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Describe the inheritance patterns of sex-linked genes.

Sex chromosomes determine whether an organism is male or female. In humans and many other species, an XX organism is female and an XY organism is male. X-linked alleles are located on the X chromosome. A female will express the phenotype of an X-linked allele coding for a recessive trait only if she inherits two copies of it, one from each parent. On the other hand, a male will express the phenotype of an X-linked allele coding for a recessive trait if he inherits it from his mother, since he has no second X chromosome to mask the recessive trait. Because sex-linked genes are expressed differently, depending on gender, sex-linked traits do not follow Mendelian inheritance patterns.

Explain how linked genes deviate from Mendelian inheritance patterns.

Deviations from Mendelian inheritance patterns also occur when genes are linked, that is, when they are located on the same chromosome. Linkage is particularly common when genes are located very close to each other on the same chromosome. Linked genes deviate from the 1:1:1:1 ratio of Mendelian inheritance, in a pattern that depends on the combination of alleles inherited by the heterozygous parent from each of its parents.

Relate the use of heredity data for linked genes to the construction of genetic linkage maps.

Genes that are located far apart from each other on the same chromosome often cross over during meiosis, producing recombinant offspring that do not look like either parent. Scientists can estimate how far apart two genes are by determining how often offspring show recombination. Using recombination rates, we can construct genetic linkage maps, which show the relative order of genes along a chromosome.

Barr body

The inactivated X chromosome in female cells so that there are not too many X chromosome proteins in the cell.

crossing over

Exchange of genetic material between homologous chromosomes in meiosis.

genetic map

Identifies the locations of genes on a chromosome.


A spore-producing structure in ascomycte fungi; component of an ascocarp; plural is asci.The pheonomenon of gene inheritance whereby genes that are on the same chromosome are inherited together despite meiotic recombinaiton events.

linkage map

Location of all known genes relative to others on a chromosome.

linked trait

A trait that is co-inherited with another trait more often than not; arises from genetic sequences that are on the same chromosome.


An adjective describing a genetic sequence or a genome that arises from the combination of genetic material; such as through meiosis; can refer to an organism synthetically modified to contain specific genes in its genome that are not naturally occuring.

sex-linked trait

Genes associated with the sex chromosomes.

wild type

Most common phenotype in nature.

X-linked gene

Most sex-linked traits are on the X chromosome, but not on the Y chromosome, because X is much larger than Y.

Y-linked gene

A trait that is inherited through the Y chromosome.

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