Sex Linkage Flashcards
Autosomal vs Sex Chromosomes. What is the difference?
Autosomal Chromosomes (Autosomes)- 1 of the 22 numbered chromosomes responsible for determining somatic characteristics
Sex chromosomes- pair of chromosomes that determine the sex (male or female) of the individual
Male- XY
Female- XX
X chromosome >1000 genes
Y chromosome 307 genes
Sex Chromosomes- How do we tell the difference between X and Y chromosomes?
1^o (Primary) Pseudoautosomal region- top of the chromosome
2^o (Secondary) Pseudoautosomal region- bottom region of the chromosome
The Pseudoautosomal region is homologous for X and Y chromosomes; they behave like an autosome and recombine during meiosis
The non-homologous region allows us to tell the difference between X and Y chromosomes as they are unique for each.
Also, the Y chromosome is physically smaller than the X.
Note: the non-homologous portion of the Y chromosome has an SRY gene (specific to males-for sex development)
What are X-linked and Y-linked traits? How does Inheritance relate?
X-linked traits- genes found on the non-homologous portion of the X chromosome
-sex linked inheritance/ X-linked inheritance
Y-linked traits- genes found on the homologous portion of the Y chromosome
- Holandric gene inheritance/ father-to-son inheritance
What are Inheritance Patterns- Autosomal vs Sex Chromosomes?
Nuclear Gene Inheritance
/ \
Autosomal Inheritance Sex Chromosomal Inheritance
- Independent Assort. - Sex-linked Inheritance (F)
- Linked gene Inheritance - Holandric gene inheritance
(M)
How are Sex-linked (X-linked) Mutations different from Autosomal Mutations?
Sex- linked Mutations show up immediately in hemizygous males as the gene trait only shows up in either the Y or X chromosome
Autosomal Mutations show up only when two mutant alleles come together in a homozygote
(Sex- linked mutations are easier to identify than autosomal mutations)
What does hemizygous/ hemizygous male mean?
An individual who has only one member of a chromosome pair or chromosome segment rather than the usual two.
Hemizygosity is often used to describe X-linked genes in males who have only one X chromosome
What is the Sex-linked (X-linked) Inheritance in Drosophila?
White eye mutation (first shown)
How does Sex-linked inheritance work?
A male needs to inherit one recessive allele to show an X- linked trait (hemizygous for sex-linked genes) while…
A female needs to inherit two recessive alleles (one from each parent) to show an X-linked trait (homozygous for sex-linked genes).
And a female that has only one recessive to be heterozygous for sex-linked genes.
Females are carriers for recessive traits
Do hemizygous males pass down the recessive traits to their sons or grandsons?
No!
Females inherit the sex-linked trait from their fathers as they receive their X chromosomes but the chromosome then gets passed down to the son from the mother, hence females are the carriers for recessive traits.
Males only pass down the Y chromosome and not the X chromosomes to their sons.
What causes Haemophilia in males?
(Human X-linked inheritance)
Haemophilia- inability to produce a factor needed for blood clotting
- Nearly all individuals with haemophilia are male
- Males inherit it from their heterozygous mothers
- If infected males reproduce, they transmit the mutation to their daughters!!!
- These daughters are mostly carriers since they usually have the other wild type allele
- Infected males NEVER transmit the disease to their sons!!!
What causes Red-Green Colour blindness in males?
(Human X-linked inheritance)
Red-Green Colour blindness
- Each cone cell of the human eye contains one of three pigments that can absorb light at a particular wavelength
- One pigment absorbs blue light, another absorbs red light and the third absorbs green light
- Gene for blue pigment is on chromosome 7 (an autosome- so it is not affected by the sex- linked trait)
- Genes for red and green pigments lie close together on the X chromosome
*Again the mother can only be a carrier
What are the differences between Autosomal vs. Sex-Linked Inheritance?
Autosomal Sex-Linked
1. No reciprocal differences 1. Reciprocal differences
may result
2. Mendelian ratios obtained 2. Mendelian ratios not
sometimes obtained
3. Traits tend not to skip a 3. Skip-generation
generation inheritance
4. No crisscross inheritance 4. Crisscross inheritance
5. Autosomal traits are 5. Recessive traits more
equally common in both frequent in males than
male and females females, dominant traits
more frequent in females
What are Reciprocal differences?
When parents are interchanged, the progeny ratios are changes as well (a concept of crossing a pair of parents with the sexes reversed)
eg. If a tall male mates with a short female vs a short male mates with a tall female
What is Crisscross Inheritance?
The X chromosomes does not pass directly from one parent to the offspring of the same sex (a boy does not get his father’s X chromosome)
A male transmits his trait to his grandson through his daughter, while a female transmits her traits to her granddaughter through her son
What is Skip-generation Inheritance?
A male parent x-linked trait does not show in the F1 but appears in the F2
What is Holandric Gene Inheritance?
- Holandric genes show father-to-son inheritance
- Holandric genes are found only in males and hence traits confined to males (found on the Y chromosome)
- eg Gene responsible for hair on ears, porcupine skin, TDF (testes determining factor, hypertrichosis)
