Sex-linked Characteristics Flashcards
What is locus
Position of a gene on a chromosome
When are genes said to be linked and why
Genes with loci on the same chromosome are said to be linked, the closer together the loci of the two genes on a chromosome the more closely they are said to be linked as they’ll stay together during independent assortment and unlikely to be separated during crossing over if close together
When is a characteristic said to be sex-linked
When the locus of the allele that codes for the characteristic is on a sex chromosome
What are the sex chromosomes in female mammals
XX
What are the sex chromosomes in male mammals
XY
Why are most sex linked characteristics X-linked
As the Y chromosome is smaller than the X chromosome and carries fewer genes so most genes on the sex chromosome are only carried on the X chromosome
Why are males more likely to have sex-linked disorders
As males only have one X chromosome they often only have one allele for sex-linked genes meaning they often only have one copy and will express the characteristic of this allele even if it is recessive while females have two copies so both alleles will need to be recessive to express characteristic of this allele. Means males more likely to show recessive phenotypes for genes that are sex- linked.
Examples of sex-linked disorders
Red-green colour blindness and haemophilia
How are chromosomes ordered
From longest to shortest so chromosome 1 likely to contain the most genes and chromosome 22 the fewest
How is red-green colour blindness caused
Happens when there is a mutation in one of the cone pigment genes on the X chromosome while the Y chromosome has no locus for the gene that controls colour blindness
What is an autosomal gene
A gene that’s loci is on a non-sex chromosome so on a numbered chromosome
What is epigenetics
Heritable changes in gene function without changes to the base sequence of DNA
