Chapter 3 Mendelian Inheritance I. Genes, Chromosomes, and Genotypes II. Germ Cells and Their Formation III. Formation of the Embryo IV.The Randomness of Inheritance V. Dominance and Epistasis VI.Sex-Related Inheritance
Chapter 3 Mendelian Inheritance Learning Objective: For students to understand basic genetic terms and Mendel s laws of inheritance and to be able to apply this knowledge to predict genetic outcomes using Punnett squares.
I. Genes, Chromosomes, and Genotypes Gene The basic unit of inheritance found on chromosomes. Chemically made of DNA, the molecule that forms the genetic code for a specific body protein. Allele A different version of the same gene (B vs. b). Originally the result of mutation. Chromosome The structural carrier of genes (roughly 1,000 genes on a chromosome). Inherited in pairs. Largely consists of long strands of DNA. Locus The specific location of a gene on a chromosome (plural is loci).
Genes Chemically composed of deoxyribonucleic acid (DNA), consisting of nucleotides : base (adenine, cytosine, guanine, thymine) sugar (deoxyribose) phosphate (phosphoric acid) The genetic code or codon AAG = Lysine
30-50 million years Evolution of Cattle: 1 million years 1/4 million years
Evolution of Cattle: Cattle have 30 pairs of chromosomes, one member from each parent. A chromosome bears approx. 1,200 genes for a total of approx. 36,000 genes. Eldridge, 1985
Chromosome numbers of Livestock species: Cattle, bison, goats 60 Horses 64 Donkeys 62 Pigs 38 Rabbits 44 Sheep 54 Chickens 78 Turkeys 80 This karyotype is for which species?
Chapter 3 Mendelian Inheritance II. Germ Cells and Their Formation
II. Germ Cells and Their Formation Mendel s Laws of Inheritance: 1) Segregation The separation of paired chromosomes bearing genes during germ formation. 2) Independent Assortment The independent separation of genes at different loci on the same chromosome during germ formation. 3) Recombination The formation of a new combination of genes on a chromosome as a result of crossing-over during germ formation.
Mendel s Laws of Inheritance: 1) Segregation The separation of paired chromosomes bearing genes during germ formation.
Mendel s Laws of Inheritance: 2) Independent Assortment The independent separation of genes at different loci during germ formation.
Mendel s Laws of Inheritance: Parental Genotypes: JJBB JJBb JjBb J J J j B B B Gametes: b b
Mendel s Laws of Inheritance: 3) Recombination The formation of a new combination of genes on a chromosome as a result of crossing-over during germ formation, which ensures equal proportions of gene combinations as observed in the offspring generation.
Mendel s Laws of Inheritance: Exception to Independent Assortment and Recombination is Linkage. Example: Rose-combed Creeper Chickens {R = Rose comb/ r = Single comb; C = Creeper/ c = Normal}
Example: Rose-combed Creeper Chickens Line A Line B Parental RRCC X rrcc lines {Rose-Creeper} {Single-Normal} RrCc F 1 cross RrCc X rrcc Backcross (Test for linkage)
RrCc X rrcc Backcross mating RC Rc rc rc rc RrCc Rrcc rrcc rrcc 22 (P) 1 4 33 (P) Rose-Creeper Rose-Normal Single-Creeper Single-Normal L = R/T (L = linkage coefficient) L = 5/60 = 8.3% (L=50% indicates no linkage)
Chapter 3 Mendelian Inheritance III. Formation of the Embryo Single Gene Effects: A trait influenced by a single gene Dominance Only 1 gene copy is needed (B_) Recessive Two gene copies are needed (bb)
III. Formation of the Embryo Single Gene Effects: Homozygous dominant (BB) Phenotype = Bay color Heterozygous (Bb) Phenotype = Bay color Homozygous recessive (bb) Phenotype = Sorrel color
Littlejohn, M.D., K.M. Henty, K. Tiplady, T. Johnson, C. Harland, T. Lopdell, R.G. Sherlock, W. Li, S.D. Lukefahr, B.C. Shanks, D.J. Garrick, R.G. Snell, R.J. Spelman, and S.R. Davis. 2014. Functional reciprocal mutations of the prolactin signalling pathway define hairy and slick cattle. Nature Communications 5, Article 5861. NC 2014
More trait examples of simply inherited traits are available at the ANSC 3335 website (Course Documents).
