Presentation on theme: "Inheritance of Single-Gene Differences – discovered by Gregor Mendel!!! I. I.Mendel: father of genetics A. A. Quick review of terminology B. B. Mendel’s."— Presentation transcript:
Inheritance of Single-Gene Differences – discovered by Gregor Mendel!!! I. I.Mendel: father of genetics A. A. Quick review of terminology B. B. Mendel’s Empirical approach II. II.Monohybrid cross A. Mendel’s Postulates to explain his data B. B. Mendel’s First “law” equal segregation C. C. Punnent Square III. III.Dihybrid cross A. A.Mendel’s Second “law” independent assortment B. B.The branch diagram & probabilities C. C.Using the testcross
I. Gregor Johann Mendel Who was this “Father of Genetics”?
Transmission genetics – link between meiosis & Mendel’s postulates Mendel determined the transmission of discrete units (genes located on chromosomes) from parent to offspring, predicting the formation of gametes Mendel determined the transmission of discrete units (genes located on chromosomes) from parent to offspring, predicting the formation of gametes Future cytological studies suggested a correlation exists between the behavior of chromosomes during meiosis and the transmission of traits Future cytological studies suggested a correlation exists between the behavior of chromosomes during meiosis and the transmission of traits
A. Terminology review Genes come in different forms = ALLELES i.e. there may be a single gene for flower color but several alleles, each producing a different color i.e. there may be a single gene for flower color but several alleles, each producing a different color Each individual has 2 alleles per gene (1 derived from mother, 1 from father) Each individual has 2 alleles per gene (1 derived from mother, 1 from father) Phenotype = expressed form of a character (what an individual looks like) Genotype = specific set of alleles carried by an individual (the actual genetic composition) Homozygous = the alleles of a gene are identical (AA) Heterozygous = the alleles of a gene are different (Aa) Dominant allele = an allele that expresses its phenotypic effect even when heterozygous… therefore AA and Aa have the same phenotype Recessive allele = An allele whose phenotypic effect is not expressed in a heterozygote… therefore (a) can only be expressed when the individual is homozygous – (aa).
Terminology cont. - Genetic Crosses Controlled mating of two specific organisms Self Cross = cross to oneself (plants, fungi) Haploid Cross = simplest, each gene present in 1 copy only (fungi) Diploid Cross = each gene present in 2 copies Homozygote cross (AA x AA), aka pure-breeding Homozygote cross (AA x AA), aka pure-breeding Heterozygote cross (Aa x Aa) Heterozygote cross (Aa x Aa) Testcross = cross with a known homozygote recessive Testcross = cross with a known homozygote recessive Backcross = hybrid offspring are crossed with one of the parents Backcross = hybrid offspring are crossed with one of the parents Reciprocal Cross = in an initial cross, if the female parent has the mutant condition & the male parent has the wild type condition - The reciprocal cross is the reverse (female is wild type & male is mutant) Reciprocal Cross = in an initial cross, if the female parent has the mutant condition & the male parent has the wild type condition - The reciprocal cross is the reverse (female is wild type & male is mutant)
B. Mendel’s success with the empirical approach Came up with an elegant model of experimental design chose a good “model” organism: Pisum sativum chose a good “model” organism: Pisum sativum restricted his examination to one or very few pairs of contrasting traits in each experiment restricted his examination to one or very few pairs of contrasting traits in each experiment took meticulous notes with accurate quantitative records took meticulous notes with accurate quantitative records
Mendel’s Empirical approach By using controlled crosses, Mendel designed experiments to determine the quantitative relationships from which laws could be discovered
Looked at contrasting characteristics of the garden pea -seed coat, seed color, petal color, pod shape, pod color, stem size, axial/terminal flowers.
