Overview Definitions Patterns of Mendelian Inheritance Non-Mendelian Inheritance
Genes: Info in chromosomal DNA Heritable traits passed to offspring Diploid (2n): Pairs of genes on pairs of homologous chromosomes
Alleles: Alternative forms of a gene One form usually dominant over other If pair is identical over many generations = true-breeding lineage Hybrid: Cross between 2 true-breeding individuals that have non-identical alleles for trait e.g. AA x aa = hybrid offspring
Homozygous: Pair of identical alleles on pair of homologous chromosomes e.g. A & A Heterozygous: Pair of non-identical alleles on pair of homologous chromosomes e.g. A & a
M locus: leaf colour Both alleles are the same = homozygous D locus: plant height Both alleles are the same = homozygous Bk locus: fruit shape Alleles are different = heterozygous chromosome 1 from tomato pair of homologous chromosomes M D Bk
Dominant allele (e.g. A): Effect on trait masks effect of recessive allele (e.g. a) Note: dominant alleles are not necessarily more common or better Homozygous dominant genotype = AA Homozygous recessive genotype = aa Heterozygous genotype = Aa
Genotype: Genes Individuals alleles e.g. Aa Phenotype: How genes are expressed Individuals observable traits e.g. green eyes
P = true-breeding parents F 1 = 1 st -generation offspring F 2 = 2 nd -generation offspring of self-fertilized or crossed (mated) F 1 individuals
Old Inheritance Theory Hereditary material from both parents mixed at fertilization e.g. red flowers + white flowers = pink flower offspring Couldnt explain obvious variation in traits +
Gregor Mendel & His Peas Viennese monk who studied botany & math Pisum sativum: garden pea Self-fertilizing (flowers produce male & female gametes that fuse to form new plant so that parent & offspring = same traits) Can also be cross-fertilized
Mendel tracked 7 traits over 2 generations
Mendels Theory of Segregation Monohybrid cross: 2 homozygous parents that differ in trait dictated by alleles of 1 gene P F 1 AA x aa Aa
After Mendel tracked 7 traits for 2 generations, he found that: F 2 : recessive forms & dominant forms of trait
Fertilization is chance event with # of possible outcomes Can calculate probabilities of possible outcomes of genetic crosses Can determine all types of genetically different gametes that can be produced by male & female parents Genetics is a science of probability
homozygous parent AAAA gametes
heterozygous parent AaaA gametes
The Punnett Square Method Allows prediction of both genotypes & phenotypes of genetic crosses A a aA
Draw Punnett square with each row & column labelled with one of possible gametes of sperm & eggs respectively Fill in genotype of offspring in each box by combining male & female gametes A A A A a a a a AAAa aAaa
Count # offspring with each genotype & convert to fraction of total # offspring To determine phenotype proportions, add fractions of genotypes that would produce given phenotype Phenotype I (dominant; AA & Aa) = + 2/4 = Phenotype II (recessive; aa) = AA = Aa = aA = 2/4 = aa = A A a a AAAa aAaa
So, for Mendels cross of F 1 offspring from monohybrid cross, he predicted: F 2 = AA, Aa, aa Phenotypic ratio = 3:1 AA + Aa = dominant phenotype aa = recessive phenotype A A a a AAAa aAaa
Since each gamete is equally likely, each of these offspring is equally likely Due to dominance we see a ratio of 3 purple:1 white
An Example Imagine you are crossing a true breeding plant with yellow peas & a true breeding plant with green peas. If yellow color is dominant: What would the F 1 generation look like? What would the F 2 look like?
PPPP PpPpPp These three all look the same! PP pPpP PpPp P ppp white spermeggs offspring genotypes genotypic ratio (1:2:1) phenotypic ratio (3:1) 1212 1212 1212 1212 1212 1212 1212 1212 1414 1414 1414 1414 1414 2424 1414 1414 Dominance creates some problems for scientists For example: How can I know which genotype I have if all I can see is phenotype?
Test cross: Individual shows dominance for trait but genotype is unknown Cross with homozygous recessive individual to see if homozygous dominant or heterozygous
If homozygous dominant:If heterozygous:
Test crosses supported Mendels predictions Mendel found that crossing F 1 purple flowers with true- breeding white flowers: F 2 = purple (Aa), F 2 = white (aa) F 1 purple flowers were heterozygous sperm p p P pp PpPp all eggs PP or Pp sperm unknown if PP if Pp egg pollen p 1212 1212 1212 P p 1212 all Pp sperm
An Example Imagine you have a plant with yellow peas but you dont know its genotype. Remember that yellow is dominant to green. What type of pea would you mate it with? Why? If the offspring are all yellow what does this tell you? Does it matter how many offspring there are?
Mendels Big Ideas Genes have alternate versions (alleles) Organisms have two particles for each gene = diploid Some alleles are dominant to others (in organisms with two different alleles (heterozygous), the dominant allele masks the recessive allele) Alleles separate during gamete formation = the law of segregation Heterozygotes produce two different types of gametes
Mendels Theory of Segregation 2n cells have pairs of genes on pairs of homologous chromosomes Members of each gene pair separate during meiosis & end up in different gametes
Applying Mendels Ideas Imagine you have mated a black guinea pig with an albino guinea pig. They have 12 offspring & all are black. What alleles are dominant in this case? How do you know? What are the parents phenotypes? Genotypes?
Now imagine a cross between a different pair of guinea pigs, one black & one albino. If they have 7 black & 5 albino offspring: What are the parents genotypes? How do you know?
Mendel performed a lot of crosses & sometimes he was tracking more than one trait at a time This let him develop one more Big Idea
Mendels Theory of Independent Assortment Dihybrid cross: True-breeding homozygous parents that differ in 2 traits dictated by alleles of 2 genes P F 1 AABB x aabb AaBb F 1 heterozygous for alleles of both genes
ab AB F 1 = 100% AaBb AaBb For P (AABB), gametes = AB For P (aabb), gametes = ab
With independent assortment, alleles for one trait are independent of alleles for another e.g. if you have A you are equally likely to have B or b This means that each of the four gametes are equally likely
During meiosis of F 1 cells, there are 4 possible combos of alleles in sperm or eggs: 1/4 AB, Ab, aB, ab With 4 different sperm & egg types, F 2 offspring of hybrid cross = 16 possible combos of gametes
AABBAABbAaBBAaBb AABbAAbbAaBbAabb AaBBAaBbaaBBaaBb AaBbAabbaaBbaabb AB Ab aB ab aBAbAB
e.g. with A = purple, a = white B = tall, b = dwarf 9/16 tall, purple 3/16 dwarf, purple 3/16 tall, white 1/16 dwarf, white Phenotypic ratio = 9:3:3:1 abaBAbAB Ab aB ab AABBAABbAaBBAaBb AABbAAbbAaBbAabb AaBBAaBbaaBBaaBb AaBbAabbaaBbaabb
An Example A true breeding plant with wrinkled green seeds was mated to a true breeding plant with smooth yellow seeds. In the first generation all the plants had smooth yellow seeds. What alleles are dominant in this case? How do you know?
Taking these (dihybrid) F 1 plants, Mendel allowed them to self-fertilize We could write the F 1 genotypes like this: SsYy x SsYy What would their gametes look like? SY Sy sY sy What would the zygotes look like? Use a Punnett Square.
P p PP P p pp PpPp eggs Pp self-fertilize pPpP 1212 1212 1212 1212 1414 sperm 1414 1414 1414 Remember that a monohybrid cross will give you a 3:1 ratio The 9:3:3:1 ratio is actually just two 3:1 ratios stacked on top of each other