Presentation on theme: "Genetics What accounts for the passing of genetic traits from parents to offspring? Are traits blended in the offspring? Or: are traits inherited."— Presentation transcript:
What accounts for the passing of genetic traits from parents to offspring? Are traits blended in the offspring? Or: are traits inherited as single, discrete and separate units (genes)?
Today, much of what we know about genetics and heredity started with the work of an Austrian monk in the 1800s—Gregor Mendel Mendel discovered the basic principles of heredity with his experiments with garden peas.
Varieties include those with distinct heritable characteristics (traits) such as flower color or plant height. Pea plants are normally self-pollinated, but can be easily cross-pollinated by the plant breeder. Therefore, the breeder can control which traits are crossed.
Pea Flowers have Petals that are closed over the Stamens and Carpels. They are not open to wind and other pollinators such as insects. Therefore, they are self-pollinating. What does this mean regarding their genetic diversity?
Mendel chose traits that were either..or He also chose plants that were true-breeding for a particular trait. (Purebred). And, since peas plants are normally self- pollinated, he knew what genes they carried.
In his experiments, Mendel crossed two plants with contrasting traits, producing offspring called hybrids. The true-breeding parents are the P (parental) generation The offspring are called the F 1 generation. When the F 1 generation plants self-pollinate, their offspring are called the F 2 generation. P
When Mendel crossed contrasting, true- breeding white and purple flowered pea plants, all of the F 1 hybrids were purple When Mendel crossed the F 1 hybrids, many of the F 2 plants had purple flowers, but some had white Mendel discovered a ratio of about three to one, purple to white flowers, in the F 2 generation
Fig. 14-3-3 EXPERIMENT P Generation (true-breeding parents) Purple flowers White flowers F 1 Generation (hybrids) All plants had purple flowers F 2 Generation 705 purple-flowered plants 224 white-flowered plants
In the first experiment, when only purple flowers were produced, Mendel thought that the white trait had possibly been absorbed and had disappeared. However, when it reappeared in the F2, he knew it had just been hidden. So, he called the purple color the dominant trait and the white color the recessive trait.
Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F 2 offspring Four related concepts that make up this model can be related to what we now know about genes and chromosomes Mendel’s garden in the abbey in Austria where he conducted his experiments.
Alternative versions of genes account for variations in traits. For example: there are two versions of the gene for flower color in peas: purple and white. These alternative versions are called alleles—each of which resides in a particular place on a chromosome (the locus).
For each characteristic, an organism inherits two alleles: one from the male parent and one from the female parent. The alleles may be the same (like the true- breeding plants), or they may be different (like the F 1 hybrids). Mendel figured all of this out without ever knowing anything about chromosomes! Homozygous Axial Homozygous terminal Heterozygous Axial
If the two alleles at a locus are different, then one of them (the dominant allele) determines what the organism will look like. The recessive allele has no noticeable affect on the appearance. Why are all of the F 1 offspring purple?
The law of segregation states that the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes An egg or sperm gets only one of the two alleles that are present in the somatic cell. Mendel figured this out without ever knowing anything about meiosis!
How do we explain the 3:1 results that Mendel got in the F2 generation? If we know the genetics of the parents, a Punnett Square can show the possible combinations of genes the offspring can inherit. Use CAPITAL letters for dominant genes, lower case letter for recessive
Homozygous —an organism with two alleles for a given trait that are identical (also called purebred) Heterozygous —an organism with two alleles for a given trait that are different. (these are also known as hybrids)
An organisms’ traits may not always reveal its true genetics. (think about those hybrid purple plants) Phenotype : what the organisms’ physical traits are—what it looks like Genotype : what genes the organism carries— what are its actual genes? Notice that the Genotype Ratio is 1:2:1 and the Phenotype Ratio is 3:1. Why are they different?
TECHNIQUE RESULTS Dominant phenotype, unknown genotype: PP or Pp ? Predictions Recessive phenotype, known genotype: pp If PP If Pp or Sperm ppp p P P P p Eggs Pp pp or All offspring purple 1 / 2 offspring purple and 1 / 2 offspring white Test Cross Technique
Mendel figured out the Law of Segregation by studying the results of crosses involving only one trait (for example, flower color). These are called monohybrid crosses.
Mendel derived his second law by studying crosses involving pea plants who differed in two traits. These are called dihybrid crosses. An organism who is hybrid for two traits is called a dihybrid.
A dihybrid cross, a cross between F 1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently
1. Figure out the genotypes of the parents. 2. Figure out the possible combinations of genes that could be in the gametes of these parents. Example: TtYy could produce 4 kinds of gametes: TY, Ty, tY, ty 3. Put the gametes into a Punnett Square & Solve 4. Figure out genotypic and phenotypic ratios.
Using a dihybrid cross, Mendel developed the law of independent assortment The L aw of Independent Assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation Law of Independent Assortment Remember that in Meiosis, it is random which direction the chromosomes go during Anaphase I and II. There are all kinds of possibilities!
Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: When alleles are not completely dominant or recessive When a gene has more than two alleles When a gene produces multiple phenotypes Why does everything have to be so complicated????
Complete dominance—when the dominant gene totally dominates over the recessive. Most of the traits Mendel studied showed complete dominance.
In incomplete dominance, the phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties
Red P Generation Gametes White CRCRCRCR CWCWCWCW CRCR CWCW F 1 Generation Pink CRCWCRCW CRCR CWCW Gametes 1/21/2 1/21/2 F 2 Generation Sperm Eggs CRCR CRCR CWCW CWCW CRCRCRCR CRCWCRCW CRCWCRCW CWCWCWCW 1/21/2 1/21/2 1/21/2 1/21/2 Incomplete Dominance in Snapdragons
In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways There isn't a blending of the traits, but rather both alleles are present in the phenotype. In this flower, both the dark pink allele and the white allele are co- dominant. Neither one dominates over the over, so the phenotype shows both alleles.
Polygenic Traits are characteristics that are affected by more than one gene. Examples: Eye Color, Skin Color Eye color comes from different genes which affect tone, amount and position of the pigments. Skin color is determined by at least 3 different genes working together to produce a wide variety of tones.
Most genes exist in more that two allelic forms. A classic example of this is human blood types. Four major blood types exist: O, A, B, and AB They are named for the presence or absence of certain carbohydrates on the surface of the red blood cells.
The gene for blood type has 3 possible alleles: I A, I B, and i Both I A and I B are dominant over i I A and I B are co- dominant to each other Type A Blood Genotypes: I A I A or I A i Type B Blood Genotypes: I B I B or I B i Type AB Blood Genotype: I A I B Type O Blood Genotype: ii
Pedigrees are family trees used to describe the genetic relationships within a family. Pedigrees can be used to determine the risk of parents passing certain conditions to their offspring