Punnett Square Method Bay versus Sorrel Mating of a BB stallion x bb mare Mare Stallion B b 100% of offspring should be heterozygous and Bay colored.
Punnett Square Method Mating of a Bb stallion x bb mare Mare b B Stallion b 1:1 genotypic and phenotypic ratio
Punnett Square Method Stallion B b Mating of a Bb stallion x Bb mare Mare B A 1:2:1 genotypic and a 3:1 phenotypic ratio b
Punnett Square Method - Roan Shorthorns Mating of a Rr bull x Rr cow Cow R R Bull r 1:2:1 genotypic and phenotypic ratio r
Chapter 3 Mendelian Inheritance IV. The Randomness of Inheritance Prediction of number of unique gene combinations in gametes and in zygotes: Gametes: Zygotes: 2 n 3 n Aa = 2 1 = 2 (A,a) Aa x Aa = 3 1 = 3 = (AA, Aa, aa) AaBB = 2 1 = 2 (AB, ab) AaBb = 2 2 = 4 (AB, Ab, ab, ab) AaBb x AaBb = 3 2 = 9 (AABB, AABb, AAbb, AaBB, AaBb, Aabb, aabb, aabb, aabb) n = 100 2 100 = 1.27 x 10 30 ; 3 100 = 5.15 x 10 47
Quantitative traits, which by definition are influenced by many genes, have a normal distribution (a bellshaped frequency distribution) due to the cumulative and independent effects of many genes.
Chapter 3 Mendelian Inheritance V. Dominance and Epistasis No dominance The phenotypic expression of the heterozygote is exactly mid-way between both homozygous genotypes (i.e., expression of both alleles; co-dominance ). Partial dominance The phenotypic expression of the heterozygote more closely resembles the expression of the homozygous dominant genotype (e.g., incomplete dominance ). Complete dominance The phenotypic expression of the heterozygote is identical to the expression of the homozygous dominant genotype. Overdominance The phenotypic expression of the heterozygote is outside the range of the expressions of the homozygous genotypes but usually more closely resembles the expression of the homozygous dominant genotype. Epistasis Inter-allelic interaction. The phenotypic expression of an allele at one locus depends on the alleles present at one or more other loci.
V. Dominance and Epistasis A linear model approach
V. Dominance and Epistasis Examples: No dominance Roan Shorthorn cattle; Calico cats. Partial dominance Comprest Herefords; HYPP in horses. Complete dominance Polled cattle; Bay colored horses; White-belted Hampshire hogs. Overdominance Fertility in F 1 cows; Sickle-cell anemia in humans; Warfarin resistance in rats. Epistasis and Palomino colors in horses.
V. Dominance and Epistasis Examples: Partial dominance Comprest Herefords Photo credit: Harlen Ritchie Photo credit: J.F. Lasley
Epistasis - In the Rabbit Gene Symbols: A = Agouti C = Color c chl = Light chinchilla Agouti A_C_E_ E = Color e = Red Rabbit video Light chinchilla A_c chl _E_ Ghost chinchilla A_c chl _ee Red A_C_ee
V. Dominance and Epistasis A horse breeder recently observed a rare Tobaloosa horse and was so impressed that he decided to produce his own. The photo reveals what resembles a Tobiano from the front-half and an Appaloosa from the rear-half of the animal. He first plans on mating a Tobiano stallion (also called Pinto) to an Appaloosa mare. His next plan is to take a Tobianomarked offspring from this mating and backcross it to an unrelated but nicely marked Appaloosa.
Punnett Square Method Trait Gene Phenotype Tobiano T Large spots or patches of color t Solid body color (no spots) Appaloosa A Solid body color (no spots) a White rump with small dark spots a) Predict the genotypic and phenotypic outcomes and also show these as ratios for the offspring from the first mating (Tobiano stallion with the genotype of Tt AA to an Appaloosa mare with the genotype of tt aa) using the Punnett Square method. Assume complete dominance at both loci. Tip: the T_A_ genotype will produce a Tobiano phenotype.
