Forms of dominance Complete Incomplete Codominance Multiple Alleles

Forms of dominance Complete Incomplete Codominance Multiple Alleles Sex-linked Traits

Complete dominance • Mendel’s garden peas showed complete dominance • Example: TT x tt = 100% Tt tall short tall http: //www. communityschoolhouse. org/pu nnettsquares. htm T t t T Tt Tt tall

Want more practice? • http: //www. emints. org/ethemes/re sources/S 00001476. shtml

Incomplete dominance • The dominant allele does not completely cover up the expression of the recessive allele • The phenotype of the offspring appears as a blend of the two For example: snapdragons red x white = pink

Incomplete dominance RR x rr Red white R r r Rr Pink Rr pink R Rr Pink Rr pink

Codominance • Two alleles are both dominant • If inherited together, both will be expressed

Codominance Black x White BB WW B W W B BW BW Blue roan

Multiple Alleles • Sometimes there is more than two forms of a trait! • Example human blood typeshttp: //nobelprize. org/medicine/educational/landsteiner/index. html

Ex: Blood type problems • What are their possible blood types if a heterozygous type A man and a heterozygous type B woman have children? I Ai x I Bi IA IB i i A B I I B I i A I i ii 25 % chance for AB, A, B, or O

Blood types and paternity • Blood type can only exclude a person from being the possible father of a child • Ex: a man accused of fathering a child has type O blood and the child has type AB ii x _____ i i The man can only pass on recessives! He could not possibly have fathered this child

Sex-linked traits • Traits that are controlled by genes inherited only on the X sex chromosome

Red-Green colorblindness

Hemophilia • a hemophilia carrier and a normal man want to know the probability of having children affected by hemophilia • http: //www. yourgenesyourhealth. org/hemo/inherited. ht m Genotype Phenotype for blood clotting XH XH Normal Y XH Y normal Xh XH Xh carrier Xh Y hemophilia XH XH XH Y Normal female Normal male XH Xh Female who is a carrier, she has normal blood clotting Xh Y Male who inherited the recessive allele located on the X sex chromosome will not produce the blood clotting factor

Polygenic Inheritance • occurs when more than one gene controls a single trait – these are sometimes called quantitative traits because the number of dominant to recessive alleles creates a wide range of observable phenotypes – Example: human height, skin color, eye color http: //www. ncbi. nlm. nih. gov

Melananin Production in skin and eyes

Human Height