II. The Monohybrid cross Hybridization = when two plants of the same species but with different characteristics are crossed (mated) to each other. Mono = dealing with one pair of contrasting characteristics P – parental generation F 1 – First filial generation F 2 – Second filial generation
Mendel’s results from the monohybrid crosses Parental F1F1F1F1 F2F2F2F2 F 2 ratio Round x wrinkled All round 5474 round 1850 wrinkled 2.96:1 Yellow x green seeds All yellow 6022 yellow 2001 green 3.01:1 Purple x white All purple 705 purple 224 white 3.15:1 Inflated x pinched All inflated 882 inflated 229 pinched 2.95:1 Green x yellow pods All green 428 green 152 yellow 2.82:1 Axial x terminal All axial 651 axial 207 terminal 3.14:1 Long x short All long 787 long 277 short 2.84:1
A. Mendel’s Postulates to explain his data 1)the existence of unit “factors” – particulate theory of inheritance Traits inherited as discrete units that remain unchanged as they pass from parent to offspring Traits inherited as discrete units that remain unchanged as they pass from parent to offspring 2)genes are in pairs, thus 2 phenotypes must be determined by 2 different alleles of 1 gene When two unlike unit factors responsible for a single character are present in a single individual, one unit factor is dominant to the other (Dominanance/Recessiveness) When two unlike unit factors responsible for a single character are present in a single individual, one unit factor is dominant to the other (Dominanance/Recessiveness) 3)the principle of segregation, genetic units segregate from each other
Points 4 & 5: 4) gametic content – the F2 3:1 ratio is based on a 1:1 segregation in a heterozygote segregation in a heterozygote 5) random fertilization – gametes are brought together for fertilization in a random manner for fertilization in a random manner
B. Mendel’s Law of equal segregation: B. Mendel’s Law of equal segregation: 1)Equal Segregation = The two members of a gene pair segregate from each other into the gametes; so half the gametes carry one member of the pair and the other half of the gametes carry the other member of the pair.
C. Using Punnett Squares in Genetic Crosses Punnett squares used for monohybrid crosses Considers only genes of interest Considers only genes of interest List sperm genotypes across top List sperm genotypes across top List egg genotypes down side List egg genotypes down side Fill in boxes with zygote genotypes Fill in boxes with zygote genotypes
Pp 1(25%) White 3 (75%) Purple Frequencies Phenotypes Genotypes Frequencies Making a Punnett Square: Heterozygous X Heterozygous Eggs of Heterozygous Plant Pollen of Heterozygous Plant 11 2 P p pP PpPP pp PPpppPPp
D. Using the testcross to determine if the parent is heterozygous The organism of the dominant phenotype is crossed to a known homozygous recessive individual
III. Mendel’s Dihybrid Cross Follows the inheritance of two different traits within the same individual. i.e. Yellow, Round x Green Wrinkled
A. Mendel’s (postulate) Second Law of independent assortment: Independent Assortment = two different genes on different chromosomes will randomly assort their alleles during gamete formation Possible gametes produced from meiosis The alleles assort independently
(Hair color) & (Hair length) Black/Brown Short/Long P: Black, short x Brown, long F 1 : all black, short F 2 : Black, short x Black, short: BbSs x BbSs
B. Probability and statistics in genetics How can we calculate the expected ratios of the phenotypes/genotypes for progeny? How can we determine if our results are significantly different from what we would expect under Mendelian principles? Pr(A) = Probability of an event A, number between 0 and 1 that measures the likelihood that A will occur when the experiment is performed Accuracy of prediction depends on sample size
Probability Rules Product rule: the probability of independent events occurring together is the product of the probabilities of the individual events Product rule: the probability of independent events occurring together is the product of the probabilities of the individual events Pr(A) x Pr(B) = Pr(A and B) Sum rule: probability of either of two mutually exclusive events occurring is the sum of their individual probabilities. Sum rule: probability of either of two mutually exclusive events occurring is the sum of their individual probabilities. Pr(A) + Pr(B) = Pr(A or B) Conditional probability: the probability that one outcome will occur, given the specific condition upon which the outcome is dependent Conditional probability: the probability that one outcome will occur, given the specific condition upon which the outcome is dependent Pr c = Pr(a)/Pr(b)
e.g. Product rule in practice If you self cross an F1 dihybrid yellow, round pea plant- What proportion of offspring will be yellow and round? Probability of producing yellow peas: ¾ (Y/Y or Y/y) Probability of producing round peas: ¾ (R/R or R/r) Therefore, Yellow-Round offspring: ¾ x ¾ = 9/16 What if you crossed pure breeding tall plants with purple flowers that make yellow round peas with short plants with pure breeding white flowers that make green wrinkled peas?
