Sex Linkage Flashcards
How is the genetic information for biological sex stored?
the genetic information for biological sex is is carried on two chromosomes.
What are the female and male sex chromosomes found in mammals?
Females: two X chromosomes
males: One X, One Y
What is meant by sex linkage?
where an allele is located on one of the sex chromosomes, meaning it’s expression is dependent on the sex of the individual.
What are X- linked genes?
Genes found on the X chromosome of the sex chromosomes.
What do you need to remember about Y chromosomes?
It is smaller than the X chromosome and carries fewer genes.
So most genes on the sex chromosomes are only found on the X chromosome.
Why are males more likely to express a recessive sex-linked allele?
Most sex-linked alleles are located on the X chromosome. Therefore males only get one copy of the allele, so will express this characteristic even if it’s recessive. Since females get two alleles, this is less likely.
What are X- linked disorders? Give some examples.
Where the faulty allele for the disorder is found on the X chromosome.
e.g. colour blindness, haemophilia.
How are sex-linked alleles represented?
Both the chromosome and the allele are represented.
Letter = chromosome
superscript = superscript
Draw a punnet square to show an epistasis cross with parental genotypes of AABB and aabb.
Genotypes BB or Bb allow expression of gene A, while genotype bb masks gene A. With this in mind, what % of the offspring will have gene A masked.
25%
Colour blindness is a sex-linked disorder caused by a faulty allele on the x chromosome.
Show how colour blindness is rarer in women than in men.
Use a genetic diagram where the chromosome is represented as X and the alleles as N and n.
The Y chromosome doesn’t have an allele for colour vision so is just represented by Y.
Females would need two copies of the recessive allele to be colour blind, while males only need one copy.
This means colour blindness is much rarer in women than in men.
In the e.g. above, there’s a 3:1 ratio of offspring without colour blindness : with colour blindness.
As can be seen in this e.g. when a female carrier and a male without colour blindness have children only their male offspring are at risk at being colour blind.
Ratio:
2:1:1 ratio of female offspring without colour blindness : male without : male with.
How will the ratio of affected to unaffected offspring change when a female carrier (X^NX^n) and a male with colour blindness (X^nY) have children.
The predicted ratio will then be 1:1 (offspring with : offspring without).
The ratio will be the same for offspring of each sex.
You only end up with this ratio for a monohybrid F2 cross with a sex-linked characteristic.
Which parent do males inherit sex linked characteristics come from?
Their mother, since the Y chromosome can only come from their father. Therefore if the mother is heterozygous for sex-linked alleles, she is a carrier and may pass the trait on.
Draw a punnet square to show a sex-linked cross with parental genotypes of X^HX^h and X^HY.
Possible phenotypes?
Normal female, carrier female, normal male, affected male.
