Key Terms & Scientists Genetics Traits Blending hypothesis Mendel Self-fertilization (true breeding) Hybrid Cross pollination Monohybrid/Dihybrid Parental generation First/second filial (generation) Genes/alleles Dominant/recessive Homozygous & heterozygous Law of Segregation & Law of Independent Assortment Probability Punnett Square Testcross Complete & incomplete dominance Codominance Intermediate inheritance Polygenic traits Multiple alleles Pleiotropy Chromosomal Theory of Inheritance Linked & sex-linked genes Sex Chromosomes Autosomes
Inheritance Genetics is the scientific study of heredity. A trait is a characteristic that is passed from parent to offspring (ex. Eye color). The blending hypothesis was once believed to be the way traits were inherited from generation to generation. Think mixing paints. This is the idea that each generation is a mix (or blend) of both parents genes (traits). This does not account for the appearance of unexpected traits. Traits are passed to offspring through chromosomes. Genes DON’T Mix!
Gregor Mendel, an Austrian Monk, (1860’s) studied the pea plant. He knew nothing of molecular biology (or chromosomes). He did NOT support the blending hypothesis, and in fact, disproved it through his studies. He is the father of genetics.
Mendel used the pea plant for 3 reasons: 1. The structure of the pea flowers allowed: self fertilization (which means the plant can breed with itself, a process called pure breeding) OR he could cross pollinate the flowers and produce a hybrid (this is an organism that receives different forms of a genetic trait from each parent, or 2 sets of DNA: 1 from each parent). 2. The rapid reproduction cycle: the pea plant reproduces about every 90 days.
3. The presence of distinctive traits allowed Mendel to observe his results easily. He studied 7 traits (we will look at 5). Traits in the pea plant have only 2 forms (there is NO intermediate or in between form; it is either/or): Purple (P) vs. white (p)= flower color Yellow (Y) vs. green (y)= pea color Round (R ) vs. wrinkled (r )= pea shape Green (G) vs. yellow (g)= pod color Tall (T) vs. short (t)= height
Mendel’s Observations: When Mendel worked with the pea plants he used 2 different groups of purebred plants, looking at 1 trait at a time. For example, he used 1 group of purebred purple flower pea plants & 1 group of purebred white flower pea plants.
He crossed these 2 groups with each other (cross pollinated them) and called them the parental generation, or P. ◦ This is a monohybrid cross (crossing 1 trait). All of the offspring had purple flowers. ◦ This generation did not show up as a blend of parents (no mix b/c they are not less purple). But, where did the white flower trait go?
He called this generation of offspring the First Filial or F1 generation (filial refers to offspring). The offspring is a hybrid of the parents. He allowed the F1 generation to self- fertilize. He called this generation the second filial, or F2 generation. The F2 offspring revealed 3 out of 4 had purple flowers and 1 out of 4 had white flowers. Again, no blending resulted. Also, the white flower trait had NOT disappeared.
Mendel performed this experiment with all 7 traits and received the same results: the offspring is not a mixture of the parents; the original traits do not disappear. In his work, all F1 revealed 1 characteristic: this characteristic is dominant. All F2 generations were in a 3:1 ratio (3 dominant: 1 recessive).
F1 generation F2 generation
Genes are sections of a chromosome that code for a trait. ◦ Most organisms have 2 copies for every gene and chromosome (1 from each parent). An allele is a distinct form of a gene. ◦ If an organism has 2 different alleles for 1 trait, only 1 allele is expressed or visible (usually).
The dominant allele is a form of a gene that is fully expressed when 2 different alleles are present. ◦ This is represented with a capital letter (and is written 1st). ◦ Ex. Purple= P The recessive allele is a form of a gene that is not expressed when paired with a dominant allele (it takes 2 recessives to be expressed). ◦ This is represented by a lower case letter & is written 2nd. ◦ Ex. White= p
The Chromosome Theory of Heredity (developed by Walter Sutton) states that the material of inheritance is carried by the genes in the chromosomes. A genotype is the genetic makeup of an organism. Ex: GG, Gg, gg or BB, Bb, or bb A phenotype is the physical expression of the genotype or the outward expression of that trait. Ex: yellow peas.
Homozygous is having 2 of the same alleles (2 identical alleles). Ex: GG or gg Heterozygous is having 2 different alleles. Ex: Gg
Mendel’s Laws: These are the Rules of inheritance: 1. The Law of Segregation: Gene pairs separate when gametes form. This means: genes (alleles) are on chromosomes; chromosomes separate during meiosis; gametes form during meiosis; therefore, genes separate when gametes form. 2. The Law of Independent Assortment: When looking at 2 traits at the same time, it is seen that traits are inherited independently from each other. Gene pairs segregate into gametes randomly and independently of each other.