P: T/T;P/P;Y/Y;R/Rx t/t;p/p;y/y;r/r F1: T/t;P/p;Y/y;R/r F2? What is the probability of having tall plants with purple flowers that make yellow peas? (T/-;P/-;Y/-;R/-) ¾ x ¾ x ¾ x ¾ = 81/256
Properties of probabilities 1.The probability of an event always takes on a value between 0 and 1 2.The probability of two events occurring together is equal to Pr(A) x Pr(B) 3.If two events A and B are mutually exclusive, then the probability the either A or B occurs is equal to Pr(A) + Pr(B)
A pure breeding black guinea pig is crossed with a pure breeding tan guinea pig. If black is dominant to tan, what will the genotype and phenotype of the F1 be? Give proportions. For the above, what would the genotypes and phenotypes of offspring from an F1 x F1 cross be? Give proportions.
The forked-line method, or branch diagram Calculate the probability of obtaining an aa; B-; C- zygote from the cross Aa; Bb; Cc X Aa; Bb; Cc. Much simpler than using the Punnent square for looking at more than one trait Genetic ratios – expressed as probabilities Based on the product rule of probability Pr(A) x Pr(B) = Pr(A and B)
Binomial expansion Used to predict the probability of an unordered combination of events each event possesses one of two mutually exclusive characteristics, eg. curly hair or straight hair each event possesses one of two mutually exclusive characteristics, eg. curly hair or straight hair the outcome for any one event is independent of the outcome for any other event the outcome for any one event is independent of the outcome for any other event Example: from a cross between two tall plants, Tt x Tt, what is the probability of having 2 dwarf plants out of five offspring?
Pr(x successes in n trials) = n! (n-x)! x! p x q n-x Equation to determine the probability of unordered events Step 1. calculate individual probabilities (p & q) p = ¼, q = ¼ Step 2. determine # of events in category x and the total # of events x = 2 n = 5 Step 3. substitute values for p, q, x in the equation from a cross between two tall plants, Tt x Tt, what is the probability of having 2 dwarf plants out of five offspring? 5! 3!2! 0.25 2 x 0.25 3 =120/12 x (0.0625)(0.015625) =0.0097 or 0.97%
The ability to taste phenylthiocarbamide is an autosomal dominant phenotype, and the inability to taste it is recessive. If a taster woman with a nontaster father marries a taster man who, in a previous marriage had a nontaster daughter, what is the probability: a. that their first child will be a nontaster girl b. that their first child will be a taster girl c. that two out of three children will be nontasters 1/8 3/8 14% (½ x ¼) (½ x ¾)
Inheritance of Gene Differences – non-Mendelian geneic interactions, part 2 I.Non-Mendelian ratios - Interactions between the alleles of one gene II.Interactions between the alleles of more than one gene A. A. Gene interaction B. B. Epistasis
A. Incomplete Dominance Two alleles (heterozygote) produce an intermediate phenotype At the molecular level, the mutant allele results in a reduced amount of functional protein 2 doses = 100% 2 doses = 100% 1 dose = 50% 1 dose = 50% 0 dose = None 0 dose = None
F 1 hybrids have an appearance somewhere in between the phenotypes of the two parental varieties F 1 is pink, an intermediate color between white and red, F 2 1:2:1 incomplete dominance
Example: Tay-Sachs disease – Homozygous recessive individuals are severely affected (death by age 3), Heterozygotes express only about 50% of hexosaminidase enzyme for lipid metabolism. Slightly affected. The closer we look, the more we find that heterozygotes are different from homozygous dominant individuals.
