Welcome Back! 2/6/18 1. A species of mice can have gray or black fur and long or short tails. A cross between blackfurred, long-tailed mice and gray-furred, shorttailed mice produce all black-furred, long-tailed offspring. Using the gene symbols G for black, for gray, S for long and s for short, what would be the genotype of a gray-furred, short-tailed mouse? 2. A. GGSS B. ggss C. ggss D. GgSs E. Ggss
Dominant Recessive
According to Gregor Mendel: Genotypes: 25% BB 50% Bb 25% bb Phenotypes: 75% Black 25% White What about grey?!?!?!?!?!
B W Genotypes: 25% BB 50% BW 25% WW B B B B W Phenotypes: 25% Black 50% Grey 25% White W B W WW
Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical (Mendelian) In incomplete dominance, the phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties Creates a mixture of traits In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways Traits appear together
Figure 14.10-1 P Generation Red C R C R White C W C W Incomplete Gametes C R C W
Figure 14.10-2 P Generation Red C R C R White C W C W Incomplete Gametes C R C W F 1 Generation Pink C R C W Gametes 1 / 2 1 / 2 C R C W
Figure 14.10-3 P Generation Red C R C R White C W C W Incomplete Gametes C R C W F 1 Generation Pink C R C W Gametes C R 1 / 2 1 / 2 C W F 2 Generation Sperm 1 / 1 2 C R / 2 C W Eggs 1 / 2 C R C R C R C R C W 1 / 2 C W C R C W C W C W
Co-Dominance R = allele for red flowers W = allele for white flowers red x white ---> red & white spotted RR x WW ---> 100% RW
Multiple Alleles Most genes exist in populations in more than two allelic forms For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: I A, I B, and i. The enzyme encoded by the I A allele adds the A carbohydrate, whereas the enzyme encoded by the I B allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither
Figure 14.11 (a) The three alleles for the ABO blood groups and their carbohydrates Allele I A I B i Carbohydrate A B none (b) Blood group genotypes and phenotypes Genotype I A I A or I A i I B I B or I B i I A I B ii Red blood cell appearance Phenotype (blood group) A B AB O
O ii O A, B, AB, O O Phenotype Possible Genotype(s) Allele (antigen) on RBC surface Can Donate Blood To Can Receive Blood From A B I A i I A I A A A, AB A, O I B i I B I B B B, AB B, O AB I A I B AB AB A, B, AB, O
Problem: Multiple Alleles Show the cross between a mother who has type O blood and a father who has type AB blood. GENOTYPES: - Ai (50%) Bi (50%) - ratio 1:1 A i Ai i Ai PHENOTYPES: - type A (50%) type B (50%) - ratio 1:1 B Bi Bi
Problem: Multiple Alleles Show the cross between a mother who is heterozygous for type B blood and a father who is heterozygous for type A blood. GENOTYPES: -AB (25%); Bi (25%); Ai (25%); ii (25%) - ratio 1:1:1:1 B A AB i Bi PHENOTYPES: -type AB (25%); type B (25%) type A (25%); type O (25%) - ratio 1:1:1:1 i Ai ii
Pleiotropy One gene being responsible for or affecting more than one phenotypic characteristic. Pleiotropy can be found in many different forms, but mainly it is viewed as causing inherited diseases and disorders. Pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease
Epistasis In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus Labrador retrievers, coat color depends on two genes One gene determines the pigment color (with alleles B for black and b for brown) The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair
c gene is epistatic to the A/a gene; controls whether or not pigment of any color will be deposited in the hair.
Tracing Traits A pedigree is a family tree that describes the interrelationships of parents and children across generations Inheritance patterns of particular traits can be traced and described using pedigrees Squares Men Circles- Females Half Shaded Carrier Full Shade - Affected
Figure 14.15 Key Male Female Affected male Affected female Mating Offspring 1st generation 2nd generation 3rd generation Ww ww ww Ww Ww ww ww Ww Ww WW or Ww ww ww 1st generation 2nd generation FF or Ff 3rd generation Ff Ff ff Ff ff ff Ff ff Ff FF or Ff ff Widow s peak No widow s peak Attached earlobe Free earlobe (a) Is a widow s peak a dominant or recessive trait? b) Is an attached earlobe a dominant or recessive trait?
Y-Linked Traits No affected females ALL sons of affected males are also affected
X-Linked Recessive
Recessively Inherited Disorders Many genetic disorders are inherited in a recessive manner These range from relatively mild to life-threatening Examples Sickle Cell Anemia Albinism Tay-Sachs Disease
The Behavior of Recessive Alleles Recessively inherited disorders show up only in individuals homozygous for the allele Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal; most individuals with recessive disorders are born to carrier parents Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair
Figure 14.16 Normal Aa Parents Normal Aa Eggs A a Sperm A AA Normal Aa Normal (carrier) a Aa Normal (carrier) aa Albino
Figure 14.UN03 Relationship among alleles of a single gene Description Example Complete dominance of one allele Heterozygous phenotype same as that of homozygous dominant PP Pp Incomplete dominance of either allele Codominance Multiple alleles Pleiotropy Heterozygous phenotype intermediate between the two homozygous phenotypes Both phenotypes expressed in heterozygotes In the whole population, some genes have more than two alleles One gene is able to affect multiple phenotypic characters C R C R C R C W C W C W I A I B ABO blood group alleles I A, I B, i Sickle-cell disease
Figure 14.UN04 Relationship among two or more genes Description Example Epistasis The phenotypic expression of one gene affects that of another BbEe BE BE be Be BbEe be be Be be 9 : 3 : 4 Polygenic inheritance A single phenotypic character is affected by two or more genes AaBbCc AaBbCc