Lecture 9 Flashcards
Lilian Hunt Morgan
found a gene in Thomas’s fly experiment linked to sex
Sex linkage and ratios
ratios will often be different to that expected if the gene is on one of the sex chromosomes
Sex linkage
genes are on a sex chromosome
usually X meaning women will have two genes and men will only have one
this makes men more likely to be recessive and means dominant allele fathers cannot have a recessive daughter
Hemizygous exposure
when men have a recessive allele on their X chromosome it will be showed despite only having one
Carriers of sex linked genes
can only be females
males cannot carry a recessive allele without it presenting in the phenotype
examples of x linked disorders
haemophilia
Duchenne muscular dystrophy
colour-blindness
Haemophilia
type A
- lack of clotting factor VIII
type B
-lack clotting factor IX
injury- leads to incomplete blood clotting and continuous bleeding consequences, easy bruising
treatment- clotting factors,
Sex chromosome systems
vary for many different species
some have diploid females and haploid males
some have one sex chromosomes (either X or XX)
X- chromosomes
-large, many genes
-genes for many biological functions
-genes on X are X-linked
Y chromosomes
small and has few genes
most genes relate to male fertility
genes on Y are Y-linked
How are two X chromosomes tolerated
one X chromosomes is randomly inactivated in every cell
X-inactivation
-gene dosage compensation mechanism
-only in mammals
-early embryo cells randomly inactivate one X chromosome
-inactivation is passed onto daughter cells during mitosis
- organism is a mosaic with patches of cells with either X inactivated
Why are sex chromosome aneuploidies tolerated
as only one X chromosome is necessary for life- loss of an X is survivable
gain of X is also survivable
Y is not essential for life
and excess Y only affects a small number of genes so all are survivable
