Sex-Linked Genetic Disorders

Question 1

how do you know if a characteristic is sex-linked

Answer 1

if the allele that codes for it is located on a sex chromosome (x or y)

Question 2

why does the y chromosome carry fewer genes than the x chromosome

Answer 2

• because its smaller

- so most genes on sex chromosomes are only carried on the x chromosome

Question 3

what is the result of men only carrying one x chromosome in terms of alleles

Answer 3

they often only carry one allele for sex linked genes

Question 4

what is the effect of men only having one allele due to having one x chromosome compared to women

Answer 4

• for men the characteristic of that one allele is shown even if its recessive
• makes it more likely than women to show recessive characteristics for genes that are sex linked

Question 5

what are sex linked disorders

Answer 5

disorders caused by faulty alleles located on sex chromosomes

Question 6

what is colour blindness caused by

Answer 6

what is colour blindness caused by

Question 7

if colour blindness is represented as X^n where n is the faulty allele, how do the chances of men inheriting colour blindness compare to women

Answer 7

• women need 2 copies of recessive allele to be colour blind
• because they have 2 x chromosomes, so if one is dominant it will be the chosen characteristic
• however, men only need one copy
• this is because men only have 1 x chromosome, so there is no dominant allele on the x chromosome to compensate for the faulty one
• this means that colour blindness is much rarer in women than men

Question 8

what is a woman with only one copy of the recessive allele that is responsible for colour blindness, if its one the x chromosome

Answer 8

a carrier

Question 9

what does being a carrier mean in terms of colour blindness

Answer 9

they arent colour blind themselves but they can pass the allele on to their off spring

Question 10

what would the genotype of a female carrier of colour blindness where the faulty allele, n, is located on the x chromosome (N indicates a healthy allele)

Answer 10

X^n X^N

Question 11

what would be the genotype of a man that isnt affected by colour blindness, where the faulty allele is n and the healthy allele is N and the faulty allele is only carried on the x chromosome

Answer 11

X^N Y

Question 12

what would be the four offspring genotypes and phenotypes if an unaffected female carrier and an unaffected male were to breed

Answer 12

• male = XN Y
• female = Xn XN
  off spring:
• XN Xn = unaffected female carrier
• XN XN = unaffected female
• Xn Y = affected male (colour blind)
• XN Y = unaffected male

Question 13

what would the ratio be of unaffected to colour blind if the unaffected female carrier and unaffected male bread

Answer 13

ratio of unaffected to colour blind off spring is 3:1

Question 14

in the example with the male and female who bred to make 4 off spring, what would the ratio of 2:1:1 be representing

Answer 14

unaffected:carrier:colour blind = 2:1:1

Question 15

unaffected:carrier:colour blind = 2:1:1

Answer 15

• 1 in 4 or 25% chance of child being colour blind

- chances rise to 1 in 2 or 50% if you know its a boy