Chapter 11~ Mendel & The Gene Idea (So, what else does a monk have to do with his time????) Chapter 11~ Mendel & The Gene Idea (So, what else does a monk have to do with his time????)

Mendelian Genetics Character (heritable feature, i.e., fur color) Character (heritable feature, i.e., fur color) Trait (variant for a character, i.e., brown) Trait (variant for a character, i.e., brown) True-bred P generation, homozygous for trait all offspring of same variety) True-bred P generation, homozygous for trait all offspring of same variety) Hybridization c Hybridization c P generation – true breeding parents to start P generation – true breeding parents to start F 1 generation is result of crossing F 1 generation is result of crossing true-breds – they are always heterozygous for trait F 2 generation F 2 generation product of crossing of F1 heterozygotes What a surprise! What is happening here??

Leading to the Law of Segregation Alleles - Alternative versions of genes account for variations in inherited characteristics Alleles - Alternative versions of genes account for variations in inherited characteristics For each character, an organism inherits 2 alleles, one from each parent For each character, an organism inherits 2 alleles, one from each parent If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance The alleles for each character segregate (separate) during gamete production (meiosis). The alleles for each character segregate (separate) during gamete production (meiosis). Mendel’s Law of Segregation Mendel’s Law of Segregation –THE TWO ALLELES FOR A TRAIT SEPARATE DURING THE FORMATION OF GAMETES, ONE TO EACH GAMETE Important! Recognize the 3:1 phenotypic ratios and 1:2:1 genotypic ratio of a straight Mendelian F2 generation!!

Genetic Vocabulary Review……. Punnett Square: predicts the results of a genetic cross between individuals of known genotype Punnett Square: predicts the results of a genetic cross between individuals of known genotype Homozygous: Homozygous: –pair of identical alleles for a character Heterozygous: Heterozygous: –two different alleles for a gene Phenotype: Phenotype: –an organism’s traits Genotype: Genotype: –an organism’s genetic makeup Testcross: Testcross: –breeding of a recessive homozygote X dominant phenotype (but unknown genotype)

The Law of Independent Assortment Law of Segregation involves 1 character. What about 2 (or more) characters? Law of Segregation involves 1 character. What about 2 (or more) characters? Monohybrid cross vs. dihybrid cross Monohybrid cross vs. dihybrid cross The two pairs of alleles sort independently of each other. The two pairs of alleles sort independently of each other. Mendel’s Law of Independent Assortment: Mendel’s Law of Independent Assortment: –INHERITANCE OF ONE TRAIT DOES NOT INTERFERE WITH THE INERITANCE OF ANOTHER TRAIT. Happens when homologous chromosomes sort independently during anaphase 1.

The dihybrid cross Important phenotypic ratio to recognize in the Important phenotypic ratio to recognize in the F2 generation: F2 generation: 9:3:3:1 9:3:3:1

Non-single gene genetics, (i.e., alleles) Recessive Inheritance: Trait expressed when dominant allele absent Recessive Inheritance: Trait expressed when dominant allele absent Ex: Tay-Sachs disease (mutation-90 possible!- in HEXA gene, ch #15: accumulation of fatty acids-gangliosides-in brain. Eastern European Jews, French Canadians Quebec, Creole in Louisiana) Ex: Tay-Sachs disease (mutation-90 possible!- in HEXA gene, ch #15: accumulation of fatty acids-gangliosides-in brain. Eastern European Jews, French Canadians Quebec, Creole in Louisiana) Incomplete dominance: appearance between the phenotypes of the 2 parents. Ex: snapdragons C R C W Incomplete dominance: appearance between the phenotypes of the 2 parents. Ex: snapdragons C R C W Codominance/Multiple alleles: more than 2 possible alleles for a gene. Each expresses Ex: human blood types I A I B I, roan coat color Codominance/Multiple alleles: more than 2 possible alleles for a gene. Each expresses Ex: human blood types I A I B I, roan coat color

Blood type – What happens when you get the wrong blood? Codominance up close

Blood type genetics problems Concept check If a man with type AB blood marries a woman with type O, what blood types would you expect in their children? What fraction would you expect of each?

Another example of codominance: Roan coat color The phenotype for heterozygous Rr horses is roan – an orangish coat color The coat is a mixture of completely white and completely red hairs.

Non-single gene genetics (non-mendelian) Pleiotropy*: (many) Pleiotropy*: (many) –genes with many phenotypic effect. –Ex: sickle-cell anemia: affect many organs; –PKU (phenylketonuria) mental retardation, skin color from inability to convert phenylalanine- treated by diet control Epistasis*: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. Ex: mice coat color Epistasis*: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. Ex: mice coat color Polygenic Inheritance*: an additive effect of two or more genes on a single phenotypic character Ex: human skin pigmentation and height Polygenic Inheritance*: an additive effect of two or more genes on a single phenotypic character Ex: human skin pigmentation and height

Pleiotropy – one gene with many effects - a closer look

Human disorders The family pedigree Autosomal Recessive disorders: Cystic fibrosis Cystic fibrosis mucus glands of the lungs, liver, pancreas, intestines) Tay-Sachs Tay-SachsSickle-cell Dominant disorders: Huntington’s Testing: amniocentesis Testing: amniocentesis chorionic villus sampling (CVS) Baby DNA from mother’s blood

A closer look at Tay-Sachs