Human GeneticsChapter 12

Pedigrees•A pedigree is a graphic representation of

human inheritance.

Squares = malesCircles = females

Shaded circles/squares = individuals showing the trait being studied.

Recessive Heredity•Most genetic disorders are caused by

recessive alleles (must be inherited from both parents)

•Traits with recessive alleles usually skip a generation.

•Examples – cystic fibrosis, Tay-Sachs disease, phenylketonuria

Simple Dominant Heredity•Only need to be inherited from one

parent.

•Examples – tongue rolling, attached earlobes, hitchhikers thumb, Huntington’s disease

Incomplete Dominance

•The phenotype of the heterozygous is intermediate.

•Ex. Red flower (RR) x white flower (rr) = pink flower (Rr)

•Ex. Curly Hair (HH) x straight hair (hh) = wavy hair (Hh)

Codominance•Causes the phenotype of both homozygous

parents to be produced in heterozygous offspring.

•Example:

black feather chicken (BB) + white feather chicken (bb) = black and white feather

chicken (Bb)

Other Genetic Patterns

•Multiple Alleles – traits controlled by more than two alleles.

•Sex determination:autosomes (44) – all the other

chromosomessex chromosomes (2) – determine sex

XX = femaleXY = male

•Sex-linked traits – traits controlled by genes located on the sex chromosome.

* usually occur more in males

•Polygenic inheritance – traits controlled by two or more genes (Many alleles)

ex. eye color and skin color

Codominance In Humans

•Sickle-cell Anemia:- most common in black Americans

(1 in 12)- the presence of abnormal shaped

red blood cells (sickle shaped or half moon)- slow blood flow, block small

vesssels, and result in tissue damage- Individuals who are heterozygous,

produce both normal and sickle shaped

blood cells. (codominance)

Multiple Alleles in Humans

•Blood Type:- Gene I has three alleles: IA, IB, i- these three alleles produce four

blood types A, B, AB, and O- blood typing is necessary before a

person can get a blood transfusion. - blood typing can also help

determine parents

Sex-Linked Traits in Humans

•Traits that are determined by genes that are carried on the sex chromosomes.

•Most are located on the X-chromosome.•Males have these traits more than

females

•Ex. Colorblindness, hemophilia

Polygenic Inheritance in Humans•Eye Color•Skin color

Changes in Chromosome Numbers•Humans normally have 44 autosomes (22 pairs)

- an extra chromosome is called trisomy- Trisomy 21 (extra 21st chromosome) =

Down Syndrome

•Changes in sex chromosomes:- If there is a Y chromosome, it is male.- If there is an X chromosome, it is female- Extra chromosomes usually mean they are sterile, but can lead normal lives