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OCR A Biology

X Allele Sex Sex Linked Chromo Chromosomes Females Males

Human sex is determined by sex chromosomes (XX for females, XY for males), and sex-linked traits are carried on these chromosomes. Males are more likely to express recessive X-linked traits, such as hemophilia, because they only have one X chromosome.

Humans have 23 pairs of chromosomes, with the 23rd pair (sex chromosomes) determining sex: XX for females and XY for males. Sex-linked characteristics are determined by genes on sex chromosomes; since the Y chromosome is smaller, males have only one X-chromosome allele, making them more susceptible to recessive X-linked traits like hemophilia, which causes slow blood clotting. Females can be carriers if they are heterozygous for the recessive allele.

Front Determining sex - chromosomes, sex-linked chromosomes, and exaple of sex-linked genetic dissorder
Back
  • humans have 23 pairs of chromosomes, 22 pairs have both members of same size, but 23rd pair (sex chromos) are different, females have XX and males have XY (X is larger)
  • some characteristics sex linked (determined by genes carried on sex chromos), Y smaller so theres a number of genes on X chromo that males only have one copy of, so recessive alleles on X chromo (not present on Y chromo) more likley to present in males as females may have the domiant allele
  • Haemophilia
    • sex-linked genetic dissorder results in slow blood clotting due to absense of protein blood-clotting factor so upon injury, leads to prolonged bleeding
    • recessive allele and only present on X chromo, so if male inherits recessive allele on X chromo, they will have the disease, but heterozygous females are carriers (dont have it but can pass onto offspring)
    • sex-linked alleles shown on sex chromo its found on e.g X^H as found on X chromo and H is the dominant healthy allele and here X^h is faulty allele

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