Types of Inheritance Patterns

Gene Linkage

Genes on the same chromosome are considered linked and are called a linkage group.

Human Sex-linked InheritanceSex-linked traits are distinguishable by their mode of transmission through successive generations of a family.

In humans it is preferable to speak in terms of X-linked or Y-linked inheritance.

Red-green color blindness was the first human trait proven to be due to a gene on a specific chromosome. Hemophilia is another common sex-linked trait.

Sex-Linked Traits

If a gene is found only on the X chromosome and not the Y chromosome, it is said to be a sex-linked trait. Because the gene controlling the trait is located on the sex chromosome, sex linkage is linked to the gender of the individual.Usually such genes are found on the X chromosome. The Y chromosome is thus missing such genes.

The result is that females will have two copies of the sex-linked gene while males will only have one copy of this gene. If the gene is recessive, then males only need one such recessive gene to have a sex-linked trait rather than the customary two recessive genes for traits that are not sex-linked.This is why males exhibit some traits more frequently than females.

Examples of Sex-linked Traits:

Red-green colorblindnessMale Pattern BaldnessHemophiliaDuchenne Muscular Dystrophy

Chromosome Mapping

A chromosome map is a linear diagram of the position of the alleles on a chromosome.

The closer together the genes are linked on the chromosome the less chance they will separate during crossing over.

The map distance is the distance between genes on a single chromosome.

The greater the distance between linked genes, the more likely they are to cross over during meiosis.

Sample Sex-linked Trait Problems

In humans, red-green colorblindness is a recessive sex-linked trait. It is found on the X chromosome, not the Y. Because, males only have one X chromosome, they have a much greater chance of having red-green colorblindness. Females would have to be homozygous recessive in order to have red-green colorblindness

Multiple Alleles and Blood Types

Pedigree

* is a tool that can determine the pattern of inheritance of a particular trait.

Multiple Alleles Within a

population, more than two alleles can exist (although any given individual only has two alleles).

The human ABO blood groups are an example of multiple alleles, and the relationship between phenotype and genotype.

There are four possible phenotypic blood types for this particular gene: A, B, AB, and O. The letters refer to two specific antigens on the surface of red blood cells.

Individuals can have the A antigen (blood type A), the B antigen (blood type B), both the A and B antigen (blood type AB), or neither antigen (blood type O).

Human blood type is determined by codominant alleles. There are three different alleles, known as IA, IB, and i. The IA and IB alleles are codominant, and the i allele is recessive.

The possible human phenotypes for blood group are type A, type B, type AB, and type O. Type A and B individuals can be either homozygous (IAIA or IBIB, respectively), or heterozygous (IAi or IBi, respectively).

Cross Type AB and a Type O blood What are the possible blood types of the

offspring of a cross between individuals that are type AB and type O? (Hint: blood type O is recessive)

Type A and type B cross Four different

genetic crosses are possible. All four crosses must be considered to determine all potential offspring. However, one cross (IAi mother x IBi father) is most informative.

Types of Inheritance patterns

Autosomal Dominant Inheritance • using chromosomes

other than sex chromosomes

• the traits would be either dominant or recessive

• whenever a recessive phenotype occurs in a child of parents who exhibit the dominant trait, the parents must be heterozygous for that trait

Autosomal Recessive Inheritance

deals with autosomes with the recessive genes being inherited