6.2.5 sex linkage

Question 1

define sex-linked

Answer 1

gene present on (one of) sex chromosomes

Question 2

describe sex-linkage in humans

Answer 2

• sex determined by 1 of 23 pairs of chromosomes (sex chromosomes)
• other 22 pairs = autosomes (each autosomal pair fully homologous - match length/contain same genes at same loci)
• XY in males
• XX in females
• A & Y chromosomes aren’t fully homologous (small part of 1 matches small part of other = match up before meiosis)

Question 4

what happens if female has abnormal allele on 1 of X chromosomes

Answer 4

likely has functioning allele of same genes on other X chromosome

Question 5

what happens if a male inherits an X chromosome with abnormal allele for particular gene from his mother

Answer 5

suffer genetic disease as doesn’t have another functioning allele for the gene

Question 6

what are males for X-linked genes

Answer 6

• functionally haploid (hemizygous)
• cannot be heterozygous/homozygous for X-linked genes

Question 7

examples of sex-linked characteristics in humans

Answer 7

• haemophilia A
• colour blindness

Question 8

describe haemophilia A as example of sex-linked characterstics in humans

Answer 8

• unable to clot blood fast enough (injuries may cause bleeding/internal haemorrhage)
• 1 of genes on non-homologous region of X chromosome codes for blood-clotting proteins
• mutated form of allele codes for non-functioning protein
• female with 1 abnormal allele & 1 functioning allele can produce enough protein to allow blood to clot normally if needed = carrier for disease
• female can pass X chromosome to son containing faulty gene = will not have functioning allele for protein on Y chromosome
• son is now sufferer from haemophilia A

Question 9

what is the inheritance pattern for haemophilia A when father doesnt have condition but mother is carrier

Answer 9

1. parent phenotypes/genotypes: XHXh (mother = carrier) x XHY (male)
2. gamete genotypes: XH Xh x XH Y
3. offspring genotypes: XHXH (normal female), XHY (normal male), XhXH (carrier female), XhY (male with haemophilia)

Question 10

describe colour blindness as example of sex-linked characteristic in humans

Answer 10

• 1 of genes involved in coding for protein involved in colour vision on X chromosome
• mutated allele of gene may cause colour blindness (red/green)
• female with 1 abnormal allele & 1 functioning allele will not suffer
• male with abnormal allele on X chromosome doesn’t have functioning allele on Y chromosome so will suffer
• inheritance pattern same as haemophilia A

Question 11

describe sex-linkage in cats

Answer 11

• 1 of genes (C) for coat colour is sex-linked on non-homologous region of X chromosome
• allele Co = orange (ginger) fur
• allele Cb = black fur
• alleles are codominant = cats with genotype Xco Xcb are tortoiseshell
• orange/black alleles contribute to phenotype, but orange allele only expressed in cells where chromosome bearing black coat colours is inactivated (& vice versa)
• male cats black or ginger (1 X chromosome)

Question 12

inactivation of X chromosomes in female mammals

Answer 12

• in every female nucleus, 1 X chromosome inactivated
• determination of which member of pair of X chromosomes becomes inactivated is random & happens during early embryonic development