Incomplete Dominance Codominance Sex Linked Polygenic Traits More Genetics Incomplete Dominance Codominance Sex Linked Polygenic Traits

Incomplete Dominance http://faculty.southwest.tn.edu/rburkett/GB%20Genetics.htm In incomplete dominance, there are two homozygous phenotypes but the heterozygous phenotype is distinctly different from either of the homozygous phenotypes.

Codominance – Blood Type http://pustakalaya.org/wiki/wp/a/ABO_blood_group_system.htm In codominance, if the allele is present it is expressed. The ABO human blood type is a good example.

an aside - Donors and Recipients Donor Blood Type Type A Type B Type AB Type O Recipient Type A - IA IA IAi Y N Type B - IBIB Ibi Type AB - IAIB Type O - ii Here is an aside about blood types and donation. Type O is referred to as the universal donor because people with this blood type do not have A or B antigens on their red blood cells. Type AB is referred to as the universal recipient because they do do possess antigens for either A or B antibodies.

Codominance Practice Problems

Sex-linked Traits mainly X , mainly recessive Muscular dystrophy Hemophilia Fabry’s disease colorblindness http://www.nlm.nih.gov/medlineplus/ency/imagepages/19097.htm Many of the sex linked traits in humans are located on the X chromosome and many are recessive. That means that females who receive an X from each parent, seldom exhibit the genetic condition but may be carriers (heterogyzotes). Males on the other hand receive only one X and it is maternal. If they receive the maternal gene, they express it as there is no other X chromosome.

Sex-linked Practice Problems

Polygenic Traits Most “phenotypic traits” are multiple gene interactions. In humans these include Eye color Hair color Skin pigmentation Height Weight Intelligence (?)

Phenotypes and Polygenic Traits http://bioserv.fiu.edu/~walterm/FallSpring/inheritance/Fall03_lecturea.htm Instead of having genes with a few alleles that produce a few possible phenotypes, polygenic traits, where multiple genes interact, produce a broad spectrum of phenotypic traits. Here is a oversimplified example of theoretical height in the population.

Eye Color in Humans More than 6 genes control eye color http://en.wikipedia.org/wiki/Eye_color Although blue eye color was thought to be a simple recessive trait, the genetics of eye color is determined by as many as 15 different genes. However two genes OCA2 and HERC2 on Chromosome 15 are major determinants of blue and brown eye color. These genes affect pigment formation (melanin) and distribution in the iris and other tissues. More than 6 genes control eye color Two genes on Chromosome 15 account for most of the eye color combinations

Hair Color At least 4 genes control hair color “Two types of pigment give hair its color: eumelanin and pheomelanin. Pheomelanin colors hair orange and yellow. All humans have some pheomelanin in their hair. Eumelanin, which has two subtypes: black or brown, determines the darkness of the hair color. A low concentration of brown eumelanin results in blond hair, whereas a higher concentration of brown eumelanin will color the hair brown. High amounts of black eumelanin result in black hair, while low concentrations give gray hair. These genes are regulated and their“regulatory genes” add another level of complexity. In addition the “pigment” genes may be additive meaning if you inherit two active eumelanin genes you may have darker hair than someone who inherited only 1 eumelanin gene.” http://en.wikipedia.org/wiki/Human_hair_color At least 4 genes control hair color

Skin Color “Human skin color ranges in variety from the darkest brown to the lightest pinkish-white hues. Human skin pigmentation is the result of natural selection. Skin pigmentation in humans evolved to primarily regulate the amount of ultraviolet radiation penetrating the skin, controlling its biochemical effects.[1] The actual skin color of different humans is affected by many substances, although the single most important substance determining human skin color is the pigment melanin. Melanin is produced within the skin in cells called melanocytes and it is the main determinant of the skin color of darker-skinned humans. The skin color of people with light skin is determined mainly by the bluish-white connective tissue under the dermis and by the haemoglobin circulating in the veins of the dermis. The red color underlying the skin becomes more visible, especially in the face, when, as consequence of physical exercise or the stimulation of the nervous system (anger, fear), arterioles dilate.[2] There is a correlation between the geographic distribution of UV radiation (UVR) and the distribution of indigenous skin pigmentation around the world.  In addition, it has been observed that adult human females are considerably lighter in skin pigmentation than males. Females need more calcium during pregnancy and lactation. The body synthesizes vitamin D from sunlight, which helps it absorb calcium. Females evolved to have lighter skin so their bodies absorb more calcium At least 4 genes are involved in skin color, controlling how much and how deep the pigments are deposited.” http://en.wikipedia.org/wiki/Human_skin_color

The Ultimate Punnett Square f two  AaBbCc  people would get married and have children, the Punnett square would look like this: The Ultimate Punnett Square If two heterozygous individuals (AaBbCc) have a child, what is the probably that their child will be very dark skinned? http://ibbio.pbworks.com/w/page/36703335/Topic%2010%3A%20Genetics%20HL If we assume a three gene, six allele model for skin color, here is the Punnett square for two heterozygous parents that predicts the probability of the skin color of their offspring. Similarly that same Punnett square illustrates the frequency of skin colors one would expect to see in a population of offspring from heterozygous parents.