B. Multiple alleles Some genes are found in three or more alleles that are different from each other e.g. white clover, coat color in rabbits
C = full coat color; dominant to all other alleles cch = chinchilla coat, a partial defect in pigmentation; dominant to ch and c ch = himalayan coat, color in only certain parts of body; dominant to c c = albino, no color; recessive to all other alleles A rabbit with chinchilla fur is mated to a himalayan. Some of their F 1 offspring have himalyan fur, some have chinchilla fur and some are albino. Name the genotypes of the parents and the genotypic ratios of the F 1 offspring.
Codominance Specific type of multiple alleles, when two alleles are equally expressed in the heterozygous individual e.g. ABO blood group Genotype Blood Type I A /I A or I A /i A I B /I B or I B /i B I A /I B AB i/iO
C. Lethal alleles Allele in an essential gene that has the potential of causing the death of an organism. Age of onset Age of onset Conditional lethal alleles Conditional lethal alleles Semilethal alleles Semilethal alleles
II. Interactions between the alleles of more than one gene Genes interact in concert with other genes and with the environment to influence a particular characteristic. Final product
I Interactions between genes produce many different phenotypes Most traits can be affected by the contributions of two or more genes Examples: morphological characteristics - Height, weight, growth rate, pigmentation
One trait, involving between two genes Bateson & Punnett 1906 studied comb morphology in chickens – found a departure from expected ratio for a single trait RPRprPrp RPRRPPRRPpRrPPRrPp RpRRPpRRppRrPpRrpp rPRrPPRrPprrPPrrPp rpRrPpRrpprrPprrpp 1 trait: comb 9/16 Walnut 3/16 Rose 3/16 Pea 1/16 Single
–9:3:3:1 with four different phenotypes R-P- (walnut), R-pp (rose), rrP- (pea), rrpp (single)
A. Epistatic interactions Often arise because two or more different proteins participate in an enzymatic pathway leading to the formation of a single product. Colorless colorless Purple Pigment precursorintermediate Enzyme C Enzyme P Expression of one gene or gene pair masks or modifies the expression of another gene or gene pair Epistatic = gene product that masks another gene. Hypostatic = the second gene product being masked by another gene. Enzyme C needed to convert the precursor into the intermediate, Enzyme P converts the colorless intermediate into purple pigment
Sweet Peas – flower color P: White x White F 1 : all purple, CcPp F 2 : 9 purple, 7 whiteCPCpcPcp CPCCPPCCPpCcPPCcPp CpCCPpCCppCcPpCcpp cPCcPPCcPpccPPccPp cpCcPpCcppccPpccpp C-P- (purple), cc or pp masks C or P (producing white flowers) homozygosity for the white allele at one gene masks the purple producing allele of another gene [EPISTASIS] CCpp x ccPP
2 genes: bw + & st +, both necessary for red eye (wild type eye color in Drosophila)
Two genes encode enzymes catalyzing successive steps in the synthesis of blue pigment (pathway). If the first step in the pathway is blocked due to a homozygous mutant (w/w) or both steps are blocked, then the flower will be white. If the second step in the pathway is blocked due to a homozygous mutant (m/m) the flower will be magenta. P: w/w;m + m + x w + w + /mm F 1 : w + /w;m + /m (all blue) F 2 : 9:3:4 9/16 Blue: w + /-; m + /- 3/16 Magenta: w + /-; m/m 4/16 White: w/w; m + /- & w/w; m/m
B- (black) bb (Brown) E- (color deposition) B/B, E/EB/B, e/e b/b, E/E The progeny of a dihybrid cross would be 9:3:4, black, brown, golden.
BEBebEbe BEBBEEBBEeBbEEBbEe BeBBEeBBeeBbEeBbee bEBbEEBbEebbEebbEe beBbEeBbeebbEebbee BbEe x BbEe Precursor MoleculeBlack brown color deposited B- bb E- ee golden (golden)
Plants of the mustard family were crossed. When a true-breeding plant with triangular seeds is crossed with a plant with ovate seeds, the F1 generation has triangular seeds. When the F1 is self-fertilized, the result is a 15:1 ratio (triangular:ovate). Explain this ratio. This is a 2 gene interaction. The presence of one dominant allele in either gene results in a triangular seed.