Punnett Square Method Mating of a TtAA stallion x ttaa mare Mare ta Stallion TA ta 1:1 genotypic and phenotypic ratios
b) The breeder keeps an offspring from the first mating that shows Tobiano markings with a genotype of TtAa. Predict the genotypic and phenotypic outcomes and also show these as ratios for the second mating (the Tobiano-marked offspring from the first mating above to a well marked Appaloosa, tt aa) using the Punnett Square method. TA Ta ta ta ta TtAa Ttaa ttaa ttaa 1:1:1:1 genotypic and phenotypic ratio
V. Dominance and Epistasis Coat Color of Horses: A = Bay a = black B = Bay b = sorrel C = Color C CR = Cremello (dilution)
Chapter 17 - Genetics Coat Color Genotypes of Horses: Solids A_B_C_ = Bay aab_c_ = black A_bbC_ = sorrel Dilutes A_B_C C CR = A_bb_C C CR = Palomino A_B_C CR C CR = Perlino A_bbC CR C CR = Cremello Color dilutions website
Punnett Square Method - x Mating (Bb CC CR ) BC BC CR bc bc CR BC BC CR bc bc CR
Punnett Square Method - x Mating (Bb CC CR ) BC BC CR bc bc CR BC BBCC BC CR BBC CR C CR bc bbcc bc CR bbc CR C CR
Punnett Square Method - x Mating (Bb CC CR ) BC BC CR bc bc CR BC BBCC Bay BC CR bc BBC CR C CR Perlino bbcc Sorrel bc CR bbc CR C CR Cremello
Punnett Square Method - x Mating (Bb CC CR ) BC BC CR bc bc CR BC BBCC BBCC CR Bay BC CR BBCC CR BBC CR C CR Perlino bc bbcc Sorrel bc CR bbc CR C CR Cremello
Punnett Square Method - x Mating (Bb CC CR ) BC BC CR bc bc CR BC BBCC BBCC CR Bay BC CR BBCC CR BBC CR C CR Perlino bc bbcc Sorrel bc CR bbc CR C CR Cremello
Punnett Square Method - x Mating (Bb CC CR ) BC BC CR bc bc CR BC BBCC BBCC CR BbCC Bay BC CR BBCC CR BBC CR C CR Perlino bc BbCC bbcc Sorrel bbc CR C CR bc CR Cremello
Punnett Square Method - x Mating (Bb CC CR ) BC BC CR bc bc CR BC BBCC BBCC CR BbCC Bay Bay BC CR BBCC CR BBC CR C CR Perlino bc BbCC Bay bbcc Sorrel bbc CR C CR bc CR Cremello
Punnett Square Method - x Mating (Bb CC CR ) BC BC CR bc bc CR BC BBCC BBCC CR BbCC Bay Bay BC CR BBCC CR BBC CR C CR Perlino BbC CR C CR bc BbCC Bay bbcc Sorrel BbC CR C CR bbc CR C CR bc CR Cremello
Punnett Square Method - x Mating (Bb CC CR ) BC BC CR bc bc CR BC BBCC BBCC CR BbCC Bay Bay BC CR BBCC CR BBC CR C CR Perlino BbC CR C CR Perlino bc BbCC Bay bbcc Sorrel bbcc CR BbC CR C CR bbcc CR bbc CR C CR bc CR Perlino Cremello
Punnett Square Method - x Mating (Bb CC CR ) BC BC CR bc bc CR BC BBCC BBCC CR BbCC Bay Bay BC CR BBCC CR BBC CR C CR Perlino BbC CR C CR Perlino bc BbCC Bay bbcc Sorrel bbcc CR Palomino BbC CR C CR bbcc CR bbc CR C CR bc CR Perlino Palomino Cremelo
Punnett Square Method
Punnett Square Method
Punnett Square Method
Punnett Square Method
Punnett Square Method
Punnett Square Method
Punnett Square Method
Punnett Square Method - x Mating ( ) Genotypes: 1 BBCC 2 BBCC CR 1 BBC CR C CR 2 BbCC 4 2 BbC CR C CR 1 bbcc 2 bbcc CR 1 bbc CR C CR Phenotypes: 3 Bay (B_CC) 6 (B_CC CR ) 3 Bay Cremello ( Perlino ) (B_C CR C CR ) 1 Sorrel (bbcc) 2 Palomino (bbcc CR ) 1 Sorrel Cremello ( Cremello ) (bbc CR C CR )
Chapter 3 Mendelian Inheritance VI. Sex-Related Inheritance Sex-Linked Genes carried on the X-sex chromosome (e.g., the classic example of calico female cats). Sex-Limited phenotypic expression of a trait is limited to one sex (e.g., milk production and semen quality). Sex-Influenced Modes of gene expression differ between males and females. For example, in certain breeds of cattle and sheep, only one gene is needed for males to express horns but two genes are needed for females to express horns.