Monogenic inheritance part 2

Question 1

How is sex determined in humans?

Answer 1

• genetically determined
• humans have 23 pairs of chromosomes of varying sizes and shapes
  
  • in 22 of the pairs, both members of the pair are the same ]
  • but the 23rd pair, known as the sex chromosomes are different
• females have two X chromosomes, males have an X and a Y chromosome

Question 2

What are the differences between the X and Y chromosome?

Answer 2

• X chromosome is large
  
  • contains many genes not involved in sexual development
• the Y chromosome is very small
  
  • contains almost no genetic information
  • carry a gene that causes the embryo to develop as a male

Question 3

What are sex linked genes?

Answer 3

• characteristics determined by genes carried on the sex chromosomes

Question 4

What is the effect of the difference between the chromosomes on the male genotype?

Answer 4

• as the Y chromosome is much smaller than the X
• there are a number of genes in the X chromosome that males only have one copy of

Question 5

What effect does the difference cause in males?

Answer 5

• any characteristics caused by a recessive allele on the section of the X chromosome, which is missing in the Y chromosome, occurs more frequently in males
• this is because many females will aso have a dominant allele present in their cells

Question 6

What is haemophilia?

Answer 6

• an example of a sex-linked genetic disorder
• patients have blood which clots extremely slowly due to the absence of a protein blood-clotting factor
• as a result injury can result in prolonged bleeding which, if left untreated, is potentially fatal

Question 7

How is haemophilia passed on?

Answer 7

• if a male inherits the recessive allele that codes for haemophilia (on their X chromosome) they cannot have a corresponding dominant allele on their Y chromosme
  
  • and so develop the condition
• as a result, the vast majority of haemophilia sufferers are male
• females who are hetrozygous for the haemophilia coding gene are known as carriers
  
  • they do not suffer the condition but may pass on the allele to their children
  • this can result in the birth of a son who suffers from haempohilia

Question 8

How are the alleles in sex-linkage shown in a genetic cross?

Answer 8

• X used to represent gene
• superscript capital letter to show the dominant allele
• superscript lower case letter to show the recessive allele
• Y used to represent the Y chromosome
  
  • no allele attached to it