Patterns of Inheritance Lecture Slides PDF

Patterns of Inheritance Dr. Praveen Babu Life Science for Engineers BIO 213  Chromosome Review Humans are diploid (2n); we have two copies of each chromosome Two members of a pair make up a homologous pair of chromosomes 23 pairs of chromo...

Patterns of Inheritance Dr. Praveen Babu Life Science for Engineers BIO 213  Chromosome Review Humans are diploid (2n); we have two copies of each chromosome Two members of a pair make up a homologous pair of chromosomes 23 pairs of chromosomes total 22 pairs of autosomes 1 pair of sex chromosomes (designated X and Y chromo’s) Homologous pairs contain the same genes, but may have different alleles or “flavors” of the same gene  Johan Gregor Mendel Mendel experimentally uncovered the fundamental principles of genetics His use of pea plants rapidly sped up the study of inheritance (20+ years for humans vs. 100 days for pea plants) He chose pea plants because: They had a number of different traits that could be studied They were self-fertilizing and had a flower structure that minimized accidental pollination Their offspring were fully fertile enabling future crosses to be made  Johan Gregor Mendel Mendel tested all available varieties of peas for 2 years to ensure that he and true-breeding lines Lines that give rise to the same traits in all offspring, generation after generation He performed experiments on a large scale (28,000 plants) to avoid artifacts His methodical, hypothesis-driven approach was the key Mendel studied seven characters that affected the seeds, pods, flowers, and stems of the plants  Genetic Terminology NOTE: genes and chromosomes are terms that Mendel never knew; coined well after his death Mendel’s inherited factors are what we know as genes Alleles are the different versions of a gene E.g., Pisum sativum pea plants have one gene for seed shape that can be either of two alleles—smooth (also known as round) or wrinkled All alleles of a particular gene will be found at a specific place on the chromosome called a locus  Plants: Single Traits 1st performed monohybrid crosses; differed only in inheritance of seed shape Crossed plants with smooth seeds and plants with wrinkled seeds & vice versa (parental generation, or P1) Seeds from this cross, the first filial (F1) generation, were ALL smooth These seeds were planted and allowed to mature and self-fertilize Of the 7324 seeds collected, 5474 were smooth and 1850 were wrinkled 74.74% smooth :and 3 Smooth 25.26% wrinkled 1 wrinkled  Plants: Single Traits Results concerning all 7 characters were the same: F1 only showed one of the two parental traits Didn’t matter which plant supplied pollen; reciprocal cross results were always same Trait not present in F1 offspring reappeared in 25% of F2 offspring Mendel showed that traits remained unchanged as they were passed from parent to offspring; not blended  Plants: Single Traits Might not be expressed, but still present and passed on Traits were inherited as separate units and don’t blend! From these experiments, Mendel was able to conclude that each parent makes an equal contribution to the genetic makeup of the offspring He also made several conclusions that hold up even today…  Conclusions: Genes Genes (Mendel called them “factors”) determine traits and can be hidden or unexpressed F1 seeds contain a gene for wrinkled that was present, but not expressed; reappeared in the F2 generation (the wrinkled seeds) Called the trait not expressed in the F1, but expressed in some of F2 a recessive trait Called trait seen in F1 the dominant trait Called this phenomenon dominance  Phenotype versus Genotype Dominance led Mendel to conclude that while they looked identical, the P1 and F1 plants differed genetically It is important to make a distinction between an organism’s appearance and its genetic makeup… Genotype: specific genetic constitution of an organism Phenotype: observable properties of an organism Which have same phenotype? P1 & F1  How Many Genes? Already concluded male and female parents contribute equally to traits of offspring Simplest explanation is that there are two alleles for seed shape gene: Smooth (dominant) = S wrinkled (recessive) = s Noted that gene number doesn’t double every generation so Mendal reasoned that gene pairs segregate  How Many Genes? Principle of Segregation: the separation of a gene pair during gamete formation; (Mendel’s 1st Law)  How Many Genes? Principle of Segregation: the separation of a gene pair during gamete formation; (Mendel’s 1st Law) Remember, Mendel figured all of this out before discovery of mitosis, meiosis, or even chromosomes!; No punnet sq’s either  Mendel’s 1st Law: Segregation Mendel’s reasoning allows prediction of F2 genotypes Mendel confirmed these predictions through five successive generations! Mendel accurately identified alternative forms of a gene or alleles; “different flavors” Organisms with identical alleles are homozygous for that gene Heterozygous – diff. alleles  Plants: Two Traits Next examined a pair of differing traits; dihybrid cross Mendel knew Smooth (S) is dominant to wrinkled (s) and Yellow (Y) is dominant to green (y) Performed the following cross: Smooth Yellow (SSYY) x wrinkled green (ssyy) What do you expect in F1 generation? Smooth Yellow (SsYy) In the F2 generation? Saw 4 different phenotypes…  Plants: Two Traits Next examined a pair of differing traits; dihybrid cross Mendel knew Smooth (S) is dominant to wrinkled (s) and Yellow (Y) is dominant to green (y) Performed the following cross: Smooth Yellow (SSYY) x wrinkled green (ssyy) What do you expect in F1 generation? Smooth Yellow (SsYy) In the F2 generation? Saw 4 different phenotypes…  Plants: Two Traits In the F2 generation? Saw 4 different phenotypes… 315 smooth and yellow 108 smooth and green 101 wrinkled and yellow 32 wrinkled and green Notice that the 3:1 ratio is still present for EACH trait 423 smooth : 