sex linked genes
Genes on the X chromosome
Absent on smaller Y chromosome
o If gene on x chromosome in MALE has RECESSIVE form of allele--->man will
express RECESSIVE CONDITION
o RECESSIVE CONDITIONS--->colour blindness & haemophilia
HAEMOPHILIA:
− Sex-linked hereditary disease
− Recessive allele on X-chromosome (Xh)
− Occurs in males
− Females are carriers (XHXh if heterozygous)
− Faulty gene cannot produce BLOOD CLOTTING FACTOR (VIII)
− Person lacks ability to stop bleeding--->take a long time for blood to clot after
injury
COLOR BLINDNESS:
− Sex-linked condition
− Occurs in males
− Females are carriers
− Recessive allele for colour-blindness carried on X chromosome (Xc)
− NORMALLY--->gene on X chromosome enables you to distinguish between red
& green
− MUTATION OF GENE--->results in inability to distinguish between red & green
− People see shades of grey & black instead of red & green
ALBINISM:
− Condition inherited in RECESSIVE form
− GENE MUTATION--->leads to lack of enzyme that synthesises pigment,
MELANIN
− RESULTS in lack of pigmentation in skin, hair and eyes
− RESULTS in poor eyesight & extreme sensitivity to sun
SICKLE CELL ANEAMIA:
− Inherited in RECESSIVE FORM
− GENE MUTATION--->affects normal HAEMOGLOBIN
− RBCs become SICKLE SHAPED (HbS HbS)--->reduces their ability to carry O2---
>cells clump together and block capillaries
− SICKLE CELL TRAIT: HbA HbS --->shows CO-DOMINANCE--->Some normal
RBCs and some sickle shaped cells
− SICKLE CELL ANEAMIA: HbS HbS --->DOUBLE RECESSIVE
Good to be heterozygous in malaria area:
- malaria parasite reproduces in RBCs
- in person with sickle cell trait, when parasite enters RBC it spontaneously becomes sickle-
shaped and parasite cannot reproduce - breaking life cycle
Genes on the X chromosome
Absent on smaller Y chromosome
o If gene on x chromosome in MALE has RECESSIVE form of allele--->man will
express RECESSIVE CONDITION
o RECESSIVE CONDITIONS--->colour blindness & haemophilia
HAEMOPHILIA:
− Sex-linked hereditary disease
− Recessive allele on X-chromosome (Xh)
− Occurs in males
− Females are carriers (XHXh if heterozygous)
− Faulty gene cannot produce BLOOD CLOTTING FACTOR (VIII)
− Person lacks ability to stop bleeding--->take a long time for blood to clot after
injury
COLOR BLINDNESS:
− Sex-linked condition
− Occurs in males
− Females are carriers
− Recessive allele for colour-blindness carried on X chromosome (Xc)
− NORMALLY--->gene on X chromosome enables you to distinguish between red
& green
− MUTATION OF GENE--->results in inability to distinguish between red & green
− People see shades of grey & black instead of red & green
ALBINISM:
− Condition inherited in RECESSIVE form
− GENE MUTATION--->leads to lack of enzyme that synthesises pigment,
MELANIN
− RESULTS in lack of pigmentation in skin, hair and eyes
− RESULTS in poor eyesight & extreme sensitivity to sun
SICKLE CELL ANEAMIA:
− Inherited in RECESSIVE FORM
− GENE MUTATION--->affects normal HAEMOGLOBIN
− RBCs become SICKLE SHAPED (HbS HbS)--->reduces their ability to carry O2---
>cells clump together and block capillaries
− SICKLE CELL TRAIT: HbA HbS --->shows CO-DOMINANCE--->Some normal
RBCs and some sickle shaped cells
− SICKLE CELL ANEAMIA: HbS HbS --->DOUBLE RECESSIVE
Good to be heterozygous in malaria area:
- malaria parasite reproduces in RBCs
- in person with sickle cell trait, when parasite enters RBC it spontaneously becomes sickle-
shaped and parasite cannot reproduce - breaking life cycle
