Patterns of Inheritance

Define pleiotropy

A single gene that affects several phenotypes

Define Barr body

An inactive X chromosome that is condensed and located in a female cell

Define hemizygous

Having only a single copy of a gene instead of the customary two copies. All the genes on the single X chromosome in the male.

Describe X-linked recessive and X-linked dominant inheritance of a trait and list the hallmarks of each mode of inheritance

In X-linked recessive inheritance the disease is never passed from father to son. Although males are more likely to be affected than females. If a female is affected, then her father must be affected. If affected males cannot reproduce, only males will be affected. All affected males in a family are in a family are related through their mothers. In X-linked dominant inheritance the trait is never passed from father to son. All daughters of an affected male are affected. All affected males have an affected mother. Males are usually more severely affected than females.

Compare complete dominance, codominance, and partial/incomplete dominance at the organismal and cellular levels

In codominance both alleles are expressed together. Ex: human blood types A, B, AB, O Partial/incomplete dominance where both alleles are expressed but incompletely. Ex: sickle cell anemia, people who are heterozygotes have both normal and sickle blood cells

Outline the process of X chromosome inactivation and predict the consequences of this inactivation in heterozygous females

In females, one X chromosome is inactivated by random selection during the blastula stage. Functionally, females have one X chromosome but structurally they have two. A heterozygous female could be a manifesting carrier due to random X chromosome inactivation throughout the body.

Describe Y-linked inheritance of a trait

Only males have any traits that result from genes on the Y chromosome so no females are affected. All male offspring of an affected father will be affected.

Describe mitochondrial inheritance and differentiate it from other modes of inheritance

Mitochondrial DNA can only be inherited from the mother, so any genetic disease the mother has will be passed on to all of her offspring.

Explain the differences between autosomal and sex-linked inheritance

Sex-linked inheritance is due to the transmission of sex genes in X and Y chromosomes. Whereas, autosomal inheritance is due to the transmission of genes in autosomes or non-sex determining chromosomes. Unlike sex-linked inheritance, autosomal inheritance affects males and females with equal frequencies. Sex-linked inheritance uses a family tree/pedigree to map the inheritance of genes.

Define genetic mosaicism

The phenotype of a multi-celled organism, which is composed of cells expressing different genotypes; for instance, women are functionally mosaic due to the X chromosome genes expressed. Ex: Calico cat

Distinguish autosomes and sex chromosomes

There are 22 homologous autosomes and only one pair of sex chromosomes XX or XY

Compare autosomal recessive and autosomal dominant inheritance of a trait

To express an autosomal recessive trait an individual must be homozygous, so people can be carriers for recessive traits without expressing them. Autosomal dominant traits cannot be hidden, if you have a dominant allele you will express the trait regardless of whether you are a homozygous or heterozygous.

Define manifesting carrier

You can have a female who is a heterozygous for a normally recessive condition who has phenotypic manifestations of the disorder due to partial expression of the recessive allele.