Variations on Mendel’s principles Incomplete dominance Co-dominance Multiple alleles Sex-linked alleles

Incomplete Dominance and Codominance 2

Incomplete Dominance  F1 hybrids in betweenphenotypes  F1 hybrids have an appearance somewhat in between (intermediate ) the phenotypes of the two parental varieties.  Example:snapdragons (flower)  Example: snapdragons (flower)  red (RR) x white (WW)  RR = red flower  WW = white flower 3 R R WW

Incomplete Dominance 4RWRWRWRW R RW All RW = pink (heterozygous pink) produces the F 1 generation W

Incomplete Dominance 5

Many genes have more than two alleles in the population Ex. three alleles for ABO blood type in humans I A, I B, i

Co-dominance  Two alleles are expressed equally in multiple alleles in heterozygous individuals.  Example: blood type  1.type A= I A I A or I A i  2.type B= I B I B or I B i  3.type AB= I A I B (equally expressed)  4.type O= ii (recessive alleles) 7

Co-dominance Problem  Example: homozygous male Type B (I B I B ) X heterozygous female Type A (I A i) 8 IAIBIAIB IBiIBi IAIBIAIB IBiIBi 1/2 = I A I B (A & B = equally expressed) 1/2 = I B i (B dominant over i) IBIB IAIA i IBIB

Another Codominance Problem 9 Example:Example: male Type O (ii) x female type AB (I A I B ) IAiIAiIBiIBi IAiIAiIBiIBi 1/2 = I A i 1/2 = I B i i IAIA IBIB i

Co-dominance Question: If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents? boy - type O (ii) X girl - type AB (I A I B ) 10

Co-dominance Answer: 11

Sex-linked Traits  Traits (genes) located on the sex chromosomes  Sex chromosomes are X and Y  XX genotype for females  XY genotype for males  Many sex-linked traits carried on X chromosome 12

Sex-linked Traits 13 Sex Chromosomes XX chromosome - femaleXy chromosome - male fruit fly eye colour Example: Eye colour in fruit flies

Sex-linked Trait Problem  Example: Eye colour in fruit flies  (red-eyed male) x (white-eyed female) X R Y x X r X r  Remember: the Y chromosome in males does not carry these traits.  RR = red eyed  Rr = red eyed  rr = white eyed  XY = male  XX = female 14 XRXR XrXr XrXr Y

Sex-linked Trait Solution: 15 X R X r X r Y X R X r X r Y 50% red eyed female 50% white eyed male XRXR XrXr XrXr Y

Female Carriers 16

Polygenic traits - A single trait may be influenced by many genes Quantitative traits skin colour, height, eye colour Fraction of population Skin pigmentation

Sex-linked disorders affect mostly males Most sex-linked human disorders are due to recessive alleles Ex: hemophilia, red-green color blindness These traits appear mostly in males. Why? If a male receives a single X-linked recessive allele from his mother, he will have the disorder; while a female has to receive the allele from both parents to be affected

Pedigree Chart: Inheritance Pattern for an X-linked Recessive Disease

A high incidence of hemophilia has plagued the royal families of Europe Queen Victoria Albert AliceLouis AlexandraCzar Nicholas II of Russia Alexis

Fetal testing can spot many inherited disorders early in pregnancy Karyotyping and biochemical tests of fetal cells can help people make reproductive decisions Fetal cells can be obtained through amniocentesis Figure 9.10A Amniotic fluid Fetus (14-20 weeks) Placenta Amniotic fluid withdrawn Centrifugation Fetal cells Fluid UterusCervix Cell culture Several weeks later Karyotyping Biochemical tests

Chorionic villus sampling is another procedure that obtains fetal cells for karyotyping Figure 9.10B Fetus (10-12 weeks) Placenta Chorionic villi Suction Several hours later Fetal cells (from chorionic villi) Karyotyping Some biochemical tests

Examination of the fetus with ultrasound is another helpful technique Figure 9.10C, D PGD - Preimplantation Genetic Diagnosis genetic analysis of embryos from in vitro fertilization (IVF) before inserting into womb