Mendelian Genetics Review Simple Dominance Codominance Incomplete Dominance Sex Linked Dihybrid Crosses

Simple Dominance  P 1 cross b/t pure breeding dominant and recessive traits yields an F 1 generation that is 100% dominant in phenotype.  Heterozygotes express dominant phenotype  A cross between the F 1 hybrids (heterozygotes) yields a predictable 1:2:1 ratio of homozygous dominant: heterozygous: homozygous recessive in F 2 offspring  F 2 monohybrid phenotypic ratio = 3 dom:1 rec.

Codominance  P 1 cross b/t pure breeding strains of two traits yields an F 1 generation that exhibits both phenotypes fully expressed (heterozygous)  A cross between the F 1 hybrids (heterozygotes) yields a predictable 1:2:1 ratio of homozygous trait 1: heterozygous 1+2: homozygous trait 2 in F 2 offspring  F 2 monohybrid phenotypic ratio = 1:2:1

Incomplete Dominance  P 1 cross b/t pure breeding strains of two traits yields an F 1 generation that exhibits the intermediate trait (heterozygous)  A cross between the F 1 hybrids (heterozygotes) yields a predictable 1:2:1 ratio of homozygous trait 1: heterozygous ½ blend: homozygous trait 2 in F 2 offspring  F 2 monohybrid phenotypic ratio = 1:2:1

Sex Linkage  Can be X or Y linked: carried only on that chromosome  X-linked recessive: expressed more in males. Why?  Y-linked: only expressed in males  X-linked recessive  Female carriers (heterozygotes) X normal males yield F 1 normal females and 50% affected males

Dihybrid Cross P 1 cross of organisms with two sets of seemingly opposite traits yield offspring that express only one of each of the traits = 100% dominant for both phenotypes. A cross of the F 1 hybrid offspring yields a predictable 9:3:3:1 phenotypic ratio for the F 2 generation.

What does it mean when these predictions DON’T occur? There is something else controlling the expression of the genes. Such as: gene linkage, epistasis, dominance series, polygenes, etc.

allele Variable form of a gene

genotype Symbolic representation of a gene combination

phenotype Expression of a gene combination

Punnett square Graphical representation of the probability of inheriting a particular trait

More Terms Generations: P 1, F 1, F 2... Heterozygous, Homozygous Testcross Monohybrid, dihybrid, multihybrid 3:1/1:2:1 monohybrid 9:3:3:1 Dihybrid Law of segregation/Law of independent Assortment

Solving Multihybrid crosses Crossing the following : AABBCCDDEEFF x aabbccddeeff P 1 All AaBbCcDdEeFf F 1 –Just like monohybrid crosses!! Crossing F 1 offspring, you can predict number of gametes and genotypes of offspring easily –Number of gametes = 2 n where n=number of heterozygous allele combinations –Probability of offspring with a “multiple” genotype = product of individual genotype probabilities

Example Crossing AaBbCcDdEe hybrids There are 5 pairs of alleles that are heterozygous 2 5 = number of possible gametes AABBccDdEeFf gives how many possible gametes? 2 3 = 8 possible gametes

Offspring Probability Examp. What is the probability of getting an offspring with the genotype AABBCcDdEe from a Multihybrid cross of AaBbCcDdEe? Treat each pair of alleles INDEPENDENTLY. Product of probabilities = overall probability ¼ AA x ¼ BB x ½ Cc x ½ Dd x ½ Ee = 1/128 probability of AABBCcDdEe genotype

DNA Deoxyribonucleic acid Deoxyribose (pentose sugar) + Phosphate backbone Nitrogenous bases: A, G, C, T Sugar + Phosphate + base = nucleotide Cells contain approximately 3 meters of DNA each

Closer look Closer look

Chromosome Supercoiled DNA/protein complex designed to “manage” the immense information packaged in cellular DNA

Genome Full complement of DNA contained within and organism

Gene Segment of DNA encoding a protein and/or regulating the expression of other genes

Sample Multihybrid Gametes and Probabilities

A pea plant is heterozygous for both seed shape and seed color. S is the allele for the dominant, spherical shape characteristic; s is the allele for the recessive, dented shape characteristic. Y is the allele for the dominant, yellow color characteristic; y is the allele for the recessive, green color characteristic. What will be the distribution of these two alleles in this plant's gametes?

25% SY 25%Sy 25% sY 25% sy

Law of Independent Assortment: Alleles of different genes are assorted independently of each other during the formation of gametes. Law of Segregation: during meiosis, alleles separate from each other

The ability to taste the chemical PTC is determined by a single gene in humans with the ability to taste given by the dominant allele T and inability to taste by the recessive allele t. Suppose two heterozygous tasters (Tt) have a large family. a. Predict the proportion of their children who will be tasters and nontasters. Use a Punnett square to illustrate how you make these predictions.

What is the likelihood that their first child will be a taster? What is the likelihood that their fourth child will be a taster?

1st child-- 3/4 4th child-- 3/4 (each child is independent)

What is the likelihood that the first three children of this couple will be nontasters?

1/4 for each child; 1/4 x 1/4 x 1/4 = 1/64 that all three will be nontasters Product rule: AND is key word

Tail or No Tail... that is the question. Tail = expressed trait P1 F1 F2 F3

Mackerel/Blotched Tabby Stripes

+ +

Inheritance Modes All white v. not all white All white dominant to not all white Some white v. no white Some white dominant to no white with variable expressivity Dilute v. not dilute Dense color dominant to dilute

continued Agouti v. non-agouti Agouti dominant epistatis Makeral v. blotched Mackeral dominant No Tail dominant over Tail