What we know… Inheritance of biological characteristics is determined by genes Principle of Dominance When there are two or more alleles for a gene, some are dominant while others are recessive Law of Segregation In sexually reproducing organisms, adult cells have two copies of each gene—one from each parent; these genes segregate when GAMETES are formed
Does the segregation of 1 pair of alleles affect the segregation of another pair of alleles?
Let’s look at Pea shape and pea color Round (R) vs. wrinkled (r) Yellow (Y) vs. green (y) Remember, there are 4 possible gametes If a parent is homozygous dominant (true-breeding dominant) for both shape and color, what will all possible gametes be? If a parent is homozygous recessive (true-breeding recessive) for both shape and color, what will all possible gametes be?
Make a test cross of:
True breeding Round Yellow Peas
True breeding Wrinkled Green peas
ry ry ry ry RY RY RY RY
What is the phenotype of the F1 offspring? What is the genotype of the F1 offspring?
What this first cross told us… All F1 offspring were heterozygous for seed shape (round) and seed color (yellow) RrYy The F1 plant was made from fusing a gamete carrying RY and a gamete carrying ry Will the dominant alleles stay together or separate when making the F2 offspring?
Now cross these hybrid (RrYy) plants on a new 4X4 Punnett Square
What does the F2 cross tell us? Are there combinations of alleles that we did not see in either of the parents? This means that the alleles for seed color separated independently than the alleles for seed shape Genes that segregate separately do NOT influence each other’s inheritance
Principle of Independent Assortment Genes for different traits can segregate independently during the formation of gametes. This principle helps account for many genetic variations in plants, animals and other organisms.
Mendel’s 4 Principle’s Inheritance of biological characteristics is determined by genes Principle of Dominance Law of Segregation Principle of Independent Assortment
Exceptions to Mendel Genetics more complicated Some alleles are neither dominant nor recessive Many traits are controlled by multiple alleles or multiple genes
Incomplete Dominance When red flowered (CRCR) plants were crossed with white flowered (CWCW) plants they made…pink flowers (CRCW) Which allele is dominant? neither Incomplete dominance: Case in which one allele is not dominant over another The heterozygous phenotype is somewhere between the two homozygous phenotypes
Codominance Both alleles contribute to the phenotype Chickens Allele for black feathers is codominant with allele for white feathers Chicken looks speckled with black and white feathers Not like the blending of dominant phenotypes… BOTH dominant phenotypes show up In humans Gene for protein that controls cholesterol levels in the blood People with heterozygous form make both types of protien
Multiple Alleles When a gene has more than two alleles NOT more than 2 alleles for a person but MORE than 2 alleles for the trait exist Coat color in rabbits A single gene for coat color At least 4 different alleles Simple dominance and make 4 possible coat colors Genes for human blood type 3 different alleles: IA, IB, I You can get different genotypes: IAIA IAi IAIB IBIB Ibi ii You can get different Phenotypes: Type A (dom) Type B (dom) Type AB (dom) Type O (recessive)
Polygenic Traits “Poly” many “-genic” genes Traits controlled by two or more genes Several genes interact to produce a trait Wide range of phenotypes Skin color Four different genes Fruit Fly eye color Three genes make the reddish brown pigment
Epistasis When the expression of one gene effects the expression of another gene Ex. Fur color in mice…controlled by 2 separate genes Gene 1 Brown fur pigment (BB or Bb) is dominant over gray fur pigment (bb) Gene 2 Coat Pigment depositing gene Dominant gene (CC or Cc) means fur will get pigment and this is determined by gene one Recessive gene (cc) means that no pigment will be deposited on fur…whether the they have the gene for black or brown fur The gene for Pigment Deposition is the EPISTATIC gene because it alters the Phenotypic ration Sd
Homework Complete a Dihybrid cross for 2 mice that are heterozygous for both Brown fur (Bb) and Pigment deposition (Pp) Predict your phenotypic ratio… What are you final phenotypic ratios? Write a paragraph explaining your results.
Genes and the Environment Genes provide the plan for development How the plan unfolds depends on the environment Example: Sunflower has genes for height and color of flowers But these traits are also influenced by climate, soil conditions and water availability
Applying Mendel’s Principles… Apply Mendel’s Principles to many organisms, including humans Thomas Hunt Morgan (1900’s) American geneticist Common fruit fly Drosophiliamelanogaster Produced offspring very quickly Single pair of flies=100 offspring Mendel’s principle’s were tested with Drosophilia and many other organisms and they applied to all of them as well
Would genes on the same chromosome be inherited together? Thomas noticed that almost every single time he crossed two flies that each had red eyes and mini wings, the offspring almost always inherited BOTH red eyes and mini wings This went against Mendel’s Principle of Independent Assortment…(review!)
Gene Linkage Thomas Hunt Morgan gave us the answer 1910 PRINCIPLE of LINKAGE 50 Drosophilia genes Seemed to contradict Principle of Independent Assortment b/c certain genes were always inherited together He grouped the fly’s genes into linkage groups
Linkage groups are made up of genes that seem to be inherited together Linkage groups assort independently but all genes in one group are inherited together Drosophilia 4 linkage groups 4 chromosomes What can be concluded?
Conclusions Each chromosome is actually a group of linked genes Mendel’s Principle of Independent Assortment holds true but an adjustment needs to be made… IT IS THE CHROMOSOMES THAT ASSORT INDEPENDENTLY, NOT THE INDIVIDUAL GENES
How Mendel missed it… What 7 genes did he study? 6 of the 7 genes were on different chromosomes Two genes were on the same chromosome but they were so far apart on the same chromosome that they assorted independently
If two genes are on the same chromosome, does that mean they are linked for ever? No, chromosomes cross over during meiosis so they can separate Crossing over produces new combinations of alleles Important for genetic diversity
Lucky Student Alfred Sturtevant 1911 Columbia University Worked in Morgan’s Lab Hypothesis If two genes are farther apart on a chromosome, the they are more likely to be separated during meiosis Experiment Measured the rate at which linked genes were separated and recombined to make a “map” of distances between genes Conclusion Recombination rates could be used to make gene maps Gene maps showed the location of a gene on a chromosome
Gene Linkage and Crossing Over The farther apart 2 genes are on a chromosome…. The more likely they are to “cross-over” The closer two genes are on the same chromosome… The less likely they are to be separated
Steps for Dihybrid cross Make a Key Trait 1: height Dominant phenotype: Tall TT or Tt (ways to get it) Recessive phenotype: Short tt Trait2: color Dominant phenotype: Purple PP or Pp Recessive phenotype: White pp Write out genotypes for each parent _ _ _ _ x _ _ _ _ Write Out Gametes for each Parent (use arrows) 4 gametes for each parent (_ _) Make Punnett Square (16) boxes Label parent one and write gametes along top Label Parent 2 and write their gametes on side Fill in each box (should have 4 letters) Tally genotypes Write out 4 possible phenotypic combinations Dominant trait 1 and dominant for trait 2:______ Dominant trait 1 and recessive trait 2:_______ Recessive trait 1 and Dominant trait 2:_______ Recessive trait 1 and recessive trait 2:________ Tally Phenotypes (should =16) Write phenotypic ratio __dd__:__dr__:__rd__:__rr__ Dominant- capital letter D Recessive- lower case d Homozygous- 2 of the same size letter If its two little letters recessive dd If its 2 big letters it is dominant DD Heterozygous- 2 different size letters (capital and lowercase) Dd ALWAYS dominant