Modern and most accurate definition of the gene: A section of DNA that determines the amino acid sequence of a polypeptide Two important concepts that are associated with the modern definition of the gene: 1) Polygene- No complex trait is likely to be produced by a single gene There is really no one gene for any complex feature Products (enzymes) from different genes can act on common metabolic or biosynthetic pathways When one gene appears to control the properties of a particular trait, it is because the enzyme encoded by that gene acts as the rate-limiting step in a particular pathway 2) Pleiotropism- Many genes (probably most) don t have a single effect and a single function Ex i) strabismus and creamy white fur with chocolate stripes in white tigers of the Cincinnati zoo, ii) resistance to malaria and sickle-shaped red blood cells in sickle cell anemia
Polygene Concept: Gene B Gene A Trait A Gene C Pleiotropism: Trait B Trait A Gene A Trait C
Lethality: Dominant- L_ = lethal, ll = normal Ex epiloia in humans Recessive: ll = lethal, L_ = normal Ex i) amaurotic idiocy (Tay-Sachs disease), ii) cystic fibrosis in humans Semi-dominant: L`L` = normal, LL` = abnormal, LL = lethal Ex i) creeper chicken syndrome, ii) familial hypercholesterolemia (FH) in humans
Epiloia (tuberous sclerosis) Is a dominantly inherited congenital disease Includes: Adenoma sebaceum (angiofibromas involving sebaceous glands) and benign tumor-like malformations in the nervous system, heart, and kidneys Epilepsy Mental retardation
LETHALITY 1 Dominant: Whenever lethal allele is expressed, individual dies Therefore no cross can be performed 2 Recessive: Ex, L = normal (dominant); l = lethal (recessive) Include ratios before gametes, phenotypes, and genotypes Circle gametes PR = phenotypic ratio; GR = genotypic ratio Pheno x x Geno LL x LL LL x Ll Gametes x x Pheno Geno PR GR Pheno x Geno Ll x Ll Gametes x Pheno Geno PR GR
LETHALITY 1 Dominant: Whenever lethal allele is expressed, individual dies Therefore no cross can be performed 2 Recessive: Ex, L = normal (dominant); l = lethal (recessive) Include ratios before gametes, phenotypes, and genotypes Circle gametes PR = phenotypic ratio; GR = genotypic ratio Pheno Normal x Normal Normal x Normal Geno LL x LL LL x Ll Gametes 1/1 L x 1/1 L 1/1 L x 1/2 L 1/2 l Pheno 1/1 Normal 1/1 Normal Geno 1/1 LL 1/2 LL 1/2 Ll PR 1 1 GR 1 1:1 Pheno Normal x Normal Geno Ll x Ll Gametes 1/2 x 1/2 L 1/2 l L 1/2 l Pheno 3/4 Normal 1/4 Lethal Geno 1/4 LL 2/4 Ll 1/4 ll PR 3:1 1 (viable) GR 1:2:1 1:2 (viable)
LETHALITY (continued) 3 Semi-dominant: One homozygous condition is normal (L'L') Other homozygous condition causes death (LL) Heterozygous condition is abnormal (LL') Include ratios before gametes, phenotypes, and genotypes Circle gametes PR = phenotypic ratio; GR = genotypic ratio Pheno Normal x Normal Normal x Abnormal Geno x x Gametes x x Pheno Geno PR GR Pheno Geno Gametes Abnormal x Abnormal x x Pheno Geno PR GR
LETHALITY (continued) 3 Semi-dominant: One homozygous condition is normal (L'L') Other homozygous condition causes death (LL) Heterozygous condition is abnormal (LL') Include ratios before gametes, phenotypes, and genotypes Circle gametes PR = phenotypic ratio; GR = genotypic ratio Pheno Normal x Normal Normal x Abnormal Geno L L x L L L L x LL Gametes 1/1 L x 1/1 L 1/1 L x 1/2 L 1/2 L Pheno 1/1 Normal 1/2 Normal 1/2 Abnormal Geno 1/1 L L 1/2 L L 1/2 LL PR 1 1:1 GR 1 1:1 Pheno Abnormal x Abnormal Geno LL x LL Gametes 1/2 L 1/2 L x 1/2 L 1/2 L Pheno 1/4 Normal 2/4 Abnormal 1/4 Lethal Geno 1/4 L L 2/4 LL 1/4 LL PR 1:2:1 1:2 (viable) GR 1:2:1 1:2 (viable)
Example Problem: Using the information from the following cross, give the numbers expected in the blanks below if you set 1600 fertile eggs from a creeper x creeper cross C'C' = normal; CC = death; CC' = creeper; PR = phenotypic ratio; GR = genotypic ratio Circle gametes and include ratios before gametes, phenotypes, and genotypes in your cross Pheno Creeper x Creeper Geno x Gametes x Pheno Geno PR GR Total chicks hatched Creeper chicks Normal chicks Chicks which did not hatch (dead)
Example Problem: Using the information from the following cross, give the numbers expected in the blanks below if you set 1600 fertile eggs from a creeper x creeper cross C'C' = normal; CC = death; CC' = creeper; PR = phenotypic ratio; GR = genotypic ratio Circle gametes and include ratios before gametes, phenotypes, and genotypes in your cross Pheno Creeper x Creeper Geno CC x CC Gametes 1/2 C 1/2 C x 1/2 C 1/2 C Pheno 1/4 normal 2/4 creeper 1/4 death Geno 1/4 C C 2/4 CC 1/4 CC PR 1:2:1 1:2 (viable) GR 1:2:1 1:2 (viable) Total chicks hatched 1200 Creeper chicks 800 Normal chicks 400 Chicks which did not hatch (dead) 400