Patterns of Heredity

Incomplete Dominance, Codominance and Sex-linked

Figure 14.2 Mendel tracked heritable characters for three generations

Genetics Vocabulary• Incomplete Dominance - a form of

intermediate inheritance in which one allele for a specific trait is not completely dominant over the other. If the trait is incomplete it will be stated in the problem.

(Some people, like incomplete dominance, use superscripts or subscripts w/ diff letters)

Figure 14.9 Incomplete dominance in snapdragon color

Genetics Vocabulary• Codominance - A condition in which

both alleles of a gene pair in a heterozygote are fully expressed, with neither one being dominant or recessive to the other. If the trait is codominant it will be stated in the problem. (When crossing codominant traits, use different capital letters) RR RW WW

Autosomes vs. Sex-chromosomes

Sex-linked traits• Most human traits are inherited by passing

autosomes – non-sex chromosomes– (just like peas)

• Sex-linked traits = inherited by passing sex-chromosomes.– Many human diseases including:

• Hemophilia & Color blindness

Designating Sex-Linked Traits

XA = normalXa = the trait (a genetic disease or abnormality)Y = Y chromosome (males only)

* X-linked traits are only on the X chromosome

Genotype Phenotype

XA XA Normal female

XA Xa Normal female, also called a carrier

Xa Xa Female with trait

XA Y Normal male

Xa Y Male with trait

• X-linked traits may also skip generations

• Males are affected more often than females because they only have one X chromosome

Sample problem

• Hemophilia is an X-linked disorder. If an affected father and a carrier mother have a boy, what is the probability that boy will have hemophilia?

What is the probability that boy will have hemophilia?

Mother’s Genes

Xa XXa

Y

XaXa XaX

XaY XY

Fath

er’s

Gen

es

Autosomal Dominant diseases

• Individuals have the trait if they have the dominant allele.– Ex: familial

hypercholesterolemia– Huntington’s disease– Dwarfism– Neurofibromatosis– Colon cancer– Kidney disease– Etc.