17.6 - Sex-linkage Flashcards
How many pairs of chromosomes do humans have, and what distinguishes the 23rd pair?
- Humans have 23 pairs of chromosomes.
- 22 pairs have homologous partners identical in appearance for both males and females.
- The 23rd pair are the sex chromosomes.
- In females, these are two X chromosomes, while males have one X and one smaller, differently shaped Y chromosome.
What are the differences between male and female sex chromosomes?
- Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY).
- The X chromosome in males is similar to that in females, but the Y chromosome is smaller and shaped differently.
How do the sex chromosomes affect gamete formation in males and females?
- Females produce gametes (eggs) that all carry a single X chromosome.
- Males produce two types of gametes (sperm): half contain an X chromosome and the other half contain a Y chromosome.
What are sex-linked genes, and why do they affect males and females differently?
- Sex-linked genes are carried on either the X or Y chromosome.
- The X chromosome is much longer than the Y chromosome, meaning there is no equivalent homologous portion on the Y chromosome for most of the X chromosome’s length.
- Characteristics controlled by recessive alleles on the X chromosome will appear more frequently in males because they lack a corresponding dominant allele on the Y chromosome to mask the effect.
What is an X-linked genetic disorder, and can you provide an example?
- An X-linked genetic disorder is caused by a defective gene on the X chromosome.
- An example is haemophilia, where blood clots slowly, leading to persistent internal bleeding.
- This condition is mostly confined to males.
What causes haemophilia, and how is it inherited?
- Haemophilia is caused by a recessive allele with an altered sequence of DNA nucleotide bases, resulting in a faulty protein necessary for blood clotting.
- mother’s that are carriers are heterozygous (X^HX^h)
- Males inherit haemophilia from their mothers because the defective allele is linked to the X chromosome.
- Females can be carriers if they inherit one normal allele (X^H) and one defective allele (X^h), but usually do not express the condition.
Why is haemophilia rare in females?
- Haemophilia is rare in females because they need to inherit two defective alleles (X^hX^h) to express the disease, and females with haemophilia historically died at puberty due to menstruation complications.
- Most females are carriers with one normal and one defective allele (X^HX^h), so they do not show symptoms.
What does it mean for a female to be a carrier of haemophilia, and how does this affect her offspring?
- A carrier female has one normal allele (X^H) and one haemophilia allele (X^h).
- She does not show symptoms because the dominant X^H allele produces enough clotting protein.
- However, she can pass the defective allele to her sons (who may develop haemophilia) or her daughters (who may become carriers).
How do males inherit haemophilia?
- Males inherit haemophilia through their X chromosome, which comes from their mother.
- Since males only have one X chromosome, if they inherit the X^h allele from their mother, they will express the condition as there is no corresponding allele on the Y chromosome to mask it.
Can a father pass haemophilia to his son?
- No, a father cannot pass haemophilia to his son because males pass their Y chromosome to their sons, and haemophilia is carried on the X chromosome.
- However, a father can pass the haemophilia allele to his daughters, making them carriers.
How is the inheritance of sex-linked traits like haemophilia tracked in pedigree charts?
- In pedigree charts, a male is represented by a square, and a female by a circle.
- Shading within the square or circle indicates the presence of the trait in the phenotype.
- These charts are useful for tracking the inheritance of sex-linked traits like haemophilia.
How has modern biotechnology improved the treatment of haemophilia?
- The production of functional clotting protein by genetically modified organisms has allowed haemophiliacs to receive the necessary protein, enabling them to lead near-normal lives despite their condition.
Red-green colour blindness is linked to the X chromosome. The allele (r) for red-green colour blindness is recessive to the normal allele (R). Figure 5 shows the inheritance of this characteristic in a family.
State what sex chromosomes are present in individuals labelled E and F?
E=XX F=XY
Red-green colour blindness is linked to the X chromosome. The allele (r) for red-green colour blindness is recessive to the normal allele (R). Figure 5 shows the inheritance of this characteristic in a family.
Interms of colour blindness, identify the phenotypes of each of the individuals labelled A, B and D.
A = not colour blind/normal vision B = not colour blind/normal vision D = colour blind
Red-green colour blindness is linked to the X chromosome. The allele (r) for red-green colour blindness is recessive to the normal allele (R). Figure 5 shows the inheritance of this characteristic in a family.
Interms of colour blindness, identify the genotypes of each of the individuals labelled G, H, | and J.
- G = X^R X^r
- H = X^R Y
- I = X^R X^R
- J = X^r Y
Red-green colour blindness is linked to the X chromosome. The allele (r) for red-green colour blindness is recessive to the normal allele (R). Figure 5 shows the inheritance of this characteristic in a family.
If individual C was to have children with a normal female (one who does not have any r alleles), determine the probability of any sons having colour blindness.
- 0%
- because sons inherit their X chromosome from their mother and she has only alleles for normal vision (X^R)
Red-green colour blindness is linked to the X chromosome. The allele (r) for red-green colour blindness is recessive to the normal allele (R). Figure 5 shows the inheritance of this characteristic in a family.
Individual J is colour blind. Assuming no history of colour blindness in either parent’s family tree, suggest how this might have occurred.
By mutation (of the R allele)
