Inheritance Patterns

Mendel’s Laws Law of Dominance: if the two alleles at a locus differ, then one, the dominant allele, determines the organism′s appearance; the other, the recessive allele, has no noticeable effect on the organism′s appearance Law of Segregation: the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes Law of Independent Assortment: each pair of alleles segregates independently of other pairs of alleles during gamete formation

Segregation Alleles segregate when homologous chromosomes separate during Meiosis I.

Nondisjunction Changes in chromosome number can occur when homologous chromosomes or sister chromatids fail to segregate during meiosis, an event called “nondisjunction.” Can occur in anaphase of meiosis I or II, resulting in gametes with abnormal numbers of chromosomes.

Independent Assortment

Linked Genes Genes located on the same chromosome tend to be inherited together Such genes are said to be “linked genes.” When genes are linked, they do not assort independently.

Crossing Over Linked genes are only separated by crossing over. The closer two genes are on a chromosome, the tighter their genetic linkage. The farther apart the genes are, the more likely it is that a crossover event will separate them.

Intermediate Inheritance (Incomplete Dominance) The heterozygote has a phenotype that is intermediate between the phenotypes of the two homozygotes. Example: Petal color in snapdragons.

Codominance The heterozygote expresses both traits at the same time. Example: roan coat color in cattle Red “Roan” White

CRCR x CWCW

CRCW x CRCW

Sex-Linked Inheritance Genes located on sex chromosomes produce different patterns in males and females. Females generally have two alleles for these genes. Males generally have only one allele. If a male inherits a sex-linked recessive allele from his mother, the allele will be expressed.

Red/Green Color Blindness

color-blind male x carrier female

Multiple Alleles For many genes, several alleles exist in the population. This expands the number of possible genotypes and phenotypes.

Example of Multiple Alleles Human blood type is determined by three alleles: A, B, & O. IA & IB are codominant. i is recessive.

Nondisjunction

Polygenic Inheritance Many genes influence a single trait

Continuous variation Most traits show a range of variation rather than distinct either/or types This occurs when multiple genes and environmental factors influence the trait’s expression Continuous variation is often described with frequency distribution tables.

Epistasis Effects of one gene override or mask the phenotype of a second gene. Epistasis is not dominance. Compare the definitions: Epistasis: One gene masks the expression of a different gene for a different trait Dominance: One allele masks the expression of another allele of the same gene

Example of Epistasis Labrador retrievers can be black, brown, or yellow. Two genes control this. One gene influences melanin production B (black) is dominant to b (brown) One gene influences melanin deposition E (full deposition) is dominant to e (reduced deposition)