Sex Determination and Sex Linked Traits
Humans have ____ pairs of chromosomes and _____ pair of sex chromosomes. Sex chromosomes
determine whether we are male or female. Females possess two X chromosomes while males
possess one ______ and one _______ chromosome.
Female: __________________
Male: ________________________
Some chromosomes act like homologous chromosomes during meiosis. Gametes receive only one
chromosome out of each homologous pair so during meiosis, gametes receive only one of the sex
chromosomes.
Female gamete = 100% chance of X
Male gamete: 50% chance X, 50% chance Y
The father determines the sex of the child by passing on either his X or his Y chromosome
The X chromosome is larger than the Y chromosome - contains more genes. Since the X and Y
chromosomes are not homologous chromosomes, they do not contain pairs of alleles for all the same
genes. The Y chromosome is unable to match some of the genes that are present on the X
chromosome. If a gene is found on the X chromosome but not on the Y chromosome it is a sex-linked
trait.
Sex linked reccessive traits are more common in males than females. Since females have two X
chromosomes, to express the X-linked recessive trait they need it on both chromosomes. This is very
uncommon.
In males, there is no gene on the Y chromosome, which can mask the X-linked recessive alleles,
therefore if a male receives a recessive allele on his X chromosome, it will be expressed.
Examples:
Red-green colour blindness (b) is a recessive sex-linked trait.
A male with normal vision marries a female with normal vision who carries the colour blindness allele.
Is there a chance that their children will be colourblind?
Parents: _________________________ (female)
x
_________________________
(male)
Eye colour of fruit flies is a sex linked trait. Red is dominant to white.
Show the cross of a white-eyed male and a red-eyed female
Parents: _____________________ (female)
x
_________________________ (male)