Genetics & Predictions: In genetics we use mathematical probability (P). If you flipped a coin what are the chances of it landing on heads? ◦ P= ½ or 50% If you flipped a coin 10X what would you expect the chances of it landing on heads? ◦ About 5 times or 50% or ½ or 1:1 (ratio) In science, we generally use the ratio.
A punnett square is used to organize & predict genetic information. Let’s use Mendel’s purebred purple flowers & purebred white flowers: PP X pp Always show the cross Set up square Genotype= 4PpAlways use ratios! Phenotype= All PurpleUse WHOLE #s (no fractions)!
Let’s cross the F1 generation. Pp X Pp Genotype= 1PP: 2Pp: 1pp Phenotype= 3 purple: 1 white Now you have some practice problems!
What happens if we have a purple flower but we don’t know if it is heterozygous or homozygous? How would we figure out what it is? We would perform a testcross. This is a cross between a recessive organism (in this case a white flower because we know the genotype) with an organism that has an unknown genotype (the organism that is showing the dominant phenotype) in an attempt to discover the genotype of the unknown. If the offspring result in a recessive organism then the unknown parent must be heterozygous.
Variations in Inheritance: Complete dominance is what Mendel saw. One trait is completely dominant (expressed) over another. Either/or; dominant or recessive. Purple flowers or white flowers. Intermediate Inheritance: Not all genes are cut and dry; one allele is not always clearly dominant over another & there are not always just 2 distinct forms in nature. Intermediate inheritance is when the heterozygous offspring has its own trait (different than either parent). This is not seen in pea plants. This includes codominance & incomplete dominance.
Incomplete dominance is when there is a heterozygote BUT neither the dominant or recessive allele is completely expressed. Look at snapdragons. A red snapdragon (RR) is crossed with a white snapdragon (rr). ◦ As you would expect, the F1 generation is Rr BUT they are not Red, they are PINK! This almost looks like the blending hypothesis, right? But it is not. Why??
Allow the F1 generation to self- fertilize. RrXRr The genotypic results are 1RR: 2Rr: 1rr The phenotypic results are 1 red: 2 pink: 1 white The original traits are NOT lost; therefore this is NOT the blending hypothesis. An example of incomplete dominance in humans is hypercholesterolemia (having too much cholesterol in the blood). endel/c14x9incomplete-dominance2.jpg
Codominance is seen when there are more than 2 alleles for 1 trait and 2 different dominant alleles are together but neither dominant alleles overpower the other. ◦ This is seen in human blood types. There are 4 blood types in humans: type A, type B, type AB, and type O. These are phenotypes! Alleles for blood types in humans are represented with the letter I. ◦ I A represents A, I B represents B, and i represents O. Codominance is human blood types is phenotypically represented by type AB and genotypically represented by I A I B.
Polygenic traits are when traits are affected by more than 1 gene. ◦ Eye color, hair color & skin color are examples of polygenic traits. Multiple alleles are when there are more than 2 alleles per trait. ◦ Again human blood types are examples.
Pleiotropy is when 1 gene affects more than 1 trait. An example of this is sickle cell anemia or sickle cell disease. This affects the shape of red blood cells (RBCs). RBCs are normally round. ◦ In sickle cell anemia, they are crescent-moon shaped (sickle shaped). ◦ This blocks normal blood flow through blood vessels causing circulatory system damage, weakness, anemia, brain damage & other organ damage.
Chromosomal Theory of Inheritance Specific genes are located on specific chromosomes, or have loci
Genetic Linkage Genetic linkage (or linked genes) genes that are located on the same chromosome. ◦ Generally, these genes will be inherited together. The closer these genes are on a chromosome, the higher the chances are that they will be inherited together. Thomas Morgan worked with fruit flies (Drosophila melanogaster) and discovered linked genes.
Sex-Linked Traits Sex chromosomes determine the sex of the organism. In humans, XX is female; XY is male. Autosomes are non-sex chromosomes. Sex-linked traits are genes that are located on the X or Y chromosomes. There are more genes on the X than the Y.
Sex-linked Traits in Humans: Colorblindness is recessive and found on the X chromosome. ◦ This is when someone cannot see red or green. ◦ More males suffer from this than females. Hemophilia is recessive and X- linked also. ◦ This causes excessive bleeding and no normal blood clotting. ◦ More males suffer from this than females.
FYI: Environmental Effects: External & internal environmental conditions can affect genetic expression. Some examples: Environmental temperature affects the Himalayan rabbit’s fur coat & the western white butterfly’s wing coloration for flight. Soil acidity affects the color of hydrangeas (acidic=blue; neutral=pink) Japanese Goby fish changes sex in response to social environment
Nature vs Nuture: Study of identical twins that were separated at birth & brought up differently revealed that there are genetic links between individuals. The results of the studies revealed that these twins had similar likes, dislikes, opinions, etc.