Mendelian Genetics

Group 2: Mendelian Genetics

Mendelian Genetics

Mendel’s Experiment:

  • Genotype is the set of genes that it carries, while its phenotype is all of its observable characteristics (what the organism looks like)

    • Homozygous (has the same 2 genes: PP or pp)

    • Heterozygous (has one of each allele: Pp)

  • Crossed white flower and purple flowers: offspring was purple flowers

  • Original thought was that purple and white would make purple - believed in blending

  • However Mendel crossed the offspring of the purple flowers from the white and purple parents and got a 3:1 ratio of purple to white flowers, which didn’t align with the idea of blending


  • We know about the dominant and recessive genes because of Mendel

  • Purple would be dominant so PP, ad white would be recessive so pp, so the offspring would be Pp. Then the offspring that have the genes Pp there would be a 25% chance that their offspring would be pp which is white (explains the 3:1 ratio) 

  • You can use a punnett square, and then each gene gets one column

  • In a dominant trait, if there is just one dominant gene, then the dominant trait is what’s shown. For example, a flower with Pp would be purple because even though it has a little p which is a recessive gene, since there is a big P, which is the dominant gene, that means the purple will show

  • In order for the recessive to show, both genes have to be the little letter (pp)

  • Make a punnett square if unsure what the offspring possibilities could be


Mendel’s Laws

  • Law 1: Law of Segregation

    • Each individual has two copies of each gene called alleles

    • During reproduction, these alleles separate so that each parent contributes only one allele to their offspring

    • The offspring inherits one allele from each parent resulting in a new combination of alleles

    • Say a parent’s genotype is Pp, the law of segregation states then that means the offspring has a 50% chance of receiving the big P allele or the little p allele

  • Law 2: Independent assortment

    • Says that one gene for example the gene that causes hitchhiker’s thumb or the gene that causes an attached earlobe, the 2 traits don’t affect each other, they are independent

    • This is different from linked genes: for example people with red hair usually have freckles, this is because they are linked genes so if you have one you’ll most likely have the other 




One and two factor crosses

  • A one-factor cross is when you are only looking at one trait, for example flower color. A problem involving a one-factor cross would be to see what happens when you cross a red flower with a white flower; you would have to see if the offspring were red, white, or some mix


  • A two-factor cross is when you are looking at two traits at once, for example flower color (red or white) and petal shape (round or pointy). A two factor cross means a person is trying to figure out how both these traits are inherited at the same time. You are trying to find the possible combinations of flower (for example the crossing of a red flower with round petals with a white flower with pointy petals what will be the possibility of the offspring)

  • Also known as a dihybrid cross


Non-Mendelian Traits

  • They don’t follow the Mendelian “rule” that having the dominant allele means the dominant trait will show 

  • Some examples of non-mendelian traits are snapdragons, human height, and speckled chickens

  • Sex-linked traits and multiple alleles are also examples of non-mendelian traits 

  • NEVER ASSUME SOMETHING IS NON-MENDELIAN UNLESS GIVEN CLUES BY THE PROBLEM THAT IT IS



Incomplete dominance 

  • Snapdragons have incomplete dominance

  • There are 3 phenotypes, red, white, or something in between which is pink 

  • The pink color is an example of incomplete dominance, it means when the dominant allele is not completely expressed when the recessive allele is present, an in-between phenotype

  • So if the recessive allele is present even when the dominant allele is there, it doesn’t follow the Mendelian rule of the snapdragon just being completely red


  • So if you have two snapdragons with Rr, then you can get red, pink, or white

  • Some people like using completely different alleles for incomplete dominance. For example they might represent Rr as RW instead (there are more ways though)

Codominance

  • Speckled chickens have codominance

  • Codominance is when the alleles work together, when both traits show up,

  • For example if you have a black chicken (BB) and a white chicken (bb), their offspring (Bb) will be speckled black and white

  • For codominance some people like expressing it with exponents C^B x C^W or as BW (more ways though)


Pleiotropy

  • A type of genetic expression in which only ONE gene affects multiple traits

  • One example of pleiotropy is sickle cell anemia; the gene mutation that results in sickle-shaped red blood cells also leads to other affected traits (including anemia, blood clots, pain, organ damage, and high blood pressure)

  • A pleiotropic effect is when one gene impacts many different traits, a change to the gene that has this effect will lead to changes in multiple traits




Polygenic inheritance

  • One example of a polygenic trait is human height 

  • A polygenic trait is when not just one gene determines the trait; there are no set pair of alleles that code for a person’s height

  • Basically when many genes code for one trait

  • Instead it's a long code of different genes, and you still inherit one allele of each of the genes from each parent, and they all work together to influence your height 

  • Another example of a polygenic trait is skin color 

  • However both of these traits can be influenced by environmental factors as well such as nutrition (height) and how much time you spend in UV rays (skin color); however environmental factors don’t change the genetics 




Epistasis 

  • When one gene depends on another gene for it to be expressed 


  • For example in this scenario, BB or Bb means the llama is black while bb means the llama is brown

  • However if the llama doesn’t have a big C gene, then NONE of the pigments (black or brown) will not show up even though the llama has the gene in the first place; no big C they will be none of the colors - this is epistasis 

  • So in this cause the C gene is the gene that controls if the llama is black or brown or if its not 

  • To find the offspring you will have to make a 16 square Punnett Square





Sex-linked disorders

  • Sex-linked as related to genetics refers to characteristics (or traits) that are influenced by genes carried on the sex chromosomes

  • In humans that USUALLY refers to traits/disorders influenced by genes on the X chromosome as it contains many more genes than the Y chromosome which is smaller

  • This is why males are more likely to be affected by sex-linked disorders since they only have one X chromosome; if a female has one mutated and one non-mutated X chromosome it can cause different, milder, or no symptoms of a disorder (so a female requires 2 mutated X chromosomes in order to have the disorder while males only need 1)

  • So females usually are carriers of the disease (with no symptoms) 

  • Some sex linked disorders are:

    • Hemophilia: Disorder where blood doesn’t clot normally. That means if you get a cut, you might bleed for longer. People this this disorder bruise more easily and can have bleeding inside their joints or brain

    • Red-green color blindness: Condition where you can’t see the difference between red and green. Might see these colors as shades of gray or brown or mix it up with other colors

    • Duchenne muscular dystrophy: Condition that makes your muscles weaker over time. Mostly affects boys. Kids with this condition might have trouble walking, running, and doing other physical activities

    • Fragile X-syndrome: A condition that causes a range of developmental problems including learning disabilities and cognitive impairment. Usually males are more severely affected than females. Affected individuals usually have delayed development of speech and language skills. *note: sometimes not considered a sex-linked disorder since it’s not passed down like a regular sex-linked disorder not really sure