133 wrinkled  Plants: Two Traits In the F2 generation? Saw 4 different phenotypes… 315 smooth and yellow 108 smooth and green 101 wrinkled and yellow 32 wrinkled and green Notice that the 3:1 ratio is still present for EACH trait 423 smooth : 133 wrinkled 416 yellow : 140 green Saw a 9:3:3:1 ratio overall  Plants: Two Traits Principle of Independent Assortment: alleles are distributed into gametes randomly during meiosis; (Mendel’s 2nd Law) Explains the inheritance of two traits (i.e., the 9:3:3:1 ratio in F2) Mendel assumed that alleles of one gene pair (Smooth vs. wrinkled) segregate into gametes independently of the alleles of other gene pairs (Yellow vs. green) Resulting gametes have all combinations of alleles Mendel used probabilities to deduce this  Plants: Two Traits The principle of independent assortment to explains the inheritance of two traits (i.e., the 9:3:3:1 ratio in F2) We know F1 generation will be all Smooth Yellow All Ss Smooth All Yy Yellow  Plants: Two Traits Only possible outcome from the P1 cross is SsYy When the F1’s are crossed, there are four phenotypic combinations now possible  Plants: Two Traits Only possible outcome from the P1 cross is SsYy When the F1’s are crossed, there are four phenotypic combinations now possible Again, we see the 9:3:3:1 ratio Mendel published his results after 10 years of work!  Meiosis Explains Mendel’s Results Mendel had no knowledge of mitosis, meiosis or chromosomes By 1900, it became obvious that gene and chromosomes has much in common  Meiosis Explains Mendel’s Results Mendel had no knowledge of mitosis, meiosis or chromosomes By 1900, it became obvious that gene and chromosomes has much in common We now know genes are located on chromosomes Each gene is located a specific site on a chromosome called a locus Humans carry their 20,000 – 25,000 genes on 23 pairs of chromosomes (22 autosomal pairs, 1 sex pair) Not all chromosomes have the same number of genes For example, chromosome 19 is the most gene dense  Modification of Mendelian Ratios Mendel was lucky! that he chose (1) diploid pea plants (2) genes on separate chrom.’s (3) traits that all showed clear dominance If he had chosen a different system, he may not have been able to make the same conclusions Linked genes, for example, do not assort independently during meiosis  Modification of Mendelian Ratios Linked Mendelgenes, was lucky! that he chose for example, do not assort independently during meiosis (1) diploid pea plants BUT, they can occasionally assort independently if a (2) genes on separate chrom.’s crossover event occurs between them (3) traits that all showed clear dominance If he had chosen a different system, he may not have been able to make the same conclusions Linked genes, for example, do not assort independently during meiosis  Incomplete Dominance Incomplete dominance: the expression of a phenotype that is intermediate to those of the parents + = Does such a phenomenon follow Mendel’s laws? While phenotypes are complex, the genotypes in these instances strictly follow Mendelian inheritance While rare in humans, in snapdragons, flower color is controlled by two dominant genes (incomplete dominance)  Incomplete Dominance Incomplete dominance: the expression of a phenotype that is intermediate to those of the parents + = In snapdragons, flower color is controlled by two dominant genes: R1 (red) and R2 (white; or “not red”) Crosses between two homozygous dominant strains Does such a phenomenon follow Mendel’s laws? (R R x R R ) yield pink flowers (R R ) because neither 1 1 2 2 1 2 allele While is phenotypes recessive are complex, the genotypes in these instances strictly follow Mendelian inheritance F1 R R (pink) 1 2 While rare in humans, in snapdragons, flower color is controlled by two dominant genes (incomplete dominance)  Incomplete Dominance R2 R2 R1 R2 R1 R1R2 R1R2 R1 R1R1 R1R2 In snapdragons, flower color is controlled by two R 1 R 1 R2 R 1 R 2 R 2 R R R2R2 2 1 dominant genes: R (red) and R (white) 1 2 Crosses between two homozygous dominant strains (R1R1 x R2R2) yield pink flowers (R1R2) because neither allele is recessive F1 R1R2 (pink) F1 x F1 R1R2 (pink) x R1R2 (pink) F2 ¼ R1R1 (red) : ½ R1R2 (pink) : ¼ R2R2 (white)  Codominance: Multiple Alleles In codominance, full expression of both alleles is seen heterozygotes Also follows Mendelian inheritance (for genotype) Not the same as incomplete dominance because unlike “red” and “not red” yields pink, codominance is when two different alleles are fully expressed Incomplete dominance: red + “not red” = pink (an intermediate phenotype) Codominance: red + “not red” = red AND white flowers on the same snapdragon plant (both phenotypes)  Codominance: Multiple Alleles Many genes have more than two alleles; we usually only carry two alleles because we’re diploid (2n) ABO blood group system comes from 3 codominant alleles of one gene (I): IA , IB , and iO Type AB individuals are universal acceptors and Type O individuals universal donors (who get the most cookies)  Inheritance Both sexes have pairs of autosomes, but different sex Patterns chromosomes (designated X and Y) Because the X chromosome has many more genes than the Y, many sex-linked traits are X-linked For example, red-green color blindness or hemophilia is X-linked recessive  Inheritance For Bothexample, red-green sexes have pairs ofcolor blindness autosomes, butordifferent hemophilia sex is X-linked Patterns recessive chromosomes (designated X and Y) Because the X chromosome has many more genes than the Y, many sex-linked traits are X-linked For example, red-green color blindness or hemophilia is X-linked recessive  Inheritance For example, red-green color blindness or hemophilia is Patterns X-linked recessive Many more patterns of inheritance exist and can be visualized using pedigrees This one shows autosomal dominance Lastly, remember that your genes AND the environment contribute to phenotypes