17.6 Sex linkage Flashcards
What type of chromosomes are the 23th pair
How do they appear for women vs for men
They are the sex chromosomes
For women, the pair the same is XX
For men, the pair is XY
Where the Y is smaller in size, and shaped differently
What can the gametes be for females and for males
For females, two X chromosomes mean that all the gametes are the same as they are all a single X chromosome
For males, they have two types of gametes: with one x chromosome and one Y chromosome
What is an example of the disease linked to sex
How is it caused
Haemophilia
Where the blood clots very slowly so there may be slow and persistent internal bleeding in joints etc
It is an X linked genetic disorder caused by a defective gene on the x chromosome
What does sex linked mean
What sex is haemophilia linked to
Any gene carried on the X or Y chromosome is sex linked
Haemophilia is most common in males because although it may unlikely occur in females, due to puberty and their periods they will die
Why do sex linked characteristics occur more often in males
The X chromosome is much longer than the Y, so for majority of the length of X chromosome, there is no equivalent homologous Y portion
Those characteristics that are controlled by RECESSIVE ALLELES on this non homologous portion of the X chromosome will occur more frequently in males
This is because there is no homologous portion on the Y Chromosome that might have the dominant allele, in the presence of which the recessive allele wouldn’t express itself
What happens when a gene is linked to the X chromosome
Males will more likely get that gene, as only one allele needs to be present, and even if it is recessive, only one is needed to make the male have the gene because there are no dominant alleles on the Y chromosome
What is the cause of haemophilia
. A recessive allele with an altered sequence of DNA nucleotide bases that therefore codes for a faulty protein which doesn’t function
. This results in the individual being unable to produce a functional protein that is required for blood to clot
What is a way to help haemophiliacs
Genetically modify organisms to produce the functional protein to give it to haemophiliacs so they can live almost normal lives
What is the probability of haemophilia occurring in males if mother is carrier and father is normal
H is allele for production of clotting protein
h is recessive allele for non production of the clotting protein
. The mother must be a carrier for it so it heterozygous X^HX^h and the father must be a normal male so it heterozygous for X^H Y as males don’t have two X chromosomes
Gametes are X^H, X^h, X^H, Y
So the offspring produced are 25% X^HX^H, so normal female
25% normal male so X^H Y
25% carrier female X^HX^h
25% haemophiliac male X^hY
Why are the alleles for clotting and non clotting proteins not shown separately
They aren’t shown separately because they are linked to the X chromosome so are X^H and X^h
There is no equivalent allele on the Y chromosome as it doesn’t carry the gene for producing clotting protein
Why does haemophilia occur in males and not females really
. Males only obtain their Y chromosome from their father, so their X chromosome must come from their mother
. As the defective allele that does not code for the clotting protein is linked to the X chromosome, males always inherit the disease from their mother
If their mother doesn’t suffer from the disease she may be heterozygous for it X^HX^h
So is a carrier because they carry the allele without showing any signs of it in their phenotype
What are the genotypes and phenotypes of a normal female with a haemophiliac male having children
So how are carrier females made
The gametes are X^h and Y for the father, and X^H X^H for the mother
So in a punnet square, the offspring produced are two X^HX^h, and two X^HY,
So 50% of offspring female carriers
and 50% of offspring are normal males
So this is how carrier females are made
Why can’t males pass on haemophilia to their sons but can to their daughters
They pass the Y chromosome to their sons but they pass the X allele onto their daughters so their daughters can become carriers for it
How would a female get haemophilia
If her mum is a carrier so has genotype X^HX^h and her dad has haemophilia so X^hY
This will make a female with X^hX^h who would have it
