Download sex-linked genes

SBI 3U
Mrs. Holterman/Ms. Powell
Name: ___________________
SEX-LINKED GENES
The determination of sex is based on the inheritance of a certain combination of
chromosomes. In humans, there are 46 chromosomes (23 pairs). Twenty-two of these
pairs are known as autosomes. These contain genes for many traits, but are not related to
the sex of the individual. The twenty-third pair is known as the sex chromosomes or
allosome. Females have a homologous pair of X chromosomes (genotype XX), while
males have the XY combination.
The X chromosome carries genes for many traits, while the Y chromosome does not
carry these genes. Therefore the phenotype of the females will be determined by the
combination of two alleles, while the phenotype of the males will be determined by the
allele present on their one X chromosome.
Characteristics of sex-linked traits:
1. The sex-linked recessive phenotype will occur more frequently in males because
they cannot hide it a second, dominant allele.
2. Females have two copies of these genes, so if one copy is the recessive allele,
they may still have the dominant phenotype.
3. Heterozygous females are called carriers since they can pass sex-linked traits to
their children.
4. A female must be homozygous recessive to display the recessive phenotype
(rare).
Example:
In humans, a disease called Duchenne-type muscular dystrophy is caused by a sex-linked
recessive allele, while the normal condition results from the dominant allele. If a female
carrier marries a normal male, what are the possible genotypes for their children?
Answer:
SBI 3U
Mrs. Holterman/Ms. Powell
Name: ___________________
Let XN = normal, Xn=Duchenne-type muscular dystrophy,
Parents:
XNXn
x
Y=male
XNY
Punnett Square:
XN
Xn
XN
XNXN
XNXn
Y
XNY
XnY
Answer: 1 in 4 children will probably have DMD. None of the girls will have it, but
half of the boys probably will.