3.4

Define the term sex linkage

when gene that controls a characteristic is carried on one of the sex chromosomes (usually the X-chromosome)

Explain why a human female can be homozygous or heterozygous for sex-linked
genes carried on the X-chromosomes but a male cannot.

* females have two X-chromosomes and therefore two alleles of the gene (which can be the same or different)
* males only have one X-chromosome so can only have one allele of the gene

What is a female carrier?

* a female who is heterozygous for a sex-linked recessive condition / has one copy of a recessive allele
* that causes a genetic disease in individuals who are homozygous for the allele
* her phenotype is normal but she can pass the allele onto offspring

Explain the inheritance of color blindness.

* sex-linked recessive, since it is only present in males and not all males have the gene for color-blindness
* Sex linked genes are almost always located on the X chromosome.
* The results in differences in the pattern of inheritance in crosses.
* Females have two X chromosomes and so will have two copies of sex-linked genes.
* Male only have one X chromosome and therefore only have one copy of a sex-linked gene that is located on the X chrokemosome.
* Because that gene is absent from the Y chromosome, there won’t be an allele for that gene on the Y, so a male will only have one allele for a gene.
* Because there will be no other allele to mask it, the allele will always be present in the male phenotype whether is dominant or recessive.

Explain the inheritance of Huntington's disease

Huntington's disease is an autosomal dominant disorder, which means that a person needs only one copy of the nontypical gene to develop the disorder.