Honors Biology Genetics - HAPPY NEW YEAR!!! Exceptions to Mendel (Incomplete dominance, Codominance, Multiple alleles, Sex-linkage, Polygenic traits)

Incomplete Dominance When two or more alleles influence the phenotype, the heterozygous phenotype is a blend of both homozygous phenotypes the trait is considered incompletely dominant. White FF Pink Ff Red ff

Incomplete Dominance: Example The trait for hair texture in humans is incompletely dominant. People with curly hair have two HH alleles People with straight hair have two hh alleles People with Wavy hair are heterozygous Hh hh Hh HH

Incomplete Dominance: Problem A wavy haired man marries a wavy haired woman, they are expecting their first child, what is the chance they will have a child with wavy hair? ____ x ____ Hh Hh H h HH Hh hh H h 50 % chance they will have wavy hair (Hh)

Codominance When both alleles for a gene are expressed in a heterozygous offspring the trait is considered codominant (there is no blending in codominant traits both traits are expressed.)

Codominance: Example Problem The trait of fur color in horses and cows follows a codominant inheritance pattern. The allele for brown fur and white fur are codominant meaning a heterozygous horse will have both brown and white furs on their body (ROAN).

Codominance: Example Problem A brown horse (R) and a white horse (r) are mated, what are the phenotypes and genotypes of their offspring? ____ x ____ Answer: 100% mixed coat RR rr R r Rr

Multiple Alleles Definition: A trait that is controlled by three or more alleles that can fit into one gene. C= full coat color, cch chinchilla, ch Himalayan, c albino

Multiple Alleles: Example Problem ABO blood type Phenotype Genotype(s) A type IAIA or IAi B type IBIB or IBi AB type IAIB O type ii Phenotype Genotype(s) A type B type AB type O type

Multiple Alleles: Example Problem If a person who is heterozygous for B type blood marries a person who is homozygous for A type blood what is the chance they will have a child with O type blood? IB i IA IAIB IAi IB i IA None of their children will have O type (ii)

Sex Linkage Refers to genes that exist only on the X chromosome. Males have only one allele for these traits females have two alleles.

Sex-Linkage: Example Ichthyosis forms scales on the skin because of the build up of dead skin cells This genetic disorder is X-linked and recessive. When a trait is X-linked it means it is only found on the X chromosome. In these problems you will need to include both the gender AND the allele A male genotype is written XY and a female genotype is XX. Ichthyosis is represented by a Xi the i represents the recessive allele for this trait.

Sex-Linkage: Example Problem If a healthy woman and a man with ichthyosis have a child, what is the chance the child will also have the disease? XI XI x XiY None of the children will have Ichthyosis but the daughters will carry the allele Xi Y XI Xi Y XI XIXi XIY

Polygenic A trait that is controlled by many genes combined together

Polygenic: Considerations Three genes help to determine skin color: gene A, gene B and gene C. if a person has all dominant alleles: AABBCC they have a very dark skin tone If they have all recessive alleles: aabbcc they will be very pale If they have half of the dominant alleles AaBbCc they are medium tone. How many different skin tones are possible? 2x2x2 = 8