosomes as well.

  3. How sex chromosomes control gender • Genes present on X and Y chromosomes: • The Y chromosome is much shorter than the X chromosome and contains only a few genes, Includes the SRY sex-determination gene • The X chromosome is much longer and contains several genes not present on the Y chromosome. Includes the genes for haemophilia and red-green colour blindness

  4. Genes present on X- chromosome • Sex Linkage: • Sex linkage refers to when a gene controlling a characteristic is found on a sex chromosome • Sex-linked conditions are usually X-linked, as very few genes exist on the shorter Y chromosome

  5. Sex-linked Conditions • COLOUR BLINDNESS AND HAEMOPHILLIA • The gene loci for these conditions are found on the non-homologous region of the X chromosome (they are not present of the Y chromosome) • X-linked recessive conditions • They have a higher frequency of being recessive and expressing the trait • Males will always inherit an X-linked recessive condition from their mother • Females will only inherit an X-linked recessive condition if they receive a recessive allele from both parents • When assigning alleles for sex-linked traits the convention is to write the allele as a superscript to the sex chomosome (usually X) • Haemophilia:  XH = unaffected ; Xh = affected • Colour Blindness:  XA = unaffected ; Xa = affected

  6. Females and Sex-linkage • As human females have two X chromosomes (and therefore two alleles for any given X-linked gene), they can be either homozygous or heterozygous • Males only have one X chromosome (and therefore only one allele) and are hemizygous

  7. Heterozygous Females • CARRIER: An individual with a recessive allele for a disease condition that is masked by a normal dominant allele. • Carriers are heterozygous and can potentially pass the trait on to the next generation, but do not suffer from the defective condition themselves • Females can be carriers for X-linked recessive conditions because they have two X chromosomes. • Because a male only inherits an X chromosome from his mother, his chances of inheriting the disease condition from a carrier mother is greater.

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