Disorders and Disorders and Other Patterns of Other Patterns of

InheritanceInheritance

Biology 1-2Biology 1-2

Inherited DisordersInherited Disorders

In humans, many genetic disorders In humans, many genetic disorders show complete dominance.show complete dominance. Recessive disordersRecessive disorders are caused by a are caused by a

recessive allele.recessive allele. Heterozygotes are Heterozygotes are carrierscarriers (have the (have the

recessive allele and can pass it on but do recessive allele and can pass it on but do not show symptoms of the disorder).not show symptoms of the disorder).

Inherited Disorders Inherited Disorders cont’dcont’d

Dominant disordersDominant disorders are caused by a are caused by a dominant allele.dominant allele. Both homozygous dominant and Both homozygous dominant and

heterozygotes show the disorder.heterozygotes show the disorder. There are no carriers.There are no carriers.

PedigreesPedigrees

A A pedigreepedigree is a is a family tree tracing family tree tracing the history of a the history of a trait in a particular trait in a particular family.family. Circle = femaleCircle = female Square = maleSquare = male Shaded= person Shaded= person

with the trait.with the trait. Unshaded = either Unshaded = either

does not have the does not have the trait or is unknown.trait or is unknown.

Incomplete DominanceIncomplete Dominance

Incomplete dominanceIncomplete dominance-the -the heterozygote has an appearance that heterozygote has an appearance that is an intermediate between the is an intermediate between the parents.parents. The heterozygote produces a phenotype The heterozygote produces a phenotype

that is a blend of the other two that is a blend of the other two phenotypes.phenotypes. Both alleles are dominant.Both alleles are dominant. Written as capital letters.Written as capital letters.

CodominanceCodominance

CodominanceCodominance-both alleles are -both alleles are expressed fully in the heterozygote.expressed fully in the heterozygote. Both alleles show completely and fully.Both alleles show completely and fully. The heterozygote has both traits.The heterozygote has both traits. Both alleles are dominant.Both alleles are dominant.

No recessive alleles.No recessive alleles. Both written as capitals.Both written as capitals.

Sex-Linked TraitsSex-Linked Traits

Sex-linked traitsSex-linked traits are genes carried are genes carried on the X or Y chromosomes.on the X or Y chromosomes. Show more frequently in males than Show more frequently in males than

females.females. Males have a single X chromosome.Males have a single X chromosome.

Receive one copy of any gene on the X Receive one copy of any gene on the X chromosome.chromosome.

Y chromosome has no equivalent gene.Y chromosome has no equivalent gene. Males show the gene on the X and cannot Males show the gene on the X and cannot

hide a recessive.hide a recessive.

Sex-Linked cont’dSex-Linked cont’d

Females have two XsFemales have two Xs A recessive on one X can be covered by A recessive on one X can be covered by

a dominant on the other X.a dominant on the other X. Are written as XAre written as XnnXXnn or X or XnnYY

n is letter of the trait.n is letter of the trait. Also includes traits that determine Also includes traits that determine

gender.gender.